3
Planetary Satellites Inside Jupiter's Orbit

Satellites are natural consequences of planetary formation processes. They can form in place around planets via condensation and agglomeration of material from circumplanetary gas and dust disks. Such disks may be an integral part of the development of large gaseous planets like Jupiter from the solar nebula. Natural satellites can also develop from shorter-lived disks produced by large impacts on a growing planet; such a process may have produced Earth's Moon. Some satellites may be captured objects—i.e., objects that formed elsewhere in the solar system but were captured into orbit around a planet by aerodynamic drag forces generated by passage through an extended early planetary atmosphere.

Planetary satellites vary widely with respect to their post-formation geologic histories, ranging from quiescent bodies that have been subjected to little since their formation (except for impacts), to volcanic bodies that continue vigorous activity to the present. They also vary widely in their endowment of H2O, ranging from objects with no discernible water to objects that are more than 50 percent H2O by mass. Aspects of a satellite's formation, subsequent geologic history, and water endowment must all be considered carefully when evaluating the object's biological potential.

ORIGIN, COMPOSITION, AND ENVIRONMENTAL CONDITIONS OF SATELLITES EXAMINED

The Moon

Of all the planetary satellites considered in this chapter, the easiest to assess from a planetary protection perspective is the Moon. Lunar meteorites have been delivered to Earth throughout its history, and several hundred kilograms of lunar samples were returned to Earth deliberately during the Apollo program. Both crews and samples from Apollo 11, 12, and 14 were subjected to an elaborate quarantine and testing regime at the Lunar Receiving Laboratory to ensure that no harmful organisms were introduced to Earth's biosphere by returning astronauts, spacecraft, or extraterrestrial material (Allton et al., 1998).1 Quarantine was ended after Apollo 14

NOTE: The planetary satellites examined in this study were selected based on scientific interest and the likelihood of possible sample return missions in the near future.

1  

 One of the reviewers for this report observed that the quarantine procedure for the Apollo missions appeared to be compromised when the command module was opened while retrieving the crew at sea.



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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making 3 Planetary Satellites Inside Jupiter's Orbit Satellites are natural consequences of planetary formation processes. They can form in place around planets via condensation and agglomeration of material from circumplanetary gas and dust disks. Such disks may be an integral part of the development of large gaseous planets like Jupiter from the solar nebula. Natural satellites can also develop from shorter-lived disks produced by large impacts on a growing planet; such a process may have produced Earth's Moon. Some satellites may be captured objects—i.e., objects that formed elsewhere in the solar system but were captured into orbit around a planet by aerodynamic drag forces generated by passage through an extended early planetary atmosphere. Planetary satellites vary widely with respect to their post-formation geologic histories, ranging from quiescent bodies that have been subjected to little since their formation (except for impacts), to volcanic bodies that continue vigorous activity to the present. They also vary widely in their endowment of H2O, ranging from objects with no discernible water to objects that are more than 50 percent H2O by mass. Aspects of a satellite's formation, subsequent geologic history, and water endowment must all be considered carefully when evaluating the object's biological potential. ORIGIN, COMPOSITION, AND ENVIRONMENTAL CONDITIONS OF SATELLITES EXAMINED The Moon Of all the planetary satellites considered in this chapter, the easiest to assess from a planetary protection perspective is the Moon. Lunar meteorites have been delivered to Earth throughout its history, and several hundred kilograms of lunar samples were returned to Earth deliberately during the Apollo program. Both crews and samples from Apollo 11, 12, and 14 were subjected to an elaborate quarantine and testing regime at the Lunar Receiving Laboratory to ensure that no harmful organisms were introduced to Earth's biosphere by returning astronauts, spacecraft, or extraterrestrial material (Allton et al., 1998).1 Quarantine was ended after Apollo 14 NOTE: The planetary satellites examined in this study were selected based on scientific interest and the likelihood of possible sample return missions in the near future. 1    One of the reviewers for this report observed that the quarantine procedure for the Apollo missions appeared to be compromised when the command module was opened while retrieving the crew at sea.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making because all the protocol requirements had been met, and samples were certified as safe by the Interagency Committee on Back Contamination. 2 Subsequently, the distribution of Apollo lunar samples came under the purview of a scientific advisory committee whose main concern was scientific preservation of samples. Since release from quarantine, there has been no restriction of sample distribution based on concerns about back contamination or planetary protection. There have been no discernible adverse consequences for researchers or for Earth's ecosystem as a result of this policy. Moreover, no evidence of lunar life has been found in any of the samples. The task group's discussion of the Moon is therefore cursory. The Moon is a large rocky body with a history dominated by impacts and volcanism. The most recent significant volcanism took place approximately 2.5 Gyr ago. There have been a number of hypotheses of lunar origin, including co-accretion with Earth, dynamically induced fission from the growing Earth, or capture from some other region of the solar system. However, the hypothesis that appears most consistent with all the available data is that the Moon accreted from debris that was excavated from Earth's mantle by a giant impact very early in Earth's history (Hartmann and Davis, 1975). One consequence of its apparently violent origin is that the Moon is highly depleted in volatiles. There is no perceptible water in lunar rocks and no geologic evidence for the former presence of liquid water at or near the lunar surface. The potential for hydrothermal systems at any point in lunar history therefore appears small. It has been hypothesized that ice may exist near the lunar polar regions (Arnold, 1979), built up as a consequence of cometary impacts on the Moon. Most H2O molecules released during cometary impact events would ultimately escape to space because of high daytime lunar surface temperatures. However, a small fraction could come into contact with cold, permanently shadowed regions