5
Comets

ORIGIN

Place of Formation

Our best understanding of the origin of comets is that they formed in the protoplanetary disk, at distances from the Sun ranging from the distance of proto-Jupiter to far beyond the distance of proto-Neptune. It is generally agreed that the planetesimals that accreted to form Uranus and Neptune, and probably also the cores of Jupiter and Saturn, were identical to comets. The only difference between those planetesimals and comets is circumstantial; i.e., some were captured into the planets and some were not. Although interstellar comets—comets that originally formed around other stars and were subsequently ejected from those stellar systems—are generally predicted to exist, it is certain that none of the comets for which orbits have been determined is interstellar. They all are members of our own solar system and have been for the 4.5 × 109 years of its existence.

Nebular Processes and Accretion

Cometary formation is not well understood, but it certainly includes the following processes. Some interstellar grains, probably only microscopic ones that formed long before the Sun's progenitor molecular cloud contracted to form the Sun, were carried directly into the protoplanetary disk and incorporated in comets. Other grains condensed from vapor as the material of the Sun's progenitor cloud was carried to higher densities and pressures. Much of the material generally thought of as rocky, such as silicate grains, may have been in solid form prior to the formation of the protoplanetary disk. More volatile species, from ordinary water ice down through very volatile species such as CO, may have provided their ices either by condensation from the vapor or from preexisting interstellar grains. This is one of the open questions in cometary research. Since models of the protoplanetary disk predict that the temperature decreases with distance from the protosun, the relative abundances of the volatile species in icy grains undoubtedly varied with distance from the protosun. A key feature of all theories for the formation of planets is that the density of the gaseous, protoplanetary disk is too low to form planets. The solid grains, whether preexisting or condensed from the vapor, are not held up by the pressure of the gas, and they sink rapidly to the mid-plane of the protoplanetary disk. This step is crucial in making the density high enough that the grains will encounter each other frequently. Initially the grains grow by aggregation or agglomeration in which the grains run into each other and stick to each other, at rates determined entirely by geometric cross sections and velocities, the sticking occurring primarily by molecular forces. When the grains become sufficiently large,



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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making 5 Comets ORIGIN Place of Formation Our best understanding of the origin of comets is that they formed in the protoplanetary disk, at distances from the Sun ranging from the distance of proto-Jupiter to far beyond the distance of proto-Neptune. It is generally agreed that the planetesimals that accreted to form Uranus and Neptune, and probably also the cores of Jupiter and Saturn, were identical to comets. The only difference between those planetesimals and comets is circumstantial; i.e., some were captured into the planets and some were not. Although interstellar comets—comets that originally formed around other stars and were subsequently ejected from those stellar systems—are generally predicted to exist, it is certain that none of the comets for which orbits have been determined is interstellar. They all are members of our own solar system and have been for the 4.5 × 109 years of its existence. Nebular Processes and Accretion Cometary formation is not well understood, but it certainly includes the following processes. Some interstellar grains, probably only microscopic ones that formed long before the Sun's progenitor molecular cloud contracted to form the Sun, were carried directly into the protoplanetary disk and incorporated in comets. Other grains condensed from vapor as the material of the Sun's progenitor cloud was carried to higher densities and pressures. Much of the material generally thought of as rocky, such as silicate grains, may have been in solid form prior to the formation of the protoplanetary disk. More volatile species, from ordinary water ice down through very volatile species such as CO, may have provided their ices either by condensation from the vapor or from preexisting interstellar grains. This is one of the open questions in cometary research. Since models of the protoplanetary disk predict that the temperature decreases with distance from the protosun, the relative abundances of the volatile species in icy grains undoubtedly varied with distance from the protosun. A key feature of all theories for the formation of planets is that the density of the gaseous, protoplanetary disk is too low to form planets. The solid grains, whether preexisting or condensed from the vapor, are not held up by the pressure of the gas, and they sink rapidly to the mid-plane of the protoplanetary disk. This step is crucial in making the density high enough that the grains will encounter each other frequently. Initially the grains grow by aggregation or agglomeration in which the grains run into each other and stick to each other, at rates determined entirely by geometric cross sections and velocities, the sticking occurring primarily by molecular forces. When the grains become sufficiently large,

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making gravitational attraction assists with both collecting and binding the grains and the process becomes known as accretion. When the grains become macroscopic, the accretion can be modeled more easily, and many investigators have studied the various mechanisms that lead to growth from macroscopic grains to planetesimals. The role of resonances and the ability of resonances to lead to characteristic sizes of planetesimals have been investigated with inconclusive results by many people. The current state of the art in this modeling is the work by Weidenschilling (1997), who finds that resonances are unimportant but predicts characteristic sizes on the order of 100 meters, the size at which the planetesimal has a sufficiently large ratio of mass to cross section that it decouples from the effects of gaseous drag. Gravitational Scattering The planetesimals that have grown large enough to escape the effects of gaseous drag can be modeled with purely gravitational physics. The results are somewhat sensitive to the assumed starting conditions, but in the