Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies Framework for Decision Making (1998) / Chapter Skim
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4 Asteroids and Meteorites
4 Asteroids and Meteorites
Pages 40-51
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From page 40... ...
asteroids; undifferentiated metamorphosed asteroids; and differentiated asteroids. The historical definitions of asteroids relate to the absence or presence of "cometary activity," which requires volatile compounds (especially water ice)
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From page 41... ...
Undifferentiated C-like types C Very low albedo, flat longward of 0.4 ,um; absorption band in UV and sometimes near 3 ,um Low albedo; C-like but brighter, more neutral Low albedo; C-like but brighter, strong UV Undifferentiated metamorphosed types Q Moderate albedo, strong absorption near 1 ,um and 2 ,um S Moderate albedo, reddish in visible, weak to moderate absorption near 1 ,um and 2 ,um B G Differentiated types S E Moderate albedo, slightly reddish linear slope V, J High albedo, like S-types but stronger and additional adsorptions High albedo, strong adsorptions due to olivine Moderate albedo, reddish in visible, weak to moderate absorption near 1 ,um and 2 ,um High albedo, flat or slightly reddish Others D Very low albedo, slightly reddish linear slope Very low albedo, reddish linear slope Carbonaceous chondrites Carbonaceous chondrites Carbonaceous chondrites Ordinary chondrites Ordinary chondrites Irons HED basaltic achondrites Brachinites Stony-irons, achondrites (?
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From page 42... ...
, velocities must once have been much lower, as in other planetary accretion zones. There are large gaps in the distribution of asteroids where planetesimals once must have existed but no longer do, owing to resonant perturbations by Jupiter (these include not only the Kirkwood commensurability gaps within the asteroid belt, but also the so-called secular resonances, and the vast volumes of space beyond 3.2 AU where asteroids are now rare and were presumably cleared out early in solar system history)
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From page 43... ...
The sampling of such asteroids by carbonaceous meteorites is likely to be biased toward those near middle-belt resonances (like the 3:1) rather than from outer belt asteroids; moreover, it is likely that the vast majority of carbonaceous chondrites come from fewer than 10 C-type parent bodies, although minor representation of a much vaster sample is likely.
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From page 44... ...
The resulting liquid water may have been eventually lost to space but, at least in the larger asteroids, remained for long enough to produce a secondary, hydrated Ethology from the primary anhydrous silicates, oxides, sulfides, and metal originally accreted by the asteroid (Zolensky and McSween, 1988~. The liquid water also reacted with the primary organic matter, producing the crop of secondary organic compounds found in carbonaceous chondrites today (Kerridge, 1993~.
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From page 45... ...
Based on their chemical and isotopic compositions, known chondrites, other than carbonaceous chondrites, are derived from at least a half dozen undifferentiated asteroids (Rubin, 1997~. Even though they show no
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From page 46... ...
that this metamorphism was caused by internal heating of the asteroidal parent bodies, perhaps by decay of recently synthesized radionuclides such as 26A1 or 60Fe, and that the most severely heated chondrites resided at the greatest depth within such an asteroid. Those ordinary chondrites that exhibit evidence for thermal metamorphism contain neither detectable organic matter nor hydrated minerals, so that two of the criteria for origin of life are not met.
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From page 47... ...
Thus, uncontained sample return from such unusual and/or unsampled bodies would have to await further investigation of their properties. For many asteroids, the requirements for life to have emerged (presence of liquid water, organic matter, and a usable energy source)
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Because the knowledge base for P- and D-type asteroids is highly speculative, the task group concluded conservatively that strict containment and handling requirements are warranted at this time. For samples returned from C-type asteroids, undifferentiated metamorphosed asteroids, and differentiated asteroids, the potential for a living entity in a returned sample is extremely low, but the task group could not conclude that it is demonstrably zero.
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From page 49... ...
The problem of association of meteorite types with asteroid types is an evolving multidisciplinary study, involving ground- and space-based remote sensing studies of asteroids; laboratory analysis of meteorite mineralogy; laboratory experimentation on issues like space-weathering processes, which affect interpretation of reflectance spectra; theoretical studies of the collisional and dynamical physical processes that liberate meteorites from their parent bodies and deliver them to Earth; and so on. A relatively new way of studying asteroids is by spacecraft flyby and rendezvous, which employs techniques not previously applied to asteroids (spatially resolved spectral reflectance maps, x-ray and gamma-ray mapping, and others)
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SUMMARY For samples returned from C-type asteroids, undifferentiated metamorphosed asteroids, and differentiated asteroids, the potential for a living entity in a returned sample is extremely low, but the task group could not conclude that it is zero. Based on the best available data at the time of its study, the task group concluded that containment is not warranted for samples returned from these bodies.
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From page 51... ...
1993. Origins of organic matter in meteorites.
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