Considering the
Earth as an Open System
Charles Breiterman
J.D., M.A.
cbreiterman@gvpt.umd.edu
This interdisciplinary
article could not have been written without the assistance of:
Michael F. A’Hearn, Ken
Conca, Charles R. Greenwell, Brian F. Rauch, and Michael A. Salay.
Abstract
This review article synthesizes research from
several disciplines by conceiving of the Earth as an open system significantly
influenced by inputs of matter from outer space. It surveys the topics of mass extinctions, transpermia,
interplanetary dust and micrometeorites, delivery of complex pre-biotic
organics molecules from space, ice ages, small comets and panspermia. The evidence, if verified by continuing
investigations, shows that the Earth was an open system just after planet
formation. Since then, the Earth has
been significantly influenced by inputs of extraterrestrial matter, but the
inputs are not critical to the sustained functioning of the system.
© Charles Breiterman
(text and original figures only)
Contents
2. Impact Events
and Supernovae: Mass Extinctions of Life
2.1 Mass Extinctions and Supernovae
2.2 Galactic
Plane Hypothesis: Large Molecular Clouds
2.3 Spiral Arms
Hypothesis: Supernovae and Mass Extinctions
3. Transfer of Life-Bearing Impact Ejecta Between
Planets (Transpermia)
3.2 Outline of Evaluation of
the Hypothesis
3.3
Weaknesses and Unexplored areas of the Transpermia Hypothesis
4. Micrometeorites and Interplanetary Dust
5. Inputs of Extraterrestrial Matter: Effects on Climate and Life
5.1 Inputs of
Extraterrestrial Matter: The ice ages and the Earth’s climate
5.3 Recent
Effects on Biosphere
7. Possible
Influences of Interplanetary Dust and Small Comets on Climate
8. Panspermia
and the Open System
Appendix I: Detailed
Analysis of Small Comets Debate
1. Introduction
This review article synthesizes research from several disciplines by
conceiving of the Earth as an open system significantly influenced by inputs of
matter from outer space.[1] While this would not be a scientific
revolution on par with the Copernican, it would nonetheless be a fundamental
paradigm shift. A university level astronomy
textbook published in 1978 stated, “Asteroids and meteoroids are other
residents of our solar system. We shall
see how, along with the comets, they may prove to be storehouses of information
about the solar system’s origin.”[2] Treating asteroids as relics has been
forcefully revised by the well-publicized findings that an asteroid impact
triggered the extinction of the dinosaurs and cleared the way for mammals. In 1985, an eminent geophysicist commenting
on mass extinction wrote of asteroids, “A fraction of these are large enough to survive the trip through
the Earth’s atmosphere.”[3] But during the 1990s, research into
interplanetary dust and micrometeorites revealed that a highly relevant
question is whether the object is small
enough to survive the trip (see section 4).
The current dominant paradigm is expressed by a recent Earth System
Science text which explains that it covers:
1. The cycles of matter
(Earth is a closed system with respect to matter);
2. The
flows of energy (Earth is an open system with respect to energy); and
3. The
web of life (Earth is a networked system with respect to life.)[4]
Yet almost simultaneously, geology professor Csaba Detre was writing
that, "Today it is obvious that the main events in the Earth's history
have cosmic origin, because the Earth is not a closed system, but a mirror of
cosmic events, an inseparable part of the Universe."[5] And professors James and Jessie Miller,
pioneers of system science, were writing that, “The Earth is an open system,
interacting with its atmosphere and with matter and energy in space.”[6]
Textbook
thermodynamics informs us that an isolated
system is one with a boundary that allows neither energy nor matter to
pass. This is a concept that exists in
the ideal realm, but a well‑insulated thermos is an approximation. A closed
system is one that allows the exchange of energy, but does not allow matter
to pass. A rechargeable battery is an
example. An open system is one whose boundary passes both energy and matter.[7]
It
is fundamental that the Earth is at least a closed system. Its boundary might best be approximated as
the upper atmosphere and the exchange of energy across the boundary is
essential to life. The input of solar
energy in the form of visible light and infrared radiation[8] is
essential for photosynthesis and a habitable planet temperature. Without these the planet would feature a
severely hobbled biosphere, atmosphere and hydrosphere. The surface would be frozen, pitch-black,
lifeless, with almost no atmospheric circulation. Life might exist up to 10 kilometers below the surface[9]
and around deep ocean hydrothermal vents,[10]
which are the only places where water would flow freely, thawed by the Earth’s internal heat. In these places chemosynthetic life, which
derives energy from chemicals such as hydrogen sulfide, currently exists and
might even exist on a frozen planet Earth.
Conversely, the escape of excess infrared radiation prevents the planet
from becoming an oven like Venus.[11] Therefore both the input and output of
energy is essential and the Earth is at minimum a closed system.
Recent
thinking has pointed to the conclusion that perhaps the Earth is an open system
with respect to matter- a direct reversal of point 1 above. Inputs of extraterrestrial matter into the
terrestrial sphere may have made a continuing vital contribution throughout the
existence of the planet. This paper
will review and assess the following research areas: mass
extinctions of life, transfer of life-bearing impact ejecta between the Earth
and Mars (transpermia), interplanetary dust and micrometeorites, delivery of
complex pre-biotic organic molecules from space, ice ages, small comets and
panspermia.[12]
2. Impact Events and Supernovae: Mass Extinctions of Life
Some
of the most solid observational evidence supporting the concept that the Earth
is an open system comes from impacts of large comets and asteroids. Large impactors account for only 5% of the
mass of the extraterrestrial matter reaching Earth, yet they appear to have had
a decisive impact on the biosphere.
Aspects of this topic that are already well-covered in textbooks and the
science press will be treated here only as necessary to elucidate the open
system concept. Exploring the causes behind
asteroid and comet impacts will lead us into the potential role of supernovae
in mass extinctions.
The
Cretaceous-Tertiary (K-T) boundary impact of 65 Ma (million years ago) was
probably the decisive factor in the extinction of the dinosaurs, allowing the
rise of mammals and ultimately the evolution of humans.[13] Impacts have been implicated as the
triggering or a contributing factor in at least three other mass extinctions in
the past 500 million years.[14] One of these was the Eifelian-Givetian mass extinction
(~380 million years ago).[15]
Another was at the Permian-Triassic boundary (251 Ma). This was the most extensive mass extinction
known, killing 90% of all marine and 70% of all land life.[16] Finally came the Triassic-Jurassic boundary
(208 Ma),[17]
which is associated with the rise of the dinosaurs.[18] There are over 150 surviving impact craters
around the globe, several with diameters in the 80 km range.[19] An ongoing project using remote sensing
aims to uncover craters previously undetectable due to the erasing processes of
erosion, sedimentation, volcanism and plate tectonics.[20]
This
evidence establishes that the Earth system has been significantly influenced by
inputs of extraterrestrial matter.
Without the impacts, the biosphere would not have turned out the way it
has. For example, humans would probably
not exist.[21] Yet this evidence does not establish that
the planet is an open system in the way that a life form is, because the planet
would still be able to sustain itself without these impacts; in fact it would
still have a thriving surface biosphere.
2.1 Mass Extinctions and Supernovae
An
alternative hypothesis links mass extinctions to supernovae that occur in
Earth’s galactic neighborhood. It
involves the input of extraterrestrial matter in the form of particles from the
supernovae, and so will be analyzed here.
The
sun is about 2/3 of the way to the outer edge of the Milky Way galaxy, as shown
in Figure 1. The location is about
27,000 light years from the galactic center.
The solar system oscillates above and below the galactic plane with an
amplitude of about 114 light years and a period of about 62 million years.
Figure
1. The Milky Way viewed
from the side. The bright, dense core
is the galactic center. “ly” stands for
“light years.” The arrows indicating
the vertical oscillation are exaggerated due to the difficulty of rendering
such a small fraction of the horizontal radius (114 compared to 27,700). After
figure 12-6, Michael A. Seeds, Horizons, 6th edition, Pacific Grove:
Books/Cole, 2000). Illustration credit:
Nguyet Mai Vuong.
The
galaxy is structured like a disk with a spherical center (consider Figures 1
and 2 together). When an object is
inside the comparatively flat disk, it is said to be in the galactic
plane. As the solar system oscillates
above and below the galactic plane, the disk tide is operative. The disk tide refers to the gravitational
force of all the matter in the galactic disk influencing the sinusoidal motion
of the solar system.
2.2 Galactic Plane Hypothesis: Large Molecular Clouds
The galactic plane
hypothesis ties the vertical oscillation cycles (shown in Figure 1) to mass
extinctions of life on Earth. One
version of the hypothesis holds that as the solar system oscillates in and out
of the galactic disk, it encounters dense clouds of interstellar gas and dust
thought to be up to 1000 light years across with a mass of up to 1 million
suns. These are known as the large molecular clouds. These molecular clouds are thought to exert
a gravitational influence on the Oort cloud, dislodging some of the billions of
comets contained there.
Various attempts have
been made to correlate the timing of the oscillations to the mass extinctions.[22] One problem is that the geological record
(fossils and sediment) contains uncertainties, and so do our estimates of when
the solar system has encountered these molecular clouds. The double uncertainty makes finding
correlations a very tricky business. As
one example of attempts to correlate terrestrial mass extinctions with
extraterrestrial phenomena, we shall examine recent research into supernovae
and mass extinctions.
2.3 Spiral Arms Hypothesis: Supernovae and Mass Extinctions
The spiral arms
hypothesis ties mass extinctions on Earth to the solar system’s journeys
through the spiral arms of the Milky Way galaxy, illustrated in figure
6. Our solar system revolves around the
galactic center with an orbital period of about 240 million years. The spiral
arms are also rotating, in the same direction but more slowly. Therefore, the solar system catches up to
and moves through the spiral arms.[23]
Figure 2. The arrows from the solar system and the
spiral arm indicate direction of rotation.
The arrow from the solar system is longer to indicate its greater
velocity. The solar system is moving
approximately 68 km/sec faster than the spiral arms. ly= light years. After figure 1 J.H. Taylor and J.M. Cordes,
“Pulsar Distances and Galactic Distribution of Free Electrons,” The
Astrophysical Journal, 411 (1993): 674-684; figure 12-6, Michael A. Seeds, Horizons, 6th edition, Pacific Grove:
Books/Cole, 2000); figure 17-19 T.P. Snow and K. R. Brownsberger, Universe:
Origins and Evolution(Belmont, CA: Wadsworth, 1997). Illustration credit: Nguyet Mai Vuong.
It
takes the solar system 770 million years to make a complete passage through all
4 of the arms.[24] In the past 500 million years, the solar
system has passed through three of the spiral arms, and 3 of the 6 mass extinctions
during this time have occurred when the solar system was in the middle of a
spiral arm:[25]
Figure 3. After
figure 2 of Leitch and Vasisht (1998).
Top Panel: The labels over the curves are the names of
mass extinctions in the geologic record, giving the % of global species loss
after subtraction of the baseline extinction rate. For example, about 25% of marine species go extinct every million
years. [26] Bottom
Panel: The labels over the curves are the names of 3 of the galaxy’s
spiral arms. Traveling perpendicular
through a spiral arm, when entering the arm, the free electron density begins
to rise, peaking in the center of the arm and tapering off as one exits. Both sets of data, top panel and bottom
panel, contain uncertainties, so the apparent correlation must be treated with
caution.
What is a free electron? Ionization is the removal of an electron
from an atom or molecule, leaving a free electron and an ion. Thus a free electron is simply an electron
that has been separated from its parent atom or molecule and temporarily exists
independently in space. The density of
free electrons is relatively high in the spiral arms because the arms contain
many regions of ionized gas. One cause
of ionization in the spiral arms is the presence of many O and B type stars,
which radiate large amounts of ultraviolet radiation, which in turn is at the
right energy level to have a high probability of liberating electrons from
their atomic orbits.
It
is in the spiral arms where the majority of the galaxy’s supernovae are
located. Each time the solar system
passes through a spiral arm it passes within 10 parsecs (1 parsec = 3.26 light
years) of at least one supernova explosion.[27] At 10 parsecs from a supernova, the Earth would
be subjected to prolonged bombardment by high energy cosmic rays at 100 times
the average level. At 5 parsecs, cosmic
ray flux would be 1,600 times normal, but such a close passage has an extremely
low probability of occurring.[28]
The
relevant supernovae produced cosmic rays are protons, electrons and atomic
nuclei stripped of electrons (mainly helium nuclei known as alpha particles).[29] These particles are matter. The supernova explosion has accelerated the
particles to near light speed and given them energies from 106 electron
volts (eV) through 1016 eV.[30] To give substance to these abstract terms, a
cosmic ray particle with 1016 eV has the same kinetic energy as a
ping pong ball tossed at about 5.6 kilometers per hour.[31]
When
the cosmic rays reach the Earth they collide with molecules, atoms and other
particles in the atmosphere.[32] These collisions shatter the participants
into subatomic components, which in turn go through successive rounds of
collisions with other particles in the atmosphere, generating a particle
cascade. The original cosmic ray
particles lose energy with every round of collisions and the vast majority
eventually become harmless. This is how
the atmosphere “absorbs” cosmic rays.
The particle cascade
triggers an electromagnetic cascade. As
discussed in section 2.1, electromagnetic radiation is photons, which are not
matter. Photons are packets of energy
with properties of both waves and particles.
In certain wavelengths photons are visible light.
Table 1. Relevant Portion of the Electromagnetic
Spectrum
|
Name |
Energy per photon |
wavelength (λ) |
|
Infrared |
Approx .001-1.5 eV |
1 millimeter (10-3
meters) – 760 nanometers (10-9 meters) |
|
visible (red-violet) |
1.5-5 eV |
760-400 nanometers |
|
Ultraviolet |
5-125 eV |
400-10 nanometers |
|
x-ray |
125-250,000 eV |
10 nanometers -5x10-12 meters |
|
gamma ray |
250 KeV-100 MeV and higher |
5x10-12 meters
and smaller |
The electromagnetic
cascade is extremely complex and only a simplified portion of the process can
be described here. When a cosmic ray
smashes into an atmospheric particle, one of the common products is a subatomic
particle known as a neutral pion (p0). This particle is known to decay almost
instantaneously into two gamma rays (extremely high energy photons):
p0 → γ + γ
As
the gamma ray radiation (photons) travels deeper into the atmosphere, it loses
energy through various processes. For
example, the Compton Effect is when a photon interacts with a free electron.[33] The photon transfers part of its energy to
the free electron. As a result the
photon loses energy, but continues to move to lower altitudes. The photons become x-rays and then
ultraviolet light.[34] Ultraviolet light is still considered to be
energetic photons, because it has around 10 times the energy of visible light.
Ruderman (1974) and Ellis and Schramm (1995) describe how energetic photons produced by the electromagnetic cascade in the vicinity of the stratosphere would set off a series of atmospheric chemical reactions that would result in severe damage to the ozone layer.[35] The reaction begins when a nitrogen molecule absorbs a photon and breaks into two nitrogen atoms, one of which is in an excited state:
N2
+ energetic photon → N +N*
N*
+ O2 → NO + O
(reaction)
NO
+ O3 → NO2 + O2
Note
that the molecule of NO (nitric oxide) precipitates the reaction but is not
consumed by it, leaving it available for additional O3 destruction.
Ellis
and Schramm calculate that a supernova within 10 parsecs would precipitate such
a reaction, causing the destruction of 95% of the ozone layer. Effects for the biosphere could include
destruction of photosynthesizing organisms which are at the base of the food
chain, leading to global mass starvation of most species. Leitch and Vasisht (1998) present evidence,
(figure 3 above), which indicates that nearby supernovae are correlated with
mass extinctions of life. Of course
this tends to support Ellis and Schramm.
