Phobos and Deimos as observed by MIRS spectrometer on board of Martian Moon eXploration (MMX) mission

Phobos and Deimos will be globally spectral characterized by MIRS (MMX InfraRed Spectrometer)  onboard of the MMX mission.  MMX is an approved mission by JAXA to explore the two Martian moons, with the aim to return sample (>10 g) from Phobos to Earth. The spacecraft will be launched in 2024, arriving into Mars orbit in 2025 and bringing back Phobos’sample in 2029. Arrived at Mars system, the spacecraft will be injected into Phobos co-orbit and then in orbit around Mars like a Martian Moon in the so called quasi-satellite orbits (QSOs). The QSOs will be settled at different altitudes in the equatorial plane of Phobos to obtain data at various Phobos surface resolution and to reduce the effect of Phobos’gravity.

MIRS will work during the three years of the MMX stay in circum-Martian space. MIRS will provide global characterization of Phobos, Deimos and will observe Mars atmosphere to study spatial and temporal changes. In particular, MIRS instrument will be able to map the Phobos’ surface from High (from 90-190 km) and Medium (from 37-84 km) altitude orbits to determine Phobos composition, to observe in detail the landing site candidates at Low altitudes (from LA:17-38 km; LB: 8-21 km; LC: 6.6-16 km) to provide necessary  information to evaluate and select the two most interesting sampling sites and will perform close observations of the two landing sites, during the Vertical  descent phase. One of the main mission goals is to decipher the origin of the moons, which will provide important constraints on the formation and evolution processes of themselves and clues on planetary formation. The origin of Phobos and Deimos is still a debated question. Past space missions revealed that the properties of these two moons shared similarities with asteroids [1]. Their red spectra, without strong absorption features, are very different from those of Martian surface, and resemble to those of primitive C-D-type asteroids and support consequently the hypothesis of the captured asteroids [2]. On the other hand, for their orbits on the equatorial plane of Mars with very low eccentricity, recent numerical simulations seem to be more in favor of in-situ formation by a giant impact on Mars [3, 4]. 

To unveil the surface composition of the two moons at high spatial resolution and select the two sampling sites, MIRS will observe Phobos and Deimos in the 0.9-3.6 μm range with a spectral resolution better than 20 nm. MIRS will acquire spectra of Phobos at a spatial resolution of about 20 m for a latitude of +/-30° during the Medium altitude survey. The different landing sites will be selected at different resolutions up to few meters for the final five landing site candidates with priority order. A higher spatial resolution less than 1 m will be reached over an area within 50 m from the selected sampling sites. The spectral radiometric absolute accuracy is expected to be of 10%, and the relative accuracy of 1%. The high SNR (>100 up to 3.2 μm) and unprecedented spatial resolution achieved by MIRS will permit to characterize the detailed composition of both the red and blue units on Phobos and to investigate the local compositional heterogeneity associated with the different surface morphology. MIRS is expected to spectroscopically detect and characterize all present signatures, like water (ice) (absorption bands at 1.5, 2.0 and 3.0-3.2 μm), hydrous silicate minerals (features at 2.7-2.8 μm, and minor overtones at 1.4 and 1.8 μm), or anhydrous silicates (bands in the 0.9-1 and 2.0 μm regions) as well also organic matter (3.3-3.5 μm), if present. A detailed characterization of the detected absorption bands, with precise measure of the band center, depth and area, thanks to high S/N ratio, will allow to constrain the surface mineralogy and species’ abundances. Spectral observations of fresh areas, like small craters will provide insights on space weathering processes. MIRS will be also able to measure the spectral radiance of the surface within the instrument footprint. The spectral thermal tail (from about 2.5 μm) will permit to derive the surface temperature, consequently MIRS will allow to derive surface temperature variation and surface thermal inertia.

For Deimos, MMX will perform multiple flight-bys. At a distance of 300 km, MIRS will observe Deimos surface at spatial resolution of 100 m, which is comparable to that for Phobos from high QSO, and it will be able to detect the same major absorption bands as observed in Phobos. The most important scientific goal is to understand whether Deimos is made of the same material as Phobos. The data will allow to detect compositional heterogeneity that could be linked to topography and to the Martian phase aspect. As in the case of Phobos, a full characterization of the bands is possible for band depth as weak as 3%.  MIRS will improve the knowledge of the surface composition especially in terms of spatially resolved spectral data. Even if Deimos is not the target for the sampling, MIRS with the other onboard instruments will considerably improve the physical and chemical knowledge of its surface, and bring new insights about the history of the Martian environment.

MIRS will be able to characterize the global composition of Phobos and Deimos surface material (and subsurface through analysis of crater ejecta). These unprecedented data will allow a better understanding of the two Martian moons. MIRS data will help deciphering whether Phobos composition is closer to primitive dark asteroids and consequently similar to carbonaceous chondrites with possible presence of organics and/or ices which will imply a capture origin or containing, even if partially, high-temperature phase materials representing  a mixture from crust and mantle of Martian silicates [5], more similar to a devolatilized  and/or hydrated Martian mantle, which would indicate a giant impact origin.  

References

[1] Pang, K.D. et al. 1978, Science 199, 64; [2] Fraeman, A.A. et al. 2014, Icarus 229, 196; [3] Rosemblatt, P. et al. 2016, Nature Geoscience 9, 581; [4] Hyodo, R. et al. 2017, Astroph. J., 845; [5] Usui, T. et al. 2020, SSRv 216, 49.