vicinity of Jupiter and Saturn most planetesimals are either incorporated into Jupiter and Saturn or ejected entirely from the solar system by encounters with those bodies. In the vicinity of Uranus and Neptune, many planetesimals are still captured into the two planets or ejected from the solar system, but there is also a significant likelihood of a gentle ejection from the planetary region outward for thousands of astronomical units, i.e., to distances that are a significant fraction of interstellar distances, where the planetesimals are still gravitationally bound to the protosun in a very extended disk. Comprehensive models have not been calculated, but the studies thus far indicate that this is the dominant source of comets in the Oort Cloud. Since the relative numbers of planetesimals at the distances of the different giant protoplanets are not well known, the relative contributions to the Oort Cloud from various regions of the protoplanetary disk also are not well known. The efficiency is much higher near proto-Uranus and proto-Neptune than near proto-Jupiter and proto-Saturn, but it is unclear whether higher relative numbers of planetesimals near proto-Jupiter and proto-Saturn might have counteracted this effect. As originally explained by Oort (1950), many dynamical simulations have shown that the effects of passing stars, galactic tides, and passages through molecular clouds convert this disk into a roughly spherical distribution that is now know as the Oort Cloud. Beyond Neptune, the process of formation of planetesimals proceeded more slowly owing to the lower densities and lower velocities. The formation of a planet was inhibited because the slow accretion to planetesimals used up all the material before one planetesimal became large enough that its gravitational cross section greatly exceeded its geometrical cross section and approached the scale of the mean separation between planetesimals. Most of these planetesimals are still present today in what is now called the Kuiper Belt, although the inner parts of this belt, say from 40 to 50 AU, have been considerably depleted by subsequent planetary perturbations. The time scale for formation of large (tens of kilometers) comets in the Uranus-Neptune region is on the order of 106 years, that for ejection to the Oort disk is probably up to an order of magnitude longer, and the time scale to convert the Oort Cloud from a disk to a sphere is on the order of 109 years. It should be noted that one expects mixing of planetesimals from one part of the protoplanetary disk to another during the stage at which the planets accrete, although the simulations are not yet accurate enough to determine how much mixing occurs. Today the orbits of comets in both the Kuiper Belt and the Oort Cloud are nearly circular, with orbital periods in the Oort Cloud being the longest at about 106 years. The comets that are seen today in the inner solar system have been delivered from both the Oort Cloud and the Kuiper Belt much more recently than the epoch of formation. Comets entering the inner solar system for the first time from the Oort Cloud can be recognized with high confidence from their orbits, the orbit having been perturbed by some passing star or possibly by a chance encounter between two comets. On the basis of an analysis of their orbits, it is possible to identify those comets that more likely came from the Oort Cloud and ones that more likely came from the Kuiper Belt, but for any individual comet we cannot be sure from which of these two reservoirs it came. Researchers also know of seven Centaurs, bodies orbiting roughly between Jupiter and Neptune, one of which is known to show cometary outgassing. These are transition objects between short-period comets and Kuiper Belt objects, although again researchers do not know confidently in individual cases whether the Centaur is arriving from the Kuiper Belt or is being ejected back toward the Kuiper Belt.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making Early Heating and Melting An important issue is whether the formation process allowed any significant internal heating. In the inner solar system, the combination of release of gravitational energy and radioactive decay leads to significant internal heating, at least for the larger bodies. For small bodies, up to the size of the largest known comet (Chiron), the accretion process is gentle enough that no significant accretional heating should occur, at least not sufficient to melt the interior ice on a large scale. Similarly, the long-lived radioisotopes that provide a large part of the outward heat flux from Earth today are insufficient to melt the interior of a small body; they would melt bodies on the order of 1,000 km and larger. On the other hand, short-lived radioisotopes, particularly 26Al, have often been invoked as sources of heating that would have been sufficient to melt the interior of a cometary nucleus if the comets formed within a few half-lives of the creation of these radioisotopes in a nearby nova or supernova. Whether this happened or not is still unclear, and experts argue both sides of the case. The highest concentrations of 26Al are seen in calcium-aluminum inclusions (CAIs), high-temperature condensates that were among the first things to condense in the inner solar system. These concentrations are an order of magnitude higher (relative to the stable 27Al) than expected from production rates in supernovae, which in fact are balanced (at least to within factors of a few) with the observed gamma rays in the interstellar medium (Timmes et al., 1995). Calculations by Prialnik and Podolak (1995; see also Yabushita, 1993; Grimm and McSween, 1989) show clearly that, with reasonable assumptions about the ratio of refractories to ice, and with an initial abundance of 26Al that is well below that found in CAIs, and even below the excess produced in supernovae, the interiors of solid cometary nuclei only 20 km in radius will be substantially melted. On the other hand, they also find that if the nucleus is porous, the process of sublimation, vapor transport, and recondensation is so efficient at carrying heat away that even much larger comets will never melt, and in fact will usually not substantially exceed the temperature at which amorphous ice crystallizes exothermically (typically in the range from 100 to 150 K). The question of whether or not cometary nuclei melted, therefore, reduces to two other questions: What was the porosity of cometary nuclei initially? and, What was the distribution of 26Al at the place(s) and time(s) of formation of cometary nuclei? MacPherson et al. (1995) argue from the variations within individual grains of chondritic meteorites that