However,
Crutzen and Brühl [1996] point out that the reaction:
N
+ NO → N2 + O
Removes NO (nitric
oxide) from the atmosphere and significantly slows the destruction of 03
molecules. For a supernova at 10
parsecs, they estimate that ozone depletion would be only 50% at the poles and
about 15% in the tropics. This would
stress life in the poles, but have little effect on the tropical biosphere.[38] Benítez et
al. (2002) follow this and speak of a nearby supernova causing, “at most, a
minor extinction.”[39]
The
spiral arms hypothesis is speculative.
Leitch and Vasisht write, “The uncertainties involved in the above
analysis are admittedly quite large, and any quantitative comparison should
naturally be regarded with caution.”[40]
3. Transfer of Life-Bearing Impact Ejecta Between Planets (Transpermia)
By definition, a thermodynamically open system
requires input and output of matter.
Meteorites that originated as rocks from the moon and Mars have been
found on Earth.[41] Rocks of Earth origin are expected to be
found on Mars as we explore the planet.
The transpermia hypothesis suggests that microbial life can travel
between planets while protected inside rocks that have been blasted off the
origin planet by meteorite impacts.
While proving this theory is the work of generations of scientists, in
the past five years interdisciplinary groups have made significant
progress.
Figure 4. Illustrated Summary of Transpermia
The input and output of rocks containing viable
microorganisms would be significant because it could mean that life originated
on another planet (probably Mars) and then spread to Earth. Or early Earth life could have been
sterilized by a meteorite impact, and then the Earth could have experienced a
“second genesis” of Mars microbes.
Similarly life could have been transferred to Mars from Earth. While the comet and asteroid impacts covered
in section 2 show inputs of matter, this hypothesis would show both inputs and
outputs of matter, and that this matter can contain viable life forms. Thus, the biosphere could be open to inputs
and outputs of life. “Given the
possibility of exchange of life among the planets by large impacts, we may have
to regard the terrestrial [rocky] planets not as biologically isolated, but
rather as a single ecological system with components, like islands in the sea,
that occasionally communicate with one another.”[42] There could have been several transfers of
life since planet formation. The
transpermia hypothesis has implications for two of the Earth’s subsystems: the
biosphere and atmosphere. An input of
life is a significant effect on the biosphere, however life has also led to
radical changes in the atmosphere. For
example, the oxygen atmosphere of Earth exists only because of photosynthetic
lifeforms. Arguably life has led to
wholesale changes in the lithosphere as well.
For example, the famous White Cliffs of Dover are carbonate deposits-
the remnants of the shells of marine microorganisms (foraminifera).[43]
Coal is the compressed remnant of decayed plant matter. We shall analyze the transpermia hypothesis
in depth, because it is vitally relevant to the “open system” theme of this
manuscript.
3.1 Summary of the Hypothesis
The “spallation” hypothesis[44]
is the dominant explanation for the first stage of how rocks travel between
planets. When a large asteroid or comet[45]
strikes a rocky planet such as Earth or Mars, the impact generates interfering
shockwaves which cancel each other in a horizontal layer near the planet
surface, perhaps only tens of meters deep.[46] This region is called the “spall zone” and
it is located around the rim of the impact crater. The rocks in the spall zone are relatively unshocked and
unheated. Below the spall zone, in the
deeper sub-surface, the rock is subject to the full pressure of the impact
shockwave- enough pressure to pulverize rocks.
The result is a very steep pressure difference between the near
subsurface and the deep subsurface. The
definition of pressure is “force per unit area.” The rock in the spall zone is acted upon by the tremendous force
underneath it. Since F=ma, and the mass
of the spall zone rock is constant, the spall zone rock is subject to an
upwards acceleration. This acceleration
is so powerful that some of the rocks reach escape velocity.[47]
According to calculations based on the spallation
hypothesis, some 25 x 109 pieces of rock have been ejected from Mars
as a result of meteorite impacts during the last 3.8 billion years.[48] Once in space, the rocks are subject to the
physics of orbital and interplanetary travel, and their journey has been the
subject of computer simulation.[49] The results show that a small minority of
the rocks from Mars crash into the Earth after a period of from less than 1
year to 20 million years.[50] The majority of the ejected rocks are lost
by falling into the Sun or Jupiter, and through collisions with other
asteroids. For example, 5.0% of ejecta
from Mars land on Earth within 10 million years, 0.5% within 1 million years
and .01% of Earth ejecta land on Mars within 1 million years.[51] Fewer Earth ejecta land on Mars first
because due to Earth’s higher gravity and thick atmosphere, it is hard for
ejected rocks to escape the planet (many re-accrete to Earth), second Mars is a
smaller target with a weaker gravitational field, and therefore sweeps up fewer
passing objects.[52] Given the large initial numbers of ejecta,
since the end of the heavy bombardment (about 4 billion years ago) some 250,000
pieces of rock have traveled from Mars to Earth in less than 1 million years. Even now, several meteorites from Mars land
on Earth each year.[53]
Microbiologists have reported that there exist
endolithic microorganisms, that is, microbes that live inside rocks.[54] Thus, Earth rocks that land on Mars may have
bacteria in them, and if there is life on
Mars, there could be bacteria inside Mars rocks that land on Earth. Two questions are, can microbes survive the
ejection from the planet and can they survive the interplanetary journey?
Microorganisms can probably survive ejection
from the origin planet and impact on the host planet. Rock from the spall zone is subject neither to significant shock
nor heating, as discussed above. Rocks
that achieve escape velocity are subject to a sudden, extreme acceleration. However, experiments show that a significant
percentage of microbes survive.
Experiments also show that a significant fraction of microbes inside
rocks could survive the shock of impact on the host planet.[55]
On Earth bacteria have been revived after being
dormant 40 to 250 million years.[56] Therefore, it is quite reasonable to explore
the idea that some of the bacteria inside the ejected rocks should survive the
interplanetary journey, particularly if it is less than 1 million years.
While in space, the rocks containing bacteria
would be exposed to several harmful factors including vacuum, extreme cold,
cosmic rays, solar ultraviolet radiation and the natural radioactivity of
rocks. While the radioactivity of the
rocks inhabited by endolithic bacteria is not an issue on Earth where bacteria
live and die in relatively short lifetimes, in becomes an issue where a single
generation of bacteria is attempting to survive in dormant state for perhaps
millions of years.
Two species of bacteria have been studied to
determine the survival rates for an interplanetary journey inside a rock. Bacillus
subtilis is representative of the class of bacteria that form
endospores. In response to
environmental stresses such as lack of nutrients or lack of water, some
bacteria go into a dormant state by transforming into an endospore. The endospore is a protective cocoon in
which the cell DNA and major components are located in the core underneath
multiple layers of protective coatings.
Endospores are known to survive total desiccation (dryness), freezing,
boiling, vacuum and radiation.[57] Deinococcus
radiodurans boasts a combination of the highest resistance to radiation
exposure of any known life-form and the ability to enter into a state known as
anhydrobiosis (“life without water”).
Anhydrobiosis is a biochemical survival strategy that is a response to
extreme desiccation in the organism’s habitat. The organism enters into a shriveled, dormant state in which it
can remain for hundreds of years, yet quickly revive once exposed to water. The sugar trehalose is associated with
anhydrobiosis and appears to replace the normal water matrix of the cell
components.[58]
One criticism of these transpermia studies is
that the researchers have chosen an easy case to prove their point that
bacteria could survive the journey. B. Subtilis and D. Radiodurans are extremely survivable bacteria. There is no guarantee that such bacteria
will be in the ejected rocks. Should we
not revise downward any survival probability they compute based on these hardy
microorganisms?
Taking into account all the danger factors, the
most comprehensive study to date systematically calculated the survival rates
for bacteria on an interplanetary journey.
For example, if Deinococcus
radiodurans bacteria were inside the rock, on average 13% would survive a
100,000 year journey and 2% would survive a 330,000 year journey.[59] Put another way, in a piece of ejecta rock
1.1 meter in diameter, it would take 1.4 million years before only 1 in a
million Deinococcus radiodurans
microbes were still viable.[60] This survival rate is thought to be
acceptable, since there are between 1 billion to 10 million bacteria per gram
of Earth surface soil.[61] Mileikowsky et al. admit that there is “much less [bacteria per gram] in, for
example, rocks.”[62]
It is a probability argument: given the vast
tonnage of rocks that have been transferred between planets, and given the
numbers of bacteria living per gram in the rocks, the probability is that some
of the bacteria did survive the interplanetary journey at least once. A 10 person team of scientists, each having
an established reputation in their specialty, concluded that Mars to Earth
transfer of viable bacteria inside meteorites is “not only possible but highly
probable,” if bacterial life has ever
existed on Mars and that Earth to Mars transfer “is also possible but at a
much lower frequency.”[63] One noted microbiologist, Imre Friedman,
concluded that given the large numbers of asteroid impacts on the Earth and
Mars during the history of life (approximately 3.7 billion years), viable
bacteria have probably been transferred at least once.[64]
3.2 Outline of Evaluation of the Hypothesis
There are several areas of the transpermia hypothesis which are currently “weak links” in the chain of reasoning. We shall analyze these carefully since the credibility of the hypothesis is important to the theme of the essay. The current state of research suggests that Mars experienced a significant shift as late as 3.1 billion years ago from an early temperate climate with liquid water on the planet surface to the current cold, dry Mars. Therefore, we shall consider whether there could have been a transfer of life for two distinct time periods: (1) between Mars and Earth prior to 3.1 billion years ago, and (2) between Mars and Earth from 3.1 billion years ago through the present.
Table 2
3.3 Current Weaknesses of the Transpermia Hypothesis
First, as discussed, not all microorganisms are
as hardy as B. Subtilis and D. radiodurans. Second, the concept of “transfer of viable
microbes” is incomplete, as it is addressed in the most comprehensive study to
date, Mileikowsky et al. (2000). It would be trivial if a transfer meant
dormant microbes sitting uselessly in a rock on the host planet surface. “Transfer” must mean the establishment of
active foreign microorganisms on the host planet, even if only briefly. The net
result of the seven weak links in the transpermia hypothesis is that current
articles overestimate the fraction of microbes that would survive
interplanetary transfer, as illustrated in Table 3.
Table 3.
No x (survival fraction f1) x (f2) ...
(f6) x (f7) x (f8) x (f9)
x (f10) x (f11) x (f12) x (f13) =
Nf
No = the original number of
microbes in a gram of rock ejected from origin planet.
Nf = the final number of
microbes per gram of rock that survive to engage in actual colonization the
host planet.
This article adds seven new probability
factors, here highlighted in blue. Only a fraction of the microbes survive each
factor. This drastically reduces the
final number of microbes that survive to colonize the host planet.
For example, assume N0=10,000,000
bacteria. If 30% survive each hazard of
the transfer between planets, then Mileikowsky et al. would calculate Nf
as 107 x .36= 107 x 0.000729=7,290 surviving
bacteria per gram of rock that lands on the host planet.
This article suggests the Nf would be 107 x .313
= 107 x 0.0000001594323= 1.6
surviving bacteria per gram of rock.
After Tables VIIIa and VIIIb in
Mileikowsky et al. (2000).
Third, Mileikowsky et al. count on a high number (N0) of bacteria in
ejecta rock, yet the reality may be different.
Hazen et al.[65]
took samples of sedimentary rock as deep as 550 meters below the surface near
Aiken, South Carolina. They found a
range of bacteria densities from less than 1000 to a maximum of 40 million
viable bacteria per gram of rock.[66] Lehman et
al.[67]
drilled a 120 meter deep core into a rocky aquifer near Kingman, Arizona. They found that the concentrations of
bacteria attached to the rock were only on the order of 10,000 per gram of
rock. The density of bacteria
populations in near subsurface rock has not been systematically studied, but it
seems clear that it varies with location.
The fact that a meteorite impact may by chance occur in an area where
there is a low density of bacteria per gram of rock reduces the probability of
the transfer of viable bacteria between planets. If 2% of the endolithic bacteria survive a 330,000 year journey,
that is as little as 200 or as many as 800,000 viable bacteria per gram of rock
landing on the target planet.
A fourth obstacle to the transpermia process is
that once a rock containing viable microorganisms lands on either Mars or
Earth, then the passengers must exfiltrate the rock. As mentioned, it would be a mere curiosity if viable bacteria
were to survive interplanetary voyages only to sit uselessly inside a rock on
the target planet.
While
it has not been covered in refereed journal articles, whether the
microorganisms can get out of the rock probably depends on whether the rock
will ever be immersed in liquid water.
The sustained presence of water would allow the microbes to seep or be
flushed out of the rock.
In
order to describe how there might be successful microorganism exfiltration from
a rock on Mars’ surface at any time from about 3.1 billion years ago through
the present, we must understand the environmental conditions on the
planet. The surface temperature ranges
from a maximum of 25° C (77° F) at the equator to a minimum of –125° C (-193°
F) at the South Pole during midwinter.
Mars has an average atmospheric pressure of approximately 6 millibars,
(roughly 6/1000 of Earth’s atmospheric pressure at sea level). This means that liquid water on the surface
sublimates to vapor very rapidly at most locations on the planet. The atmosphere is 95.3% CO2, 2.7%
Nitrogen, 1.6% argon, with all other gases including O2 and H2O
making up the remaining .4%.[68] There is enough water on Mars to cover the
globe with an ocean that is 3 meters deep by the low estimate,[69]
and 0.5-1 kilometer deep by the mainstream estimate.[70] For comparison, Earth’s water would cover
our globe to a depth of about 2.7 kilometers.
Almost all Mars’ water is in the subsurface and frozen at the polar ice
caps, which are composed of both CO2 ice and H2O
ice. At any given time, there is only
enough water vapor in the Mars atmosphere to fill a pond.[71] Instruments of the Mars 2001 Odyssey orbiter
have found frozen water in the near subsurface- between 1 foot and 1 meter
deep. In the belt between the
lattitudes 60-75°, the instruments detected twice the volume of water in Lake
Michigan. The regolith (soil) in this
region is frozen water mixed with rock- up to 50% H2O.[72] The depth of the frozen water below the
surface appears to depend upon lattitude.
The closer to the equator, the warmer the climate. With no permafrost in the warmer regions,
the water would find itself in liquid state and would sublimate into the
atmosphere as vapor. Therefore, it is
thought that the soil in the tropical regions of Mars (within 30° lattitude of
the equator) is dessicated to a depth of tens or hundreds of meters.[73]
Figure 5. Illustration of Mars Odyssey Findings
Photo courtesy NASA: http://photojournal.jpl.nasa.gov/catalog/PIA03803
With this background, we can now discuss
specific mechanisms by which microorganisms could exfiltrate rocks that land on
Mars. Several of the Mars meteorites
recovered on Earth show evidence of chemical changes brought about by the
circulation of liquid water through their interiors.[74] We have a high degree of confidence that
prolonged exposure to water did not happen on Earth because the falls of these
particular meteorites were observed and they were recovered quickly.[75] The oldest of these meteorites is 1.3
billion years old.[76] The
foregoing means there was liquid water flowing for a prolonged time on Mars
within 1.3 billion years, and it also means the phenomenon is widespread on the
planet, because these SNC meteorites just happened to be blasted off a random
location of Mars by a random impact. We
have some observations that could explain how the SNC meteorites were exposed
to water on Mars, and how microbes could exfiltrate a meteorite once it landed
on Mars.
The Mars Pathfinder (1997) landed in the Chryse
Planitia region in a flood channel known as Ares Vallis. The mainstream view among Mars specialists
is that such channels were formed by multiple episodes of large floods of
liquid water. Flood channels on Mars
are up to 2000 km long and 200 km wide.[77] Available information on Ares Vallis is
that it is 25 km long and 1 km deep.[78]
Figure 6. Ares Vallis: Flood Channel Imaged By Viking Orbiter
Photo courtesy NASA:
http://history.nasa.gov/SP-441/p36.htm
The geophysical process that caused the floods
is unclear. It may be that the gradual
rise of the Tharsis Plateau caused groundwater to flow downward into the Valles
Marineris, which is collapsed terrain to the south of the landing site. Tharsis is a volcanic region, so otherwise
frozen groundwater could have been thawed by geothermal heat. Stresses on the underground acquifer would
build up over tens of millions of years, leading to a flood surge that relieved
the stress. There may have been
multiple flooding episodes.[79]
Figure 7. Ares Vallis: Pathfinder Landing Site as seen from the Lander
Camera
“The effects of the floods
that carved Ares Vallis are seen everywhere at the site. ... As with the rocks found in terrestrial
streambeds, some rocks at this site appear to be aligned in the direction of
the flow down Ares Vallis. These rocks
may have originated elsewhere (upstream) and were washed to their current
location by the flood. Not all the
rocks are aligned. Some rocks are
randomly distributed around this site and ... are most likely derived from the
nearby impact craters and are presumably the same composition as the
plains.” (Boyce, Smithsonian Book of
Mars, at 136-7).