on the floors of lunar craters, becoming permanently trapped there. Bistatic radar results from the Clementine mission have been interpreted as providing some evidence for lunar polar ice (Nozette et al., 1996), although Earth-based radar results have called these conclusions into question (Stacy et al., 1997). More recently, results from the Lunar Prospector mission have provided strong evidence for the existence of at least modest amounts of lunar polar ice. The discovery has little biological significance, however. Lunar polar ice passes directly from solid to vapor upon impact, and back to solid upon condensation, never existing in the liquid phase. As noted above, many samples of lunar rocks and soils were returned to Earth by the U.S. Apollo and Soviet Luna programs. None have been found to contain any evidence of past or present lunar biological activity. The Satellites of Mars Phobos and Deimos, the two natural satellites of Mars, are small, irregularly shaped rocky objects. With maximum dimensions of 27 km (Phobos) and 15 km (Deimos), they are more similar in size and shape to asteroids than to the other much larger planetary satellites discussed in this chapter. Phobos and Deimos are notoriously difficult objects to observe from Earth. Spacecraft observations clearly show that they have very low albedos (about 0.05). Mariner 9 and Viking spacecraft spectrophotometric data from 200 to 700 nm show few features other than a dropoff in reflectance shortward of about 400 nm (Pang et al., 1978). These data are broadly consistent with a composition similar to those of undifferentiated carbonaceous meteorites and asteroids. This interpretation is equivocal, however, based as it is on a lack of observable spectral features. Ground-based near-infrared (IR) spectra obtained for Deimos out to 3 µm show a match that is closest to the C-type and P-type asteroids (Bell et al., 1993; Murchie and Erard, 1996). The densities of the martian satellites have been constrained by spacecraft flybys to be approximately 2.2 ± 0.5 g/cm3 for Phobos and approximately 1.7 ± 0.5 g/cm3 for Deimos (Duxbury and Callahan, 1982). These low densities are reminiscent of the C-type asteroid Ceres. The densities probably cannot be taken as good indicators of composition, however, owing both to their large uncertainties (i.e., ± 0.5 g/cm3) and to the possibility that the satellites have significant internal porosities. 2    The Interagency Committee on Back Contamination was established by NASA in 1966, in cooperation with the U.S. Public Health Service, the Departments of Agriculture and the Interior, and the National Academy of Sciences to advise NASA on measures necessary for the prevention of contamination by lunar samples returned to Earth.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making Images of Phobos and Deimos show a geologic evolution that has been dominated by impacts. Craters are the primary morphologic features on both satellites. These craters show a variety of forms, but this variety appears to arise primarily from the degree to which the craters have been degraded by or filled with ejecta from subsequent impacts (Thomas et al., 1992). Individual impact ejecta blocks have been observed on both satellites; morphologic evidence suggests that crater ejecta has been transported both ballistically and by downslope motion (Thomas and Veverka, 1980). The most complex geologic features on the satellites are sets of subparallel topographic grooves on Phobos, but the bulk of the geologic evidence indicates that even these are simply a consequence of fracturing produced by Phobos' large Stickney impact (Thomas et al., 1979), perhaps modulated by tides. Spacecraft images therefore show no clear evidence for any geologic activity other than as a result of cratering within the martian satellites. The presence of Stickney suggests the possibility that Phobos and/or Deimos may have been catastrophically disrupted, and reassembled in Mars orbit, by one or more even larger impacts. The degree to which such events reorganized the satellites' original structure, and the timing of such events, are difficult to evaluate. It is important to recognize, however, that the presence of Phobos and Deimos in orbit near Mars results in much more effective reaccumulation of debris and mantling by a thick regolith than would be true for asteroids of equivalent size in heliocentric orbit. The origin of Phobos and Deimos is unclear. Both lie in orbits that are low in inclination and nearly circular. Because of their apparent spectral similarity to primitive asteroids, it has been suggested that one or both may be captured objects that formed in the outer portion of the asteroid belt (Hunten, 1979; Lambeck, 1979; Cazenave et al., 1980). However, a capture-based origin presents serious dynamical difficulties (Burns, 1992). The present orbit of Deimos poses the most serious problem. Solid-body tidal interactions between Phobos and Mars have caused substantial evolution of Phobos' orbit from its ''original" one (e.g., Burns, 1977). Indeed, this orbital acceleration of Phobos will cause it to impact Mars in the geologically near future. Phobos' orbit therefore in principle could have undergone some significant evolution from an initial capture orbit (which presumably would have been far more elliptical). Deimos, however, lies far enough from Mars that it has undergone essentially no orbital change due to tidal interactions over its history. Its present near-circular, near-equatorial orbit is hardly a likely one for any plausible capture mechanism. Even for Phobos there are significant dynamical arguments against a capture origin. In order to have evolved from an early postcapture orbit of significant inclination and eccentricity to its present one, the orbit of Phobos is very likely to have crossed that of Deimos for a significant period (Burns, 1992). A destructive collision with Deimos would therefore have been difficult to avoid. Considering these dynamical arguments, a more likely scenario may be that Phobos and Deimos co-formed with Mars and are composed of undifferentiated material left over from the martian formation process. Overall, the limited spectral data and poorly determined densities of Phobos and Deimos are broadly consistent with their being similar to C-type or P- and D-type asteroids. The dynamical arguments discussed above are not easily reconciled with this interpretation, however, since such asteroids are most commonly found in the outer portions of the main belt. Taken together, then, the available data do not allow for a definitive answer about the compositions and internal histories of the martian satellites other than that they may have been broadly similar to those of similar-sized asteroids of carbonaceous, undifferentiated, and/or unknown composition. It is not known whether or not there was ever any liquid water or hydrothermal activity within Phobos and Deimos. There is no evidence to support the idea, but the uncertainty regarding the bodies' composition does not exclude the possibility. If any aqueous activity ever did take place within Phobos, it clearly would have been very early in the objects' histories, during the brief interval when the thermal effects of short-lived radionuclides like 26Al were being felt. As is discussed in more detail in Chapter 4, the upper meters of an asteroid are subjected to continuous bombardment by galactic cosmic rays and solar flare protons. In addition, all portions of an asteroid are subjected to the radiation produced by long-term decay of natural radionuclides such as U, K, and Th. The only plausible exception would be an object with void spaces that were filled with ice containing biological materials and that, by virtue of ice's lack of radionuclides, shielded these materials from radiation. Phobos and Deimos are dark and lie considerably closer to the Sun than do most typical P- and D-type

