During their residence in the solar system, cosmic dust particles are exposed to radiation from solar flares and galactic cosmic rays. The dose from these sources is estimated to exceed most organisms' tolerance to radiation when exposed on time scales of 105 years (see Clark et al., 1998). Thus, IDPs with residence times in the solar system of 105 to 107 years would have been radiation sterilized. As is discussed in Chapter 5, a possible exception would be IDPs derived from dust that intersects the orbit of Earth from a cometary coma or tail and for which lifetimes, and thus exposure to lethal radiation, could be as short as days or months.

Samples of cosmic dust for analysis of composition have been obtained from meteoroids collected in the stratosphere by aircraft and Long Duration Exposure Facility (LDEF) satellite and from micrometeorites obtained by the melting and filtration of large quantities of polar ice (Bradley and Brownlee, 1991; Maurette et al., 1991; Maurette, 1998).

Antarctic micrometeorites ranging in size from 100 to 400 mm, as well as smaller particles collected by aircraft and the LDEF satellite, are related primarily to the carbonaceous chondrites, and within this group mostly to the CM chondrites (which account for about 2 percent of meteorite infalls to Earth). Carbonaceous chondrites such as the Murchison meteorite are rich in organic compounds, including many organic compounds, such as amino acids, associated with terrestrial biochemistry (see Cronin et al., 1988). As mentioned above, some IDPs are also derived from comets, which contain abundant simple organic components (see Table 5.1), but whose inventory of complex organic molecules is less well known than is that of the carbonaceous meteorites. Some of the cometary organic compounds such as HCN, aldehydes, and ammonia are involved in the abiotic synthesis of more complex molecules, including amino acids and some of the bases present in DNA and RNA (see, e.g., Miller and Orgel, 1974). Thus, it is not unreasonable to expect that compounds important in biochemistry may also be present in comets, although this conclusion likely is dependent on whether at some time during a comet's history liquid water was present either on its surface or in its interior.

NATURAL INFALL OF DUST TO EARTH

Based on measurements of the impact craters on the LDEF satellite, the rate of accretion of cosmic dust on the present-day Earth is estimated to be 4 ± 2 × 1010 g/yr (Love and Brownlee, 1993). A somewhat higher infall rate of 7 to 25 × 1010 g/yr has recently been estimated from osmium isotopes (Sharma et al., 1997). Direct measurements of the flux of micrometeorites reaching Earth's surface (Maurette et al., 1991; Hammer and Maurette, 1996; Taylor et al., 1996a), and comparison with the IDP preatmospheric flux at 1 AU (Love and Brownlee, 1993), indicate that micrometeorites in the 50- to 500-µm size range deliver to Earth's surface about 2 × 1010 g of extraterrestrial material each year. This annual flux of IDPs is similar in magnitude to that of larger objects (1- to 10-m meteorites and 1- to 10-km asteroids and comets) averaged over longer time scales (Ceplecha, 1992). Most of the IDP mass is in the form of micrometeorites with sizes of approximately 200 mm. The infall rate of IDPs appears to have varied over geologic time (Farley, 1995) and may have been as much as a factor of 10 higher 500 million years ago in comparison to the present-day flux (Schmitz et al., 1997).

It has been predicted that approximately 99 percent of the micrometeorites larger than 100 µm are completely melted upon atmospheric entry and that only small particles of less than 20 µm are not heated to at least 160 °C (Brownlee, 1985; Love and Brownlee, 1991, 1993). These theoretical calculations are critically dependent on the particles' size and on their velocity and angle during entry. Because particles of less than 20 µm make up only about 10-3 percent of the IDP or micrometeorite mass, only about 4 × 105 g/yr of the cosmic dust escapes heat sterilization during atmospheric entry. In addition, calculations of helium loss from IDPs in the size range from 5 to 150 µm suggest that only 0.5 percent of the mass of particles (approximately 2 × 108 g/yr) are heated below approximately 600 °C (the temperature at which helium is released) during delivery to Earth's surface (Farley et al., 1997). These predictions thus suggest that only some of the smallest IDPs (particles smaller than 20 µm), which make up only a minor fraction of the original IDP mass flux, escape heating to the temperatures of greater than 160 °C considered necessary for biological sterilization (Microbiology Advisory Committee, 1993) during atmospheric entry. However, this conclusion must be considered somewhat tentative because IDPs are exposed to peak temperatures on time scales of a only few seconds (Love and Brownlee, 1991). Some proteins may be



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