Spectral evidence for a carbonaceous chondrite surface composition on Deimos

Abstract

The good match between the reflectance spectra of Phobos and Ceres, an asteroid inferred to have a composition analogous to carbonaceous chondrites because of its low density, ∼2.2g cm⁻³ (ref. 1), and spectral signatures that include a water of hydra-tion band centred at 3.0 µm (refs 2,3), has led to the conclusion⁴ that the surface composition of the inner martian satellite is similar to that of carbonaceous chondrites. Pollack et al.⁵ independently reached the same conclusion by comparing Phobos' spectral albedo with reflectance spectra of meteorites and basalts. Mass and density estimates (∼2g cm⁻³) for the satellite were made based on the perturbations on the orbit of Viking 1 by Phobos, during a series of close encounters⁶. The density of Phobos is consistent with the same composition as that inferred for its surface. Among materials expected to be common in the Solar System only ices or water-rich carbonaceous chondrites (20% H₂O by weight in type C1) have densities as low as Ceres and Phobos⁷. The compositional results have forced a re-evaluation of hypotheses concerning Phobos' origin. Scientific opinion now seems to favour an origin by capture of a body that came too close to Mars⁸. The analysis of data on Deimos reported here reveals a spectral reflectance very similar to that of Phobos, indicating a surface composition analogous to that of carbonaceous chondrites.