Organic detection in the near-infrared spectral Phobos regolith laboratory analogue in preparation for the Martian Moon eXploration mission

Context. The Martian Moon eXploration mission (MMX) of the Japanese space agency (JAXA) is scheduled to take off in September 2024 to explore Phobos and Deimos - the two martian moons - by in situ observations, but also by a sampling and returning regolith samples to Earth. The origins of Phobos and Deimos are still unknown and their understanding is one of the main goals of the MMX mission. In one scenario, Phobos could be a captured asteroid, as the Phobos spectrum is similar to dark D-type asteroids. Aims. For the present work, we considered the hypothesis of Phobos being a captured D-type asteroid, and we investigated the detectability of organics on Phobos using laboratory spectral analogues. Methods. We synthesised a near-infrared spectral analogue of Phobos composed of olivine (77 vol.%, 50-125 mu m), hyperfine anthracite (20 vol.%, <1 mu m), and organic tholins (3 vol.%, similar to 400 nm) by measuring the reflectance spectrum from 0.4 to 4.75 mu m with the SHADOWS spectrogonio-radiometre developped at IPAG. The best spectral match for a Phobos regolith analogue was chosen based on its reflectance level and spectral slope similarities to Phobos' observed spectrum. Several samples were then prepared by adding a different volume content of organic matter (Titan tholins). We monitored the 3 mu m band attributed to N-H bands stretching modes absorption due to the amine function in the tholins, so as to assess the detectability of the NH-rich organics on Phobos. Results. We have demonstrated that the organic compounds become detectable for more than 5 vol.% in the mixture. We further studied the observation geometry effects on the absorption band depth and found no significant effect except at large phase angles (>80 degrees). These results will be useful to interpret the data of the MMX Infrared Spectrometer (MIRS) onboard the MMX spacecraft, which will measure the spectral reflectance of Phobos from 0.9 to 3.6 mu m.