Asteroids and Meteorites
Asteroids, like comets, are the remnant population of planetesimals—those small primordial bodies from which the planets accumulated. Common asteroid types are described in Table 4.1. Generally, the asteroids considered are those that are relic planetesimals formed in and beyond the asteroid belt (which is located between 2.2 and 3.2 AU from the Sun), as far away from the Sun as the Trojans, which orbit at Jupiter's distance. Those formed in more distant locations are usually called, or at least thought of as, comets. This chapter examines the origin, composition, and environmental conditions of three major classes of asteroids: undifferentiated, primitive (C-type) 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) near or at the surface of the object. Watson et al. (1963) showed that water ice readily sublimates in periods of time short compared with the age of the solar system out to Jupiter's distance. Therefore, objects made of a primordial mixture of ices and refractories formed at asteroidal distances from the Sun may be expected to have lost surficial ices and be dormant "asteroids," whereas objects deflected into the inner solar system from much more remote storage locations (the Kuiper Belt, Scattered Disk, and Oort Cloud) retain such volatiles on or near their surfaces and show "cometary" activity (comae and tails) until they have lived for some thousands of years in the inner solar system.
While, for this reason, absence of near-surface volatiles does not guarantee an absence of volatiles at depth within asteroids, other factors determine whether or not asteroids have or have had appreciable quantities of volatiles. Inside some solar distance, volatiles may never have condensed within planetesimals. It remains a matter of conjecture and current research where this boundary existed (e.g., was Earth formed "wet" [e.g., Dreibus and Wänke, 1987] or were all of its volatiles derived from late-accreting planetesimals [Chyba, 1987], e.g., comets, from much farther out?). In addition, subsequent thermal evolution—perhaps dependent on body size and/or distance from the Sun—undoubtedly dried out some asteroids; perhaps it dried out most asteroids. Other processes (e.g., efficient megaregolithic overturn of rubble-pile asteroids by repeated collisions) might also have allowed volatiles to sublimate. Spectral reflectance studies and other remote-sensing techniques show that some asteroids likely are composed of a dry suite of minerals (e.g., metallic bodies). Although it is conjectural, most researchers would not be surprised if many asteroids from the middle of the asteroid belt out to the Trojans were to look briefly like comets following rare catastrophic, disrupting collisions, which would expose buried volatiles. (A minority but significant fraction of objects called asteroids, especially those in Earth-approaching orbits, may be dormant or dead comets.)
For nearly two centuries, it has seemed reasonable to the scientific community that meteorites might come from asteroids. But as late as the 1970s, there were no known physical mechanisms for transporting asteroid fragments to Earth. For instance, collisions sufficient to produce such drastic orbital changes would instead vaporize the asteroidal material. The problem is now solved, except for the details (Wisdom, 1985). Throughout the asteroid belt, there are zones where resonant gravitational perturbations by planets, primarily Jupiter and Saturn, form dynamically chaotic zones. When inter-asteroidal collisions near the boundaries of these zones send fragments into them, orbital eccentricities are rapidly increased and the fragments cross the orbits of the other planets—including Earth's. Fragments reach Earth primarily from the inner parts of the asteroid belt, especially near the 3:1 commensurability with Jupiter (those in the outer belt reach Jupiter first and are generally ejected from the solar system). Earth-crossing asteroids often continue to have aphelia in the asteroid belt, and thus they continue to suffer collisions with main-belt asteroids. The smaller fragments that encounter Earth as meteorites result from a multi-generational collisional cascade and come both directly from the resonances in the main belt and from cratering and collisions involving Earth-crossing asteroids.
A few meteorites are from the Moon and Mars (Warren, 1994). Some meteorites may be from comets (Campins, 1997) or from more distant asteroids, but none have so far been identified as likely candidates, and there are physical reasons that mitigate against that (the high velocity and weak strength of comets result in upper atmospheric disintegration of incoming cometary meteoroids; efficient dynamical mechanisms for delivering asteroidal debris from regions beyond the 5:2 resonance have not been identified). The collisional cascade also generates finer materials as small as interplanetary dust, which is transported by Poynting-Robertson drag and other radiation forces (Burns et al., 1979). The relative contribution of comets and asteroids to particles of various sizes in the interplanetary dust complex is not well known, but both sources contribute a significant fraction (Bradley et al., 1988).