the grains had long histories in the protosolar nebula before they were incorporated into planetesimals, during which time reprocessing of grain material occurred (ending after many half-lives of 26Al in the formation of chondrules from some of the grains that might otherwise have been CAIs), and that the 26Al decayed away within the grains but before accretion of planetesimals. Wood (1996), on the other hand, has argued that drag effects would lead to short dynamical lifetimes of grains falling into the Sun and that chondrule formation would have been far easier in the first million years of the nebula than later, thus concluding that most of the 26Al must have decayed after accretion. Whichever viewpoint one takes, the arguments refer to the situation at the asteroid belt. Independent of evidence for extreme heterogeneity in the abundance of 26Al on small spatial scales, there is good evidence for a non-uniform distribution of 26Al, or at the very least for non-uniform heating in response to the decay of 26Al, as a gradient across the asteroid belt. For example, Grimm and McSween (1993) argue that the systematic variation of asteroidal taxonomy with heliocentric distance is due to the increase of the accretional time scale with heliocentric distance such that the outer parts of the asteroid belt accreted after most of the 26Al had decayed. Although the greater density of solids in the protoplanetary disk just outside the ice boundary (between the asteroid belt and Jupiter) would enhance accretion relative to the rate just inside the boundary, it is clear that accretion would proceed more slowly in the regions in which the comets primarily accreted than in the asteroid belt where the chondritic parent bodies accreted. Evidence cited below in the subsection on chemical composition argues against the differentiation that would have occurred with global melting since highly evolved comets show chemical composition similar to that of comets newly arrived from the Oort Cloud. Furthermore, one of the widely discussed mechanisms for driving outbursts of comets, particularly at large distances from the Sun, is the exothermic crystallization of water ice. If cometary nuclei had substantially melted, there could not be any significant reservoir of amorphous ice. In fact, Prialnik et al. (1987) find the evidence for amorphous ice sufficiently strong that they use this to argue that the comets must have formed after the 26Al decayed. Finally, it is noted that according to the best estimates the density

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making of cometary nuclei is on the order of 0.3 g/cm3. Although this number is very poorly determined and applicable primarily to evolved cometary nuclei, it clearly suggests a high porosity that would effectively inhibit melting even in the presence of 26Al. There is also no empirical evidence favoring the existence of excess 26Al in comets, nor is there any evidence for differentiation beyond that expected from vapor transport through porous ice. Therefore, it is concluded that it is very likely, although not firmly proven, that cometary nuclei never melted. COMPOSITION Physical Characteristics When most comets are observed from Earth, the brightness of the coma, the gas and dust driven from the nucleus by sublimation, often overwhelms the brightness of the nucleus. Thus far, direct images are available of only one cometary nucleus, 1P/Halley, as well as a direct measurement via a stellar occultation of the size of one more, P/Chiron, which is actually a Centaur, and fairly reliable indirect measurements of the sizes of a handful more. In addition, granting the assumption that the reflectivity of the nucleus is known (actually a bad assumption), there are another dozen or two comets for which nuclear sizes have been determined. The radii so determined range from about 300 m to 90 km (for P/Chiron) (Meech, 1997) with radii of a few kilometers being the most common. Brandt et al. (1996a,b) have argued that this is purely a selection effect in discovering comets and that the predominant size is really much smaller than this. The recent comet Hale-Bopp, one of our more spectacular visitors in several decades, is thought to have a nucleus with a radius of about 20 km, although this number is uncertain by a factor of two. The smaller comets appear to be typically prolate, with axial ratios on the order of 2:1. The larger comets, P/Chiron and P/Schwassmann-Wachmann 1, show little variation in brightness as they rotate and are therefore generally thought to be spherical. It is unclear whether this difference (1) is a random fluctuation from small-number statistics, or (2) reflects a different origin (such as formation by fragmentation of larger, spherical nuclei to make smaller, irregular ones), or (3) reflects a different evolutionary history (such as outgassing near the Sun with seasonal and rotational effects leading to mass-loss preferentially from certain areas), or (4) is due to the greater importance of gravitation in shaping the larger bodies (an analogous transition to spherical shapes for asteroids occurs at a larger size and is thought to reflect the importance of gravity). The known Kuiper Belt objects and Centaurs, if they have reflectivities similar to those of known cometary nuclei, are all much larger than the known cometary nuclei. Other than Pluto, the known Kuiper Belt objects are all still too small to have been heated by the long-lived radiogenic species. It is not possible at this time to say confidently whether the size distribution of comets in the inner solar system reflects the size distribution of the bodies in the Kuiper Belt and in the Oort Cloud. One expects that the loss of mass as short-period comets pass near the Sun, typically on the order of 1 m of material per perihelion passage, should lead to shrinkage of the nuclei, but the size of the net effect is tied up closely with models for the evolution of comets discussed below. The irradiation by galactic cosmic rays is less than that for comets in the Oort Cloud due to shielding by the heliosphere. There may still be sufficient radiation to break every chemical bond in the outermost layer, but the issue has not been studied because comets arriving in the inner solar system ''for the first time" from the Kuiper belt, unlike those originating from the Oort Cloud, have not been observed. Collisional evolution is likely to have fragmented some large bodies in the Kuiper Belt, but collisions are expected to be negligible in the Oort Cloud. The key characteristic of cometary nuclei is the outgassing, the observation of which provides the empirical definition used by the International Astronomical Union (IAU) to distinguish a comet from an asteroid. Ice in the cometary nucleus sublimes directly to the vapor phase, perhaps at the surface if ice is exposed but more likely from a subsurface layer of ice. The vapor expands rapidly into the vacuum of space, dragging grains ("dust") from the surface as it goes. The pictures of comet 1P/Halley (Keller et al., 1986) show jets of dust which make it clear that, at least for this one comet, the release of dust is concentrated in a relatively few active areas covering a modest fraction (15 percent) of the total surface. It is not directly known whether the gas also is released only from these active areas or whether the gas only drags dust from the surface in these areas. The pictures of Halley do not have sufficient resolution to address the question of whether ice in the active regions is subliming at the surface as opposed to subliming at a modest depth and percolating through a mantle. Comparison of observed water

