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

Abstract

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 µm), hyperfine anthracite (20 vol.%, <1 µm), and organic tholins (3 vol.%, ∼ 400 nm) by measuring the reflectance spectrum from 0.4 to 4.75 µm 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 µm 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 • ). 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 µm.

Grain size effects on the infrared spectrum of mineral mixtures with dark components: New laboratory experiments to interpret low-albedo rocky planetary surfaces

“…Here, we summarize the method and procedures to evaluate all the selected features: the NIR slope, 2.7 µm OH-stretching intensity, position of the Christiansen feature (CF), and the variation in Reststrahlen band (RB) and Transparency feature (TF). The NIR slope between λ 1 = 1.5 µm and λ 2 = 2.5 µm was evaluated using the formula from Wargnier et al (2023a) converted to microns:…”

Development of a new Phobos spectral simulant: spectral properties from visible to the mid-infrared range

“…In this work, we will present the updates of the simulant proposed ★ E-mail: antonin.wargnier@obspm.fr in Wargnier et al (2023) and the comparison with existing spectroscopic data of this new laboratory spectral simulant of Phobos, in support of the entire MMX mission. In particular, the development of this new simulant will be pivotal for the calibration and future data interpretation of the MMX Infrared Spectrometer (MIRS, Barucci et al 2021).…”

“…Nowadays, no analog material or simulant achieved a perfect match of Phobos spectra in terms of slope in the near-infrared nor features in the mid-infrared (Fraeman et al 2012;Miyamoto et al 2021;Landsman et al 2021;Poggiali et al 2022;Wargnier et al 2023). The University of Tokyo developed two simulants (UTPS, Miyamoto et al 2021) based on the different hypotheses on Phobos origin.…”

“…UTPS analogs are reasonable simulants for optical characteristics of Phobos surface with a spectral slope in the near-infrared close to the slope of Phobos. Our previous study (Wargnier et al 2023) started a process of improvement of the spectral match in the near-infrared with data collected on Phobos. The reflectance level and the spectral slope in the near-infrared match well with the Phobos red-unit simulant; but the change of slopes between visible and near-infrared was not reproduced.…”

“…For the silicate part, we chose to use olivine ((𝑀𝑔, 𝐹𝑒) 2 𝑆𝑖𝑂 4 )) which has been detected on several asteroids. Olivine used in this study was retrieved from Brazil (Wargnier et al 2023). We used coals to mimic the carbon-and organic-rich compounds in our Phobos simulant.…”