TABLE 4.1 Common Asteroid Types
The early history of asteroids is shrouded in uncertainty. It is believed that a planet never accreted in the asteroid belt because of the influence of massive Jupiter. Certainly, the inclinations and eccentricities of asteroid orbits today mean that asteroids typically collide at velocities of 5 km/sec, which results in cratering and catastrophic fragmentation, not accretion. Yet for growth to the sizes of the larger asteroids (hundreds to nearly 1,000 km in diameter), 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). Perhaps collisions (and close gravitational encounters, if they were big enough) by those now-vanished bodies and others in Jupiter's accretion zone (Jupiter-scattered planetesimals) pumped up the velocities of the remaining main-belt asteroids. Alternatively, jovian resonances, which migrated through the asteroidal region during the late stages of Jupiter's growth, may have done so while asteroidal planetesimals were accreting, clearing out and/or pumping up the velocities of many asteroids (Ruzmaikina et al., 1989). The asteroids are also depleted by collisional fragmentation, but recent research suggests (but not yet definitively) that collisions are inefficient at disrupting asteroids (Asphaug et al., 1998) and probably did not play more than a minor role in reducing the original planet's-worth of mass in the asteroidal region to the roughly 0.01 percent that remains today.
Whatever the precise scenario for asteroids' origin, the asteroid population has been roughly what it is like today for at least 4 billion years, with the largest bodies never much larger than the 950-km Ceres. The asteroids have been gradually evolving by collisions ever since. Sufficiently energetic collisions break asteroids into pieces, imparting the fragments with velocities that exceed escape velocity so that they go into similar but separate heliocentric orbits (groups of asteroids in similar orbits are called "families"). A majority of collisions, however, lack the energy to disrupt an asteroid, yet are more than sufficient to shatter its constituent rocky materials. Therefore, most asteroids (at least those larger than a few kilometers in diameter) are expected to be shattered into gravitationally bound "rubble piles" (Melosh and Ryan, 1997). Exceptions may be the much stronger remnant metallic cores of differentiated bodies (i.e., bodies that once melted to the degree that metal sank into their cores and the least-dense silicates erupted onto their surfaces as lava).
It should be noted that the size distribution of asteroids is such that most of the mass (hence collisional kinetic energy of impacts) is in the largest objects. Thus the greatest damage is done on the largest spatial scales, resulting in coarse rubble, analogous to the lunar megaregolith. While surficial regoliths exist on the larger asteroids, which may be analogous to the fine-grained lunar regolith, asteroid interiors are not "gardened" at fine scales but rather are jumbled about. Therefore, while a significant fraction of meteorites have portions that contain solar wind gases and cosmic-ray tracks, indicating that they were once, for a brief period, near the surface of their parent body, much of the other material within asteroids must be expected to have never been close enough to the surface to be sterilized by cosmic rays, despite the collisional jumbling.
Meteorites bear witness to the collisional processes. Many are rocks (referred to as breccias) whose properties reflect the collisions that break, shatter, and weld together the asteroidal rocks. While localized melting sometimes occurs, melting has affected only a tiny fraction of meteoritic materials and has probably never been sufficient to differentiate all, or even a substantial part, of any asteroid (Keil et al., 1997). There is evidence that some asteroids were heated to very high temperatures (see below), but it all happened very early in their history, perhaps by the decay of 26Al within the first million years or so.
Even the remaining asteroidal parent bodies of meteorites show evidence of high temperatures. Not only were some asteroids melted to the point of geochemical differentiation (forming metallic cores, mantles, and basaltic crusts), but many others were also metamorphosed at temperatures only a few hundred degrees shy of melting (McSween et al., 1988), and even many of the most primitive, least evolved asteroids show signs of early high temperatures. For instance, most carbonaceous chondritic meteorites experienced extensive aqueous alteration early in their parent body's history (Zolensky and McSween, 1988). However, present-day asteroids are too small to have had, or to have maintained, hot regions after the early period of formation. (The only meteorites known with young formation ages are now understood to come from Mars.) Asteroids are occasionally dislodged into orbits that approach the Sun, which would heat them; however, such bodies are likely to be destroyed within a few
million years of such orbital change (e.g., by diving into the Sun or crashing into a planet). There is simply no viable scenario for maintaining warm temperatures within an asteroid over solar system history.