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making asteroids. They are therefore warm objects, and typical subsurface temperatures are far too high for ice to persist for long periods, at least in near-surface environments. It is unlikely that ice-filled voids are present today within the upper reaches of Phobos and Deimos that are accessible to sample return missions. However, given the evident large-scale cratering on these satellites, and the possibility of past reorganization of interior materials by catastrophic disruption and reassembly, it is conceivable that materials long-protected from natural radioactivity might be available for sampling. Because any biological materials would have to date from Phobos' and Deimos' earliest histories, the effects of radiation would be substantial in most cases. Considering just the radiation produced at all locations within the satellites by chondritic abundances of U, K, and Th, the time to accumulate a total dose of 18 Mrad (sufficient to eliminate the most radiation-resistant microorganisms known) is 900 million years. For the upper few meters of the satellites the situation is considerably more severe. A total dose of 18 Mrad is produced at a depth of 1.5 m by galactic cosmic rays and solar flare protons in just 15 million years. The radiation levels are sufficient to inactivate any organisms in or on the satellites, except in rare cases where regolith processes have moved any long-protected materials from depth to within range of sampling techniques. It is possible that the processes of disruption/reassembly and regolith turnover have been so efficient that Phobos and Deimos have been devolatilized for aeons and all materials will have been bathed in lethal radiation, rendering them harmless. The most plausible way for biological material not formed within the bodies themselves to reach the surface of Phobos or Deimos would be in materials ejected from Mars as a result of impacts on the planet's surface. It is clear that some small fraction of ejecta from impacts on Mars are transferred to the planet's satellites. Three factors, however, reduce concern about the possibility of hazards from this source of contamination: (1) any martian material sampled from the satellites would have arrived there as a result of random impacts, not from the special environments that might be sampled by dedicated sample return missions to Mars itself; (2) Viking data have demonstrated that the surface and near-surface of Mars are hostile to biological materials; and (3) SNC meteorites, sampled by random impacts, have not been found to be hazardous on Earth. The Galilean Satellites of Jupiter The four galilean satellites are large objects in low-inclination, low-eccentricity orbits around Jupiter. Their substantial size and the regularity of their orbits provide evidence that they formed via condensation and agglomeration from a primordial circumjovian nebula. A major uncertainty regarding their formation is the gas density in the nebula at the time that accretion took place. Plausible hypotheses range from essentially gas-free scenarios (e.g., Safronov et al., 1986) to ones in which gas densities were high enough that gas drag was a dominant dynamical process (e.g., Hayashi et al., 1985). Regardless of the gas/solid ratio during accretion, however, the present bulk compositions of the satellites must to some degree reflect the pressure and temperature conditions in the material orbiting Jupiter at the time that condensation occurred. Simple consideration of the condensation sequence of materials from a circumjovian nebula of solar composition suggests that the galilean satellites' "original" (i.e., post-accretional) composition was some mixture of H2O ice and roughly chondritic-composition non-icy material (Lewis, 1971, 1972). Other ices are not expected to have been present in substantial quantities owing to insufficiently low temperatures. In the presence of a radial temperature gradient in the circumjovian nebula, the ratio of icy to non-icy condensed material would have depended on the distance of formation from Jupiter. This scenario is supported by the progression in the densities of the galilean satellites, which decrease regularly with increasing distance from Jupiter. The satellites may be thought of most simply, then, as chondritic/silicate bodies in bulk composition, with substantially varying additional endowments of H2O. The satellites' densities indicate just how substantial the variation in H2O endowment is. Io's density is about 3.5 g/cm3, consistent with a rocky or rock-metal composition that is lacking even in water of hydration. Europa's density is about 3.0 g/cm3, consistent either with hydrated silicates or with dehydrated silicates and a substantial quantity of free H2O. Ganymede and Callisto have densities of about 1.9 and 1.8 g/cm3, respectively, indicating bulk compositions that approach 50 percent H2O by mass.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making The satellites also show an enormous degree of variability in the extent of geologic activity they have undergone since their formation (Smith et al., 1979a,b). Callisto has a heavily cratered surface that provides evidence for little or no internal activity. Ganymede exhibits two distinct types of geologic terrain: dark, heavily cratered terrain that is similar to Callisto's (though with somewhat lower crater density), and brighter, younger "grooved terrain" that has undergone intense tectonic deformation. Europa's surface is bright and icy, with extreme tectonism and a crater density far lower than anything found on Ganymede. Io has no impact craters, and there is evidence of widespread, ongoing volcanic activity. Several factors may help account for the observed variability in geologic history among the galilean satellites. Among them, the most important is probably the variability of tidal heating. The three inner galilean satellites, Io, Europa, and Ganymede, currently participate in a Laplace orbital resonance (a commensurability in their mean motions that causes repeated alignments of the satellites at identical points in their orbits). The mutual gravitational perturbations of the