6
Cosmic Dust

Cosmic dust particles (generically referred to as interplanetary dust particles, or IDPs) are derived from a variety of sources, including interstellar grains and the debris from comets, asteroids, and the planets and possibly their satellites. The dominant sources of IDPs on Earth are asteroids and comets, with a small, but still not negligible, amount of dust coming from all other sources combined. The comets certainly are a major contributor at a level ranging from 10 percent to 50 percent; the remainder of the IDPs come primarily from the asteroids. The dust from comets was lifted from the surface of the nucleus by outflowing gas and, except for having been exposed to the solar wind, is essentially unprocessed because it was an integral part of the cometary nucleus. The dust from other sources, however, is processed, most of it having been created in collisions. In the case of asteroidal dust, an impact of a small body onto a typical small asteroid is sufficient to eject considerable debris from the gravitational field of the asteroid. It is thought that particles with diameters greater than 50 µm are derived mainly from asteroids, whereas those with diameters less than 30 µm come from comets (Love and Brownlee, 1991).

Larger collisions are required to give dust enough energy to escape from planets or satellites. Such dust grains have experienced significant spike heating in the impact event that ejected them from the parent body. A possible exception is the dust around Jupiter generated from micrometeorite impact ejecta from some of its satellites, including Europa. However, the amount of this jovian satellite-derived dust is estimated to be small in comparison to captured interplanetary dust (Colwell et al., 1998).

The residence times of IDPs in the solar system depend on several factors (see Nishiizumi et al., 1991). Poynting-Robertson drag forces IDPs into Earth-crossing, inwardly spiraling circular orbits around the Sun. The IDP lifetimes owing to the Poynting-Robertson effect are dependent on particle size; for the IDPs in the range of sizes reaching Earth's surface, lifetimes are estimated to be 105 to 106 years, although chaotic orbits may decrease this value significantly. Another factor influencing particle lifetimes is the degree to which they are subject to high-velocity disruptive collisions. This factor depends on particle velocity, which decreases with increasing distance from the Sun. How the Poynting-Robertson drag and the disruptive collisional effects influence average IDP lifetimes is difficult to evaluate, but it is likely that in combination these effects act to shorten IDP residence times in the solar system. Measurements of galactic and solar cosmic-ray-generated radionuclides have enabled direct estimates of IDP solar system residence times of 105 to 107 years, somewhat longer than those projected on the basis of theoretical considerations (Nishiizumi et al., 1991). An exception would be the shorter residence time of dust recently ejected from a cometary nucleus, which might infall to Earth as the comet's coma or tail intersects Earth's orbit, as discussed in Chapter 5.