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making outgassing rates with sizes of nuclei, where known, and use of a theoretical model for the sublimation suggest that most comets, although there are exceptions, are inactive over most of their surface. Such a comparison does predict that Halley should be outgassing water over roughly 15 percent of its surface, quite comparable to the active fraction observed for dust. Many comets appear on this basis to be active over less than 1 percent of their surface (A'Hearn et al., 1995). This result is generally interpreted to mean that a mantle has built up on the surface which has choked off the sublimation by sealing in the gas, although it is unclear whether simple, consolidated rubble is involved as opposed to some type of material that is sealed by bonding. The interior of cometary nuclei is almost totally unstudied observationally or experimentally, which is of course one of the reasons for returning samples. It is clear that the material of some, and probably all, cometary nuclei is structurally weak, the upper limit to the tensile strength of P/Shoemaker-Levy 9 before disruption being 104 dyn/cm2 (Sekanina, 1995), i.e., orders of magnitude lower than the tensile strength of ice, which in turn has a tensile strength far below that of rock. The only estimates of the density of cometary nuclei are based on models for the nongravitational acceleration that are rather poorly constrained. The best estimates are that the bulk density of some short-period comets is on the order of 0.3 g/cm3 (e.g., Rickman, 1991), which implies a highly porous structure, but some would argue that the uncertainty associated with this number could be as large as an order of magnitude in either direction. It is also unclear whether the inferred porosity refers to the mixture of rock and ice in the nucleus or instead is dominated by the depletion of ice by sublimation from a large fraction of a nucleus which originally had much lower porosity. Nevertheless, it seems likely that cometary nuclei, even before depletion of the ice by sublimation, are porous bodies with low density and strength. Chemical Composition Because there has never yet been any in situ sampling of a cometary nucleus, and since remote sensing of cometary nuclei has never yet shown definitively any identifiable spectral signature (there are reports of such detections but none that would be considered definitive), current knowledge of the composition of cometary nuclei comes entirely from studies of the coma—by remote sensing for many comets and by in situ measurements for only comet Halley (missions to fly through comets P/Giacobini-Zinner and P/Grigg-Skjellerup returned minimal information relevant to nuclear composition). It is therefore convenient to separate the discussion into substances that are volatile at 1 to 2 AU from the Sun and substances that are not. The volatiles are clearly dominated by water, although it is unclear whether the ice in cometary nuclei is predominantly amorphous or crystalline. Theory predicts that ice deposited at very low temperatures should be amorphous, and the transition from amorphous to crystalline ice, which is exothermic, has often been invoked as the cause of outbursts in comes. As noted elsewhere, it is not known whether ice is exposed at the surface of nuclei or is exclusively in subsurface layers. Our knowledge of the abundances of more complex molecules has been increasing dramatically in recent years, beginning with the in situ measurements at comet Halley just over 10 years ago and advancing since then in parallel with the advances in infrared and millimeter-wave technology at each newly discovered bright comet. Table 5.1 lists the known molecules in comets, some of which are thought to be present as parent molecules in the nucleus whereas others are thought, for example, to be the result of destruction of more complex parent molecules after release from the nucleus. These include many organic molecules that can be formed abiotically in the interstellar medium. A major research interest at present is trying to sort out which of these molecules might have been preserved from the interstellar medium and which, e.g., because of different abundance ratios, must have been formed by chemical processes in the protoplanetary disk. These other volatiles probably sum to no more than 20 percent water based on abundances measured thus far. The results from the ion and neutral mass spectrometers on the missions to comet Halley show unambiguously that larger, presumably complex, molecules are present in the outflowing gas, but the exact composition is not known. There have been suggestions of a variety of polymerized species (e.g., Huebner et al., 1987), but no such species have been unambiguously identified. The interpretation of these volatile species is complex and subject to selection effects. Three examples show the complexity. The intensive observations of relative abundances in comet Hale-Bopp (Biver et al., 1997) provide the first strong evidence that previous perihelion passages have differentiated the near-surface ices. This

