Phobos: an intriguing open question orbiting Mars
Dr. Maurizio Pajola, ARC NPP Research Fellow
UNIVERSITIES SPACE RESEARCH ASSOCIATION
Since Mariner 9 observation in 1971, the inner Martian satellite Phobos has been the subject of several photometric and spectroscopic studies, ranging from the near‐ultraviolet (NUV) to the near-infrared (NIR). Despite 40 years of observations, Phobos origin is still controversial. The first hypothesis is that Phobos is a captured main-belt asteroid belonging to a C or a D‐type asteroidal family; the second hypothesis is that Phobos is an object reaccreted in Martian orbit, coalescing from the planet's debris ejected from the Mars surface after a collision with a large planetesimal.
Through this presentation we will show the Rosetta/OSIRIS Phobos observations obtained on 2007 February 24 and 25. The comparison of our reflectance spectra (250–1000 nm) shows that the spectra of Phobos are all redder than the mean D‐type asteroid spectrum, but within the spectral dispersion of other D‐types objects. To complement this result, we performed an investigation of the conditions needed to collisionally capture Phobos in a way similar to that proposed for the irregular satellites of the giant planets. We then performed a spectral modeling of the surface of Phobos providing a composition of Tagish Lake meteorite samples and magnesium‐rich pyroxene glass. The overall shape of this model between 0.25 and 4.0 μm, shows curvature and albedo level that match both the OSIRIS, Murchie et al. (1996) and Rivkin et al. (2002) data. This large interval fit is encouraging and adds weight to our model providing additional support for compositional similarities of Phobos and D-type asteroids. Eventually the Phobos work in progress (NPP main topic) will be presented.
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