Mars Pathfinder spectral measurements of Phobos and Deimos: Comparison with previous data

The Imager for Mars Pathfinder (IMP) acquired four spectra of parts of the sub-Mars hemispheres of Phobos and Deimos. The measured region of Phobos is expected to be a mixture of the two spectral units identified on that satellite from Phobos 2 data, and the IMP spectra of Phobos are intermediate to the two units as expected. The derived geometric albedo is consistent with the value for that part of Phobos determined from Viking imagery. The IMP spectrum of Deimos is generally consistent with previous measurements acquired from the ground and from the Hubble Space Telescope (HST), but the signal-to-noise ratio is lower than that of the Phobos data. The spectral contrast between the two moons is similar to that seen in HST and Phobos 2 data. Mars Pathfinder measurements therefore substantiate recent results which indicate that Phobos and Deimos are not, as previously believed, analogous to C-type asteroids. They also provide some indications for an absorption near 700 nn, perhaps like that seen in other low-albedo asteroids. Both Martian moons are redder than most asteroids, and most closely resemble two analog materials believed to have undergone very dissimilar histories: primitive D-like asteroids, and highly space-weathered, mafic-rich assemblages, such as are present in lunar mare soils.