MMX landing sites photometric properties using Mars Express SRC observations

The Martian moons, Phobos and Deimos, have debated origins. The JAXA mission Martian Moon eXploration (MMX) aims to clarify their origins through remote sensing observations of both moons and a sample return from Phobos. The mission will launch in October 2026. The MMX landing sites, from which samples will be collected, must be carefully selected to ensure that the sampled material is representative of Phobos while also having grain size and porosity compatible with the sampling mechanism. The compositional and physical properties of the regolith inferred from photometric observations [1] can help assessing the suitability of each landing site candidate. High-resolution images of Phobos have been acquired during Mars Express (MEx) flybys for more than 20 years and were recently calibrated [2]. In this study, we intend to use these images to analyze the regional photometry of the potential landing sites, as a global analysis of Phobos had already been done [3].

We analyzed data taken by the Super Resolution Channel (SRC) of the High-Resolution Stereo Camera (HRSC). HRSC is a push broom camera with nine filters in total, five being panchromatic and four color filters (blue, green, red, and infrared), and the photometric analysis of Phobos data was already done and published by a previous study [2]. SRC is a framing camera made of one panchromatic filter centered covering the 400 - 900 nm region. [4,5]. We have analyzed the photometry of 10 potential landing sites identified so far by JAXA on the basis of landing feasibility and safety for the MMX spacecraft. For each landing site, the SRC data cover a wide phase angle range (0.5-90°), including the opposition effect. The  spatial resolution ranges between 7 and 65 m/px, and with an average value of 42 m/px.

We first simulate the observation conditions for each image, meaning the incidence, emission, and phase angle, as well as the longitude and the latitude. Then, we coregister the images using ElastiX python library [6]. The coregistration was a difficult process as with the higher resolution of the images, there was a slight rotation difference between the observations and simulations, hence the need for a non-linear translation. The coregistration algorithm computes two transformations based on  the original image. The first one is a rigid transform to  a translation  and rotation to the image. Then, a similarity transformation for a more  precise rotation. Once those transformations are applied, we extract a displacement map for individual pixels and we apply this map to simulations.

Our next step is to apply disk-resolved Hapke model [7] for the different landing sites investigated. From the derived photometric parameters, such as the opposition surge amplitude and width, and single-scattering albedo, we will infer surface properties including brightness, roughness, and surface porosity for each candidate landing site analyzed and we will compare their properties to those derived from the global analysis of Phobos [3]. This study will help MMX team in the selection of the final landing site.

Acknowledgment: This work has received support from France 2030 through the project named Académie Spatiale d'Île-de-France (https://academiespatiale.fr/) managed by the National Research Agency under bearing the reference ANR-23-CMAS-0041, as well as the Centre National d’Etude Spatial (CNES).

Reference:

[1] Sato et al. (2014), JGR, 119(8)

[2] Wargnier et al. (2025), A&A, 703, A289

[3] Fornasier et al. (2024), A&A, 686, A203

[4] Jaumann et al., PSS, 55, 928-952

[5] Oberst et al. PSS, 56, 473-491

[6] Klein et al. (2010), 19, 1, 196-205

[7] Hapke (2012), Cambridge University Press