Photo courtesy NASA:
http://photojournal.jpl.nasa.gov/catalog/PIA02406
If one of the rocks in the image above happened to be a fragment of a meteorite of Earth origin that by chance fell into this plain on Mars, then the repeated floods would provide an opportunity for the rock to be immersed in water, which would allow the bacteria to escape. The rock might take 1 million years to get to Mars, then sit on the dry surface of Mars for another 1 million years. When the flood comes and the rock is flushed with water, the dormant microorganisms might have their chance to get out and into a new environment. But, the microbes would not be safe yet.
A fifth unexplored obstacle to transpermia is that on the Mars of any time in approximately the past 3.1 billion years, the microbes must somehow get underground for numerous reasons: (1) the surface of Mars is exposed to intense solar UV radiation since Mars has no ozone layer;[80] (2) the surface is exposed to solar wind particles since Mars has no magnetic field;[81] (3) the surface of Mars is bombarded by cosmic rays since the Mars atmosphere is not dense enough to stop them;[82] and (4) the surface soil (called the regolith) may be destructive of organic molecules[83] and hence destructive of living organisms.[84] Mainstream researchers arrive at this last point because the Viking landers sampled the surface soil of Mars to a depth of 10 cm and did not detect any organic molecules.[85]
Yet the flood channels may provide an opportunity for the microbes to get underground. The Ares Vallis flood channels where Pathfinder landed “can be traced on the surface for a short distance into the basin, where they disappear under the lava plains. Remarkably, these buried channels are so large that their gravity signature can be traced for several hundred kilometers farther into the basin from where they disappear under the flood plains.” [italics added][86] So if a meteorite from Earth landed in a flood channel such as Ares Vallis, the rock would have a chance of being immersed in water during one of the periodic floods. Dormant microbes in the rock would have a chance of being flushed out of the rock into a water medium, and being swept under the lava plains. Once the microbes are under the lava plains, they are protected by rock from the UV radiation, solar wind particles and cosmic rays.
If the Earth microorganisms make it under the lava plains, it leaves only one problem: would the regolith under the lava plains still be lethal to microorganisms from Earth? According to the mainstream approach to the lack of organic molecules in the Martain regolith, the answer appears to be that they might be safe, although more data is needed from upcoming Mars missions.
The mainstream view begins with the reaction
between the sun and atmosphere. On
Mars, solar UV radiation is not blocked by an ozone layer and therefore is
quite strong. UV radiation breaks
apart atmospheric H2O molecules to form H and OH radicals. The OH radicals combine with each other to
form H2O2 (hydrogen peroxide) and other oxides, such as
MnO2 (manganese peroxide) and TiO2 (Titanium dioxide),[87]
“some of [which] are a death sentence for organic molecules.”[88] Hydrogen peroxide is destructive of living
tissue and microorganisms. For example,
if you use hydrogen peroxide to disinfect a wound, it kills both the germs and
the tissue near the wound. The hydrogen
peroxide is thought to condense every night on the Martian surface, and be
broken down every day by sunlight.[89] However, the concentration of H2O2
on the Martian surface is estimated to be at most 250 ppm. There exist soil bacteria from Earth that
can survive 30,000 ppm.[90] Also, the H2O2 would
be highly diluted by the flood. And
note that since OH radicals are formed by UV radiation and have a lifetime of
only about 1 second, the process of H2O2 formation would
not continue under the lava plains. MnO2
is found in soils on Earth, which have bacteria concentrations up to a billion
per gram of soil; hence it is tolerated by Earth microorganisms. At any rate,
the authors of the study proposing the existence of MnO2 in the Mars
regolith never argue that MnO2 would destroy organics; only that MnO2,
in the presence of UV radiation and water vapor, participates in a reaction
that releases O2 in the amounts seen by the Viking Gas Exchange
Experiment.[91] No exposure to UV would occur under the lava
plains. TiO2 is a catalyst
in the breakdown of organic compounds to inorganic ones, but only in the
presence of UV radiation.[92] The mineral feroxyhyte (δ-FeOOH) has
been suggested as a possible explanation for the lack of organics, and indeed
it may be present in quantity in the regolith.[93] However, feroxyhyte is only weakly
destructive of organic matter.
Finally, Mills (1977) suggested that friction between sand grains in
Martian dust storms would generate electrostatic glow discharge. Glow discharge is known to destroy organic
molecules and kill microorganisms. The
frequency of dust storms on Mars indicates that glow discharge would
periodically sterilize the surface.[94] However, Zent and McKay (1993) rejected this
hypothesis and electrostatic discharge would not penetrate the rock shield
provided by the lava plains. Under the
lava plains, it appears that the microorganisms would be safe from the adverse
conditions that exist on the surface of Mars, although data from future Mars
missions is needed.[95]
There is a second process by which microbial
arrivals to Mars might exfiltrate the ejecta rock and get underground. From the surface floods and near surface
groundwater, water sublimates into the atmosphere and is deposited at the polar
caps as frost. Over time, the thickness
of the frost at the caps grows to the point where its weight causes melting and
sustained water running off the edges of the ice caps.[96] If a meteorite of Earth origin landed near
the edge of the ice cap, it would have a chance of being immersed in this
meltwater from the polar caps. There is
only a probability of this happening, but the whole transpermia argument is a
probability argument. Dr. Steve
Clifford of the Lunar and Planetary Institute proposes that this meltwater
“soaks into the ground, migrating downward and towards lower lattitudes,
recharging the groundwater system” all over the planet. This is a global
hydrological cycle.[97] If the microbes can “soak into the ground”
with the water, they can get into that hypothetical global subsurface liquid
water aquifer and colonize the planet.
The hypothesis continues that the upper few kilometers of the planet
consists of highly fractured rocks (called megaregolith) which contains a
tremendous amount of interconnected pore space which holds liquid water.[98] It is this tremendous amount of liquid water
that sometimes causes the massive floods such as at Ares Vallis. However, we must note that since Mars has
only a mean global temperature of -58° C (-72° F), most of the surface is
permafrost to a depth of 1 km or more.[99] So the global hydrological cycle which
Clifford proposes for the most part exists in the region at or below 1 km deep. However there may be an exception in surface
areas of the planet warmed by geothermal heat due to Mars’ molten core and
continuing volcanic activity.[100] The near subsurface (tens of meters deep and
lower) in these areas may contain liquid water and may be connected to the
hypothesized global hydrological cycle.
Microorganisms have been recovered at analogous locations in Antarctica.[101]
A sixth problem currently facing the transpermia
hypothesis is that the bacteria that can survive the journey through space may
not be the same as the bacteria that can survive on the target planet.[102] Deinoccocus
Radiodurans and Bacillus Subtilis
are the bacteria that researchers have demonstrated could survive the
journey. Yet
these organisms are probably not capable of surviving on Mars. D.
radiodurans is “strictly aerobic”[103]- it
requires oxygen to survive. This alone
probably rules out its survival on Mars, since the atmosphere contains less
than 1% O2. B. Subtilis is capable of anaerobic
respiration (utilizing CO2 in this case).[104],[105] Its survival on Mars is therefore not ruled
out on this basis.
On Mars, if any life exists, it is
thought to constitute a quite limited, primitive ecosystem, probably
underground, utilizing volcanic heat and the water thought to exist tens of
meters or more below the surface all over the planet, as discussed above. The life that we might find would literally
live off the CO2 in the atmosphere and the minerals that compose the
planet’s crust.
Unfortunately, D. radiodurans and B. Subtilis
both require organic materials that have been produced by other microorganisms
and life forms. D. radiodurans has been found in rotting meat, animal feces, cattle
hairs and skin, creek water, house dust, used towels and underwear, air from
‘clean room’ laboratories, among other places. This bacterium “require[s] complex media for growth … rich
organic environments …”[106] The native habitat of Bacillus Subtilis is thought to be soil, “B. subtilis is abundant in soil and is probably transferred from
soil to other associated environments.”[107] We are talking about Earth soil, which is
usually extremely rich in organic nutrients, and not Mars soil, which may be
completely devoid of organics (see above).
Both D. radiodurans and B. subtilis require a somewhat developed
ecosystem because both are organotrophic
and heterotrophic.[108] They derive energy from organic molecules
(such as acetate, glucose and tryptophan) and they derive carbon[109]
from organic molecules that have been in turn produced by other organisms. Thus they are not likely candidates for the
very limited ecosystem that might exist on Mars.
Additionally, the reader will note
that neither D. radiodurans nor B. Subtilis is commonly found in
endolithic habitats. The transfer of
impact ejecta hypothesis calls for rocks to be ejected by meteorite impact and
for bacteria to be in these rocks.
True, since some soil is most likely blown by the wind or tracked by
animals onto rocks and some of that penetrates into cracks and fissures in
rocks, some B. subtilis spores or
live bacteria should be found in rocks.
But it is not the high density that would optimize the probability of
successful transfer of life. As for D. radiodurans, a close relative, D. radiopugnans, was found living about
3 millimeters into the surface of rocks in Antarctica.[110] However, that is far too shallow for optimal
protection during interplanetary transfer.
And, these bacteria were living in a symbiotic relationship with lichen. They were either the decomposers of this
small ecosystem- they lived off the dead and waste matter of the fungi and
algae- or they lived off organic matter produced by the other organisms. Therefore, once again Deinococcaceae needed a rich organic environment to survive. Such an environment is unlikely to exist on
Mars. In summary, based on their known
profiles, the microorganisms that Mileikowsky et al. rely upon are not likely to be present in the ejecta rock,
and once they arrive on Mars, they would not be likely to survive.
A bacterium such as Acidithiobacillus ferrooxidans is much
more likely to survive on Mars. A. ferrooxidans is capable of both
aerobic and anaerobic respiration. In
anaerobic mode, it can derive energy from various sources. It can use elemental sulfur (S0)
as a source of electrons, with ferric iron (Fe3+) as the terminal
electron receptor.[111] It can also use diatomic hydrogen (H2)
as an electron source, with either S0 or Fe3+ as terminal
electron receptor.[112] All of these modes of respiration are
relevant to possible life on Mars.
Current data suggest that the crust and soil of Mars are rich in iron
and sulfur,[113]
so food sources for the S0/ Fe3+ respiration pathway may
be there.[114] And some researchers believe that life in
the Martian subsurface might utilize H2 as an electron source, with
CO2 as the terminal electron acceptor.[115] The H2 would be dissolved in
water, otherwise due to Mars’ low gravity it would tend to escape into
space. Most Earth microorganisms spend
most of their lives in aqueous environments where they would have access to
dissolved H2 and CO2.
Additionally, A. ferrooxidans
is autotrophic. This means that it can
derive carbon directly from CO2 and fix that carbon into the organic
molecules it needs to carry on its life processes. (This is known as carbon fixation.) As such, it would be a primary producer of a microbial ecosystem-
it is a microbe that other microbes depend upon to produce the simple organic
molecules that they need to synthesize more complex ones- such as enzymes,
amino acids and proteins.[116] A.
ferrooxidans, then, is the type of organism that might be able to survive
in whatever limited Martian biosphere may exist. It would live off the materials in the planet’s crust, in
subsurface environments made temperate by Mars’ geothermal heat,[117]
in water made liquid by that heat, and be able to fix carbon from the abundant
Martian CO2 (the CO2 would be dissolved in water). A.
ferrooxidans is a mesophile, growing at temperatures of 20-45° Celsius.
Figure 8. Left:
Acidithiobacillus Ferrooxidans magnified 30,000 times showing internal
structure. Courtesy of Henry Lutz
Ehrlich from his textbook Geomicrobiology,
2nd edition, (New York: Marcel Dekker, 1990).
Right: A.
Ferrooxidans magnified 5,000 times, exterior view. Courtesy of www.koreaearth.net.
Acidithiobacillus
ferrooxidans may be an endolithic bacterium and hence could be present in
rocks that are blasted off the planet by a meteorite impact. A.
ferrooxidans is found on Earth in the drainage from mining operations. Research seems to have focused on the
remarkable metabolism of this bacterium rather than its natural habitat. Yet the fact that it is found near mines-
areas where humans drilled into rock- suggests that it may live inside rock.[118] And the fact that it derives energy from
materials in rocks- sulfur and iron- is another indication that it may be an
endolithic bacterium.
Once on Mars, A. ferrooxidans would eventually deplete that planet’s supply of
nutrients unless it could latch onto some geochemical cycle that replenishes
these nutrients. In other words, if it
were using Fe3+ as the terminal electron acceptor, eventually there
would only be ferrous iron (Fe2+) left on the planet. Fenchel, King and Blackburn (1998) propose
an abiotic, light-driven cycle whereby Fe2+ could be transformed to
Fe3+.[119] A similar cycle would need to be in place
for the other food sources. Once
microorganisms become involved in the cycle, it becomes a biogeochemical cycle, and life is on course to transform the planet
as life has done on Earth. One should
note that this sort of thing has probably happened before, because the first
Earth life forms had to find a way to replenish their food supply. Today the most prominent cycle is when
photosynthetic organisms consume CO2 and generate O2,
while aerobic respirers consume O2 and generate CO2.
Unfortunately, A. ferrooxidans is much less likely to survive the journey through
space to Mars. It is neither spore
forming nor capable of anhydrobiosis.
The quandary is that the microorganism which can both survive the
journey through space and survive on Mars may now be unknown to science. This is not fatal to the transpermia
hypothesis. As a rule of thumb,
microbiologists reckon that science has only identified 0.1% of the microorganisms
existing on Earth.[120] Yet even if one or several species of
microbe exists that can survive during the journey through space and then on
Mars, what is the probability that this or these species is present in the
rocks that happen to be ejected by a meteorite? This is another reason why the
probability of successful transfer during the past 3.5 Ga is much lower than
Mileikowsky et al. conclude.
The above evaluation of current weaknesses of
the transpermia hypothesis has been applied to Mars from 3.1 billion years ago
through the present. On Earth from 3.1
billion years ago through the present, successful exfiltration from the
meteorite would be rather easy since there has been abundant surface water. A rock from Mars landing on Earth has high
probability of finding itself in a river, lake or ocean, or experiencing
rainwater relatively quickly. This will
provide an opportunity for any microorganisms inside the rock to be exposed to
water and seep out or be flushed out of the rock. At first glance, there would be no need to get underground
since the surface of the planet has been hospitable to life.[121] If it were necessary to get underground,
streams often flow into caves or become subterranean.
Even if hypothetical bacteria from Mars survived
a journey to Earth and exfiltrated the rock, the challenge of survival on Earth
would remain. Earth has had an oxygen
atmosphere for the past 2.5 billion years.
Any Mars bacteria, which have existed only in a CO2
atmosphere, might find oxygen toxic. Second,
any Mars bacteria would be arriving at a planet already teaming with life that
has exploited every available niche and is superbly adapted to its
environment. Would Mars bacteria that
arrive on Earth be able to survive against fierce competition from native Earth
microorganisms? The reality might be
the exact contrary of the “War of the Worlds” scenario. There is still a chance of lateral gene
transfer. Lateral (or Horizontal) gene
transfer (LGT) is the exchange of genetic material among microorganisms. If the Mars microbes die on Earth and they
use DNA to transmit their genetic information, then their DNA would be in the
environment and possibly able to be absorbed by Earth bacteria in a process
known as transformation. Transformation
occurs when an organism, usually bacteria, absorbs and incorporates free
floating genetic material from the environment it inhabits.[122] This scenario would mean that genetic
information from Mars could be incorporated into Earth life forms.