satellites upon one another sum as a consequence of these alignments to produce significant orbital eccentricities. The eccentricities in turn can lead to significant heating. Satellites close to their primary body can exhibit significant tidal deformation. The size of a satellite's tidal bulge varies with distance from the primary body. In addition, for a synchronously rotating satellite in an eccentric orbit, the orientation of the bulge varies with orbital phase. Both variations cause flexure of the satellite, dissipating strain energy as heat. This tidal heating is a very strong function of mean orbital radius. A consequence of this strong dependence is that Io is subjected to overwhelming tidal heating that powers its volcanism. Europa may undergo just enough heating to allow liquid water to exist beneath its surface, and Ganymede's tidal heating could have been geologically significant only during possible former periods of higher orbital eccentricity. Callisto is unlikely ever to have undergone significant tidal heating. The galilean satellites are therefore an extremely diverse group of objects in terms of their endowment of H2O, the fraction of that H2O that has been liquid, and the quantity of liquid that could have reached the surface. The variability in their biological potential is correspondingly large. Io Io is the innermost of the galilean satellites. It has a radius of more than 1,800 km, making it slightly larger than the Moon, and has a density of approximately 3.5 g/cm3, indicating a composition dominated by rock and possibly metal. Io is the most volcanically active known body in the solar system. Its volcanism is the result of intense tidal heating (Peale et al., 1979; McEwen et al., 1998). The volcanism takes a number of forms. The most dramatic are gas-driven eruptions that produce spectacular eruptive plumes that reach heights as great as 300 km above the surface (Strom et al., 1979). The plumes apparently develop when volatile material, either SO2 (Smith et al., 1979c) or S (Reynolds et al., 1980), comes into contact with hot silicates, vaporizing energetically. Other volcanic eruptions produce surface flows that exhibit the familiar morphology of low-viscosity lavas, but that show colors ranging from black through various shades of orange, greenish-yellow, and white. The coloration is similar in some respects to that of quenched high-temperature allotropes of sulfur (Sagan, 1979), leading to the suggestion that the flows are composed predominantly of sulfur. Solid sulfur is too weak to support much of the topography exhibited by the flow units, however (Clow and Carr, 1980), and so a more plausible explanation may be that basaltic volcanism dominates, with coloration added by a small admixture or coating of sulfur and sulfur compounds. Given the high level of volcanic activity on Io, it is abundantly clear that the geothermal energy that would have been needed to drive hydrothermal activity there is present. What appears to be lacking, however, is water. No spectroscopic evidence for H2O has been found on Io. Instead, the dominant volatiles on Io appear to be sulfur compounds. Allotropes of elemental sulfur may be responsible for some of Io's coloration, as noted above. SO2 has been clearly shown to be present via infrared spectroscopy (e.g., Nash, 1983; Howell et al., 1984). Abundant sulfur and oxygen ions are present in the jovian magnetosphere, with Io identified as the source (Bagenal and Sullivan, 1981). Io is also associated with a Jupiter-encircling torus of neutral atomic Na, K, O, and S (Brown et al., 1983), derived from magnetospheric interactions with Io's surface materials. Nowhere, however, is there evidence for H2O.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making From a biological perspective, another important point regarding Io is that the radiation environment to which it is subjected is more intense than that of any other large solid body in the solar system. Jupiter has a powerful magnetosphere, and charged particles trapped in the magnetosphere continually bombard Io's surface at high flux levels and with high energy. This radiation would serve as a powerful inhibitor of biological activity at the surface of Io. This radiation would be less intense (but not zero) for the other jovian satellites. Finally, because essentially all of the material at the surface of Io is volcanic, a good case can be made that all accessible material on the satellite has been heated at some point in time to temperatures too high for organic molecules to be preserved. Overall, the prospects for biological activity at and below Io's surface appear extremely poor. While abundant biologically useful energy is present, there is no evidence for H2O ice at the surface, and no evidence for the former presence of solid or liquid H2O beneath the surface. Europa Europa is one of the most interesting objects in the solar system. Its radius of about 1,600 km makes it slightly smaller than the Moon. It has a density of about 3.0 g/cm3, meaning that it is probably mostly silicate and metal by mass but may also contain a significant endowment of a lower-density material, probably H2O. Its surface composition is known from infrared spectroscopy to be dominated by H2O ice (Pilcher et al., 1972). Ultraviolet spectroscopic data also suggest that a small amount of sulfur is present; this sulfur was probably implanted from the jovian magnetosphere, with Io as its original source (Lane et al., 1981). The thickness of Europa's H2O-rich outer region may be substantial. If the density of the non-icy component is the same as that of Io, then this thickness is at least 100 km. Recent Galileo determinations have suggested that Europa's degree of central condensation could be still greater than this, and the thickness of the H2O region correspondingly larger (Anderson et al., 1997). A crucial question from a biological standpoint is whether or not any of the subsurface H2O on Europa is liquid. Europa participates in the same three-body Laplace orbital resonance as Io, and as a result is also subjected to tidal heating. However, the heating is much less intense. For homogeneous tidal heating, the heating rate decreases as the sixth power of the orbital radius, other factors being equal. Tidal heating therefore has not been the dominating factor in the evolution of Europa to the extent that it has on Io. Unfortunately, the extent to which tidal heating has affected Europa's interior is poorly understood. The reason for this is that tidal heating calculations depend on several uncertain material parameters. The most important of these are the flexural rigidity and the specific tidal dissipation function of the satellite's materials. While uncertainties in these parameters are comparatively unimportant on Io due to the overwhelmingly large magnitude of the heating there, they are important in evaluating Europa's evolution. Another major source of uncertainty has to do with the long-term rheological properties of Europa's ice and their variation with depth. Heat can