Near infrared reflectance spectra of Phobos with the ExoMars-TGO/NOMAD-LNO spectrometer

Abstract

The composition of Phobos is poorly understood and its origin is still an open question [1]. There are two possible scenarios for its formation. Either Phobos is an asteroid captured by Mars, or it formed in-situ in a disc after a giant impact on Mars. The capture scenario is based on surface analysis, which shows a different composition compared to the Martian surface. However, the low density of Phobos suggests a high porosity and/or a significant amount of water ice, which could be the result of re-accretion of debris in Mars' orbit, favouring the in-situ formation scenario [2]. Therefore, if the Martian moon is formed by a giant impact on Mars, its composition will reveal the original conditions on Mars and provide insights into the formation of the planet and its young environment. On the other hand, if Phobos is a captured asteroid, its material will clarify the transport process of volatile components. For all these reasons, the acquisition of new nadir observations is essential to better characterise the composition of Phobos, which is the key to clearly explaining its origin. As part of the payload of the 2016 ExoMars Trace Gas Orbiter (TGO) mission, the Nadir and Occultation for MArs Discovery (NOMAD) instrument [3] has been observing the Martian atmosphere [4]. Albeit being mainly conceived for trace gases investigation, in nadir mode (NOMAD-UVIS and NOMAD-LNO), the spectrometers’ suite can also measure surface features with a high spectral resolution in the UV and near-IR domain [5]. By focusing on the diffraction orders determining the instantaneous spectral ranges of LNO, these nadir observations can in principle be used to search for new spectral absorptions. In addition to the study of Mars, NOMAD is providing new nadir observations of Phobos [6]. This work focuses on the analysis of near-infrared observations using the NOMAD-LNO data set. For the near-infrared data, BIRA-IASB, with the participation of the ROB, has carried out several tests to correctly calibrate the instrument and determine the optimal observation window. Since March 2023, evaluation of the new Phobos near-infrared data, which include extended observing rates and modified diffraction order combinations, has been underway. At the time of writing, 29 observations of Phobos are available, using different diffraction order combinations. Data acquisition is concentrated on the search for carbonates (2.3 µm to 2.5 µm) and the hydrated mineral feature around 2.7 µm (2.5 µm to 2.8 µm). Due to the very low SNR of these observations, efforts have been made to reduce uncertainties: use of dark detector arrays to remove the background signal, geometric investigations to take account of illumination conditions and the combination of various NOMAD-LNO spectra. Ongoing analysis on NOMAD-LNO observations will be presented, with a quantitative comparison with previous spectral observations of Phobos [7-9] and laboratory measurements [10]. NOMAD is providing additional observations that remain crucial to planetary science. New data are indeed challenging our understanding of the composition, origin and formation of Phobos. In addition, this work is also preparing for the next Japanese mission, the Martian Moons eXploration (MMX) mission, in which the MMX spacecraft will observe and land on Phobos to collect surface samples to be returned to Earth for detailed observations of the Martian moon in particular using data collected by the infrared imaging spectrometer MIRS [11]. This work also contributes to the preparation of the next ESA Hera mission, which is scheduled to make a Mars flyby with observations of Martian Moons in spring 2025.  Acknowledgements ExoMars is a space mission of the European Space Agency (ESA). The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB- BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493, 4000142490), by the Spanish MICINN through its Plan Nacional and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1, ST/Y000234/1 and ST/X006549/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant numbers 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and BELSPO BrainBe SCOOP Project. The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709).