Having considered exfiltration and refuge
underground for the Mars and Earth from 3.1 billion years ago through the
present, we now turn to scenarios for the early Mars and Earth. Exfiltration of the rock landing on Mars
seems more probable during the early period of Mars’ history, up to perhaps as
late as 3.1 billion years ago, when there was significant surface water.[123]
An Earth rock arriving on Mars of that period had a probability of landing in
liquid water or crashing through the surface of an ice covered lake to the
liquid interior. The presence of liquid
water would allow the microorganisms to seep out or be flushed out of the
rock.
Mileikowsky
et al. make use of an established line of research suggesting that the
early Mars, circa 3.8 billion years ago, had an atmosphere composed mainly of
CO2 with about 3 bars of atmospheric pressure at the surface
level. Such a relatively thick CO2
atmosphere would have caused a greenhouse effect which would have raised Mars’
mean global temperature higher than the freezing point of water (273 K)-
probably to 280 K.[124] According to Mileikowksy et al., there are distinct signs that during this epoch there were
rivers, lakes and perhaps even an ocean on Mars.[125]
Gradually, the CO2 would have been
removed from the atmosphere, mainly by the geochemical weathering of rocks.[126] The era of liquid water on the surface of
Mars would have continued until perhaps 3.1 billion years ago, at which time
the CO2 pressure would have been about .5 atm and the temperature
would have been almost always below the freezing point of water. Still, at 3.1 billion years ago, the mean
global temperature would still have been far higher than today’s value of -58°
C (-72° F).[127]
During this same time on Earth, Mileikowsky at al. assert, atmospheric
conditions may have been similar. They
appeal to research suggesting that circa 3.8 billion years ago the Earth
atmosphere was mostly CO2 and CO, with 5-10 bars of pressure and a
surface temp of about 85° C.[128]
Mileikowsky
et al. note that since conditions on Earth and Mars were similar circa 3.8
Ga, this was the time when life from either planet would have had the best
chance of successfully establishing itself on the other. This period was also a time the amount of
impacts was relatively high, leading to a relatively high number of ejecta
rocks. Taking these two factors
together, Mileikowsky et al. imply
that circa 3.8 Ga was the time when there would have been the greatest chance
of “two-way microbial traffic”[129] Would the travelers survive? Life from either world would have been able
to respire anaerobically on the other.
Moving from an atmosphere of 3 bars to one of 5-10 bars, or vice-versa,
probably would not be a serious obstacle for many species of microbes. “One may speculate that a good proportion of
the organisms will survive” the change in atmospheric pressure, write
Mileikowsky et al..[130]
Yet there are some problems with the
argument. First, the atmospheric
composition they propose for the early Earth and Mars is based on peer-reviewed
articles, but the area of research is unsettled. Second, there is not quite proof that life emerged on Earth by
3.8 billion years ago. The earliest
verified evidence of life is 3.77 billion year old carbon signatures in rock
from Isua, West Greenland.[131] This does not mean life did not exist
earlier. Because Earth has such active
geological processes, it is extremely difficult to find Earth rocks that have
not been altered to an extant that erased any biological signatures in the
rock. On Mars, the meteorite ALH84001
offers only suggestive evidence that life existed 3.8 billion years ago on that
planet. However, if life ever did
emerge on Mars, it could have emerged earlier than on Earth. The emergence of life on Earth is thought
to have been delayed because the early Earth of 4.5-4.0 Ga was subject to a
number of devastating large meteor impacts which would have vapourized surface
water and ripped off the early atmosphere.
Mars may have been spared the most violent impacts because due to its
smaller size impact velocity would be lower.[132]
Third, the temperature difference between the
planets poses a big problem. The
difference between a temperature of near 0°C on Mars and 85° C on Earth would
be devastating for any known microorganism.
The exposure of microorganisms to drastic and rapid temperature change
is known as heat shock and cold shock, respectively. The dominant microbes on Mars would have been psychrophiles,
which grow optimally at 0-20° C. The
common microbes on Earth meanwhile, would have been hyperthermophiles, which
grow optimally at 80-113° C. For the
Mars to Earth scenario, Dr. Alberto Macario, one of the world’s leading
authorities on heat-shock, noted that there is no specific data on how
microorganisms respond to such a drastic temperature shock. “One may speculate that a jump from 0 to 80°
C may be too much for most, if not all species with an optimal growth
temperature near 0° C ... they will not survive.”[133] For the Earth to Mars, scenario, Dr.
Richard Cavicchioli, one of the leading authorities on cold shock response,
started with the assumption that there are perhaps 107 endolithic
microbes per gram of meteoritic rock that lands on Mars. In an e-mail he wrote that given the large
initial number of cells, “I expect some would survive.”[134] In order to provide more data, an experiment
subjecting various species to cold and heat shocks of the required magnitude
would be useful. Simply subjecting
microbes to temperature shocks to see how many survive seems like a high school
science experiment, yet it is necessary for competent investigators to acquire
this data in order to evaluate the transpermia hypothesis. Note that given the reproductive rapidity of
bacteria (see section 8.2 this paper), only one microbe need survive in order
to populate the planet. Yet the argument of this paragraph, that large numbers
of bacteria will be killed due to the temperature shock, once again lowers the
probability of successful transfer of life.
The temperature shock would be an issue for successful transfer both
prior to and after 3.1 billion years ago and so constitutes a seventh
weakness/unexplored area of the transpermia hypothesis.
One way of surmounting the obstacle of planetary
temperature difference is through volcanic activity. On early Mars, volcanic activity was quite common and
extensive. For example, the Tharsis
region contains twelve volcanoes larger than 54 miles across.[135]
On
Earth, Antarctica is the region with the most “Mars like” climate (cold and
dry) and in several locations long-term volcanic activity warms the ground
surface (Broady 1993). For example, at
Deception Island the monthly average air temperature ranges from –11° C to + 3°
C. Yet near volcanic vents on this
island, “ground temperatures as high as 70 to 100° C have been recorded.”[136] At Mount Erebus and Mount Melbourne, the
highest recorded ground temperatures are 59 and 47° C, respectively. Several varieties of thermophilic bacteria
(grow at 40-80° C) have been isolated from the surface of these two mountains.[137] If a meteorite struck a volcanic region on
Mars in the time period circa 3.8 billion years ago, thermophilic microbes
might have been living in surface rocks, and might have reached Earth via the
spallation process. Such thermophilic
microbes would at least have a higher chance of survival than Martian
psychrophiles. A similar situation
would hold for Earth hyperthermophiles present in ejecta rock that landed in
Martian volcanic terrain.
There are several factors that Mileikowsky et al. did not consider in their
comprehensive review of the transpermia hypothesis. These factors greatly reduce the probability that there has ever
been a successful transfer of microorganisms between the Earth and Mars. By “successful” we mean that the microbes
exited the rock that served as their interplanetary transportation and survived
on the host planet. Successful transfer
of life may have occurred, but at significantly lower probability than
concluded by Mileikowsky et al.
4. Micrometeorites and Interplanetary Dust
Some 95% of the mass of extraterrestrial matter
reaching the surface of the Earth is “space dust.”[138]
The Infrared Astronomy Satellite’s observations indicate that micrometeoroids
and interplanetary dust particles (both hereinafter abbreviated as IDP) pervade
the solar system.[139] The
main sources of IDP are dust vented by comets and debris created by collisions
between asteroids.[140] For example, Halley’s comet generated
roughly one hundred million tons of particles during its last flyby of the sun
in 1986.[141] The majority of asteroidal IDP is generated
by collisions between asteroids in the asteroid belt. Kortenkamp et al.
(2001) estimate that it is “likely” that the dust in the vicinity of Earth is
75% asteroidal and 25% cometary in origin, while noting that a systematic study
is needed.[142]
IDP is microscopic. Micrometeorites range in size from 50-400 micrometers (microns),[143]
while interplanetary dust is about 50 angstroms-40 microns in diameter.[144] The different names for the particles are
solely due to their different sizes.[145] The typical particle is a heterogenous
composite of material found in terrestrial rocks such as olivine and pyroxene,[146]
certain elements such as iridium generally found in meteorites but not in
terrestrial rocks,[147]
organic carbon based compounds and amino acids (10%)[148]
and radiogenic isotopes formed in space due to exposure to solar radiation.[149] It is the iridium and radiogenic isotopes
which identify the particles as exogenous to Earth.
Figure 9. Interplanetary dust particle as viewed at
4,800 magnification. The bar on the
bottom of the photo indicates a length of 1 micron. This
particle was collected in the stratosphere using a high-altitude aircraft. Photo
courtesy Scott Sandford/NASA.
Five forces compose the total force acting on
any individual IDP while it orbits the sun.[150] Solar gravity, Poynting-Robertson drag,[151]
and solar wind drag,[152]
cause the particles to drift towards the sun, and during the journey some are
swept up by the Earth’s gravity and enter the atmosphere. Simultaneously, radiation pressure[153]
and the Lorentz Force[154]
move most particles smaller than 1 micron in diameter away from the sun and out
of the solar system. IDP of less than
1 micron in diameter does reach the Earth, and this portion of the inflow
probably originated from comets emitting dust as they pass very close to Earth,
or between the Earth and the sun. In
the former case, the dust would be caught immediately by Earth’s gravity, while
in the latter case the dust is pushed outward from the sun, but some of it is
caught by the Earth’s gravity as it passes 1 AU.
Kortenkamp et
al. (2001)[155]
found that a typical IDP of asteroidal origin has a lifetime of 50,000 years as
it journeys from the asteroid belt to Earth-crossing orbit. Liou et
al. (1998)[156]
performed a computer simulation of dust particles of cometary origin, modeling
the orbits over 400,000 years. At the
end of that period, 673 particles had either fallen into the sun or been
ejected from the solar system, while 427 were still in an orbit bound to the
sun’s gravity.
Currently, about 30,000 metric tons per year of
IDP cross the orbit of the Earth, are caught by the planet’s gravity and enter
the atmosphere.[157] There are spikes in the flux of IDP due to
asteroid collisions and comet activity.
These are caused by the gravitational pull of the sun, the gas giants
and nearby stars. These large bodies
influence the orbits of asteroids, resulting in collisions, and pull dormant
comets out of their positions in the Ort cloud and Kuiper belt and towards the
sun. Asteroid collisions may result in
an accretion spike of 107 tons per year for 10,000 years.[158]
A portion of the particles, perhaps 50%, survive
atmospheric entry heating and reach the surface.[159] A much higher ratio of IDP survive entry
than do their larger cousins, the meteoroids and asteroids. This is because an object is fully
decelerated if it encounters its own mass of air molecules and therefore will
not burn up but rather drift to the surface of the Earth. Given the high ratio of surface area to mass
of these particles, they have a greater chance of decelerating.[160] Over geologic time, the yearly flux has
accumulated. If we use an estimate of
15,000 tons per year accretion rate,[161] and
assume that the rate has been constant since the end of the heavy bombardment[162]
(4 billion years), this yields 6 x 1013 tons of accumulation. For comparison, this is about twice the mass
of Mars’ larger moon Phobos, which is estimated to be 2.73 x 1013
tons.[163] However, the mass of accumulated material is
still far smaller than the mass of the moon, estimated to be 7.35 x 1019
tons.[164]
5. Inputs of Extraterrestrial Matter: Effects on Climate and Life
5.1 Inputs of Extraterrestrial Matter: The ice ages and the Earth’s climate
During the 1920s, the Serbian mathematician
Milutin Milankovitch identified cyclical changes in the Earth’s orbit which
lead to variations in the amount of solar radiation reaching the Earth’s
surface. He correlated these
astronomical cycles to ice ages and warm periods on the Earth. There are three types of orbital
changes. First, the orbit of the Earth
is not circular, but elliptical, and the eccentricity
of this ellipse varies over a 100,000 year cycle. Second, there is a tilt or obliquity
to the Earth’s axis relative to the plane of the solar system, and this cycles
over a period of 41,000 years. Third,
at any given obliquity, the Earth’s
axis is wobbling like a top over a 21,000 year cycle and this is called precession.[165]
The Milankovitch cycles have been investigated
and refined since the 1920s.[166] One landmark article (Hays, Imbrie and
Shackleton, 1976), compares the Earth’s climate history to its orbital history
and concludes that the 100,000 year eccentricity cycle has been the predominant
driver of the ice ages, although precession and obliquity do explain some
climatic variance. The authors do not
specify how changes in the delivery of solar radiation to the atmosphere
translate into changes in climate: “We avoid the obligation of identifying the
physical mechanism of this response…”.[167]
The quotation at the end of the last paragraph
is not a failing of the authors; rather it reflects the limits of current
scientific knowledge. The Milankovitch
cycles explain only a part of the phenomenon we seek to understand. Between an ice age and a warm period there
is a 5 degree Celsius difference in the average global temperature. Yet the change in solar energy reaching the
Earth due to orbital eccentricity accounts for only a .15 degree Celsius
temperature change.[168] The rest of the 4.85 degree C temperature
change is thought to be due to climatic feedback effects, but there is no
consensus understanding on how these work.[169] Nor do we know what triggers (forces) ice
ages. One of the leaders in the field
opined in 1998, “We have but simple theories for the ultimate forcing of
climate change, even in recent times.”[170]
The Milkanovitch cycles are extremely prominent
in ice ages research because we can be sure that this small piece of the puzzle
is accurate. Since science knows so
little about how the ice ages start and terminate, when a new piece of knowledge
becomes available, such as IDP, scientists in various fields hope that the new
information will resolve long standing dilemmas.
Muller and MacDonald (1995, 1997) argue that for
the ice ages of the past 1 million years, the cause may have been IDP.[171] They note that for the past 1 million years
there has been a 100 thousand year climate cycle. As stated above, this had been thought to match the eccentricity
cycle, but Muller and MacDonald report that the inclination of the orbital
plane of the Earth has also been shown to happen in a 100,000 year cycle.[172] This is a fourth characteristic of the
Earth’s orbit: the plane of the Earth’s orbit varies from being perfectly level
with the sun.[173] Muller and MacDonald present a series of
elegant graphs that match the ice age predictions of the orbital inclination
theory to the actual data on the climate cycle, and argue that the inclination
theory is the best fit to the climate data:
Image © 1997 American Association
for the Advancement of Science. Reprinted with permission from R.A. Muller and G.J. MacDonald, Science 277: 215-218 (1997). http://www.sciencemag.org
Figure 10. Column A represents the predicted ice age frequency based on three theories. First is an eccentricity based model, second is a precession based model, and third is Muller and MacDonald’s orbital inclination based model. The orbital inclination model clearly and cleanly predicts an ice age every 100,000 years. Moving to columns B, C and D, 18O is an isotope that is a proxy for global ice levels. A spike in the δ 18O level indicates an ice age. The orbital inclination model is the best match for the empirical data in these three columns. See footnote for a detailed explanation of this figure.[174]
What about an increased angle of inclination
causes an ice age? Orbital inclination
has no significant effect on solar insolation.[175] They hypothesize that the Earth’s orbital
plane is tilting into a massive stream extraterrestrial dust and that the dust
is the trigger mechanism of the ice ages.