of course be transported within Europa's icy crustal materials by conduction. Under certain circumstances, however, it can also be transported by solid-state convection. Factors favoring convection include low viscosity, a large base-to-top temperature difference, and a large icy layer thickness. Convection can be a much more efficient heat transport mechanism than conduction, and so whether or not a model predicts that liquid exists beneath Europa's surface can depend in large part on whether or not convective instability is predicted. This is in turn related to the poorly known relationship between ice viscosity and temperature over convective time scales and strain rates. As a consequence of these various sources of uncertainty, the subsurface structure of Europa is poorly known. Some calculations have suggested that tidal heating has been sufficient to melt most of the H2O within Europa and to keep it molten to the present (e.g., Cassen et al., 1979; Squyres et al., 1983; Ojakangas and Stevenson, 1989). In such models, the tidal heating and rheological parameters are such that solid-state convection does not occur, and tidal heating is able to maintain a thin ice crust over a thick liquid layer. If this is the case, Europa has a rock-metal interior, a thick "ocean" of liquid water, and a relatively thin outer shell of H2O ice. Other models make parameter choices that reduce tidal heating and/or emphasize the importance of solid-state convection in the ice as an effective agent of heat transport, and conclude that the ice surrounding Europa's rock-metal interior is completely frozen (e.g., Cassen et al., 1980; Ross and Schubert, 1987). Plausible intermediate possibilities involve a thin layer

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making of water beneath a thick ice cover, an ice cover of irregular thickness, or water that is present only in small localized regions. The photogeologic evidence for liquid water under Europa's ice cover is significant but equivocal. Europa's impact crater density is very low, indicating geologically recent resurfacing (Smith et al., 1979b; Carr et al., 1998). The mechanism by which this resurfacing has occurred, however, is unclear. Surface extrusions of either liquid or warm, mobile ice may have been significant. A major role could also have been played by simple viscous relaxation of topography. There is also substantial evidence for crustal deformation on Europa. The satellite's surface is laced with an intricate pattern of fractures, ridges, and other lineations, ranging in size from tens of kilometers wide and more than 1,000 km long down to the smallest scale seen in Voyager and Galileo images. Two lines of evidence suggest that only a very thin near-surface region behaved in a brittle fashion as this tectonism took place. One is the narrow widths of most grooves and ridges, which are consistent with brittle deformation being limited to the upper 1 km or so (e.g., Golombek and Banerdt, 1989). The other is the substantial evidence that small blocks of crustal material have rotated and translated laterally with respect to one another, moving over easily deformed material that lies immediately below (e.g., Schenk and McKinnon, 1988). Whatever lies just below Europa's surface clearly is therefore substantially softer and more mobile than cold ice. High-resolution Galileo images have further fueled speculation that liquid water could be present within Europa. The most dramatic of these clearly show local regions that have been disrupted by material rising up from beneath Europa's surface. Crustal blocks in some cases have been "rafted" by this material, fragmenting, rotating, and translating with respect to one another. The Galileo images show this crustal disruption to be widespread, geologically young (perhaps as recent as 10 million years), and involving fluid-like behavior within a few kilometers of the surface. It is impossible to determine from the imaging data alone if this activity involved liquid water or solid-state convection of ice (Carr et al., 1998; Pappalardo et al., 1998a). The gravity data from several encounters suggest a layer of water or water ice from 100 to 200 km thick (Anderson et al., 1997). Reynolds et al. (1983) considered the possibility that significant biologically useful energy could be released in Europa's putative ocean. Europa's surface experiences substantial insolation. However, even under the most optimistic assumptions about the frequency of ice fracturing and the transparency of refrozen materials, only a trivial amount of sunlight can penetrate into subsurface liquid. Hydrothermal energy, however, could be another matter. Radionuclides in Europa's silicates produce heat, and radiogenic heating is augmented by tidal energy dissipation in the silicate-metal interior. It is not clear whether or not this rate of heat production has been sufficient to allow magmatic activity to persist within Europa to the present. The rate of heat production is significantly greater than it is for the Moon, which is not volcanically active at the present, and significantly less than it is for Earth, which is active. However, the higher heating rates in the past were clearly sufficient to have produced magmatic activity in Europa's silicates at some point. This argument is reinforced by the discovery by Galileo of Europa's high degree of central condensation and the likelihood that it has an iron core (Anderson et al., 1997). The interaction of magmatism in Europa's rocky material with the H2O immediately above it could have led to the development of local hydrothermal systems within Europa whether the overlying H2O was dominantly liquid or not. While much remains to be learned about Europa, several factors relevant to its biological potential are clear. One is that hydrothermal systems are likely to have operated within a few hundred kilometers of Europa's surface at some point in its history. Another is that material from beneath the surface has reached the surface in the geologically recent past. When these factors are considered together, it is clear that Europa is an important object from the standpoint of exobiology. Of all the solar system bodies other than Earth, only Mars appears to have a potential for past or present life that is comparable to Europa's. Indeed, investigation of Europa's biological potential forms much of the rationale for continued investigation of the satellite. In future exploration of Europa, a premium will be placed on sampling resurfaced regions that may contain materials brought up recently from deep below Europa's surface. Spectral data from Galileo indicate the possibility of salts included in the surface ice of Europa (McCord et al., 1998). Such materials could carry the chemical signature of subsurface hydrothermal activity. If recent hydrothermal systems on Europa have supported life, they could also carry frozen evidence of such life.