Synthesis of a Spectral Phobos Regolith Analog in Preparation of the MMX Mission

International audiencePhobos and Deimos – the two martians moons – have still unknown origins. A first hypothesis for their formation is the accretion of Martian material from a giant impact between Mars and a protoplanet (Craddock et al., 2011). The other hypothesis proposes that the two moons are primitive D-type captured asteroids (Murchie et al. 1999, Fraeman et al. 2014), which are known to be carbon-rich, highly processed, mainly found in the outer main belt or in the Jupiter Trojans and possibly linked to comets (Vernazza and Beck, 2017). Hence, for the captured asteroid hypothesis, organic matter is expected to be largely present on Phobos (Bardyn et al., 2017). The Martian Moon eXploration mission will answer the question of the origins by a sampling of the Phobos’ surface but also by in situ observation with the MIRS spectrometer (0.9-3.6 µm) currently under construction at LESIA -Observatoire de Paris -PSL. MIRS should be able to detect minerals and organic matter features (Barucci et al., 2021), which will give precious clues for the Phobos’origin.In this context, we developed a spectral Phobos analog to study the feasibility of organic detection in preparation for the MMX mission. We first searched to reproduce the peculiar spectral signature of Phobos in the NIR with low reflectance, no absorption features and a red spectral slope (Fraeman et al., 2012). Measurements were performed at IPAG using the SHADOWS spectrogonio-radiometer (Potin et al., 2018) which allows reflectance measurements between 0.5 and 4.5 µm. We found a good correspondence – in terms of reflectance but also spectral slope between 1.3 and 2.5 µm – with the Phobos spectrum for a mixture of olivine (50-100 µm), anthracite (< 1 µm) and Titan tholins (~ 400 nm) in volume fraction of 77%, 20% and 3% respectively. Then, different amounts of Titan tholins were added to assess organics’ detection. We obtained a clear detection for 5% of organic compounds. We also look at the geometric conditions on some spectral parameters such as spectral slope, reflectance, and 3 µm-band depth. We obtained spectra on one mixture for phase angles ranging from 10° to 130°. No phase effect on the organic matter’s absorption band was detected for phase angles smaller than 80°. A phase reddening and a strong forward and back-scattering were also observed