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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making 6 Cosmic Dust Cosmic dust particles (generically referred to as interplanetary dust particles, or IDPs) are derived from a variety of sources, including interstellar grains and the debris from comets, asteroids, and the planets and possibly their satellites. The dominant sources of IDPs on Earth are asteroids and comets, with a small, but still not negligible, amount of dust coming from all other sources combined. The comets certainly are a major contributor at a level ranging from 10 percent to 50 percent; the remainder of the IDPs come primarily from the asteroids. The dust from comets was lifted from the surface of the nucleus by outflowing gas and, except for having been exposed to the solar wind, is essentially unprocessed because it was an integral part of the cometary nucleus. The dust from other sources, however, is processed, most of it having been created in collisions. In the case of asteroidal dust, an impact of a small body onto a typical small asteroid is sufficient to eject considerable debris from the gravitational field of the asteroid. It is thought that particles with diameters greater than 50 µm are derived mainly from asteroids, whereas those with diameters less than 30 µm come from comets (Love and Brownlee, 1991). Larger collisions are required to give dust enough energy to escape from planets or satellites. Such dust grains have experienced significant spike heating in the impact event that ejected them from the parent body. A possible exception is the dust around Jupiter generated from micrometeorite impact ejecta from some of its satellites, including Europa. However, the amount of this jovian satellite-derived dust is estimated to be small in comparison to captured interplanetary dust (Colwell et al., 1998). The residence times of IDPs in the solar system depend on several factors (see Nishiizumi et al., 1991). Poynting-Robertson drag forces IDPs into Earth-crossing, inwardly spiraling circular orbits around the Sun. The IDP lifetimes owing to the Poynting-Robertson effect are dependent on particle size; for the IDPs in the range of sizes reaching Earth's surface, lifetimes are estimated to be 105 to 106 years, although chaotic orbits may decrease this value significantly. Another factor influencing particle lifetimes is the degree to which they are subject to high-velocity disruptive collisions. This factor depends on particle velocity, which decreases with increasing distance from the Sun. How the Poynting-Robertson drag and the disruptive collisional effects influence average IDP lifetimes is difficult to evaluate, but it is likely that in combination these effects act to shorten IDP residence times in the solar system. Measurements of galactic and solar cosmic-ray-generated radionuclides have enabled direct estimates of IDP solar system residence times of 105 to 107 years, somewhat longer than those projected on the basis of theoretical considerations (Nishiizumi et al., 1991). An exception would be the shorter residence time of dust recently ejected from a cometary nucleus, which might infall to Earth as the comet's coma or tail intersects Earth's orbit, as discussed in Chapter 5.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making 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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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making exceptionally resistant to high temperatures and may resist short-term exposure to temperatures in excess of 160 °C (Brown et al., 1990; Taylor et al., 1996b). Direct examination of micrometeorites collected in the Antarctic has shown that the proportion of unmelted micrometeorites studied was much larger than predicted by models describing frictional heating of micrometeorites upon atmospheric entry (see, e.g., Hunten, 1997; Maurette, 1998). A source of this discrepancy may be that the average density of IDPs is less than the value of 2 g/cm3 assumed in the theoretical predictions. Recent radar-based observations of comet Hyakutake indicate that particles ejected from its coma consist mainly of ''fluffy" grains with densities of less than 1 g/cm3 (Harmon et al., 1997). Also, recent flyby observations of the C-type asteroid 253 Mathilde indicate that its density (1.3 ± 0.1 g/cm3) is about half that of CM chondrites (Veverka et al., 1997). Most of Earth's annual infall of micrometeorites and IDPs lands in the oceans, where their soluble components would dissolve and accumulate if they were not consumed by organisms or destroyed by geochemical processes. Recent analyses of Antarctic micrometeorites have shown that the current global flux of exogenous amino acids from micrometeorites is roughly 3 × 105 g/yr (Brinton et al., 1998). The maximum period of accumulation in the oceans would be 107 years, the length of time it takes for the total oceans to pass through hydrothermal vents where all dissolved organic components would be destroyed (Bada et al., 1995). Thus the concentration in the present oceans of extraterrestrial amino acids generated by the infall of cosmic dust would be on the order of 0.1 parts per billion. This interpretation is consistent with analyses of filtered seawater (K.L.F. Brinton and J.L. Bada, Scripps Institution of Oceanography, unpublished results) indicating that the concentration of extraterrestrial amino acids in the modern oceans is below the limit of detection—of less than a few parts per billion—of the analytical method used. These results suggest that although cosmic dust has been constantly accreted by Earth during modern times, organic compounds associated with this extraterrestrial debris do not apparently accumulate to measurable concentrations. POTENTIAL FOR A LIVING ENTITY TO BE IN OR ON RETURNED SAMPLES OF COSMIC DUST Because interplanetary dust particles are derived from a variety of sources, including interstellar grains and debris from comets, asteroids, and possibly planetary satellites, IDPs cannot be viewed as a distinct target body. As a result, the assessment approach used in this study does not lend itself readily to evaluation of the biological potential of IDPs. Instead, the task group considered the potential source(s) of the IDPs being sampled. For the purposes of this study, IDPs are viewed as originating from either a single identifiable parent body or multiple sources. Particles collected near a particular solar system body are viewed as originating from that body, possibly including grains recently released from that body. Thus, the potential for a living entity to be present in returned samples, and the associated containment requirements, are regarding as being the same as those for the parent body. On the other hand, IDPs collected in the interplanetary medium may represent a mixture of dust originating from many parent bodies. Because IDPs in the interplanetary medium are exposed to sterilizing doses of radiation, no special containment requirements are warranted. An additional consideration is whether the process of collecting IDPs in returned samples results in spiked heating of the sample to the temperatures considered necessary for biological sterilization. If so, then no special containment is required, regardless of the source of the IDP. SCIENTIFIC INVESTIGATIONS TO REDUCE THE UNCERTAINTY IN THE ASSESSMENT OF COSMIC DUST An important issue with regard to the collection of IDP and micrometeorite material in space is the temperature to which the material is heated during collection. In the STARDUST mission, aerogel will be used to capture solid particles from comets and asteroids and from the interplanetary medium, as well as charged particles from the solar wind. The estimated capture velocities are in the range of 6 km/s for the cometary particles and within a factor of two of that for most other interplanetary particles. It is possible (albeit unlikely) that an incidental