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making Ganymede Ganymede is the largest of the galilean satellites, and the largest satellite in the solar system. With a radius of some 2,600 km, it is larger than the planet Mercury. Ganymede's density is about 1.9 g/cm3, meaning that it is probably roughly 50 percent H2O by mass. Infrared spectroscopy demonstrates that H2O ice constitutes a major fraction of Ganymede's surface material (Johnson and Pilcher, 1977). Galileo data have shown Ganymede to have a moment of inertia consistent with a large degree of central condensation (Anderson et al., 1996a). Combined with the finding of an intrinsic magnetic field (Kivelson et al., 1996; Schubert et al., 1996), it appears likely that Ganymede has a metallic core, silicate mantle, and outer ice shell several hundred of kilometers thick. Without the ice, Ganymede would resemble Io (Anderson et al., 1996b). A major factor that must be considered when examining the geologic evolution of Ganymede's interior is the complex phase diagram of H2O (e.g., Shoemaker et al., 1982). Ice has a number of different crystalline forms, or polymorphs, that are stable over different temperature and pressure ranges. The familiar form, ice I, is stable at Ganymede's surface. However, at depths of hundreds of kilometers within the satellite, polymorphs other than ice I will be present. These polymorphs all have densities significantly greater than those of ice I. Ices VI and VII, possibly important deep within the satellite at various times in its history, also have elevated melting temperatures. The various polymorphs also have substantially differing rheological properties. This factor, plus the heats of phase transformation among the various polymorphs, can act to resist solid-state convection across phase boundaries. Convection across phase boundaries is unlikely to have been inhibited at all times, however, and deep through-going solid-state convection cells could have existed during some periods in Ganymede's history (Schubert et al., 1981). Liquid water is unlikely to be present in significant quantities in Ganymede today, due both to the heat-transporting efficiency of solid-state convection and to the very low current rate of tidal heating. Ganymede's surface shows substantial evidence for geologic activity early in its history (Smith et al., 1979a,b). Some parts of Ganymede are relatively dark and heavily created, suggesting that they are composed of ancient mixtures of ice and silicates that have been subjected to little geologic activity other than impact cratering. Among the most intriguing geologic features in these old terrains are the "crater palimpsests" (Smith et al., 1979b). These are circular features of very little relief that are apparently vestiges of large ancient impacts. Their mechanism of formation has not been firmly established. However, one plausible model is that ancient impacts that penetrated through Ganymede's cold outer crustal regions allowed warm, mobile, and buoyant subsurface ice to be extruded to the surface, spreading outward to form the circular palimpsest deposits (Thomas and Squyres, 1990). Coupled with the possibility of deep through-going solid-state convection, this model opens the possibility that ice from very deep within Ganymede was extruded to the surface during palimpsest formation. Other regions of Ganymede are brighter and less heavily cratered. Evidence for tectonism in these regions of "grooved terrain" is widespread. The tectonism appears to have been dominantly extensional, producing a geometrically complex arrangement of grooves that are probably the surface expression of grabens and other fault-fracture features (e.g., Shoemaker et al., 1982). A variety of explanations have been offered for this tectonism, all related to subsurface phase changes among the various forms of H2O possible in the deep interior (e.g., Squyres, 1980; Shoemaker et al., 1982; Kirk and Stevenson, 1983). While the crater density in the grooved terrain tends to be lower than that in the darker terrain, the dominant mechanism of resurfacing is unclear. Following the Voyager mission, interpretations of Ganymede's bright terrain focused largely on the role of "cryovolcanism"—extrusion of liquid water or warm, mobile ice to Ganymede's surface. The prevailing hypothesis at that time was that the bands of bright terrain represented broad, down-dropped grabens that had been filled in with cryovolcanic deposits (e.g., Parmentier et al., 1982). In fact, extrusion of liquid rather than warm ice seemed plausible, given the lack of any observed viscous flow features in the Voyager images. Recent Galileo images have called into question the idea of cryovolcanism in Ganymede's grooved terrain. High-resolution images of the Uruk Sulcus region in particular have showed such intense deformation at small scales that the destruction of preexisting craters there seems attributable entirely to tectonism (Belton et al., 1996; Pappalardo et al., 1998b). While it is tempting to extrapolate this conclusion to the rest of Ganymede, the limited

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making spatial distribution of the high-resolution Galileo coverage makes this impossible. At this point, then, the satellite-wide role of cryovolcanism in the resurfacing of grooved terrain remains uncertain. Because Ganymede's rocky-metallic interior lies so far below the surface, little study has been performed on its detailed evolution. However, if Ganymede is fully differentiated, then the size of its non-icy interior is comparable to that of Europa's. This being the case, there is also a distinct possibility that magmatic activity has occurred there. In fact, Galileo's discovery of Ganymede's intrinsic magnetic field leaves open the possibility that deep magmatic activity in Ganymede could persist to the present (Kivelson et al., 1996). It is therefore likely that hydrothermal activity has taken place at Ganymede's silicate-ice boundary, albeit many hundreds of kilometers beneath the satellite's surface. Ganymede's biological potential is probably lower than that of Europa. Hydrothermal activity near the silicate-ice boundary could have occurred through at least some of the satellite's history, and through-going convection in the ice layer could have transported frozen hydrothermal fluids to near-surface regions. Whether or not subsurface material was extruded to the surface in formation of the grooved terrain is doubtful, but it remains a possibility. Such extrusion is certainly a possibility in the case of the crater palimpsests. Cryovolcanic deposits at the surface of Ganymede, if they exist, are likely to be hundreds of millions to billions of years old, and many hundreds of kilometers removed from any potential hydrothermal sites. Callisto Callisto is similar to Ganymede in its size (radius of approximately 2,400 km) and density (approximately 1.8 g/cm3). This similarity makes it difficult to explain why the satellites are so different in appearance. In contrast to Ganymede, Callisto shows negligible evidence for resurfacing or tectonism (Smith et al., 1979a,b). Instead, the satellite's history has been dominated