Most meteorites found on the surface of Earth come from meter-scale bodies that have been liberated from larger bodies (whether in Earth-approaching orbits or in the main asteroid belt) within the last hundreds of thousands to hundreds of millions of years. Such meteorites might often be expected to be sterilized by radiation, depending on how long they have orbited the Sun as small, independent bodies. At rare intervals (thousands to many millions of years), it is possible for Earth to be impacted by very large bodies, which are hundreds of meters to kilometers in size or larger (e.g., Gehrels, 1994). Such objects are unaffected by Earth's atmosphere, and the projectile material is vaporized, melted, or otherwise severely damaged upon impact with Earth's surface, in ways probably inimical to the survival of life that might be contained within them. At intermediate sizes and frequencies of impact, however, there are bodies meters to tens of meters in size that impact Earth quite frequently and may deliver materials, which were never subjected to cosmic radiation, fairly gently to the surface of Earth (at terminal velocity).
In the discussion of different types of asteroids and meteorites in this chapter, it is assumed that there exists a fraction of asteroidal materials that were never subjected to a lethal dose of cosmic radiation and thus, may contain dormant life. However, the possibility remains that natural radiation from long-lived radionuclides may be sufficient to have sterilized remnant life even in the buried, shielded portions of asteroids (see Chapter 1; also Clark et al., 1998). If this is the case, then many of the potentially hazardous situations discussed have been precluded. The task group notes that most asteroids contain nonvolatile material of roughly cosmic composition at small spatial scales and that their interiors would have been uniformly subjected to such low-level radiation. The exception would be portions of strongly differentiated objects that are strongly depleted in radioactive elements or portions of undifferentiated objects that contain pockets of ice.
The association between asteroids of different types and various kinds of meteorites has been determined incompletely and not without controversy. The following general summaries should be sufficient for the purpose of this report. Because of the collisional mixing process among asteroids, which results in the presence of a small percentage of lithic fragments (xenoliths) from other bodies, almost any asteroid might be expected to contain a small component of another type (see Chapter 2). This factor is not considered in what follows, which considers only the indigenous materials of the different types of asteroids and meteorite parent bodies.
UNDIFFERENTIATED, PRIMITIVE (C-TYPE) ASTEROIDS
C-type asteroids are very black objects, typically reflecting only 3 to 5 percent of incident sunlight. They have reflectance spectra that are relatively neutral in color throughout the visible and near-infrared, except for a prominent absorption feature (present in only some of them) near 3 µm, owing primarily to water of hydration (Jones et al., 1990). These features are roughly typical of laboratory spectra of carbonaceous chondritic meteorites (Feierberg et al., 1981). There are minor discrepancies in matching meteorite and asteroid spectra, and also minor variations among asteroid spectra that have resulted in additional classes of asteroids (G- and B-types are lumped in with the C-type asteroids).
Although it is not rigorously proved, it is likely that the C-type asteroids (which are overwhelmingly the most abundant type in the main belt, especially the middle and outer parts) are represented in various meteorite collections1 by carbonaceous chondrites (Feierberg et al., 1981). 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. Because the relevant spectral reflectance data lack many of the pronounced absorption features that are diagnostic of composition, the comparison
between carbonaceous chondritic meteorites and C-type asteroids is less robust than that for some other spectral types.
Most C-type asteroids observed today are probably fragments from collisions among earlier generations of somewhat larger asteroids, although the actual size distribution in the primordial asteroid belt is uncertain. The properties of carbonaceous chondrites suggest that the diameters of their parent objects are on the order of 100 km, but significantly larger objects cannot be ruled out. Note that the largest C-type today, Ceres, is nearly 1,000 km in diameter.