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making TABLE 5.1 Chemical Species Observed in Comets Volatile Atoms Refractory Atoms Diatomics Triatomics Polyatomics Ions Isotopic Variants H Na CH H2O H2CO H2O+ HDO C Mg C2 CO2 NH3 CHO+ HDO N K CN NH2 CH3CN CH+ H13CN O Al CO HCN CH3OH CO+ 13CN S Ca CS C3 C2H2 CO2+ 13C12C He Si NH HCO? CH4 N2+ H13CN   Ti OH H2S C2H6 OH+ HC15N   V S2 HNC H2CS O+ HC15N   Cr SO OCS HNCO C+ C34S   Mn   SO2 HC3N       Fe     HCOOH       Co     CH3OCHO       Ni             Cu           means that individual observations are not necessarily representative. The mechanism for this differentiation is qualitatively understood, having been predicted more than a decade earlier, and it involves vapor transfer through porous materials rather than melting. Several groups have studied this phenomenon over more than a decade, and the effects are typically important over mantle depths on the order of meters, except for the release of trapped gases in the crystallization of amorphous ice which can be important at much larger depths. Comparison of easy-to-measure species in very many comets has shown that there are systematic differences among comets: A'Hearn et al. (1995) showed that the carbon-chain radicals, C2 and C3, are strongly depleted relative to the average in a subset of comets. Although the deficit is correlated with various parameters, the best interpretation is that these species are depleted in a large fraction of the comets that originally came from the Kuiper Belt but not in those that came from the Oort Cloud, thus suggesting a chemical boundary somewhere in the Kuiper Belt such that comets formed outside that boundary do not contain as much of the (unknown) parent molecules of C2 and C3. On the other hand, A'Hearn et al. (1995) also showed that the abundances of these, of CN, and of NH show no correlation with any dynamical parameters that statistically indicate the number of perihelion passages a comet has undergone. Since material on the order of meters is lost during each perihelion passage, the material from dynamically old comets is being released from deep in the interior of the original comet. The lack of any correlated differences suggests that global differentiation through widespread melting did not occur, despite arguments (Fomenkova et al., 1992) that some individual grains in comet Halley show signs of being differentiated. Finally, it is noted that the species normally thought of as volatile are now known to come, at least in part, from more complex molecules that are only marginally volatile. The best defined example is that of CO, which amounted to roughly 15 percent water in Halley. It is now clear from the in situ studies (Meier et al., 1993) that only a small percentage of CO is actually derived as a parent molecule, i.e., from frozen CO in the nucleus. The remainder results primarily from the dissociation of formaldehyde, H2CO, which is itself derived from a much more complex molecule that is probably a polymerized organic compound that is solid when released from the nucleus and that gradually warms in the coma until it vaporizes. Refractory species in comets are much less well understood than are the volatiles. From remote sensing it is known that many comets contain silicates, although the exact composition of these silicates is unknown in most cases. In some comets, with no obvious correlation to history or formation, the silicates are known to include crystalline olivine, but in other comets this is clearly not present. It is also known that there are refractory grains, or possibly very large molecules, containing C-H bonds. The in situ measurements at Halley revealed the presence of three classes of refractory grains—silicates of various sorts, CHON (composed predominantly of C, H, O, and

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making N), and mixed (silicates + CHON) (Jessberger and Kissel, 1991). Only the atomic compositions of these grains are known, and not the molecular or crystalline composition. Even the atomic composition is not known with high precision. The CHON grains presumably are composed of refractory organic molecules and may be, at least in part, the grains that provide the extended source of H2CO and CO discussed above. Combining the atomic abundances in both the volatiles and the refractories, the overall abundances match the solar abundances reasonably well, except for the very volatile species—nitrogen, hydrogen, and the noble gases. Nitrogen forms relatively few compounds, and so most of it would likely be in the very volatile N2, while hydrogen is so much more abundant than anything else in the Sun that its most abundant compound must be the very volatile H2. This suggests that most of the material in comets is accounted for in a way that is consistent with condensation of ices in a very cold place. In general, it is concluded that comets certainly contain abundant, simple, organic molecules and that there is probably a large suite of complex organic molecules in comets. PAST AND PRESENT ENVIRONMENTAL CONDITIONS In outlining the environmental history of comets, it is convenient to separate Oort Cloud comets from Kuiper Belt comets. A comet in the Oort Cloud for 4.5 Gyr has been irradiated continuously by galactic cosmic rays without any shielding by the heliosphere. This irradiation is sufficient to break every chemical bond in the outer 10 meters or so of cometary material (depth depending on density), as has been known for many years (e.g., Moore et al., 1983). Laboratory experiments show that this irradiation leads to formation of a variety of highly volatile species, including many free radicals, and the irradiation has therefore been widely invoked to explain the anomalous photometric behavior of comets arriving for the first time from the Oort Cloud, wherein they are apparently actively releasing material at very large heliocentric distances, long before they are first discovered. The temperature of these comets in the Oort Cloud is on the order of 10 K; occasional nearby supernovae transiently heat the outer layers to perhaps 30 to 40 K (Stern and Shull, 1988). As the comet approaches the inner solar system for the first time, part of the heavily irradiated layer is stripped off by explosive sublimation of the supervolatiles created by the irradiation, but laboratory experiments also show that at least in thin samples a sticky residue of "yellow stuff" or "brown stuff" always remains, even up to room temperatures. The extent to which the heavily irradiated layer is removed is thus unclear, but it is very unlikely that it is totally removed. The comet is also warmed as it approaches the Sun for the first time, but the time scale of this warming is such that as the comet rounds perihelion the heating does not have time to penetrate very deeply. Although estimates of the thermal conductivity and heat capacity vary by orders of magnitude, a typical estimate has the thermal wave (the heat pulse up to hundreds of degrees) penetrating to a depth comparable to the amount of material released during the perihelion passage. As an Oort Cloud comet is captured by planetary perturbations into orbits of shorter and shorter period, decreasing in steps from millions of years to, for example, the 75-year period of Halley, the accumulation of many thermal