This theory has received observational support from a series of analyses
of ancient sediment which have shown that there is a 100,000 year periodicity
in the accretion rate of IDP, with a sudden increase in the amplitude of the
cycle beginning 1 million years ago and continuing today.[176] Muller (1997) concluded, “As far as we know,
none of the present climate change models include the effects of dust and
meteors. And yet our data suggests that
such accretion played the dominant role in the climate for the last million
years.”[177]
Muller does not identify the precise atmospheric
or combined mechanism by which the extraterrestrial dust influences the
climate, but Hays, Imbrie and Shackleton (1976) also do not specify a
mechanism. K.A. Farley contends that
aerosol cooling cannot be the mechanism at work.[178] He maintains that even at the peak of the
100,000 year cycle there is not enough extraterrestrial dust to cause an aerosol cooling effect.[179]
To test Muller and MacDonald’s Hypothesis, we
would need to know if such a stream of dust exists; we would need observational
data. Kortenkamp, Dermott and Grogan
have used all available data to identify the location and movement of the solar
system’s IDP within the orbit of Jupiter.[180] Additionally, they made use of several
computer models to run simulations of the interaction of the IDP with the
planets. The found not streams of dust,
but layered bands of dust. The dust
orbits the sun in these bands, and is falling towards the sun due to
Poynting-Robertson drag and solar wind drag.
It takes about 50,000 years for the orbit of a typical particle from the
asteroid belt to reach Earth’s orbit.
Figure 11. The Earth is embedded
in several layered bands of Interplanetary Dust. The widest band of dust (light color) is from the Eos asteroid
family. The narrower central dust bands
are from the Themis and Koronis asteroid families. After Kortenkamp et al.
(2001) Figure 2.
As a first step of analysis of the inclination theory, Kortenkamp et al. note that it doesn’t matter if the Earth’s orbital inclination varies with respect to the plane of the solar system. Muller and MacDonald were looking at the wrong thing. What matters is how the Earth’s inclination varies with respect to the bands of IDP in the inner solar system. Kortenkamp et al. find that the Earth’s inclination with respect to these dust bands does NOT vary in a smooth 100,000 year periodicity:
Figure
12. The “I (symbol)” stands for the
inclination of the Earth’s orbital plane.
The circle with embedded + is the symbol is that used by astronomers to
represent the planet Earth. Image © 1998 American Association
for the Advancement of Science. Reprinted with permission from S.J. Kortenkamp and S.F. Dermott, Science 280: 874-876 (1998).
In the diagram, the dashed line is the limit of the dust bands. When above the dashed line, the Earth’s orbital inclination has taken it outside the dust bands and it is receiving no dust. However, in this phenomenon, there is simply not the periodicity Muller and MacDonald are looking for.
The computational models of Kortenkamp et al. did show a smooth 100,000 year periodic relationship between the Earth’s capture rate of the IDP and the Earth’s orbital eccentricity. The capture rate of IDP is anticorrelated with the eccentricity of the Earth’s orbit. When the Earth’s orbit is least eccentric, the IDP capture rate is highest. This is demonstrated in the following graphs:
|
Figure
13. “e (symbol)” stands for the eccentricity
of the Earth’s orbit. “p” stands for
the capture rate (1=100%) of the IDP that crosses the Earth’s orbit. The dashed vertical lines are added to help
compare the eccentricity to capture rate.
At the leftmost vertical line, eccentricity is high, but capture rate of
dust from both the Themis/Koronis and Eos families is low. Same for the rightmost dashed line. So graphs D and E are telling us the
percentage of IDP from the Themis/Koronis (D) and Eos (E) asteroid families
that actually enters the Earth’s atmosphere. Image © 1998 American Association
for the Advancement of Science. Reprinted with permission from S.J. Kortenkamp and S.F. Dermott, Science 280: 874-876 (1998).
|
Even without a correlation coefficient for the
eccentricity vs. capture rate, we can see from the graphs that the correlation
is not perfect. One reason is that
there is some interference from the Earth’s orbital inclination cycle. For
example, the authors note that at 500,000 years ago and 900,000 years ago the
Earth’s orbital inclination took is outside of the Themis and Koronis dust
bands, so the accretion rate of IDP which had those dust bands as its source
dropped off at that time, despite the low eccentricity of the Earth’s orbit.
Overall, Kortenkamp et al.’s data and modeling do not support the Muller and MacDonald
hypothesis. Rather, the accretion rate
of IDP seems to be anticorrelated with the Earth’s orbital eccentricity in a
100,000 year cycle. Yet the Earth’s
eccentricity varies only slightly over the 100,000 year cycle: .005 ≤ e ≥ .0607 and is
currently at .0167.[181] With such a slight variation in
eccentricity, what is the mechanism driving the change in the capture rate of
IDP? Kortenkamp et al. do not venture in this direction. And, does the 100,000 year eccentricity cycle match the ice age
cycle? It is of similar amplitude but
50,000 years out of phase. Kortenkamp et al. give some preliminary reasons why
this might be so, but leave it to other disciplines to apply their relevant
expertise. Finally, they model only
asteroidal dust, while cometary dust and an undetermined amount of interstellar
dust[182]
are present as well.
Kortenkamp and Dermott present their own
hypothesis linking IDP to the ice ages.[183] They note that our current knowledge
suggests that many asteroids are clumps of rocky rubble held together by their
own weak gravity. If an asteroid of 10
km – 100 km radius was broken apart in a collision with another asteroid, that
would “instantaneously liberate,” as they put it, a mass of IDP up 10,000 times
greater than the normal amount in the solar system. Such an event would occur once every few tens of millions of
years. It would lead to about 107
tons per year of IDP hitting the stratosphere,[184] the
same amount of dust pumped into the stratosphere by a volcanic eruption.[185] This would continue for about 10,000 years
as the space dust is gradually captured by Earth’s gravity.
The idea is less difficult to believe if one
works through some accessible calculations.[186] A “rubble pile” asteroid has a density close
to that of water, which is 1 g/cm3 at 4 degrees Celsius. The densities of some common materials are
given below:
Table 4
Mars’ smaller moon Deimos has an approximate
radius of 6 km and a density of 1.7 g/cm3. It is close enough to a rubble pile, and it is smaller than 10
km, so it should serve as an adequate approximation for our calculations. The mass of Deimos is 1.8 x 1012
metric tons. So such a small object is
still in the trillion ton range. That
is how there is such a mass of dust in a rubble pile asteroid with a radius as
small as 10 km. With 1012
tons of dust being cleared out of the asteroid belt over 104 years,
that is 108 tons per year available to hit the Earth. Kortenkamp and Dermott must be saying that
10% of the available dust would be captured by Earth’s gravity.
Returning to the hypothesis, we have 107
tons per year of dust entering the stratosphere, the same amount of dust
emitted by a volcanic eruption. Recall
that this is some 3 orders of magnitude higher than the 30 x 104
tons that the Earth currently receives.
This lasts for 10,000 years.
Since volcanic eruptions are known to cool the climate, 10,000 years of
such effects “may lead to substantial changes in the Earth’s climate lasting
for many millions of years.”
Additionally, some of the debris would be in the kilometer size asteroid
range, which material would arrive in an Earth-crossing orbit towards the
latter end of the dust input. The
cumulative result of the rubble input could be a gradual mass extinction, the
authors suggest.
Kortenkamp and Dermott’s hypothesis would seem
to merit investigation by climatologists.
It might be worthwhile to run computer global climate models using
variables altered as suggested by the following discussion. Most volcanic aerosols of terrestrial origin
settle back to the ground in days or weeks.
Only a small fraction reach high enough altitude to remain aloft for
extended periods of time. Yet every
single particle of asteroidal dust would be injected into the upper atmosphere
and so remain airborne for months.[187] The recent Pinatubo eruptions involved far
less material and caused a measurable cooling.[188] Even if the dust influx caused cooling of
only a few tenths degree, that could be enough of a trigger because the
cause-effect relationship is probably non-linear: sometimes a small change in
the amount of dust leads to a big change in the temperature.[189]
One possible causal sequence is that a large
spike of asteroidal dust could contribute a significantly greater supply of
cloud condensation nuclei, thus increasing the numbers of clouds, which would
increase the albedo of the Earth, which could in turn lead to an ice age.[190] Cloud condensation nuclei are small
particles from .2 microns up to >1 micron in size which pervade the
atmosphere. To be effective, the nuclei
must be hygroscopic – water attracting.
Water vapor condenses around these particles, leading to cloud
formation. Without condensation nuclei,
relative humidities of several hundred percent would be required before vapor
condensation could begin.[191] Several questions would need to be
investigated in order to evaluate this hypothesis. We know the baseline size distribution of IDP (see section
3). But we are here dealing with
exceptional events that cause a spike in the baseline rate. Do these collisions deliver enough particles
at the sizes conducive to cloud formation?
Are the particles conducive to the condensation process? The answers might differ with each asteroid
collision because different bodies have unique characteristics.
The possible involvement of extraterrestrial
matter in glacial cycles suggests that inputs of matter have significantly
influenced the atmospheric component of the Earth system, and in turn, the
biosphere. The research may have some
impact on the debate over global warming.
Professor Stephen H. Schneider of Stanford writes, “…we cannot forecast
the future accurately without understanding and modeling a great deal of the
Earth’s past.[192]
… We also need to understand and be able to model the factors that might induce
changes in climate, factors we call forcings.”[193] Mark H. Thiemens, professor of chemistry
says, “One always hears the argument, ‘Isn’t this all part of a natural cycle?’
To
answer this question, you really want to have a large scale record.”[194] IDP may be a necessary component to such an
understanding.
In Cometary
Impacts and Ice-Ages (2001),[195] F.
Hoyle and C. Wickramasinghe discuss the impact of comet and asteroid impacts on
ice age cycles. They note that there
should be no way for the Earth to emerge from an ice age due to positive
feedback. As more glaciers form, the
Earth’s albedo increases, which means that more heat from the sun is reflected
back into space. This makes the Earth
colder, which causes more glaciers to form.
A major event would be needed to disrupt this process.
Hoyle and Wickramasinge contend that the
presence of water vapor in the atmosphere is actually more important to the
maintenance of a warm Earth than the amount of carbon dioxide. This is because, compared to carbon, water
vapor absorbs a larger part of the spectrum of infrared radiation (heat in
radiation form) that otherwise would escape back into space. An increased concentration of water vapor in
the atmosphere would therefore increase the mean temperature at the Earth’s
surface. As
they put it, “The impact of a kilometer-sized object, (or a slightly smaller
asteroid) into a major ocean appears essential to the ending of an
ice-age.” Such objects have undoubtedly
impacted into the open ocean, leaving no crater yet kicking up enormous amounts
of water vapor which would persist in the atmosphere for a period of months to
years and cause a spike in the Earth’s temperature. This increase in the temperature would be the trigger that ends
the ice age.
Hoyle and Wickramasinghe point to several
incidents of very rapid temperature increases in the climate and fossil
records. For example, ice cores from Greenland and Antarctica (dated to about
14,600 and 11,500 years ago) show a temperature rise of some 12-15 degrees C in
only a few decades. Fossil insect
records show that the mean summer temperature in Britain rose at least 10
degrees C during the same period, “an essentially decisive indication of a
catastrophic event as its cause.” This
article presents a hypothesis that might stimulate field researchers to gather
empirical data in order to evaluate it.
A major weakness of the hypothesis is that the authors
make no mention of volcanism. For
example, they write, “Left to itself, it is hard to see how anything internal
to the Earth could ever break the stable grip of an ice-age.” Yet Hoffman and Schrag have outlined how CO2
released from volcanoes could have been the primary factor in causing the Earth
to emerge from the most severe, overwhelming ice age known as “snowball Earth.”[196] This episode occurred approximately 600
million years ago. During the snowball
Earth episode, volcanoes emitted CO2 at their regular rate. Normally, this input of CO2 is
counteracted by the removal of an equal amount of CO2 by the
chemical weathering of silicate rocks.
The first step of weathering is that CO2 combines with H2O
to form carbonic acid (H2CO3) which then attacks the
silicate rocks. It is this first step
that removes the CO2 from the atmosphere.[197] However, during snowball Earth most of the
planet’s H2O was in solid form, so there was a diminished amount of
liquid H2O available to start the silicate weathering process. With the continued input of CO2
from volcanoes, over hundreds of years the global average temperature slowly
rose, ending the snowball earth episode.
Even for the majority of ice ages, volcanism
must play a greater role than Hoyle and Wickramasinghe concede. In these less severe glaciations, a great
portion of the planet’s H2O would still be ice, so weathering of
silicate rocks would be less and there would be buildup of CO2 from
volcanoes. Impacts into the open ocean
could be a primary trigger ending some or even many ice ages, but not the only
factor.
5.2 Effects on Origin of Life
A line of research extending back at least to
Chamberlin and Chamberlin (1908)[198]
examines whether, just after planet formation, sources such as meteorites and
IDP delivered to Earth organic compounds necessary to the origin of life. The current state of research is that while
a significant quantity of organic matter was delivered, we do not know if it
was necessary because we don’t know
how life started.
It is a puzzle that life seems to have emerged
remarkably quickly after the end of the heavy bombardment. It is generally accepted in the relevant
disciplines that until 4 billion years ago, the frequent catastrophic impacts
of multi-kilometer sized objects would have sterilized the surface of any life
that had managed to emerge. An adequate
number of scholars go even further to say that that the bombardment continued
until 3.8 billion years ago.[199] Yet the oldest sign of life has been
reliably dated at 3.77 billion years old.[200] The latest time for life’s emergence is 3.55
billion years ago, based on clearly recognizable fossils of microorganisms.[201] So the current state of research suggests
that life started almost as soon as it possibly could – perhaps less than 100
million years after the end of the heavy bombardment.
The meteorites, comets and IDP which rained down
upon the Earth during the heavy bombardment, (4.5-4 (3.8) billion years ago),
may have delivered complex pre-biotic molecules that “jump started” the
emergence of life.[202] The rate of input of extraterrestrial matter
during that time was 100-1000 times higher than at present. Such a rate over 500 million years would
have left a substantial accumulation of organic matter at or near the surface
of the Earth.
Some chemistry background is necessary in order
to understand the findings. The element
carbon is thought to be essential to complex life. A carbon atom (symbol C) has the ability to form very strong
bonds with other carbon atoms, so that very long chains of carbon atoms are
possible. A carbon atom can bond with
up to four atoms of any type, so that any of the remaining bond sites can
accomodate other atoms such as hydrogen (symbol H). Very large and complex compound molecules are possible. Silicon (Si) has also been mentioned as a
possible basis for life, but the C-C bond is much stronger than the Si-Si bond,
so that much more durable carbon-based molecules are possible.
When scientists looking at this chemistry of
early life speak of “organic matter” or “organic compounds” they mean molecules
based on carbon atoms which are useful in life processes or serve as the
building blocks of such useful compounds.[203] These compounds are carbon with some
combination of the elements hydrogen (H),oxygen (O) and nitrogen (N) attached
to their bonding sites.[204] RNA and DNA are complex organic compounds
which in addition to the above elements also contain phosphorus (P). Hydrocarbons such as methane (CH4)
and benzene (C6H6) are considered organic compounds. So
are the aldehydes, such as formaldehyde (H2CO). These are building block compounds and they
are found in outer space. For example,
the simple sugar ribose (C5H10O5) can be
formed from five molecules of formaldehyde.
Directly useful for living things are the carbohydrates (which include the sugars, starches and
cellulose), with a representative formula being that of glucose, C6H12O6. Vitamin C has the formula C6H8O6. The amino acids (the building blocks
of proteins) are organic compounds. The
simplest amino acid is glycine with the formula C2H5NO2.
Amino acids have a general
structural formula of H2N-CH-COOH-R, with R being a term that
represents a variety of substituent groups ranging from a single H atom or a CH3
group, to much more complex chains.
Several lines of observational and experimental
evidence support the idea that the heavy bombardment was a significant
exogenous source of organic matter. Two
specific types of meteorite, the CI1 and CM2 carbonaceous chondrites, are on
average 3% organic matter by mass.
Since the universe is 85% hydrogen, 14% helium and 1% all the other
elements, this is a significant concentration of C, N, O and other atoms. During Halley’s Comet close flyby of Earth
in 1986, it was measured to be composed of 25% organic matter, which may be
representative of comets. Another source says comets are 15% organic matter.[205] IDP, which is a mixture of both meteoritic
and cometary dust, contains about 10% organic matter on average.[206]
In studies of the Murchison, Murray, Orgeuil and
Tagish Lake meteorites, over 70 (out of about 100 total known) amino acids have
been found. Amino acid precursors such
as pyroglutanic acid have been found. A
sugar, dihydroxyacetone has been found.