Based on meteoritic evidence, C-type asteroids were accreted from dust, whose chemical composition was close to that of average protosolar-system material for the nonvolatile elements, plus volatile material in the form of water ice (and possibly even more volatile ices) and organic matter (Bunch and Chang, 1980). The chemical and physical form of that organic matter is unknown, but at least some was probably present as molecular fragments within ice mantles coating dust grains, such structures being predicted to form in dense interstellar clouds such as that which gave birth to the solar system (Greenberg, 1984). In this sense, C-type asteroids may be similar in composition to the nonvolatile fraction of comets.
Carbonaceous-chondrite petrology reveals that at least some C-type asteroids were heated to above the melting point of water ice shortly after accretion (Zolensky and McSween, 1988). The heating agent is not known but may have been the decay of freshly synthesized 26Al and/or 60Fe, or possibly inductive heating caused by a strong early solar wind. 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 lithology 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). The duration of aqueous activity is unknown; theoretical estimates range up to 108 years, but not longer (Grimm and McSween, 1993).
Some C-type asteroids actually show spectral evidence for hydrated minerals on their surfaces, whereas others do not. It is not clear whether those that do not show the water of hydration feature were dehydrated at moderately high temperatures, or if they were never warmed to the melting point of ice (or never incorporated water) in the first place.
Undifferentiated asteroids, especially those C-types rich in organics and that exhibit evidence of warm temperatures giving rise to aqueous alteration, are plausible candidate environments for the origin and sustenance of life. However, as explained above, these conditions lasted only for a transient period of time near the beginning of planetary history. Despite numerous false alarms, no convincing evidence for ancient organisms has ever been found in carbonaceous meteorites. However, it is clear that conditions for the origin of life—namely, presence of liquid water, organic matter, trace elements, and an energy gradient—were at least transiently met on some, possibly most, C-type asteroids.2 The extent to which the population of meteoritic organic compounds matched that needed for production of a self-replicating system is not currently known, but with that caveat, there appear to be no grounds for concluding that emergence of life in a C-type asteroid was precluded. However, the epoch of liquid water must have ended more than 4 Gyr ago (Grimm and McSween, 1993), and so the question is whether or not any life that was formed could have survived. Clearly, as argued above for all asteroids, portions of C-type asteroids have been shielded from external cosmic radiation. Within the interiors of such bodies, there is a low level of radioactivity from long-lived radionuclides that probably would sterilize dormant entities surrounded by nonvolatile material of cosmic composition. But material embedded in pockets of ice, which may well exist within some C-, P-, and D-type asteroids, could have been protected from much of that radiation, and so there is the prospect that dormant life may have survived for the ensuing aeons.
Numerous carbonaceous chondrites have been found to contain organic compounds, some of which were apparently synthesized directly on the parent body during a period when aqueous conditions existed (Cronin and Chang, 1992). However, there is no evidence that prebiotic chemistry advanced beyond the synthesis of some of the monomeric molecules thought to be important in the origin of life. There is a rich history of claims of finding evidence of life in a variety of meteorites (see Appendix C), but these indications have all eventually been found to be either artifacts or the result of terrestrial contamination. Thus, although meteorites containing extraterrestrial organic compounds have fallen on Earth throughout its history, there is no evidence that this process has ever inoculated Earth with any extinct or extant organisms.
It is likely that Earth receives considerable cosmic dust from C-type asteroids, but this material has probably been sterilized during its transport time to Earth, although very rarely—much more rarely than for comets—dust might land on Earth very shortly after its liberation from an unsterilized portion of an Earth-crossing C-type asteroid. The dominant delivery mechanism of potentially unsterilized C-type material is by meteorites and by the infrequent impact of larger C-type projectiles.
In addition to C-types, there are P- and D-types located predominantly near and beyond the outer edge of the main asteroid belt (D-types predominate among the Trojans, at Jupiter's distance from the Sun). There appear to be no meteoritic analogs for these asteroid types, consistent with their virtual absence in the inner parts of the asteroid belt for which dynamical transport processes have been identified. On the other hand, it is probable that rare fragments of P- and D-type asteroids occasionally reach Earth. It is assumed that P- and D-types are even more primitive than C-types, although this concept is difficult to test. Conceivably their colors have more to do with the state of their surfaces (owing to greater distance from the Sun, lesser collisional environment) than to their interior compositions. For purposes of this report, it is reasonable to consider P- and D-type asteroids as being similar to C-types. But caution is warranted as their nature is truly only a matter of speculation. Given that it is also not proven that a significant portion of P- and D-type material reaches Earth as part of the apparently nonhazardous natural influx, uncontained sample return from such objects should proceed only after some of the unknowns have been resolved.
