2. Impact Events and Mass Extinctions

The most empirically verified evidence in support of the concept that the Earth is an open system comes from impacts of large objects (bolides) such as comets and asteroids. Since most of this topic is well-covered even in introductory textbooks it shall be treated here with apologies and only as necessary to elucidate the open system concept.

Large bolide impacts account for only 5% of the mass of the extraterrestrial matter reaching Earth. The Cretaceous-Tertiary (K-T) boundary impact of 65 million years ago was probably the trigger event for the extinction of the dinosaurs, allowing the rise of mammals and ultimately the evolution of humans. Bolide impacts have been implicated as the triggering or a contributing factor in at least six mass extinctions in the past 400 million years,[13] including the mass extinction at the Permian-Triassic boundary (251 million years before present.). This was the most extensive mass extinction known, and it is the one that cleared the way for the dinosaurs.[14] There are over 150 surviving craters around the globe, several with diameters in the 80 km range.[15] An ongoing project using remote sensing aims to uncover craters previously undetectable due to the erasing processes oferosion, sedimentation, volcanism and plate tectonics.[16]

This evidence establishes that the Earth system has been significantly influenced by inputs of extraterrestrial matter. Things would not have turned out the way they have in the sense that humans and even the dinosaurs might never have existed. 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.

It is useful to consider the competing explanations for why these bolide impacts occur, because they illustrate some possible connections of the Earth to the cosmos. Probably the standard explanation for asteroid impacts is that the gravity of Jupiter or Saturn periodically yanks a large asteroid from the asteroid belt and puts it on a course that intersects the orbit of the Earth. Going a bit further, there is a proposal that a yet unobserved Planet X disturbs the Kuiper belt of comets,[17] dislodging some into an intercept course with Earth. It is these comets which cause mass extinctions when they collide with the planet. A third explanation is the galactic plane hypothesis. Our solar system in fact is not stationary, but moves vertically in the Milky Way galaxy as the galaxy itself rotates in space. During its journey, our solar system encounters dense clouds of interstellar gas and dust.[18] These interstellar clouds are thought to exert a gravitational influence on the Oort cloud of comets,[19] dislodging some of the billions of comets contained there. Galactic cycles drive evolution on Earth. A fourth proposal is the Nemesis hypothesis. A yet unobserved companion star to the sun, probably a red dwarf with a highly elliptical orbit, periodically passes near enough Oort cloud to yank some from their orbits and send them towards Earth.[20]

2.1 Outputs of Life-Bearing Meteorites From Earth

By definition, a thermodynamically open system also requires output of matter. There are few, if any, theoretical objections to the possibility that some rocks of Earth origin have reached Mars and other planets in the solar system.[21] We know for certain that the process works the other way- at least 12 meteorites of Mars origin have been collected.[22] If a meteorite in the size range of the Chicxulub bolide hit the Earth, the impact would eject a variety of substances into the atmosphere, and some of the rocks would be at escape velocity (11 km/sec). A fraction of these would reach other planets in the solar system after a voyage that in theory would take about 10 million years. We know that bacteria exist in the interior of rocks,[23] we know that bacteria are extremely hardy, being able to survive high doses of radiation and extremes of temperature, and we know that bacteria reproduce extraordinarily rapidly.[24] Any bacteria in ejecta rocks would be relatively protected by the rock itself. If even one bacterium survived in the interior of a rock ejected from Earth, upon arrival at another planet, that bacterium could exfiltrate the rock, find a medium in which it could survive and begin reproducing.[25] The scenario considered most likely is that a piece of ejecta of Earth origin might have landed on Mars during the period billions of years ago when it appears that Mars had surface water and a thicker atmosphere than now. This rock contained Earth bacteria which took root on Mars, so that the Earth seeded Mars with microscopic bacterial life forms, which is thought to be as far as life ever got on Mars. The scenario could also work the other way- early Mars seeded the early Earth with microscopic life, which took root and flourished on Earth.