nearly completely by impacts. All of the major geologic features seen in both Voyager and Galileo images can be explained in terms of impact-related processes. This applies even to sets of subparallel furrows, which are due to faulting but which are clearly concentric with features of major impacts and caused by them (McKinnon and Melosh, 1980). The general lack of nonimpact processes on Callisto does not rule out the possibility that subsurface material has risen to the surface in some locations. Although distinct crater palimpsests like those on Ganymede are not generally observed, it is clear that large impacts have caused excavations to substantial depths in the satellite. The largest impact scars do not retain deep crater-like topography, and so have clearly undergone postimpact modification that has included upward viscous flow of materials that were originally deep below the surface. The floors of the largest impact scars could therefore contain materials that were once many tens of kilometers beneath the satellite's surface. Galileo gravity data show that although Callisto is somewhat differentiated, it is much less so than Ganymede (Anderson et al., 1996a). The lack of a magnetic signature also distinguishes it from Ganymede (Khurana et al., 1997). If correct, this finding may mean that early heating of Callisto was less severe than that for Ganymede, so that heat transport processes (probably including deep solid-state convection) were more able to keep pace with heat production and help to inhibit differentiation. This finding is consistent with the lack of tectonism on Callisto, since the ice-ice phase changes associated with differentiation can produce substantial surface extension and tectonism (Squyres, 1980). Callisto's apparent lesser degree of differentiation, and particularly its minimal resurfacing and tectonism, are significant from a biological perspective. These findings imply that silicates, rather than being concentrated near the satellite's center as they are in Ganymede, may be distributed more nearly uniformly throughout it. If a rocky or rocky-metallic interior is poorly developed within Callisto, silicate magmatism and hydrothermal activity are less likely to have taken place there. One conceivable mechanism by which biological materials could have reached the surface of Callisto would be transfer by ejecta from an impact on another body in the Jupiter system, like Europa, that has some biological potential. However, such material would constitute such a trivial volume of Callisto's surface material that the odds of sampling it would be negligible. This applies to the other planetary satellites as well.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making POTENTIAL FOR A LIVING ENTITY TO BE IN OR ON SAMPLES RETURNED FROM PLANETARY SATELLITES For samples returned from the seven planetary satellites discussed above, the answers to the questions posed in Chapters 1 and 2 can be summarized as follows. Does the preponderance of scientific evidence3 indicate that there was never liquid water in or on the target body? The evidence indicates that there was never liquid water on the Moon and Io. For Phobos and Deimos, there is no evidence that liquid water ever existed, but owing to uncertainty about the bodies' composition, the possibility cannot be excluded. There is evidence for the potential for liquid water on Europa and, to a lesser extent, on Ganymede and Callisto. Does the preponderance of scientific evidence indicate that metabolically useful energy sources were never present? Biologically useful energy may be present on Io. It is likely that hydrothermal systems operated within a few hundred kilometers of Europa's surface at some point in its history and that material from beneath the surface has reached the surface in the geologically recent past. Because Callisto is apparently less differentiated than Ganymede, for example, a biologically useful energy source is much less likely on Callisto than on Ganymede but is still possible. 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? Because of the lack of data, it is uncertain that there was never sufficient organic matter (or CO2 or carbonates and an appropriate source of reducing equivalents) in or on the planetary satellites discussed in this chapter. 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)? Because of its volcanism, Io is likely to have been subjected to sterilizing temperatures. It is as yet undetermined whether sterilizing temperatures were reached for the other six planetary satellites discussed in this chapter. Does the preponderance of scientific evidence indicate that there is or was sufficient radiation for biological sterilization of terrestrial life forms? The effects of galactic cosmic rays, solar flare protons, and radiation from natural radionuclides on Phobos and Deimos have been substantial and are sufficient for biological sterilization of the satellites. Io is subjected to intense sterilizing radiation from Jupiter's magnetosphere. 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? Because of the lack of data, it is uncertain whether there has been natural influx to Earth of material from the planetary satellites examined in this chapter. This direct evidence, along with the absence of indirect evidence for recent or past liquid water on the Moon, indicates that the potential for a living entity to be present in samples returned is negligible. Phobos and Deimos 3    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. 4    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 have been radiation-sterilized by natural radioactive sources and are too warm and too fractured to enable life-protecting ice to persist in interior pockets, and so the potential for a living entity to be present in returned samples is negligible. However, Phobos and Deimos could be subject to cross-contamination by ejecta from Mars. It is uncertain whether such material would constitute such a trivial volume of the surface of Phobos or Deimos that the odds of sampling it would be negligible. The potential for a living entity to be present in samples returned from Io is negligible because of the lack of water in any form and the additional sterilizing influence of jovian magnetospheric bombardment on near-surface materials. There is evidence of liquid water beneath the icy crust of Europa, first surmised from Voyager data and reinforced by Galileo data. A readily available energy source (i.e., heat) may be present because of the Laplace orbital resonance relationship among Io, Europa, and Ganymede. Accordingly, the task group found that there is a potential for a living entity to be present in samples returned from Europa. Similarly, the task group found that there is a potential for a living entity to be present in samples returned from Ganymede. Because Callisto lacks an adequate source of energy to melt ice today and because there is no direct evidence that Callisto contained liquid water in the past, the potential for a living entity to be present in or on a