UNDIFFERENTIATED, METAMORPHOSED ASTEROIDS
Undifferentiated, metamorphosed asteroids are those that were heated to temperatures of less than 1,000 K so that minerals did not segregate in a macroscopic way, but are also dehydrated (if ever hydrated in the first place) and were probably subject to temperatures at which biological materials could not survive. The most common meteorites on Earth, the ordinary chondrites, are fragments of such asteroids. These meteorites are known to be undifferentiated because their bulk elemental compositions are similar to the nonvolatile elements in the solar system (e.g., Wasson, 1985).
There has been a long-standing dispute about which main-belt asteroids to associate with these common meteorites (Chapman, 1996). In all probability, some of the S-type asteroids (the most abundant asteroid type in the inner third of the asteroid belt) are undifferentiated but metamorphosed objects analogous to ordinary chondrites, although a few researchers deny this. Other S-types may be examples of differentiated asteroids of various kinds (see below). In general, the spectra of S-types show more diagnostic absorption features than do spectra of C-types. Their slightly reddish spectra show the clear presence of such silicate minerals as olivine and pyroxene. Relying solely on spectral reflectance data, however, it is not always possible to decide unambiguously whether an asteroid observed telescopically has experienced global differentiation or not. In addition to the minerals olivine and pyroxene, the reddish slope of the spectra suggests the presence of metallic iron; together, these minerals constitute the dominant materials in undifferentiated ordinary chondrites. However, the same minerals are also found, in different proportions, in certain differentiated meteorites, and in addition the observed asteroid spectra do not exactly match those obtained in the laboratory from ordinary-chondrite specimens. Because the ordinary chondrites are the most abundant variety of meteorite falling on Earth today, which argues in favor of an abundant type of parent asteroid, it is commonly, though not universally, believed that the spectral differences between ordinary chondrites and some S-type asteroids are due to "space-weathering" of the asteroid surfaces, and that the ordinary chondrites are, in fact, derived from asteroids of spectral type-S (Wetherill and Chapman, 1988).
This issue will be further addressed by the Near Earth Asteroid Rendezvous (NEAR) mission, but in the interim it seems reasonable for present purposes to equate S-type asteroids with ordinary chondritic material, since those S-types that are actually differentiated asteroids have natures, and have undergone histories, even less hospitable to life. After all, the ordinary chondrites come from some main-belt asteroids, even if they are rare, and the mineralogy that the ordinary chondrites have in common imposes quite firm constraints on their prebiotic chemical history.
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
evidence for igneous differentiation, most ordinary chondrites exhibit the effects of prolonged heating to temperatures sufficient to cause metamorphism and even, in some cases, incipient partial melting. It is generally believed (McSween et al., 1988) that this metamorphism was caused by internal heating of the asteroidal parent bodies, perhaps by decay of recently synthesized radionuclides such as 26Al 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. However, a small number of ordinary chondrites, known as unequilibrated ordinary chondrites (UOCs), show minimal evidence for metamorphism and in a few cases contain evidence for modest degrees of aqueous alteration (Alexander et al., 1989) and traces of organic matter (Yang and Epstein, 1983). Thus, some UOCs strictly satisfy the criteria for emergence of life, but the amount of water was apparently very limited (it may have been vapor rather than liquid), and there is no evidence for the kind of complex organic chemistry needed for development of self-replicating systems. Consequently, it seems highly unlikely that life could have originated on a UOC parent asteroid and, by extension, on any undifferentiated but metamorphosed, i.e., S-type, asteroid.
As mentioned above, the NEAR mission should help to resolve the question of whether ordinary chondrites are derived from S-type asteroids, thereby reducing that element of uncertainty, but otherwise, barring the fall of a UOC unusually rich in organics and hydrated minerals, it is unlikely that our understanding of the biological potential of undifferentiated asteroids is likely to improve in the foreseeable future, except through sample-return missions.