waves (each lasting a few months only) allows the heating to penetrate more deeply. The equilibrium temperature that would be reached at the center of Halley's nucleus after an infinite number of perihelion passages is roughly 130 K, but the estimates of thermal inertia (a combination of conductivity and heat capacity) are so wide ranging that it is not possible to know at this stage how close the interior has come to reaching that temperature. Kouchi et al. (1992), for example, have argued that the thermal conductivity is so low that the center of Halley, and of all short-period comets for that matter, is still at the temperature the material had in its original place of formation and that the thermal waves have accumulated only to depths of meters. The surface layers of a comet at 1 AU from the Sun, however, are routinely heated to temperatures on the order of 300 K at every perihelion passage. For a comet like P/Halley, this periodic heating certainly occurs hundreds of times, and it could easily be thousands of times. Note that the total since the first heating, however, is very short compared to the age of the solar system, probably less than 107 years and certainly less than 108 years. It is on this time scale that a comet will be ejected entirely from the solar system or will collide with a planet. Prior to collision with a planet, the typical comet from the Oort Cloud has probably undergone no collisions except with microscopic grains of interplanetary dust. The short-period comets that have undergone this history are, at least statistically, identified from their present orbital characteristics as Halley-family comets.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making The evolution of comets from the Kuiper Belt is somewhat different. Whereas the comets from the Oort Cloud evolve toward short-period comets primarily by lowering the aphelion distance, the comets from the Kuiper Belt, with orbits of much lower eccentricity, tend to lower the perihelion and aphelion distances together. This leads to smaller extremes in the temperature fluctuations, but in other respects the evolution is similar. The comets in the Kuiper Belt start out with equilibrium temperatures of several tens of Kelvins, warmer than the equilibrium temperature in the Oort Cloud, but still very cold. Because the density of objects in the Kuiper Belt is higher than that of objects in the Oort Cloud and because the relative velocities are also higher, collisions can occur in the Kuiper Belt. In contrast to the asteroid belt, where the densities are comparable but the relative velocities are higher, the collisions in the Kuiper Belt are relatively gentle. The effect of the collisions depends on the unknown size distribution of objects in the Kuiper Belt, but reasonable simulations (Davis and Farinella, 1997) suggest that the collisions may have created regoliths of meters on Kuiper Belt objects (KBOs); whether there have been body-shattering collisions, as in the asteroid belt, is still unclear. A KBO is captured, initially by planetary perturbations or by chance KBO gravitational scattering and later exclusively by planetary perturbations, into successively smaller orbits. As with Oort Cloud comets, the process is a random walk in energy so that the decrease in orbital size is not monotonic. Unlike the situation for Oort Cloud comets, however, the process for KBOs typically involves successive steps in which perihelion or aphelion is close to the orbit of one of the giant planets, this thereby imposing a sort of quantification on the evolution that is different from that of Oort Cloud comets, for which the evolution is more truly random. The thermal wave at perihelion is just like that for Oort Cloud comets, heating the surface to a heliocentric-distance-dependent value on the order of 200 to 300 K for a period of months once every orbital period. Because the eccentricities are lower, the amplitude of the thermal wave is smaller; that is, the minimum temperature at aphelion is higher, and this presumably allows the interior regions to approach equilibrium faster than for an Oort Cloud comet, although Kouchi et al. (1992) have argued that even comets from the Kuiper Belt retain their primordial temperatures at their centers. The short-period comets that underwent this history are, at least statistically, identified from their present orbital characteristics as belonging to the Jupiter family. Because of the random fluctuations in cometary orbital evolution, it is not yet possible to determine with confidence the origin of any single comet or even to derive reliable estimates of the fraction of Jupiter-family comets that might be interlopers from the Oort Cloud, or conversely the fraction of Halley-family comets that might be interlopers from the Kuiper Belt. Fractions as large as 10 percent are plausible. DELIVERY OF SAMPLES TO EARTH It is generally agreed that Earth has received samples of comets. The size distribution of comets and of near-Earth asteroids suggests that large impacts, such as the Chicxulub crater in the Yucatan, have been caused predominantly by comets (E. Shoemaker, U.S. Geological Survey, private communication, 1996), and estimates of collision rates on various bodies (Weissman, 1994) similarly suggest that comets are a significant, but uncertain, contributor to the impact flux on Earth. Meteorites from asteroids lose material by ablation as they penetrate Earth's atmosphere, but the interiors remain cool. The passage of a large cometary body through Earth's atmosphere is much less understood. Calculations by Chyba et al. (1990) and Chyba and Sagan (1992) suggest that some cometary material is preserved intact in impacts of 1- to 100-m cometary nuclei, but the calculations are equivocal. Comets are also a significant contributor to interplanetary dust, both the large "dust" in meteor showers and the small dust that may be more than 50 percent cometary, of which a small but not negligible fraction arrives at Earth's surface without having undergone heating above 160 °C. Our sampling of stratospheric dust is insufficient to unambiguously separate cometary dust from asteroidal dust. Statistically, the particles that have been strongly heated on entry are probably cometary and those that are weakly heated are asteroidal, but there are also likely to be cometary particles in the weakly heated group and, although less common, asteroidal particles in the strongly heated group. The samples that have been delivered to Earth are subject to selection effects. The samples delivered as interplanetary dust are selectively the grains that are small enough to have been lifted off the cometary nucleus by

