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Spectral evidence for a carbonaceous chondrite surface composition on Deimos


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.

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Pang, K., Rhoads, J., Lane, A. et al. Spectral evidence for a carbonaceous chondrite surface composition on Deimos. Nature 283, 277–278 (1980).

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