Phobos mineralogical interpretation from 0.25 to 4.0 μm

On 24 February 2007, the OSIRIS-WAC and NAC instrument onboard the Rosetta spacecraft observed Phobos. We present a disk-integrated compositional analysis of the Martian satellite. The area studied in this work covers from 126°W to 286°W in longitude and from 86.8°N to 90°S in latitude and it was observed with a phase angle ranging from 18.87° to 18.95°. Previous investigations of the composition of the surface of Phobos made use of spectral matching. In our study we undertake spectral modeling of the surface of Phobos albedo using a radiative transfer code based on the slab approximation. The code calculates the albedo of a powdered surface from the optical constants of candidate materials. After scaling the OSIRIS data to match a previously obtained visible albedo measurement, we provide spectral modeling of the surface of Phobos in the wavelength range between 0.25 and 4.0 mum, using OSIRIS and the Rivkin et al. (Rivkin, A. S. et al., 2002, Icarus, 156, 64) IRTF results. We provide two models, the first one fitting better the OSIRIS data and yielding a composition that includes organic carbonaceous material, serpentine, olivine, and basalt glass. The presence of organic material makes this model questionable as it produces a strong absorption at 3.0 mum not seen in previously obtained Phobos data. Our second model was fit to the OSIRIS data extended in wavelength to include the Rivkin et al. (2002) data. We modeled the OSIRIS data using Tagish Lake meteorite optical constants, and our best fitting model in this case consists of Tagish Lake and magnesium-rich pyroxene glass. The overall shape of the second model spectrum fits over a large wavelength range and shows that both curvature and albedo level match the OSIRIS and Rivkin et al. (2002) data much better than the first model did. While both models contain complex organic molecular material, Tagish Lake seems to have a composition and/or molecular structure that yields a better fit, particularly in view of the fact that it is a naturally occurring material rather than a synthetic one as the tholin. This gives weight to this model making it our most promising fit for Phobos. Since Tagish Lake is commonly used as a spectral analog for D-type asteroids, this provides additional support for compositional similarities of Phobos and D-type asteroids. All these factors are consistent with Phobos being a captured D-type asteroid as previously suggested by Rivkin et al. (2002) and Pajola et al. (Pajola, M. et al., 2012, MNRAS, 427, 3230) who presented an updated scenario based on the collisional capture mechanism of Phobos.