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making the outflowing gas. The samples that are delivered in large impacts are systematically from a larger, but not well-determined, range of sizes. If they came from fragmentation of KBO-sized bodies, they should represent both the surfaces and the interiors of these bodies. If, on the other hand, they are part of a continuous size distribution at formation, they probably started not much larger and represent a certain segment of the size distribution, perhaps too small for early radioisotope heating to matter but still large enough to survive entry through the atmosphere. Interestingly, the impact or even the near miss of a large cometary nucleus is invariably accompanied by the arrival of large amounts of dust that had been released from the comet within the days immediately prior to the impact. Dust at sizes of up to 10 or even 100 µm thus rains down intermittently on Earth with little or no heating. The classically cited case is the passage of Earth through the tail of comet Halley in 1910. Weissman (1990) has estimated that a 10-km crater is formed by a comet hitting Earth every 105 years and, for typical impact velocities, this corresponds to a 0.5-km comet. While such a large impact may heat the entire mass of incoming material to above sterilization temperatures, it is more important to consider the near misses. The ratio of the number of passages of Earth through the coma of a comet to the number of impacts of a comet onto Earth is roughly the ratio of the cross section of the coma to the cross section of Earth. If the coma has a radius of 106 km, this ratio of cross sections is 104. In other words, Earth passes through the coma of 104 comets for every cometary nucleus that hits Earth—once every several tens of years. Comet IRAS-Araki-Alcock in 1983 was marginally close enough for Earth to be considered inside the coma, but comet Lexell in the 18th century was certainly close enough, and, as noted above, Earth passed through the tail of Halley's comet in 1910. It is estimated that a typical 0.5-km comet will have a water release rate of 1027 molecules per second and a dust release rate of 2 × 104 g/s. If Earth passes through the coma at 105 km from the nucleus, it will encounter a dust column of 0.1 g/km2, and at 106 km it will encounter 0.01 g/km2. With simple scaling and ignoring factors of a few for focusing effects, the former will occur roughly every 10 3 years and the latter on the order of every 10 years. The anecdotal cases cited above suggest that these numbers may be somewhat high, but not by more than an order of magnitude, and probably are consistent within small-number statistical fluctuations. The implication is that the total material swept up by Earth is on the order of 0.1 tons per year. Although this is less than the dust provided by direct infall from the interplanetary medium, this material from the coma is all recently released from the nucleus. If only a fraction of the debris in the coma (limited to sizes <50 µm) of only a small fraction of the comets approaching Earth (those with very small relative velocities) succeeds in reaching the surface of Earth without being heated to >160 °C, significant amounts of recently released, unsterilized dust from comets have been received over time. However, this is episodic, and it seems unlikely, for example, that such an event has occurred in the last several hundred years (the encounter velocity of comet Lexell was 20 km/s). Taking into account this dust coupled with the likelihood of preserved material from 1- to 100-m impactors (the rate of which is very uncertain), it is clear that Earth has received episodically, over time, unsterilized cometary material. POTENTIAL FOR A LIVING ENTITY TO BE IN OR ON SAMPLES RETURNED FROM COMETS Current understanding of cometary nuclei indicates that they probably never exhibited the conditions under which life is thought to have evolved on Earth. The one possible exception would be early heating, in the first few million years after formation, by now-extinct sources of radioactivity. If that process was important, and as discussed above this is unlikely, then comets might have contained liquid water in significant amounts for significant periods of time. This is the only accepted scenario in which comets would have contained liquid water, a prerequisite for the formation of life. Some cometary environments would clearly destroy life. In particular, the irradiation by galactic cosmic rays of most bodies outside the heliosphere, particularly including Oort Cloud comets, would destroy any preexisting life in the outermost tens of meters, and the temperatures are so low that life could not form. The low temperatures of comets in the Kuiper Belt would not allow life to form, although it is not known whether preexisting life could survive. The short interval of heating in perihelion passages of long-period comets is not sufficient to allow formation of life. The Centaurs, which are transition objects between the Kuiper Belt objects and Jupiter-family,

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making short-period comets, are warm enough that some very volatile ices vaporize. Chiron was very active at aphelion (17 AU) prior to its actual discovery (Bus et al., 1998), but the generally accepted view is that cometary material is sufficiently porous that the volatile substances escape as vapor rather than remaining trapped as a liquid. The task group's conclusions regarding the potential for life on comets are summarized as follows in its answers to the key issues raised in Chapters 1 and 2. Does the preponderance of scientific evidence1 indicate that there was never liquid water in or on the target body? The scientific evidence regarding all scales of cometary bodies that have been studied indicates that there was never liquid water in cometary nuclei. Does the preponderance of scientific evidence indicate that metabolically useful energy sources were never present? Although free radicals and other unequilibrated chemical products could exist in the presence of liquid water for a short time, it is uncertain whether they could serve as metabolically useful energy sources. 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)2 in or on the target body to support life? There was and still is a large quantity of organic material present in cometary nuclei. 