A Review of Lunar Sample Studies and Their Application to Studies of the Terrestrial Planets

covered by water ice [Pilcher et al., 1972]. No evidence of gaseous absorption was detected [Fink et al., 1973], but the occultation experiment on Pioneer 10 indicated the presence of a very thin atmosphere on Io [Kliore et al., 1974a, b]. This satellite was also found to be the source of intense sodium emission [Brown, 1973] and seems to be surrounded by a cloud of sodium atoms whose distribution, origin, and mode of excitation are all current topics of active research [Trafton et al., 1974; McElroy et al., 1974; Fanale et al., 1974; Wamsteker et al., 1974]. A new satellite in the Jovian system was discovered by Kowat [1974]; it appears to be a member of the middle group with direct orbits. The Doppler tracking data from the Pioneer 10 spacecraft have been analyzed to derive masses for the Galilean satellites. The results indicate