Sugar alcohols such as glycerol and ethylene glycol and sugar acids
(dicarboxylic sugar acids and deoxysugar acids) have been detected. These molecules have the general formula CnHnOn
are components of RNA, DNA and cell membranes, as well as being energy sources
for life forms. A list of all the
organic molecules found would run for several pages.[207]
How do rather complex organic molecules form in
meteorites? Astrochemists suggest
various pathways. Only the most simple
can be covered here. Photolysis is the breaking down of a
substance by the energy in UV light. In
a laboratory, photolysis under simulated interstellar conditions (temperature
of 10° Kelvin) of simple non-organic molecules such as CO, NH3 and H2O
forms simple sugar related organic compounds such as ethylene glycol, glycerol
and glyceric acid.[208] One standard theory of meteorite formation
is that such compounds condensed onto grains of interstellar dust, with then
agglomerated to form the parent bodies of today’s meteorites. (The meteorites in existence today are
fragments of the original much larger parent bodies).
Another pathway might be the “formose
reaction.” In a laboratory, when
formaldehyde (H2CO) is placed in a water solution, the mixture
condenses to produce “a variety of hydroxylated compounds” (molecules with the
group –OH). These are mostly sugars and
sugar alochols having up to 7 carbon atoms- significantly larger molecules than
the mixture started with. “Several
lines of evidence suggest that the formose reaction would have been possible on
parent bodies of carbonaceous meteorites.”[209] Indeed, the authors of the various
scientific papers consider it unexceptional that these sorts of reactions could
occur. It is common knowledge among
astronomers that formaldehyde is present in interstellar dust clouds. This is based on observations in the radio
and infrared range. Additionally the
physics is understood of how formaldehyde molecules might clump onto
interstellar dust grains and then the interstellar dust grains would experience
low energy collisions with one another which would result in them sticking to
each other. Eventually a large ball of
interstellar dust would develop, and would be of sufficient size to have some
gravity. Once the object has a
gravitational pull, not only do dust grains collide with it by chance, but they
are actually attracted to it and have a greater probability of being retained.[210] Finally, it is apparent from the study of
carbonaceous chondrite meteorites that at some point the parent asteroids went
through a phase in which they were heated by the proto-sun to the point where
water became liquid. This is why the
formaldehyde is put in a water solution in laboratory studies of the formose
reaction.
Findings made during the past 2 years are even
more suggestive of an exogenous origin of life. Dworkin et al. (2001)
started with a mixture of H2O, CH3, OH, NH3
and CO, all in gas form. This reflects
the components of interstellar molecular clouds. They froze this to 15K in a vacuum in order to simulate
interstellar conditions. They then
exposed it to UV radiation. Under
actual interstellar conditions, a nearby star would be the source of such
radiation. The mixture was warmed to
room temperature and water was added.
When analyzed, the contents had formed into complex organic compounds
and also vesicular (containing small hollow cavities) droplets. The researchers added dye to the mixture,
and the dye was incorporated into the interior of the vesicle: the dye became trapped in the hollow
cavities at the center of the vesicle.
Dworkin et al. remark in their
paper that these vesicles could be the precursors of cell membranes. If these types of vesicles were present in
meteorites that fell during the heavy bombardment, they could have trapped
complex organic matter (such as amino acids) inside, with the result being
complex organic matter distinct from its environment.[211]
The research of Dworkin et al. received almost immediate observational confirmation with
the analysis of the Tagish Lake meteorite, which fell in January, 2000. The Tagish Lake meteorite was quickly
recovered under conditions that minimize the possibility of terrestrial
contamination.[212] Inside the meteorite were found the usual
organic compounds (1.3% by mass) and also, “numerous hollow, bubble-like
globules.”[213] Figure 14 allows visual comparison of the
“vesicles” generated by Dworkin et al.
to the “globules” found in the Tagish Lake meteorite. Nakamura et al. (2002)
remark that the globules and vesicles are “strikingly similar.”[214] They hypothesize that these types of structures
are formed in outer space, incorporated into asteroids and perhaps comets by
the usual processes by which these bodies form, and then survive atmospheric
entry and impact to be deposited on the Earth.
Interestingly, Maurette et al.
(1995) found similar vesicles during analysis of IDP.[215]
Figure
14. On left is Dworkin et al. image. It is
vesicles formed from molecules typically found in interstellar molecular clouds
after exposure to ultraviolet light for 15 minutes. On right is Nakamura et al.
image (actual globules found in Tagish lake meteor). The scale of the image on left is 100 times
the size of the image on the right. Left:
©
2001 National
Academy of Sciences, U.S.A.. From: J.P. Dworkin, et al., “Self-assembling Amphiphilic
Molecules: Synthesis in Simulated Interstellar/Precometary Ices,” Proceedings
of the National Academy of Sciences, 98 (2001): 815-819. Right: © 2002 Cambridge University Press, International
Journal of Astrobiology. From: K.
Nakamura et al., “Hollow Organic Globules in the Tagish Lake Meteorite as
Possible Products of Primitive Organic Reactions,” International Journal of
Astrobiology 1 (2002): 179-189.
Once all this organic matter was deposited at or
near the surface of the Earth, most researchers conclude that it became more
complex. The foundation of the entire
line of thought is the Miller-Urey experiment of 1953. Miller discharged electric sparks (simulated
lightning) into a receptacle containing that period’s best approximation of the
constituents of the earth’s early atmosphere- hydrogen, water vapor, methane
and amonia. From these simple
chemicals, organic compounds including amino acids were formed.[216]
Chyba and Sagan (1997) conclude that in the
likely case of a CO2 rich early Earth atmosphere, 50% of the organic
matter on Earth was contributed by exogenous inputs after the end of the heavy
bombardment, while 50% came from terrestrial production mechanisms.[217] While there is currently apparently not a
consensus among researchers about precisely which source contributed what
quantity of organics, these general numbers appear to have been followed.[218] We cannot say the 50% of organic matter
contributed by sources external to the Earth system was necessary to the formation of life because we don’t know how life
started. We can only say that current
findings are highly suggestive. The
science is solid, but the picture is incomplete.
5.3 Recent Effects on Biosphere
Other research suggests the possibility that the
delivery of extraterrestrial matter may have stimulated and enriched the
Earth’s biosphere in the geological recent past. Culler et al. (2000)
conduct an analysis of lunar soil samples returned by the Apollo 14 mission.[219] They found that the rate of meteorite impact
declined from the heavy bombardment until about 400 million years ago. This is in accord with standard theory. However, they also find that from 400
million years ago to present, the rate of impact increased by a factor of
4. The impact trend for the Earth would
be very similar to that of the moon due to their proximity, except that the
Earth would have approximately double the rate since it is a larger target and
has stronger gravity. Culler et al.
note that their data indicating a pronounced spike in extraterrestrial
input is “roughly coincident with the ‘Cambrian explosion’[220]
of complex life on Earth. ... It is possible that the increased debris influx
had a net stimulating effect on biotic diversity ...” Culler et al. show
correlation, which is not alone sufficient to prove causation.[221] The trend they highlight would also indicate
a similar increase in the quantity of IDP reaching the Earth. This is because an increased cratering rate
could be due to both asteroids and comets, both of which are associated with
IDP.
Does the current gentle dusting of IDP have any
impact on the ecology of the planet?
IDP is a heterogeneous composite of varieties of minerals and elements
as small as 50 angstroms.[222] How long does it take the whole particles to
be broken down by weathering and other processes on Earth, so that their
constituent parts are available for the uptake into the biosphere? If we know this period, can we then estimate
what the interval should be between an increase in the accretion rate and a possible
enrichment of life on Earth? One
relevant finding is ‘cryoconite’ from Greenland. In Greenland relatively pure samples of extraterrestrial dust are
found in melting glacial ice. The
physics of the glacial flow causes terrestrial and extraterrestrial dust
particles to concentrate as a sediment in pools of water on the surface of
melting glaciers. The dust is encased
in a cocoon of algae and bacteria. This
entire complex is the cryoconite. The
only apparent nutrient source is the dust.
However, it is not determined if the algae and bacteria are living off
the extraterrestrial or terrestrial dust or both. [223] Most soil scientists, biologists and
ecologists don’t know about IDP, so have not been alert to its possible
influences.[224] We shall return to this issue when we
discuss the panspermia hypothesis.
6. Small Comets
If small comets exist and behave in the way
proposed, it would go a long way towards establishing that the Earth is an open
system. These comets would, over
geologic time scales, be responsible for supplying much of the planet’s
water. Without this extraterrestrial
input, the Earth would be a barren planet.
Proposed by Frank, Sigwarth and Craven in 1986,
this hypothesis holds that the Earth is struck by millions of water-laden comets
per year with an average size of 12 meters in diameter. The comets are composed mostly of water
snow. As they approach the Earth they
become a cloud of water vapor at approximately 1287 km altitude. This cloud of mist loses its coherence and
enlarges while it descends towards the Earth.
Frank et al. argue that small
comets have, over geologic time, contributed much of the Earth’s oceans. It is the supply of water from small comets
that continually replenishes the water lost to subduction to the interior of
the planet and prevents the surface of the Earth from becoming dry and barren.[225]
There have been several dozen articles for and
against small comets. For example, one
group of scientists used an optical telescope to search for small comets.[226] They found nothing, although their results
did not rule out the existence of the phenomenon. Frank and Sigwarth conducted their own optical search using the
same telescope and they found small comets.[227] Both results were published in a prestigious
peer reviewed journal and so cannot easily be discounted. I evaluated the debate by focusing on one
detection method and reading the sequence of articles very carefully. By focusing in depth on a specific facet of
the debate, I hoped to reach definitive conclusions which, although limited
only to one detection method, would nevertheless be revealing.
In 1997, a group of radar scientists and
astronomers (Knowles et al.)[228]
conducted a radar search for small comets.
They used the United States Naval Space Surveillance System radar. This radar has transmitter and receiver
sites across the southern United States and a central data processing facility
in Virginia. It is designed to catalog
and track essentially all the world’s Low Earth Orbit satellites. The authors state that this is the best
available radar in the world for locating small comets because it searches an
extremely wide volume of space with very high sensitivity. Some narrow aperture radars, (radars
designed to search a small volume of space very carefully), are more sensitive,
but these might miss the small comets because they do not have a wide enough
field of view.
During the approximately 1 month of searching
with the radar, some 12,000 unidentified targets were observed. Of these, none fit the theoretical
characteristics of small comets. 50
were signal noise, and the rest were either new satellites, moving too slowly
to be small comets or were in gravitationally bound orbits (not unbound
infalling objects).
Frank and Sigwarth were given the opportunity to
reply to the negative radar results.
Knowles et al. then replied to
the reply. This author has carefully
analyzed the exchange and found that none of Frank and Sigwarth’s objections
are valid. This analysis is included as
Appendix I. The results of the Naval
Space Command radar observations seem to be strong empirical evidence against
the existence of small comets.
7. Possible Influences of IDP and Small Comets on Climate
Dr. David Deming explores the possibility that
interplanetary dust and small comets provide the necessary carbon and water for
a vibrant biosphere on Earth.[229] His ideas are unorthodox yet relevant to the
Earth-as-open-system concept. Deming
challenges the prevailing theory of balanced carbon and water cycles. According to Deming, the current state of
scientific knowledge clearly indicates that far more carbon and water is
carried down into the mantle by subduction than is released back into the ocean
and atmosphere by volcanic outgassing.
There is a 56-90% deficiency for
carbon, and it is worse for water.
Although too much carbon results in a runaway greenhouse effect,
atmospheric carbon is necessary in order to prevent sun’s heat from escaping
back to space. Water is of course
essential to life. With such a rate of
carbon and water loss, the surface of the planet should be extremely cold and
dry- inhospitable to a vibrant biosphere.
Deming explores the possibility that extraterrestrial sources-
interplanetary dust and small comets-
balance the carbon and water cycles.
Deming cautions that “what is being proposed is … a working hypothesis
subject to being tested and possibly disproved.” (p. 34).
Deming notes the Muller and MacDonald article
theory that the 100,000 year orbital inclination cycle may cause the Earth to
tilt into and out of a stream of interplanetary dust. He proposes that this dust stream adds carbon to the atmosphere,
which causes warming. An ice age occurs
when the Earth tilts out of the dust
stream because the carbon content of the atmosphere falls so that the planet
retains less heat.
Deming spends much of the article addressing
small comets. These are thought to be
10% carbon and 90% water. Specifically,
we need .6-3 x 1011 kg of carbon per year to balance the carbon
cycle, and small comets are supposed to supply .2-1 x 1011 kg of
carbon per year. We need 1-2 x 1012
kg of water per year to balance the water cycle, small comets are thought to
supply .2-1 x 1012 kg per year.
Given all the uncertainties, the area of overlap is satisfactory for
Deming to hypothesize that small comets are the missing input.
Deming concludes, “Life on Earth may be balanced
precariously between cosmic processes which deliver an intermittent stream of
life sustaining volatiles from the outer solar system or beyond, and biological
and tectonic processes which remove these same volatiles from the atmosphere by
sequestering water and carbon in the crust and mantle.” Deming seeks to use new information to fill
gaps in the existing knowledge.
Although small comets appear to be the perfect fit, the case for them is
very weak at the current time.
8. Panspermia and the Open System
Noted astrophysicist/mathemeticians F. Hoyle and
N.C. Wickramasinghe are the latest proponents of the panspermistic
hypothesis. The idea has been
entertained by scientists at least as far back as Isaac Newton.[230] Of course, just because an idea is old
doesn’t mean it is a good idea. Hoyle
and Wickramasinghe make four main points in their writing:
1) Life in the form of
microbes (bacteria and viruses) pervades the universe in the interior of
comets, meteorites and in clouds of interstellar dust.
2)
Life
from outer space was delivered to the early Earth so that the origin of Earthly
life is extraterrestrial.
3)
Extraterrestrial microbes have continued to
be delivered to Earth in the form of comet dust and interstellar dust filtering
through the atmosphere to the Earth’s surface.
4) This extraterrestrial
life has served as a constant source of fresh DNA for the biosphere of Earth
and has been a crucial driver of evolution.
Most
of the ideas of Hoyle and Wickramasinghe are expressed as verifiable
hypotheses. We shall address each
theoretical contention and its empirical verification. What follows is, of necessity, a brief
coverage of a lifetime of work by two extremely keen minds who benefited from
frequent collaboration of co-authors from applicable disciplines. A professor of geochemistry in 1990 reviewed
the relevant findings regarding points 1 and 2 above and concluded, “… it is
felt that a panspermistic approach to the initiation of life processes on Earth
would not appear outside the mainstream of contemporary scientific thought.”[231] While not advocating panspermistia, this
section intends to distill a logical and coherent presentation of Hoyle and
Wickramasinghe’s ideas.
Hoyle and Wickramasinghe envision the Earth as an open system. Genesis either occurred on Earth and was spread to the rest of the universe or, life started somewhere else and eventually extraterrestrial bacteria gained a foothold on Earth. Hence, at the outset, a crucial input or output of matter occurred. If continued input of extraterrestrial microbes is essential to the evolution of life, then inputs of matter are essential to the continued viability of the biosphere subsystem.
8.1 Life and (near-life) in the form of bacteria and viruses (together called microbes) pervades the universe in the interior of comets, meteorites and clouds of interstellar dust.
The absorption spectra of interstellar dust and
comets provide evidence that these objects contain living microbes (bacteria
and viruses). Different materials
absorb different parts of the electromagnetic spectrum. For example, a green shirt absorbs all color
wavelengths except green. A
spectrometer is an instrument used for measuring what wavelengths of the
spectrum are absorbed or emitted by an object.