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)? Evidence suggests that, except at the very surface layers, cometary nuclei were never heated to sterilization temperatures. Does the preponderance of scientific evidence indicate that there is or was sufficient radiation for biological sterilization of terrestrial life forms? For dynamically new comets (e.g., those from the Ort Cloud), the outer 10 meters have probably received sterilizing doses of radiation. The best estimates of the radiation environment of the deep interior indicate that the time for sterilizing doses of radiation is of the same order of magnitude as the age of the solar system. Thus, it cannot be concluded that the deep interior of all comets is radiation sterilized by cosmic radiation. For short-period comets, any irradiated material is entirely lost during each perihelion passage, so that the newly exposed material at the surface may not be subject to sterilizing doses of radiation. 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? Evidence suggests that unsterilized cometary dust has been delivered episodically to the surface of Earth. It is unlikely that a living entity could exist on comets, but the possibility cannot be completely ruled out except in a few cases, such as in the outer layers of Oort Cloud comets entering the solar system for the first time. Thus, the potential for a living entity to be present in samples returned from all comets is considered to be extremely low, but the task group cannot conclude that it is necessarily zero. Although the task group concluded that containment is unlikely to be required for samples returned from these bodies, this conclusion is less firm than that for the Moon, Io, and some IDPs. This conclusion needs be reexamined on a case-by-case basis at the time that missions to these bodies are planned. 1    For the purposes of this report, the term "preponderance of scientific evidence" is not used in a legal sense but rather is intended to connote a nonquantitative level of evidence compelling enough to research scientists in the field to support an informed judgment. 2    For the purposes of this report, CO2 or carbonates and an appropriate source of reducing equivalents is equivalent to "organic matter" to accommodate chemolithoautotrophs.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making SCIENTIFIC INVESTIGATIONS TO REDUCE THE UNCERTAINTY IN THE ASSESSMENT OF COMETS The key questions regarding the potential for life on comets are whether liquid water was ever present and what the thermal evolution of comets was. Searches for variations of abundances from one comet to another, correlated with the size of the comet or with its dynamical history, could increase our understanding about whether global differentiation occurred. Yet, this is a lengthy process, because the number of comets observable at any given time is small and the effects of global differentiation must be separated from effects owing to place of formation and near-surface differentiation on recent previous perihelion passages. Alternative searches for global differentiation could involve in situ sampling to very large depths. Currently projected in situ sampling on the Rosetta and DS4/Champollion missions will reach only the near-surface layers, although the CONSERT experiment on Rosetta will provide some tomographic sampling of certain properties of the interior. These missions will be very important in separating the near-surface differentiation from the global differentiation and thus simplifying the interpretation of the data from remote sensing, but they will not sample deeply enough, other than with CONSERT, to address global differentiation and in fact are not likely to reach sufficient depth to completely determine the near-surface, recent differentiation. Missions that would more directly sample the deep interior of a cometary nucleus would be extremely valuable in addressing whether liquid water was ever present. Missions that return dust samples from comets to Earth will assist in determining whether or not liquid water was present on very local scales on cometary nuclei, although this depends on understanding both how much the minerals are metamorphosed on capture and how minerals metamorphose on time scales up to millions of years in the presence of water vapor. The study of 26Al in cometary grains will be important in understanding how much 26Al was present but will not alone determine whether the 26Al was present after the body became large enough to trap heat and melt water. That question could be addressed only by looking at the hydration of other minerals, for example, by identifying clay minerals. Detailed mapping from spacecraft of the gravitational field of cometary nuclei could lead to tight constraints on the degree of central condensation and thus on the degree of differentiation that might have occurred, to help in clarifying the past presence or abundance of liquid water in the interior. Other combinations of observational and theoretical studies of cometary nuclei might aid significantly in addressing the following relevant, although perhaps less central, questions: (1) What is the present size distribution of cometary nuclei and of KBOs and of Centaurs? this information would assist in understanding whether kilometer-sized comets are fragments of larger bodies. (2) What is the true collisional history of KBOs? (3) What is the actual distribution of shapes of cometary nuclei? This information would assist in constraining the strength of nuclei against collapsing to spherical shapes and thus in determining the sizes at which collisional fragments can no longer be recognized. (4) What should be the observable effects of melting the water ice in a cometary nuclei? Should there be strong differentiation leading to abundance differences or gas/dust differences as a function of dynamical age? If observable effects can be predicted reliably, this item could become central to the issue. SUMMARY Cometary nuclei are predominantly icy bodies that most likely have never melted on large scales. For most of their 4.6-Gyr lives they have been at temperatures below 100 K, and those in the Oort Cloud have been at temperatures on the order of 10 K. The surface layers have undergone transient heating lasting months to temperatures on the order of 300 K (dependent on heliocentric distance) at previous perihelion passages. It is extremely unlikely that life could exist on comets, but only in a few cases can the possibility be totally ruled out, such as in the outer layers of Oort Cloud comets entering the solar system for the first time. The least likely place for life to exist in a comet is in the interior of a very large nucleus or in a smaller nucleus that was produced by fragmentation of a very large nucleus sometime after any liquid water had solidified. Even this possibility is remote. The task group concluded that cometary nuclei are unlikely to contain organisms capable of self-replication.

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