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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