The (Infrared) view of Phobos and Deimos, synergy between data from laboratory and remote sensing to understand the enigmatic nature of the Martian moons.

Introduction: Phobos and Deimos importance as scientific target is growing up in the latest years due to an increasing interest in the planetary community on the origin and evolution of the two Martian moons. While the debate over whether these are captured asteroids [1] or the result of a catastrophic collision [2] is still ongoing, JAXA is preparing to launch in 2026 the Martian Moons eXploration (MMX) [3] sample return mission to study the two moons in detail and to collect a sample on the larger moon, Phobos, to be brought back to Earth in 2031. Along the spacecraft payload, a pivotal instrument to characterize the surface of the martian moons will be MMX InfraRed Spectrometer (MIRS) [4] giving access to the mineralogical composition of the surface and the possible presence of volatiles. The community's interest on these targets is confirmed by the decision of ESA to allow a fly-by of the moon Deimos with ESA's Hera spacecraft during its journey to the binary asteroid Didymos-Dimorphos. The probe acquired several images of the moon and Mars with its instruments, including the Hyperscout spectrophotometer and the TIRI thermal camera. In this growing interest, it becomes essential to exploit all the data we possess to prepare and support future explorations; in this light, the Mars Express (MEx) mission with its OMEGA and PSF spectrometers and HRSC multifilter camera represents an essential dataset for the study of the two martian moons. The other side of the problem, interpreting the effect of physical properties such as grain size, composition, porosity and space weathering, can only effectively approached with laboratory measurements. Measuring analog material such as meteorites and they primary component, minerals, can open the door to more complex and meaningful interpretation of the remote sensing observations.

Methods: In this contribution we will show how laboratory data are essential to data interpretation. In particular, the effect of grain size and composition, approached as single problem is already a promising way to remote sensing interpretation. But when these two features are studied in combination we have a key factor in understanding the physical processes and characteristics of a surface. In detail, we will show how selected range of grain size (i.e. < 100 µm or 100-200 µm) affects some of the main features of the infrared spectrum (slope, 2.7 µm band and Christiansen features, Reststrahlen bands and Transparency features in the mid-infrared range). Moreover, we will prove how a realistic grain size distribution derived from boulder and regolith size frequency can change the final outcome of the spectrum. These physical properties will also be studied through their link with composition mixing two or more components. Special attention will be focused on how which variable most influences the final spectrum and whether various types of environment (vacuum, temperature, irradiance...) can affect the final result. The presentation will go through the reanalysis of some data from the ESA Planetary Science Archives of the MEx mission, in particular we will show data from OMEGA and HRSC instrument, to fully exploit the potential derived from decades-long observations of moons and the planet Mars. These instruments, along with PFS thermal spectrometer, offer a unique point of view on the Martian moons allow to investigate the mineralogical nature of the surface.

Results and conclusion: We will show results from some recent lab work [5,6] in particular how grain size and dark material abundance can alter the slope or the depth of some fundamental bands like the OH-stretching at 2.7 µm. Moreover, these laboratory data will be used for the interpretation of recently published observations from various missions such as MEx-OMEGA [7] and TGO-NOMAD [8] and others. Moreover, additional MEx mission data from the ESA PSA [9] archive will be shown and compared with previous observations and laboratory data. In particular data from recent years will be exploited ton increase our dataset on different spectral range.

The goal of this work is to show the deep link that exists between remote sensing interpretation and laboratory measurements and how new advances can also be achieved by reanalysis of archival data. In the renewed interest in Martian moons we believe that a step forward on laboratory analogs can be made, leading in a big step forwards a more confident comparison with real surfaces. Therefore, only if the remote sensing data are combined with laboratory experiments on minerals and meteorites we can aspire to obtain an essential base knowledge for any future analysis aiming to interpret the nature of these enigmatic and elusive objects.

Acknowledgments: G.P. work on remote sensing data was supported by ESA Archival Research Visitor Programme. G.P. and J.R.B. acknowledge support from Italian Space Agency ASI-INAF agreement 2022-1-HH.0. M.A.B. and A.W. acknowledge support by Centre National d’Etudes Spatiales (CNES).

References: [1] Hyodo R. et al. (2018). ApJ, 860, 150 [2] Higuchi A. and Ida S. (2017). AJ, 153, 155 [3] Kuramoto K.et al. (2022) Earth Planets Space, 74, 12 [4] [5] Poggiali G. et al. 2023a, Icarus, 394, 115449 [6] Poggiali G. et al. 2024, A&A 685, A14 [7] Pajola M. et al. 2025. A&A, (accepted) [8] Ruiz Lozano L. et al., (2024) EPSC2024-925 [9] Besse S. et al. 2018. PSS, 150, 131-140