returned sample is extremely low, but the task group could not conclude that it is zero. Callisto could be subject to cross-contamination by ejecta from another body in the Jupiter system (e.g., Europa) that has some biological potential. However, such material would constitute such a trivial volume of Callisto's surface material that the odds of sampling it would be negligible. On the basis of available information about the Moon and Io, the task group concluded with a high degree of confidence that no special containment is warranted for samples returned from those bodies beyond what is needed for scientific purposes. For samples returned from Europa and Ganymede, the task group concluded that strict containment and handling requirements are warranted. For samples returned from Phobos, Deimos, and Callisto, the potential for a living entity in a returned sample is uncertain because of potential cross-contamination. While the task group concluded that containment is not warranted for samples returned from Phobos, Deimos, and Callisto, these conclusions are less firm than that for the Moon and Io. These conclusions need to be reexamined on a case-by-case basis when missions to these bodies are planned. SCIENTIFIC INVESTIGATIONS TO REDUCE THE UNCERTAINTY IN THE ASSESSMENT OF PLANETARY SATELLITES Outlined below are investigations that would help to improve some of the uncertainty associated with assessment of the biological potential of some of the planetary satellites inside Jupiter's orbit before samples are returned. Phobos and Deimos Compositional remote sensing of Phobos and Deimos (e.g., x-ray and gamma-ray analysis for elemental chemistry, infrared analysis for mineralogy) could help to show whether these objects are primitive, undifferentiated bodies (perhaps akin to P- and D-type asteroids) or whether they are more evolved bodies with a correspondingly lower biological potential. Additionally, it would be especially useful in this context to analyze Phobos and Deimos for the presence of organic matter. Europa Europa is the focus of an intensive, ongoing investigation by the Galileo spacecraft. Unless compelling evidence is found that Europa does not now have, and never had, liquid water, special containment procedures will be needed until the question of the existence of a Europan biota is answered definitively by a sample return or in situ mission. Orbital geophysical sensing of several sorts (imaging, laser/Doppler geodesy, radar sounding) has the potential to resolve the question of whether or not there is a global "ocean" of liquid water beneath Europa's icy surface. If compelling evidence were found for subsurface liquid, remote and in situ determinations of the composition of recently resurfaced regions could provide information about the nature of that liquid.

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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making Ganymede Ganymede is also the focus of an ongoing investigation by the Galileo spacecraft. It seems likely that a sample return or in situ mission would be mounted to Europa before a mission to Ganymede. Improved determinations of Ganymede's moment of inertia could help to determine the extent to which the satellite is differentiated, and compositional remote sensing might provide evidence of whether or not liquid was involved in any of Ganymede's resurfacing. Callisto One of the primary arguments against use of special containment procedures for samples from Callisto is that the satellite appears to be largely or even completely undifferentiated. Improved determinations of Callisto's moment of inertia could verify this interpretation and thereby strengthen this case. SUMMARY The task group considered the possibility of sample return from the major satellites of the innermost five planets. These include the satellite of the Earth (the Moon), satellites of Mars (Phobos and Deimos), and satellites of Jupiter (Io, Europa, Ganymede, and Callisto). Many samples of lunar rocks and soils were returned to Earth by the U.S. Apollo and Soviet Luna programs. None has been found to contain any evidence of past or present lunar biological activity. Because of this direct evidence, and because there is no indirect evidence for recent or past liquid water on the Moon, the potential for a living entity to be present in returned samples is negligible. For samples returned from Phobos and Deimos the potential for a living entity in a returned sample is considered to be very low, but it cannot be expressed definitely because of possible cross-contamination from Mars. Although containment is unlikely to be required for samples returned from these bodies, this conclusion is less firm than that for the Moon and Io. There are no special containment procedures warranted for samples returned from Io, which is volcanically active, lacks liquid water, and has been heat sterilized (i.e., at temperatures greater than 160 °C). Although the task group does not recommend special containment procedures for samples returned from Callisto, its assessment depends on preliminary evidence suggesting that liquid water was not present in the past or present environment, but this assessment is less certain than, for example, that for Io or the Moon. Unlike samples from the Moon and Io, samples returned from Europa should be contained. The evidence for a liquid water ocean beneath the icy crust, first surmised from Voyager data and reinforced by Galileo data, coupled with a readily available energy source from its role in the Laplace orbital resonance relationship, makes Europa a prime target in the search for extraterrestrial life. Likewise, strict containment and handling procedures are required for samples returned from Ganymede. While there is no evidence for liquid water beneath the icy crust at present, it cannot be ruled out that liquid water once existed, if only at great depth. Evidence for possible whole-satellite convection, coupled with a readily available energy source from its role in the Laplace orbital resonance relationship, also makes Ganymede a target in the search for extraterrestrial life REFERENCES Allton, J.H., J.R. Bagby, and P.D. Stabekis. 1998. Lessons learned during Apollo lunar sample quarantine and sample curation. Adv. Space Res. New York: Elsevier Science. Anderson, J.D., E.L. Lau, W.L. Sjogren, G. Schubert, and W.B. Moore. 1996a. Gravitational constraints on the internal structure of Ganymede. Nature 384:541–543. Anderson, J.D., W.L. Sjogren, and G. Schubert. 1996b. Galileo gravity results and the internal structure of Io. Science 272:709–712. Anderson, J.D., E.L. Lau, W.L. Sjogren, G. Schubert, and W.B. Moore. 1997. Europa's differentiated internal structure: Inferences from two Galileo encounters. Science 276:1236–1239. Arnold, J.R. 1979. Ice in the lunar polar regions. J. Geophys. Res. 84:5659–5668. Bagenal, F., and J. Sullivan. 1981. Direct plasma measurements in the Io torus and inner magnetosphere of Jupiter . J. Geophys. Res. 86:8447–8466.

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