In astronomy, one method of observation is to image a distant object
using a spectrometer while recording which wavelengths are absorbed. This yields an absorption profile. This profile is then checked against a
database of absorption/emission profiles of thousands and elements and
molecules. The database has previously
been accumulated by researchers working under laboratory conditions. The
researchers measure pure samples of known materials to determine their baseline
emission/absorption profile. Allen and
Wickramasinghe (1981), report that spectroscopic observations of a cloud of
interstellar dust in front of GC IRS-7[232]
yield an absorption profile closest to the bacteria Escherichia coli (E. coli)
(from 2.9-3.6 micrometers.). They say
that they tried various combinations of minerals, elements and organic
compounds, and E. coli was the best match.[233] Figure 15 below is a re-evaluation of that
finding adding 1989 data from other researchers.
Illustration © Kluwer Academic
Publishers and used with kind permission.
Figure 15. The graph above
demonstrates the methodology involved.[234] Data points (dark points and open circles)
of remotely observed infrared flux
across a wavelength range are matched to a model of E.coli flux at the same
wavelengths (the continuous line). The
E.coli model is derived from laboratory data.
Hoyle and Wickramasinghe (1999) reevaluate the
1981 findings in light of new observational data.[235] They add data from independent observations
in 1989 and 1994. They find that the
new data are also a very good match to the E. coli absorption spectrum. They generate a graph which represents the
absorption characteristics of the interstellar dust between stars IRS 6E and
IRS 7 for the 3.3-3.55 micrometer
wavelengths. On this graph they
superimpose a curve of the absorption characteristics of a mixture of E. coli
and a virus called TMV (Tobacoa Mosaic Virus). The curve of the bacteria-virus
mixture absorption is a quite good match to the data points of the interstellar
dust absorption.[236] Hoyle and Wickramasinghe conclude that, at a
minimum, there is complex organic matter in the interstellar dust. But there is no known non-biological
mechanism for how such complex organic matter could be created. So they conclude that there is biological
activity that generates the complex organic matter and that in fact the organic
matter whose absorption spectra we are seeing in the interstellar dust is
bacteria closely related to E. coli and the Tobacoa Mosaic virus. We have seen in section 4.2 that abiotic
processes do produce organic molecules in outer space. Many researchers would assert that Hoyle and
Wickramasinghe are detecting these organics.
Similar methods have demonstrated that the absorption
spectrum of “galactic infrared source OH 26.5 + .6” matches that of dessicated
(dried) cellulose (whose spectrum was measured in a laboratory). This is contended to be another piece of
evidence that biological processes are occurring or have occurred in outer
space.[237] Many researchers had thought that the
spectrum of infrared radiation passing through the Trapezium nebula over the
wavelengths 8-35 microns matched the spectrum that laboratory models predicted
would be produced by silicon. Hoyle and
Wickramasinghe find that the nebula spectrum more closely matched the spectrum
that would be produced by diatoms- a water borne microorganism that incorporates
silicon into various components of its structure.[238]
Wickramasinghe et al.[239]
conducted Earth-based spectroscopy of the coma of Halley’s Comet during the
1986 flyby.[240] Over a specific range of infrared
wavelengths (3-4 microns), the emissions characteristics of the comet coma are
very close to the emissions characteristics of E. coli bacteria that have been
heated to 320 degrees K. This is
probably a reasonable temperature approximation of the temperature conditions
in the coma. The Earth based
observations were conducted on March 31.
The European comet intercept probe Giotto measured the temperature of
the outer nucleus sometime in the month of March, returning a temperature of
330 degrees K.[241] Any material in outer space is also subject
to intense radiation. To better
approximate the conditions in the coma, the authors subjected a sample of E.
coli to a radiation dose of 1.5 megarads and then heated the sample to 320
degrees K. The infrared emissions
characteristics of this irradiated sample were even closer to that of Halley’s
comet. The authors conclude this
empirical finding is evidence that some of what is expelled from a comet is
bacteria from its interior.
Based on the empirical findings, Hoyle and
Wickramasinghe argue that primitive life forms extant in outer space would be frozen
by the cold and dried by the absence of moisture in outer space. If this hypothesis is accurate, could the
microbes survive? Is the bacteria
floating outer space dead? They contend
that based on what we know about the extreme survivability of terrestrial
bacteria, the bacteria in outer space could be alive or dormant.[242]
Clarke et
al. (1999) lay down some parameters which help evaluate panspermistic
ideas. They explain that the major
obstacles to survivability of bacteria in small bodies in outer space are
three: intense radiation, intense cold
and absolute absence of water.[243] They note that, given the hardihood of some
species of Earth bacteria, it is conceivable that some species could survive
any one or even two of these factors, but the combination of all three would be
insurmountable. Given the intensity of
radiation, bacteria would have to be several meters below the surface of any
object in order to have enough shielding to survive. Asteroids and moons of different planets may contain life at
appropriate depths below the surface.
The authors conclude that comets would be the most likely place where
life would be found, presumably bacteria in spore form. The bacteria could be several meters under
the crust of the comet, encased in ice.
The factor of radiation would be absent, and since the bacteria would be
encased in ice, moisture would be locked into the spores. For their part, Hoyle and Wickramasinghe
suggest that bacteria could survive in the interior of an interstellar dust
cloud. These bacteria would be shielded
from radiation by the outer part of the cloud.[244]
When Clarke
et al. speak of the hardihood of bacteria, they are referring to evidence
of extremeophile microbes on Earth. It
is worth examining some of these findings in order to understand how life could
possibly survive in outer space if protected within a comet. It is well documented that there are
bacteria which live in boiling water (hot springs), bacteria that eat sulfur,
bacteria that live deep below the Earth in high heat and pressure, and
organisms that live in the deep ocean around hydrothermal vents. Vents are openings in the sea floor from
which heat and various chemicals seep out from the interior of the planet. These organisms survive in almost total
darkness, under great water pressure, using heat and various chemicals as their
energy sources.[245] Overmann
et al. (1992) found bacteria in seawater that can photosynthesize at .0005%
of the light intensity at the sea surface.[246] That is the light intensity at a distance of
447 AU from the sun. Pluto is at a mean
distance of 39.5 AU.[247] This finding relaxes constraints on how far
out in space photosynthetic life could survive, holding other factors
aside.
One specific extremeophile is Deinococcus radiodurans. This bacteria was first discovered in 1956
by a researcher working on preserving food by irradiating it. Meat that had
been irradiated spoiled anyway, and the culprit turned out to be D. radiodurans. Since its discovery, it has been shown to
survive radiation of up to 1.5 million rads (1500 times the dose needed to kill
a human) and if frozen, it can survive up to 3 million rads. It can also survive extended periods without
water (dessication).[248] Radiation in space is up to 18 million rads.[249]
When adverse conditions develop, many bacteria
protect themselves by becoming dormant.
The dormant forms are known as endospores and cysts. Endospores, for example, have barely
detectable metabolic activity and are extremely resistant to the effects of
heat, desiccation (drying), cold, chemicals and radiation.[250]
Recent discoveries about the longevity of bacterial spores make more plausible
the scenario that dormant bacteria could survive for eons inside small bodies
in outer space. Vreeland et al. successfully cultured bacterial
spores from a 250-million-year-old salt deposit in New Mexico.[251]
By what conceivable scenario might bacteria have
ended up in the interior of a comet or in the inner part of a cloud of
interstellar dust? I have not found
Hoyle and Wickramasinghe’s explanation comprehensively stated in any one
place. Their ideas draw on our
discussion in section 4.2 of how surprisingly complex organic molecules are
produced abiotically in outer space, since amino acids have been found in the
interior of meteorites. Hoyle and
Wickramasinghe assert that in the clouds of interstellar dust the process goes
even further- the most primitive forms of life are actually created in clouds
of interstellar dust, and this life can survive in the relatively protected
interior portion of the cloud. The life is, however, detectable from Earth [as
in the several articles described above] because it is not buried under several
meters of rock, as Clark et al. say
is necessary. The interstellar dust
condenses into a solar nebula- a disk of gas and dust that undergoes further
condensation to form a star, planets, asteroids and comets. A comet is mostly water, but up to 25%
organic compounds, (discussed in section 4.2).
Hoyle and Wickramasinghe assert that some percent of these organic
compounds is actually primitive life forms.
The comet forms around a core of radioactive material, such as the
isotope 26Al.[252] [See the text accompanying figure 10 for an
explanation of isotopes.] This is an
aluminum isotope that is a fairly powerful heat source and has a half-life of 7
x 105 years. The heat from
the radioactive core maintains water around the core in liquid form.[253] So early in the life of a comet, we have a
heat source, water and nutrients (the complex organic molecules). This is all that is thought to be needed to
support primitive life.[254] “Microorganisms are extremely versatile,
chemically. They can live on a very
simple medium with little more than water, carbon and nitrogen sources, and
some simple organic compounds for their energy. Most can synthesize their need for vitamins and amino acids.”[255]
As the heat from the core and the supply of nutrients diminishes, the primitive
life goes into a dormant state. We
already know that bacteria can be revived after 250 million years. One of the main points of the landmark Gold article is that, “The chemistry of
life we now know need not be the one associated with its essential orgin.”[256] Hoyle and Wickramasinghe hypothesize that
the earliest forms of primitive life could be revived after billions of years.
8.1.1 Verification
Hoyle and Wickramasinghe have made spectral
findings suggestive of the existence of microbes in space. Other scientists have reasoned that it might
indeed be possible for microbial life to exist in the interiors of small bodies
in outer space. It remains to verify
the remote observations and theoretical reasoning with up-close
observations. Most proposals focus on
comets. A number of missions are
currently planned that will yield relevant data.
The ideal situation for investigation of comets
would be for researchers to be in physical possession of a supply of material
from a comet nucleus so that they can conduct a virtually unlimited amount of
tests as ideas occur to them. The idea
is to land on the surface of a comet, drill into the comet and retrieve a core
sample several meters long. Successful
return of the sample to Earth would enable scientists to study cometary
material with the extensive tools available in an Earth-based laboratory. Such study would determine whether
microscopic life exists in comets, provided that terrestrial contamination
could be ruled out. There are many
technological obstacles to a sample return mission.[257]
While we wait for a nucleus sample return
mission, there are several missions to comets that are either launched or near
launch:
Stardust: Launched 1999. The probe will fly into the tail of a
comet, collect comet dust using a material called aerogel, and return samples
to Earth. If the aerogel comes back
with bacteria on it, that would certainly indicate that comets do harbor life,
provided terrestrial contamination can be ruled out. However, Clarke et al.
(1999)[258] caution that this mission could be
self-sterilizing because the comet dust will strike the aerogel at 6.1 km/sec,
instantaneously converting kinetic energy to extreme heat.[259]
Contour: Launched 2002. The mission involves close flyby (100
kilometers) of a comet nucleus and performing spectrometry similar to Earth
based work done on interstellar dust by Wickramasinghe et al. in order to determine constituents of the comet. [Note: This probe was launched in July, 2002
and lost in August, 2002].
Rosetta: Launch 2004. Itinerary is for prolonged close up
observation of a comet (as close as 16 kilometers), landing on the nucleus and
conducting in-situ examination of the surface of the nucleus. However, Rosetta will not dig into the
interior of the comet. As Clarke et al. concluded, the microbes, if there
are any, would be found in the protected interior of the comet.
Deep
Impact:
Launch 2004. A probe will approach a
comet and launch a projectile at the nucleus.
The impact will excavate a deep crater in the nucleus, revealing the
deep interior of the comet. The probe
will use remote sensing instruments to look inside the crater and analyze the
ejecta in order determine the constituents of the comet. Observations will be made from as close as
500 kilometers.
Even if it is proven that bacteria and/or viruses exist in the interior of comets (step 1), it would still remain to be proven (2) that these bacteria/viruses can enter the Earth’s atmosphere and (3) that they can reach the surface of the Earth as viable lifeforms, and (4) that they are incorporated into the biosphere, or that they started the biosphere (genesis).
8.2 Life from outer space was delivered to the early Earth so that the origin of Earthly life is extraterrestrial.
Mainstream research is pursuing the hypothesis
that organic matter and cell-like structures rained down from space upon the early
Earth. (See sec. 4.2, where ample
citations are provided). One area where
Hoyle and Wickramasinghe go further is to assert that living microorganisms
rained down, and this is how life started on Earth. If the empirical research discussed in section 7.1 above
establishes that comets contain dormant microorganisms, then it becomes
possible that these organisms were raining down upon the Earth in relatively
large quantities during the heavy bombardment from 4.5-4 billion years ago.
In the panspermistic scenario, when a comet
travels through the inner solar system, dormant bacteria are released from the
nucleus into the coma and tail of the comet, where some of it reaches the Earth
surface in viable condition. Once
vented from the tail of a comet, any microbes would be floating in space and
subject to the extremes of temperature, the vacuum and the ionizing radiation
discussed by Clarke et al.. These factors would render even hardy spores
non-viable relatively quickly. However,
in the current epoch, perhaps 4 times a century,[260] the
Earth passes through a recently formed comet tail, which yields an
instantaneous input of about 2,000 tons of dust into the stratosphere.[261] In such conditions, the microbes would not
have to survive for long in outer space before entering the atmosphere. During the heavy bombardment, there were
perhaps 100-1000 times as many comets making close approaches to Earth. This left many more opportunities for the
Earth to accrete fresh comet dust.
An essential step in the logic of the
panspermistic argument is establishing that life could be transferred between
solar systems. Over the age of our
solar system (4.6 billion years) some 2 x 1014 comets have been
ejected from the solar system by gravitational perturbations of Saturn, Uranus
and Neptune, passing stars, galactic tides and giant molecular clouds (see
section 2). These comets are now
independent of the solar system and travel in the disk plane of the Milky Way
galaxy.[262] Many of these comets probably pass through
other solar systems and would be a prime vehicle for panspermia if comets do
contain dormant microscopic life-forms.
Conversely, Zheng and Valtonen (1999) calculate that over the past 4
billion years, thousands of comets originating in other solar systems have passed
entered our system, and hundreds of these have collided with the Earth.[263]
Another essential step in validation of Hoyle
and Wickramasinghe’s assertions is proof that microorganisms in a dormant state
could survive entry into the Earth’s atmosphere. Several means have been proposed. First, if a particle is very small in size and has a high ratio
of surface area to mass, the particle will be fully decelerated if it encounters
its own mass of air molecules. If this
occurs, the particle will not burn up in the atmosphere, rather it will drift
or waft to the surface of the Earth.[264] Second, Clarke et al. (1997) state that, “It is well known that due to the process
of ablation,[265]
competent meteoroids centimeters in size can readily traverse the atmosphere
without appreciable melting of their interiors.” Protected in the interior could be microorganisms. Third, if small particles enter the
atmosphere at an oblique angle and low velocity relative to the Earth (if they
are moving at approximately the same speed of the Earth), there will be minimal
entry heating. Survival of
microorganisms in the interior of the particle is probable.[266]
Once the microorganisms have landed on the
surface, how would the dormant microbes populate the Earth? A plausible scenario draws upon firmly
established scientific knowledge. When bacteria find themselves in a new
environment, they first go into a lag phase.
The microorganisms increase in size while they synthesize the enzymes
and components needed for reproduction.
Then the bacteria begin to reproduce.
Under favorable conditions, C. perfringens, one of the fastest-growing
bacteria, has a doubling time of 10 minutes, while that of E. coli is 20
minutes. M. tuberculosis has a slow
doubling time of 12-16 hours.[267] If even one bacterium survived entry into
the atmosphere, given a doubling time of 1 hour, after 24 hours there would be
16,777,216 bacteria on the planet, (although environmental conditions might not
be optimal and there is almost never a 100% survival rate).
A significant number of scientists in
space-related disciplines expect to find that microorganismic life is pervasive
in the universe.[268] If so, there are three basic choices for
genesis. Either life started de novo
somewhere in the universe and subsequently spread to the rest of the universe,
or genesis occurred on Earth and then spread to the rest of the universe, or
genesis has occurred in multiple places.
8.3
Extraterrestrial microbes have continued to be delivered to Earth in the
form of comet dust and interstellar dust filtering through the atmosphere to
the Earth’s surface.
