3. Micrometeorites and Interplanetary Dust

Some 95% of the mass of extraterrestrial matter reaching the surface of the Earth is "space dust."[26] The Infrared Astronomy Satellite's observations indicate that micrometeoroids (MM) and interplanetary dust (IPD) pervade the solar system.[27] The main sources of MM and IPD are dust vented by comets and debris created by collisions between asteroids in space. [28] For example, Halley's comet generated roughly one hundred million tons of particles during its last flyby of the sun in 1986.[29] The particles drift into the inner solar system due to solar and planetary gravity,[30] Poynting-Robertson drag,[31] and solar wind drag, [32] where some encounter the Earth.

Currently, about 40,000 metric tons per year of MM and IPD cross the orbit of the Earth and enter the atmosphere.[33] There are spikes in the flux of MM and IPD 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.[34]

MM and IPD are microscopic. MM range in size from 50-400 micrometers (µm),[35] while IPD is about 50Å -40 µm in diameter. [36] The different names for the particles are solely due to their different sizes. The particles are studied under electron microscopes at up to 7000x magnification. Detailed analysis of the particles has been made possible by the development in the 1990s of mass spectrometers of enhanced sensitivity.[37] The typical particle is a heterogeneous composite of material found in terrestrial rocks such as ferro-magnesian minerals,[38] certain elements such as iridium generally found meteorites but not in terrestrial rocks,[39] organic carbon based compounds and amino acids (10%)[40] and radiogenic isotopes formed in space due to exposure to solar radiation.[41] It is the iridium and radiogenic isotopes which identify the particles as exogenous to Earth.

A portion of the particles, perhaps 50%, survive entry and reach the surface.[42] A much higher ratio of MM and IPD 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.[43] To get an idea of how unexpected this is for science, consider that geophysicist David M. Raup, one of the most open-minded scientists with respect to the extraterrestrial input of matter, wrote in 1985 of meteorites, "A fraction of these are large enough to survive the trip through the Earth'satmosphere." MM and IPD have simply upset the established knowledge of only a short time ago.

Over geologic time, the yearly flux has accumulated. If we use an estimate of 20,000 tons per year accretion rate,[44] and assume that the rate has been constant[45] since the end of the heavy bombardment (4 billion years), this yields 8 x 10¹³ tons of accumulation. For comparison, this is about 3 times larger than the mass of Mars' larger moon Phobos, which is estimated to be 2.73 x 10¹³ tons. [46] However, the mass of accumulated material is still far smaller than the mass of the moon, estimated to be 7.35 x 10¹⁹ tons. [47]