Differentiated asteroids are inferred to be objects, or fragments of objects, that were once heated to the point of partial melting and geochemical segregation of materials. Vesta is a classic example of a largely intact differentiated body. As demonstrated by McCord et al. (1970), Vesta is covered with basalts (represented on Earth by the so-called HED basaltic achondritic meteorites). It is presumed (and, to some degree, observed [see Binzel et al., 1997]) that the basaltic crust overlies an olivine mantle on Vesta. Presumably Vesta has an iron core.
While Vesta is apparently unique as an intact, differentiated asteroid, many other asteroids look like pieces of a smashed-up Vesta, or fragments of smaller differentiated bodies. These include some so-called M-type bodies (the largest of which is 250-km-diameter 16 Psyche) that are apparently iron cores, or fragments of cores, from the interiors of preexisting bodies like Vesta; ambiguous inferences of metallic composition from spectral reflectance studies are confirmed, in a few instances, by high (metallic) reflectances of radar echoes (Ostro, 1993). There are other classes of asteroids, including small objects (V- and J-class) that may be fragments of Vesta's crust (Binzel and Xu, 1993), the monominerallic (olivine-rich) A-type asteroids, and the E-type asteroids (iron-poor enstatite) that probably represent mantles or crusts of such differentiated bodies. Their spectra are certainly not compatible with being undifferentiated. In addition, as described in the previous section, some (or even most) S-types may be differentiated bodies, as well.
Generally speaking, the various kinds of metallic, stony-iron, and achondritic stony meteorites are believed to be derived from these, and analogous, kinds of asteroids, generally located in the inner to middle parts of the asteroid belt. In general, these materials have been subjected to long-term heating well above 1,000 K, and water has not been present. They seem to be even less likely to harbor biological materials than are the undifferentiated but metamorphosed asteroids discussed in the previous section.
Until recently, some meteorites classified as achondrites were known to have had a much more complex history, including young ages, and evidence of being derived from unusually large asteroids (not identified in space) where ongoing environments conducive to life could not be ruled out. These achondrites, the so-called SNC meteorites, are now understood to come from Mars—a very large "asteroid," indeed, and beyond the purview of this report. As with Mars (and Earth), differentiation in and of itself does not preclude possible biological activity. It cannot be totally ruled out that there were other large asteroidal objects, not now being sampled by the ever-growing suite of collected meteorites, that might have had conditions leading to the origin and presence of life. However, remnants of any such objects are evidently very uncommon among meteorites striking Earth today.
POTENTIAL FOR A LIVING ENTITY TO BE IN OR ON SAMPLES RETURNED FROM ASTEROIDS
There are a few researchers who maintain that meteorites are only a very selective sample of the asteroids and, indeed, that the proportions of extraterrestrial materials striking Earth vary dramatically with time (Halliday et al., 1990). While these ideas are not widely supported, it is prudent to remain aware that generalities deduced from studies of meteorites and the likely associations of certain meteorite types with common asteroid types may not strictly apply to any particular asteroid. With this caveat, it can generally be stated that sample return from asteroids of the types sampled by the known meteorites evidently presents no known biological threat. Furthermore, both the differentiated and the undifferentiated-but-metamorphosed asteroids (e.g., M-, S-, V-, J-, and Q-types) have histories that appear to preclude the origin of life in the first place. For C-types to harbor dangerous biological materials (not so far identified in C-type meteorites), that material must have survived aeons since conditions suitable for replication ended.
While P- and D-type asteroids (and perhaps other rare, anomalous asteroid types) may be presumed to have histories similar to those of the C-types (and other types) discussed above, the asteroid population is evidently diverse and some mysteries remain. 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) were probably met very early in their history. Although the known meteorites derived from such asteroids reveal no evidence of biological activity, those meteorites cannot be regarded as having sampled the entire population of such asteroids. Similarly, although the natural meteorite influx has apparently had no deleterious effect on terrestrial biology, it is not certain that samples of every asteroid type have fallen on Earth. Furthermore, although natural radioactivity present within the asteroidal/meteoritic material would have been adequate to sterilize any dormant organisms possibly present within the lithic fraction of such objects, if pockets of relatively pure water ice were to exist within an asteroid of this type, attenuation of the natural radiation field within that ice could in principle have permitted survival of putative dormant organisms.