Hoyle and Wickramasinghe contend that
extraterrestrial microorganisms have continued to reach the surface during the
entire existence of the planet Earth.[269] If validated by observational evidence, the
Earth would be currently and significantly an open system-not just 4 billion
years ago. These primitive microbes, if
found, may have similar DNA to strains of microbes on Earth because life on
Earth is derived from these samples.[270] However, a high degree of genetic change and
diversification may have taken place in the eons since some of these
microorganisms reached this planet. (See end section 9.3.1 page 45). When extraterrestrial microbes fall onto a
planet with an already thriving biosphere, they are incorporated into the
ongoing ecology. “Although there would
be a large fraction that perishes, of those that do not, the various
environments on the Earth pick up the types best suited for replication under
the conditions that locally prevail.”[271]
If bacteria do exist in comets, it is a
plausible hypothesis that the Earth is, even in the current epoch, showered
with viable extraterrestrial bacteria whenever the Earth passes through the
tail of a close approach comet.[272] (See section 7.2). Several projects are collecting data to evaluate the idea. High altitude balloon experiments are
conducted by a consortium of the Indian Space Research Organisation (ISRO) and
the Inter-Universities Centre for Astronomy and Astrophysics in Pune, India. Wickramasinghe is affiliated with this
project. In November, 2000 a balloon
was sent to an altitude of 16 km, and collected samples of living bacteria. As high as that is, it is still possible
that the microbes collected could be Earthly bacteria kicked up into the
stratosphere by air currents. The
bacteria collected in the balloon’s sampling device were, according to
Wickramasinghe, “a hitherto unknown strain." "It is so different from anything we've seen before that
there are only two possible explanations." The first is that "organisms have been lifted from the Earth
to great heights in the skies and have somehow multiplied there and changed
over time." The second is "that this is an example of primitive alien
life." To eliminate the first possibility,
it would be necessary to sample from a higher altitude.[273]
On January 20, 2001, the same group sent a
balloon aloft which collected samples from an altitude of 41 km of two species
of living bacterial cells and on species of fungus. The findings were reported in a peer-reviewed journal.[274] According
to the article, this is the first time that any life has been discovered so
high in the atmosphere. That assertion
may be incorrect.[275] The authors
maintain that the samples do not originate from the surface of the planet or
lower atmosphere:
Convection currents lead to mixing of ground level
particulates in the air that can be carried relatively easily into the
troposphere, but temperature inversions beyond 15 km lead to barriers through
which very few aerosols can penetrate. Whenever rare events such as volcanic
eruptions loft particles above 30 km, particles larger than a few microns fall
back quickly to the ground under gravity. The isothermal temperature regime
between 15 and 25 km effectively stops the ascent of particulates, and the
rapidly rising ambient temperature gradient at higher levels should make the
upper stratosphere almost impervious to the transport of aerosols from the
ground.
The authors state, “The bacterial material
... can be regarded as forming part of the 100 tonnes per day input of cometary
material known to reach the Earth.”
This entire manuscript gives the reader the tools to evaluate that
statement. We can say that it is quite speculative, since science has no
samples of comet dust that contain bacteria.
Indeed, the discovery of such comet dust would prove the existence of
extraterrestrial life. It would rank among the most significant discoveries in
the history of science.
NASA’s Ultra Long Duration Balloon Project would be a means of testing and reproducing the findings of life forms in the upper stratosphere. This balloon is projected to be capable of attaining a minimum altitude 32 km for 100 days. The first use of the balloon will be for research into cosmic rays. At some point in the future the balloon could be equipped to collect samples of the stratosphere in a search for bacteria.[276]
9. Conclusion
Considering the Earth as an open system has been
a useful device allowing the review of a large body of research. The open system/closed system concept is
thermodynamics and what makes an open system distinct is that it allows matter
to cross its boundary. Proceeding on
that basis, several conclusions are in order.
First, the Earth does allow matter to pass
across its boundary, the atmosphere, but in a sense it is trivial. Imagine that a 1 micron particle of
interplanetary dust burns up while entering the atmosphere. The particle has kinetic energy as it enters
the atmosphere at high speed. The
kinetic energy is turned into heat energy by the friction between the particle
and air molecules. The heating of the
air molecules is a minute gain in energy to the Earth system.
A harder test is whether the exchange of matter
is essential. The biologist and systems
scientist von Bertalanffy provides a framework for analysis. He analyzed living organisms as open
systems.[277] Warm blooded animals are able to receive
thermal energy (heat) from direct solar radiation (sunshine), convection (warm
air), and conduction (another warm body).
They can shed excess heat by perspiring, panting and increasing the flow
of blood to the body surface so its warmth can be lost to the cooler ambient
air. The latter process is facilitated
by stretching out in order to maximize the exposed surface area. Warm blooded animals are of course open to
matter in that they ingest nutrients and excrete wastes. They are self-regulating open systems which
must exchange energy and matter with their environment in order to
survive. Without the matter exchange
the life-form cannot get enough energy to maintain its state of being an
organized system and it returns to disorder (entropy)- it perishes, liquifies
and eventually becomes dust.
We can conclude that the Earth was an open
system just after its inception. The
hypothesis that during the heavy bombardment complex pre-biotic organic
molecules accelerated and perhaps were necessary for genesis would mean that an
exogenous input was significant for the emergence of the biosphere. And without the biosphere, the atmosphere
would probably be far different, because early chemosynthetic bacteria may have
altered the atmosphere, making it suitable for photosynthetic life, which then
further altered the atmosphere by producing oxygen.[278] If the first life was not native to Earth
(transpermia or panspermia), then exogenous matter would be necessary for the
existence of the biosphere.
Second, inputs of exogenous matter probably have
had and continue to have a significant effect on the Earth system. The K/T boundary impact alone indicates
this. There has been progress in tying
other mass extinctions of life to impacts.
Nearby supernovae may be responsible for minor extinctions. The work of Kortenkamp and Dermott links
astronomical cycles to possible atmospheric causes of the ice ages.
Third, the pre-biotic organics theory opens the
possibility of redating planet formation, and this has implications for the
open system idea. The current standard
theory is that the Earth was formed 4.5 billion years ago, followed by a heavy
bombardment that lasted another 500-700 million years.[279] Yet without the delivery of the pre-biotic
molecules during the heavy bombardment, there might have been no genesis, or it
would have taken very much longer.
Without the inputs from the heavy bombardment, the Earth could be all
but missing its biosphere and have a different (non-oxygen) atmosphere. Perhaps we should define planet formation as
complete only after the heavy
bombardment. Then we could say that
since planet formation, the Earth has been a closed system, although inputs of
exogenous matter appear to have significantly influenced the system.
Fourth, a problem for the Earth as open system
argument is that after the system is in motion, it can sustain itself without
exchanging matter with outer space. It
is not like a life-form or an internal combustion engine which require matter
to sustain the process. (The life-form
requires the input of food, the engine requires the input of fuel. Both food and fuel are repositories of
energy). If there had been no mass
extinctions due to large asteroids and comets and no minor extinctions due to
nearby supernovae, then there might not be humans, but there would be large
land animals. If there had been no 4x
spike in meteoritic bombardment perhaps causing the Cambrian explosion then
there might be a less thriving biosphere, but a biosphere with surface life
nonetheless. Even if an impact into an
ocean is one way the Earth emerges from ice ages, CO2 emissions from
active volcanoes would probably eventually lead to a warming. Arguably, an ice age Earth still has 4
functioning subsystems: for example, the biosphere extends to Antarctica. Humans survived and advanced during the last
ice age. And in no case would there be
any outputs of matter in the form of waste products whose excretion is
essential to the viability of the Earth as a functioning system. So the situation is not one of exchange, or
interdependence, but of dependence where the Earth is usually on the receiving
end.
The individuals quoted in the introduction seem
to envision a universe of significantly open, interdependent systems within
systems. It is an enchanting
thought. The current scientific
evidence doesn’t quite support the view.
Yet the open system perspective remains a legitimate source of creative
hypotheses, which are the engine of scientific advancement provided they are
subject to rigorous standards of proof.
Perhaps one young scientist will read this manuscript and notice
something in his or her research that would otherwise have gone undetected.
Appendix I: Detailed Analysis of Small Comets Debate
The key to sensing objects with radar is the
radar cross section (RCS). This is the
size of the scanned object as perceived by the radar. It is dependent upon the size, density, type of material,
smoothness of surface of the object, and other factors. For example, stealth technology is about
reducing the amount of radar waves reflected back to the receiver so that an
airplane appears to be the size of a pebble on the radar scope. For the altitude of 1000-3000 km, the naval
radar can detect objects with a radar cross section as small as .1 meter2. For comparison, this is the same radar cross
section as a locust.[280]
Knowles et
al. calculate the average radar section of a small comet to be .4 m2. This figure is based upon careful review of
the theoretical characteristics of small comets, as explained in various
articles by proponents of the small comet hypothesis. Small comets range in physical diameter from 5 to 20 meters, but
because they are porous, of low density, and composed mainly of water snow,
they have very small radar cross sections.
In various articles, Frank and Sigwarth state
that small comets break up by an altitude of 1000 km [Frank and Sigwarth, 1988,
1993], but they also report detecting evidence of small comets at 10,000
km. [Frank and Sigwarth, 1997]. Accordingly, Knowles et al. run two searches of the data. First, they ask whether, any small comets were detected at 1000-3000
km altitude, where the radar has a .1 meter2 sensitivity. Then they ask whether any small comets were
detected between 3000-20,000 km, where the average sensitivity of the radar is
.5 meter2. Again, Knowles et al. calculate the average radar section
of a small comet to be .4 m2.
In the lower altitudes (1000-3000 km) the radar
sensitivity is far below that of the average cross section of a small comet, so
if there are any small comets, a good portion should be detected. In the higher altitudes of 3000-20,000 km,
the radar would not be sensitive to the average small comet, but many of the
larger diameter comets would be observable by the radar. However, the result of the radar search was
that absolutely no small comets were detected.
During the approximately 1 month of searching
with the radar, some 12,000 unidentified targets were observed. Of these, none fit the theoretical
characteristics of small comets. 50 were
signal noise, and the rest were either new satellites, moving too slowly to be
small comets or were in gravitationally bound orbits (not unbound infalling
objects).
Frank and Sigwarth reply
“Comment on ‘A search for small comets with the Naval Space Command Radar’”
In a later issue of the journal in which Knowles et al. was published, Frank and
Sigwarth were given space to respond to the findings.[281] They argue that the whole point is that
comets are hard to detect, and the Naval Space Command radar is simply not one
of the instruments that can detect them.
Their criticisms shall here be evaluated point by point.
In §2 of the their reply, Frank and Sigwarth
claim that Knowles et al. use a
higher dust content for the small comets than that documented by Frank and
Sigwarth. Frank and Sigwarth assert
that using a higher dust content raises the predicted radar cross section (RCS)
of the small comets the radar is searching for. This way, Knowles et al.
can say that the naval radar has the capability of seeing them, yet they do not
show up, so they do not exist.
This contention by Frank and Sigwarth is
apparently incorrect. In the Knowles et al. article, the authors do run some
calculations of what the small comet RCS would be if they had 10% dust
throughout the comet.[282] The RCS would be much larger. But they expressly state that these
calculations are for comparison purposes only.
Knowles et al. specifically do
not use the 10% dust RCS predictions in assessing whether small comets were
detected.
In §3, Frank and Sigwarth contend that “The model of the comets used by Knowles et. al employs the small density for
the water snow, .02 g/cm3, in order to yield the largest radar cross
section.” By calculating that small
comets have a relatively large RCS, Knowles
et al. would then be able to report that the comets should have been visible
to the radar, but were not detected, so they do not exist. Alternatively, if the calculations were to
show that the comets should have a very small RCS, then it would be easier for
Frank and Sigwarth to argue that the comets are simply not detectable by the
naval radar and that is why they were not detected.
This contention by Frank and Sigwarth is
false. Knowles et al. expressly state that raising the density raises the cross
section (and by implication, using the lower density of .02 lowers the cross
section): “The above computations were made with the minimum snow density given
by Frank and Sigwarth [1993]. Raising
the snow density to .1 g/cm3 (their upper limit) approximately
doubles the RCSs….”[283] Knowles
et al. use the lowest density, yielding the lowest predicted cross section
for the small comets. This lowers the
bar that the small comet theory needs to overcome. Because they have such a small predicted RCS, the expected
frequency of detection is lower, so fewer small comets need be identified in
order to validate the theory.
In §3, Frank and Sigwarth object to the
assumption that small comets are perfect spheres. Knowles et al. had made
this assumption “for computational ease.”[284] It is indeed a fact that the RCS of a
dielectric[285]
sphere exhibits extreme oscillations.[286] About half the time, the sphere has a large cross section, but the other half of
the time the cross section drops precipitously, sometimes to near zero. This characteristic would make it more
difficult to observe small comets by radar, and would go some distance towards
explaining why the naval radar did not detect any. Frank and Sigwarth assert that in nature, small comets are
probably lumpy spheres, so that it would have been better to model them as an
ensemble of spheres of diameter x.
This objection has merit, but Knowles et al. demonstrate that it did not
affect the results. In the original
article, Knowles et al. noted that
their model did yield large oscillations in the predicted RCS of small comets.[287] There they ascribed the oscillations to
other sources, but they did expressly note their existence and apparently took
the existence of oscillations into account in calculating the expected
detection frequency of small comets.
Furthermore, in their reply to Frank and Sigwarth’s commentary, Knowles et al. calculate the RCS for small
comets as an ensemble of spheres. The
result is the same type of oscillations as with the perfect sphere model. Knowles
et al. based their computations on a model of the radar characteristics of
objects that are an ensemble of spheres.
The model was developed by independent third party researchers and
tested at a highly-regarded radar research facility at the University of
Florida, where it was found to yield accurate predictions of RCS. (p. 22,610).
In §4, of their commentary, Frank and Sigwarth
argue that Knowles et al.’s
assumption of a perfect sphere shape for small comets has the effect of raising
the predicted RCS- thus lowering the bar for disproving the existence (“If they
have such a big RCS, why didn’t we see them?”). They again state that Knowles et al. should have modeled small comets
as ensembles of smaller spheres. They
say, “Note that small comets which are characterized by surfaces with smaller
bosses than 5-meter diameter will have small RCSs relative to those shown in
Figure 2.” (p. 22,606). Modeled this
way, small comets would have a small predicted RCS, so that the predicted
detection rate would be very low.
It
takes some explanation to understand Frank and Sigwarth’s terminology. A “boss” is a feature of medieval European
cathedral architecture. It was a
decorative hemisphere placed over the intersection of a structural joint in
order to hide the structural components.
They are stating that small comets are lumpy spheres, and that the
structure is a core with hemispheres clumped around it.
The problem with Frank and Sigwarth’s quoted assertion
is that it is inconsistent with their own figure 2 on the very same page (p.
22,606). This figure shows that small
comets with diameters 7 meters or larger should be detectable by the naval
radar. A 7 meter comet is unlikely to
have bosses of 5 meters each. Therefore
surfaces with smaller bosses than 5 meters are already included in their figure
2 and figure 2 says that the naval radar should be able to detect the typical
small comet.
In §5, Frank and Sigwarth claim that the size
distribution of small comets used by Knowles
et al. is skewed towards a search for larger size small comets than
actually exist. They state that the
typical diameter of a small comet is 8 meters in diameter. Yet according to Frank and Sigwarth’s own
graph (fig. 2, p. 22,606), a comet 8 meters in diameter has a RCS above .1
meters2 and thus would have been detectable by the naval radar
within the altitude of 1000-3000 km.
Knowles et al. used perhaps the most appropriate
radar system on Earth to search for small comets and found none. Frank and Sigwarth were given the
opportunity to critique the research, but none of their objections appears
valid. The results of the Naval Space
Command radar observations seem to be strong observational evidence against the
existence of small comets.