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 similar in size and shape to typical asteroids. The origin of Phobos and Deimos is unclear. Both lie in orbits that are low in inclination and nearly circular. Overall, the limited spectral data and poorly determined densities of Phobos and Deimos are broadly consistent with their being similar to C- or P-type asteroids. It is possible that Phobos and Deimos may have experienced liquid water early in their history, but their primitive chemical composition would have led to potentially sterilizing levels of radiation since then. It is unlikely that ice-filled voids, which might have attenuated that radiation, are present today within the upper reaches of Phobos and Deimos that are accessible to sample return missions, because typical subsurface temperatures are far too high for ice to exist in equilibrium. On the other hand, biological materials, if any, could conceivably have been protected in ice pockets at depth for considerable periods of time, transported more recently to near-surface environments by catastrophic collisional disruption, and subsequently reassembled in Mars orbit and by near-surface processes of regolith turnover. Although sampling of any resulting, potentially hazardous material is very unlikely, it cannot be categorically ruled out.

Although it is clear that some small fraction of ejecta from impacts on Mars will be transferred to the planet's satellites, such material would be present only in a random (rather than targeted) sampling from the surface of Mars (which is known to be hostile to biological materials) and would be similar to SNC meteorites already striking Earth (which have been found not to be hazardous). Thus, the potential for a living entity to be present in returned samples is extremely low, but the task group could not conclude that it is necessarily zero.


Io is the innermost of the galilean satellites, with a radius of more than 1,800 km, making it slightly larger than the Moon. Its composition is dominated by rock and possibly metal. Io is the most volcanically active known body in the solar system. Although abundant biologically useful energy is present, there is no evidence for the present or past existence of solid or liquid water at or beneath the surface. 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 higher than the maximum tolerated by any organic material. Io is also exposed to Jupiter's powerful magnetosphere; 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. Because of the lack of water in any form and the additional sterilizing influence of jovian magnetospheric bombardment on near-surface materials, the potential for a living entity to be present in returned samples is negligible.


Europa is one of the solar system bodies that appears to have a potential for past or present life. Europa has a radius of about 1,600 km, slightly less than the Moon's, and it is probably mostly silicate and metal by mass. It has an upper layer, on the order of 100 km deep, composed of liquid and/or solid water. There is evidence of liquid water beneath the icy crust, first surmised from Voyager data and reinforced by Galileo data. Accordingly, the task group found that there is a potential for a living entity to be present in samples returned from Europa.


Ganymede is 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 probably roughly 50 percent water by mass. Although there is no evidence of the current presence of liquid water beneath the icy crust, the past presence of liquid water, even if only at a great depth, cannot be ruled out. 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. It is doubtful, but still possible, that subsurface

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