Based on the task group's current knowledge of the origin and composition of asteroids, the answers to the assessment questions employed in this study are as follows:
Does the preponderance of scientific evidence3 indicate that there was never liquid water in or on the target body?
There is unequivocal evidence for liquid water active within at least some C-type asteroids approximately 4.5 Gyr ago. A minor fraction of S-type asteroids may have experienced a transient episode of aqueous activity, but the great majority of S-types have never seen liquid water. Liquid water can also be ruled out for M-, V-, and E-type asteroids. For P- and D-type asteroids there is no evidence one way or the other regarding the presence of liquid water.
Does the preponderance of scientific evidence indicate that metabolically useful energy sources were never present?
There is no evidence one way or another regarding the presence of metabolically useful energy sources in other asteroid types.
Does the preponderance of scientific evidence indicate that there was never sufficient organic matter (or CO2 or carbonates and an appropriate source of reducing equivalents)4 in or on the target body to support life?
In most asteroids (especially C-types), there was some (or even an abundance) of organic matter. In others, especially the metamorphosed and differentiated asteroids, there was not.
Does the preponderance of scientific evidence indicate that subsequent to the disappearance of liquid water, the target body has been subjected to extreme temperatures (i.e., >160 °C)?
There is meteoritic evidence that some C-type asteroids have experienced temperatures above 160 °C following aqueous activity, but a substantial fraction have not been so heated. In general, there has been no source of sterilizing heat to raise the temperatures of most asteroid materials since primordial epochs, except for the localized heating due to impacts.
Does the preponderance of scientific evidence indicate that there is or was sufficient radiation for biological sterilization of terrestrial life forms?
The interiors of undifferentiated asteroids would have experienced sterilizing doses of radiation from the decay of natural radionuclides during the 4.5 Gyr since cessation of aqueous activity. (The exception is the possibility that localized pockets of ice might have shielded some materials from such radiation within C-type, but not metamorphosed, asteroids.) While sources of radioactivity may be diminished within portions of differentiated asteroids, the previous history of these bodies precludes the origin of life within them in any case.
Does the preponderance of scientific evidence indicate the natural influx to Earth, e.g., via meteorites, of material equivalent to a sample returned from the target body?
Meteorites and interplanetary dust particles (IDPs) have delivered samples of many asteroids to Earth. Some of those meteorites (although virtually no IDPs) would have been protected from sterilizing doses of radiation from galactic and solar cosmic rays while in transit to Earth. Therefore, some C-type material certainly regularly arrives on Earth unsterilized and has not been observed to have adverse effects. Whether or not the C-type material so received is representative of a particular target body, however, is uncertain.
Very little is known about P- and D-type asteroids. It is plausible that they are like C-type asteroids and dormant comets derived from the Jupiter zone. But caution is warranted as their nature is truly only a matter of speculation. Because of this lack of information, the potential for a living entity to be present in returned samples cannot be determined and, therefore, is considered conservatively by the task group as possible at this time. Another reason for caution is that very few P- and D-type asteroids are in orbits that can readily deliver materials to Earth, and most (like the Trojans) are in orbits so distant and so dominated by Jupiter's gravity that they are precluded from communicating directly with Earth, and so the natural influx of P- and D-type materials is very small or, most likely, zero.
Undifferentiated, metamorphosed asteroids as well as most differentiated asteroids5 are dry and have been heated to very high temperatures. A minor fraction of S-type asteroids may have experienced a transient episode of aqueous activity, but the great majority of S-types have never been exposed to liquid water. Like C-type asteroids, S-types would have experienced sterilizing doses of radiation from decay of natural radionuclides during the 4.5 Gyr since cessation of any aqueous activity. Thus, for S-type asteroids and other non-C/P/D-like asteroid types, the potential for a living entity to be present in returned samples is negligible. However, there is clear evidence in meteorites that substantial cross-contamination of material from one asteroid to another has occurred. It is uncertain whether such material would constitute such a trivial volume of the surface material on asteroids that the odds of sampling it would be negligible.
