Phobos and Deimos surface composition: search for spectroscopic analogues

Abstract

Phobos and Deimos, the two satellites of Mars were largely studied in the past using ground-based telescope and spacecraft data, although most of the data were obtained by opportunity observations performed by Mars dedicated orbiters. Despite the data available so far, the main composition of the two moons is not yet fully understood. The possible presence of hydrated minerals along with mafic minerals olivine and pyroxene seems to be the most plausible interpretation, but more investigations are needed. MIRS spectrometer onboard the future JAXA MMX sample return mission will help to unveil the open question on the composition of Phobos and Deimos. In this work we review past spectroscopic observations of the Martian moons, both from ground observatories and spacecraft dataset, aiming at better understanding the constraints in interpreting the Mars satellites composition and at identifying the best spectroscopic analogs. We also present new laboratory measurements on mineral mixing and meteorites to match the satellites spectral behaviour. New measurements were acquired at INAF-Astrophysical Observatory of Arcetri and IPAG laboratories at room conditions exploring different geometries and the results obtained set new constraints for future laboratory measurements. Our preliminary results confirm that the surface of Phobos and Deimos can be associated with samples characterized by an higher presence of dark components (e.g. amorphous carbon) or minerals produced by space weathering (e.g. Fe0 and FeS-bearing materials). Presence of dark component could be also totally responsible for the reduced hydrated band observed on the moons without invoking dehydration or OH-implantation on anhydrous surface.

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

“…While we mainly aimed to have a spectral analogue, we tried to make it fairly representative of the composition of a small body, for example with olivine and py-roxenes (Zolensky et al 1993;Crovisier et al 1996;Beck et al 2014). Such mafic minerals are expected on Phobos based on the observations made so far (Poggiali et al 2022) and they are common on non-differenciated asteroids, or asteroids covered by basaltic materials. For this study, olivine from Brazil and augite from the Czech Republic were used.…”

“…The proposed existing spectral analogues of Phobos in the visible and near-infrared are rare meteoritic samples and include Tagish Lake meteorite -which is a common analogue for ultra-primitive D-type asteroids (Hiroi et al 2001;Potin et al 2021;Hiroi et al 2022) -heated Murchison meteorite (700K during one week, Fraeman et al 2012), or other meteorites related to D-type asteroids (Hiroi et al 2005;Marrocchi et al 2021). More recently, Poggiali et al (2022) proposed some developments regarding the search for a spectral analogue of Phobos.…”

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

“…On the other side, some works have investigated the effect of adding dark and/or opaque components (e.g., Clark 1983;Cloutis et al 1990) finding mostly effects on band area ratios and band widths. In Poggiali et al (2022), we performed some preliminary experiments to investigate the mixtures of Martian simulants and carbonaceous chondrites simulants as effects of complex mixtures with different albedo components. Similar experiments were carried out in the attempt of matching Phobos spectrum by Wargnier et al (2023a,b).…”

Ultraviolet and Visible Reflectance Spectra of Phobos and Deimos as Measured by the ExoMars‐TGO/NOMAD‐UVIS Spectrometer

“…However, such narrow absorption bands could be missed by multispectral imagers or could be confused with noise in the higher spectral resolution, lower signal to noise data. We can use previous observations in this and other wavelength regions to constrain possible surface mineralogy (such a review was recently presented by Poggiali et al (2022)) and investigate the implications of these observations in providing new information about the origin of the martian moons. In Section 2, we present the UVIS observations and their main characteristics and Section 3 describes the calibration process.…”