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making interplanetary dust particle may be collected that is not represented in the vast quantity of particles that rain down on Earth every day, for example, one derived from a planetary satellite. It is assumed that during capture by a spacecraft, temperatures in the range of 400 to 500 °C would be attained (although this would be as a result of pulse heating lasting less than 1 second or so). Thus any organisms, as well as organic compounds, present would likely be destroyed. However, the effect of short-pulse heating on organisms and organic compounds is not well understood. Recent experiments involving the exposure of amino acids to temperatures as high as 1,000 °C for a few seconds have found, surprisingly, that some amino acids survive exposure to high-temperature pulsed heating (D.P. Glavin and J.L. Bada, Scripps Institution of Oceanography, unpublished results). Clearly, research should be carried out with a number of organic compounds and heat-tolerant organisms to further evaluate the effect of pulsed heating at temperatures greater than the assumed sterilization temperature of 160 °C. SUMMARY Cosmic dust represents a valuable source of material for evaluation of the composition and characteristics of objects throughout the solar system. Because IDPs cannot be assessed in a fashion similar to the planetary satellites and small solar system bodies examined in this study, the source of IDPs is important. IDPs sampled near a parent body are treated as samples collected from that body. Alternatively, IDPs collected from the interplanetary medium are subject to sterilizing doses of radiation, and therefore no special containment is required. If the IDP is exposed to spiked heating resulting in extreme temperatures during sample collection, then no special containment is required regardless of the source of the IDP. Cosmic dust particles have collected on Earth throughout its history. The accumulation of the various components of IDPs and micrometeorites has apparently had no known adverse effects on Earth's biota. Thus it would seem unlikely that a returned sample of this type of extraterrestrial material would pose any kind of threat to Earth's biota or biogeochemical cycles. However, the possibility cannot be ruled out that future sample recovery missions will sample objects that are not represented in the present-day IDP and micrometeorite flux to Earth. REFERENCES Bada, J.L., S.L. Miller, and M. Zhao. 1995. The stability of amino acids at submarine hydrothermal vent temperatures. Origins Life Evol. Biosphere 25:111–118. Bradley, J.P., and D.E. Brownlee. 1991. An interplanetary dust particle linked directly to type-CM meteorites and an asteroidal origin. Science 251:549–552. Brinton, K.L.F., C. Engrand, D.P. Glavin, J.L. Bada, and M. Maurette. 1998. A search for extraterrestrial amino acids in carbonaceous Antarctic micrometeorites. Origins Life Evol. Biosphere, in press. Brown, P., P.R. Liberski, A. Wolff, and D.C. Gajduse. 1990. Resistance of scrapie infectivity to steam autoclaving after formaldehyde fixation and limited survival after ashing at 360°C: Practical and theoretical implications. J. Infect. Dis. 16:467–472. Brownlee, D.E. 1985. Cosmic dust: Collection and research. Annu. Rev. Earth Planet. Sci. 13:147–173. Ceplecha, Z. 1992. Influx of interplanetary bodies onto Earth. Astron. Astrophys. 263:361–366. Clark, B.C., A.L. Baker, A.F. Cheng, S.J. Clemett, D. McKay, H.Y. McSwenn, C. Pieters, P. Thomas, and M. Zolensky. 1998. Survival of life on asteroids, comets and other small bodies. Origins Life Evol. Biosphere, in press. Colwell, J.E., M. Horanyi, and E. Grun. 1998. Capture of interplanetary and interstellar dust by the Jovian magnetosphere. Science 280:88–91. Cronin, J.R., S. Pizzarello, and D.P. Cruikshank. 1988. Organic matter in carbonaceous chondrites, planetary satellites, asteroids and comets. Pp. 819–857 in Meteorites and the Early Solar System, J.F. Kerridge and M.S. Matthews (eds.). Tucson, Arizona: University of Arizona Press. Farley, K.A. 1995. Cenozoic variations in the flux of interplanetary dust particles recorded by He-3 in deep sea sediments. Nature 376:153–156. Farley, K.A., S.G. Love, and D.B. Patterson. 1997. Atmospheric entry heating and helium retentivity in interplanetary dust particles. Geochim. Cosmochim. Acta 61:2309–2316. Hammer, C., and M. Maurette. 1996. Micrometeorite flux on the melt zone of the west Greenland ice sheet. Meteoritics Planet. Sci. 31:A56–A57. Harmon, J.K., S.J. Ostro, L.A.M. Benner, K.D. Rosema, R.F. Jurgens, R. Winkler, D.K. Yeomans, D. Choate, R. Cormier, J.D. Giorgini, D.L. Mitchell, P.W. Chodas, R. Rose, D. Kelley, M.A. Slade, and M.L. Thomas. 1997. Radar detection of the nucleus and coma of comet Hyakutake (C/1996B2). Science 278:1921–1924.

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