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. 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. However, this conclusion is less firm than the task group's same conclusion for the Moon, Io and certain IDPs and should be reexamined at the time of mission planning on a case-by-case basis.
For differentiated asteroids, despite the high temperatures that brought about differentiation of these bodies, there is no evidence that liquid water was present for the development of life. Liquid water can also be ruled out for M-, V-, and E-type asteroids, and so the potential for a living entity to be present in samples returned from these asteroids is similarly negligible. However, there is clear evidence in meteorites that substantial cross-contamination of material from one asteroid to another has occurred. Differentiated asteroids are known to be subject to cross-contamination, although it is uncertain whether such material would constitute such a trivial volume of the surface material on asteroids that the odds of sampling it would be negligible. Although the task group concluded that containment is not warranted for returned samples from differentiated asteroids, this conclusion is less firm that for the Moon and Io and should be reexamined at the time of mission planning on a case-by-case basis.
SCIENTIFIC INVESTIGATIONS TO REDUCE THE UNCERTAINTY IN THE ASSESSMENT OF ASTEROIDS
Given the possibility that many types of asteroids harbored environments suitable for the origin and sustenance of life early in their history, the chief questions that can readily be addressed by further research fall in two areas: (1) assessing the environments during subsequent aeons that may or may not have permitted dormant life to survive and (2) assessing questions concerning the association between meteorites that have been well studied on Earth (and represent material delivered by the natural influx to Earth) and specific asteroid types and specific asteroids that may be visited by spacecraft.
Although microorganisms could be readily protected from sterilizing external radiation sources if they were deeply buried, the task group's analyses suggest that the natural decay of long-lived radionuclides, in proportions represented by cosmic abundances, are roughly near the (uncertain) threshold levels of potential survivability in a dormant state (Clark et al., 1998). It would help to reduce uncertainty if the actual variety of radiation levels experienced within small bodies could be better assessed. A variety of heterogeneities within small bodies have been hypothesized (large pockets of ice within voids in a "rubblized" asteroid, degrees of local segregation of radioactive materials away from others that might increase the range of dosages within a body, and so on). Various spacecraft experiments in the vicinities of small bodies could help to better assess large-scale internal porosities and heterogeneities. In addition, studies of meteorites directed toward assessing the ranges of exposure to radiation due to small-scale heterogeneities would narrow current uncertainties.
In assessing the necessity to engineer containment for a sample return mission to a particular body, it is important to learn more about two questions: (1) To what degree do existing varieties of meteorites represent the range of materials likely to be present on asteroids of the "type" or "types" of asteroids believed to be parent bodies for those meteorites?, and (2) To what degree of certainty is it known that the particular target asteroid (of a known type) is like typical asteroids of that type? The validity of approaching a particular target based on the understanding developed from meteoritical studies depends on addressing these questions.
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) but which can be expected to be applied to only a modest number of asteroids in the foreseeable future. Thus, continued application of telescopic studies to a broader variety of asteroids must continue. Improved ground-based radar studies should dramatically enhance our understanding of the distribution of metal among asteroids. Although there are no narrow, focused lines of research that promise immediate, dramatic advances, a broad-based research program on asteroids and meteorites and on the processes that affect them should continue to narrow our uncertainties about associations among asteroids and meteorites.
The question of whether a particular asteroid (which itself may well not have been sampled by the recent natural influx onto Earth) is like meteoritic material associated with its asteroidal type can best be addressed by a
precursor mission or a mission phase to a sample return mission, that enhances our detailed knowledge about that body's composition and heterogeneity in ways analogous to the goals of the NEAR mission studies of Eros. Such investigations are likely to confirm (or deny) that a potential target body for sample return in fact resembles what we believe we know about the body from Earth-based data. Such precursor studies may not be necessary for bodies whose associations with meteorite types are quite clear (e.g., the association of Vesta with HED, or HED-like, meteorites), whereas they should be required for bodies (e.g., P- and D-type asteroids) whose association with known meteorite types is obscure.
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. However, this conclusion is less firm than it is for the Moon, Io, and some IDPs and should be reexamined at the time of mission planning on a case-by-case basis. 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.
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