To constrains the origin of Phobos and Deimos by MIRS/MMX imaging spectrometer

The JAXA MMX (Martian Moon eXploration) mission will be launched in October 2026 to the Martian system to carry out three years observations of the Martian moons, and to bring back to Earth at least 10 g samples from Phobos. The primary goal [1] of the mission is to understand the origin of the Martian moons: Phobos and Deimos, for which two major hypothesis are still debated.  Are the two satellites captured asteroids after the Mars formation ? Or are they results of a giant impact with Mars ? Laboratory analysis of returned samples will be key to answer these fundamental questions. The mission objectives in studying Martian moons are also to enlarge our knowledge of the Martian system and to constrain the processes of planet formation.

The onboard instruments [1] will allow the right selection of the two Phobos sampling site selection. The cameras (OROCHI, TENGOO and Cam-T) [2] will allow the safe selection for the landing sites, while the imaging spectrometer MIRS (MMX InfraRed Spectrometer) [3,4] and MEGANE (Mars-moon Exploration with Gamma rays and Neutrons) instrument [5] will be the key instruments for the compositional characterization, global chemical and mineralogical mapping, and to support the landing site selection.

At the arrival of the spacecraft into Mars system in 2027, MMX will observe Deimos by several fly-bys, and then it will be injected into Phobos co-orbit like a Martian Moon in orbit around Mars, in the so-called quasi-satellite orbits (QSOs). To obtain global mapping at different spatial resolutions with the cameras and MIRS, and to characterize the selected landing sites, the QSOs will be settled in the equatorial plane of Phobos with several decreasing altitude (High, Medium and Low), from 100km to 7km. The mapping at different resolutions by MIRS will be essential for a global characterization and evaluate the properties and geologic context of different materials to select the best landing sites. Observations at specific local times and phase angles will be also performed to study the surface temperature and its spatial and temporal variations. From these measurements, the surface thermal inertia of Phobos will be derived at the instrument spatial resolution. The multiple flybys of Deimos will allow to compare its surface composition to that of Phobos with observations at similar resolution.

The MIRS  spectrometer [3,4], designed to accomplish the MMX’s scientific objectives, is a push-broom imaging spectrometer working in the spectral range 0.9 -3.6 µm. The measured spectral resolution is 21 nm up to 3.2 µm with SNR ≥ 100 in the region up to 3.2 µm. The instantaneous Field of View (iFoV) is 0.325-0.350 mrad, and the field of View (FoV) is +/-1.65°.

MIRS will characterize spectroscopically the global surface material distribution of Phobos at spatial resolutions better than 20 m, up to few meters at low altitude for the preselected landing sites and up to few cm for the two selected sampling sites. MIRS is expected to detect all possible present signatures (up to band depths of 3%) on the obtained spectra like anhydrous or hydrous silicate minerals, characterize the presence of water (ice) and to detect the presence of organic materials.  The MIRS performances will allow to investigate on the composition like their aliphatic or aromatic nature, whether they are nitrogen-bearing or pure carbonate. MIRS will be able to measure the spectral radiance of the surface within the instrument footprint.  Grain sizes and porosity will be constrained by thermal Inertia. Space weathering processes will be also investigated thanks to resolved spectral observations of small fresh craters and their ejecta. MIRS data associated with the on-board cameras and MEGANE instrument will give new insights on the surface characterization of these two moons. The unprecedent spatial resolution of MIRS will allow to search for heterogeneities that could be linked to Mars surface composition and it will allow to detect exogeneous material if present on the surface of Phobos. This will provide constrains on the transport mechanisms between Mars and the two moons and the eventually dichotomy between the sub and anti-martian hemispheres of Phobos.

MIRS will be used to support the selection of the two landing sites based on their grain size and composition, and will be able to observe during its descent phase down to an altitude of 400m. MIRS will be able to determine the surface composition of both Phobos and Deimos. This will allow us to decipher the origins of the two moons given that: i) the detection of phyllosilicates as well as of organics on their surface would imply that the origin of Phobos and Deimos is by a capture process, while ii) the detection of anhydrous silicates or spinels may suggest a high-temperature origin by impact, leading to a depletion of volatile elements and incorporation of Martian rocks (crust and mantle). In the assumption that Phobos formed in a giant impact, simulations [6] show that the impact could be much less energetic than the Moon-forming impact, with temperatures of the order of 2000 K. This temperature remains sufficiently high to produce a depletion in water, phyllosilicates and organic. Consequently a detection of presence of these materials (even in small quantities) by spectroscopy on the surface of Phobos will confirm the capture hypothesis.

High-accuracy laboratory analysis of volatile elements and isotopic ratios of returned samples from the two regions will give the final answer on the origin of Phobos.

Acknowledgements: MIRS is built at Paris Observatory in collaboration with CNES, four other French laboratories (LATMOS, LAB, OMP, IRAP), and close collaboration with JAXA and MELCO. MMX is developed and built by JAXA, with contributions from CNES, DLR, ESA and NASA.

References :

[1] Kuramoto K. et al. (2022) Earth, Plan. and Space, 74, 12.

[2] Kameda et al. (2021) Earth, Plan. and Space, 73, 218.

[3] Barucci M.A. et al. (2021) Earth, Plan. and Space, 73, 211.

[4] Barucci M.A. et al. (2025) Progress in Earth and Planetary Science, in press

[5] Lawrence et al.  (2019) Earth Space Science, 6, 2605.

[6] Charnoz S. et al. (2025) Icarus, 434, 116462