The spectrum of Phobos from Phobos 2 observations at 0.3-2.6 microns: Comparison to previous data and meteorite analogs

The surface of Phobos has been proposed to consist of carbonaceous chondrite or optically darkened ordinary chondrite ('black chondrite'). Measurements of Phobos's spectrum are key evidence for testing these hypotheses. Disk-integrated measurements were obtained by the Mariner 9 UV spectrometer, Viking Lander cameras, and groundbased observations. In 1989 disk-resolved measurements of Phobos and Mars were obtained by three instruments on Phobos 2: the KRFM spectrometer, which covered the wavelength range 0.32 - 0.6 microns; the ISM imaging spectrometer, which covered the wavelength range 0.76 - 3.16 microns; and the VSK TV cameras, whose wavelength ranges overlap those of KRFM and ISM. Here we report analysis of the Phobos 2 measurements completed since earlier results were reported. We validated calibration of the Phobos measurements using observations of Mars for reference, and compared them with pre-1989 measurements. We also combined spectra from the three detectors to produce an integrated spectrum of Phobos from 0.3 - 2.6 microns. Phobos 2 results agree well with previous measurements, contrary to some reports. The general shape of the spectrum is consistent with both proposed analogues. However position and depth of the previously unobserved 1 micron absorption are more diagnostic, and indicate the composition of typical surfaces to be more consistent with black chondrite.