Deimos photometric properties: Analysis of 20 years of observations (2004-2024) by the Mars Express HRSC camera

¹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, CY Cergy Paris Université, 5 place Jules Janssen, Meudon 92195, France
² LATMOS/IPSL, CNRS, Universitéy Versailles St-Quentin, Université Paris-Saclay, Sorbonne Universitéy, 11 Bvd d’Alembert, Guyancourt 78280, France
³ Laboratoire d’Astrophysique de Marseille (LAM), France
⁴ Institute of Space Science, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany

^★ Corresponding author: antonin.wargnier@obspm.fr

Received: 17 May 2025
Accepted: 5 September 2025

Abstract

Aims. The goal of this study is to analyze the photometric properties of Deimos using observations obtained by the Mars Express (MEX) mission while aiming to improve the photometric properties and provide new insights into the texture and composition of the surface of the smallest Martian moon. The findings also support the Martian Moon eXploration mission (MMX) observations.

Methods. We analyzed the data obtained by the High Resolution Stereo Camera (HRSC) and the Super Resolution Channel (SRC) on board MEX. The HRSC data, obtained through the use of four filters (blue, green, red, and IR) have a spatial resolution ranging from 390 to 800 m/px. In comparison, the SRC panchromatic data have a resolution ranging from 85 to 300 m/px. The SRC data are of particular interest due to their coverage of a wide range of phase angles, including the opposition effect of Deimos (0.06–138°). Observations of both HRSC and SRC cover only the Mars-facing side of Deimos. As the SRC camera was never absolutely calibrated before and during the MEX mission, we performed the first absolute calibration of the SRC using observations of Jupiter and stars. We then performed a disk-integrated and disk-resolved photometric analysis using the Hapke model.

Results. The Deimos surface is dark and predominantly backscattering. The single-scattering albedo (SSA) value (between 6.8% and 7.5%, depending on the model) is similar to the one derived from Phobos. The Deimos phase curve shows a strong opposition effect due to shadow hiding, with almost no effect of the coherent-backscattering process. The amplitude and the half-width of the shadow-hiding opposition surge were found to be 2.14 ± 0.14 and 0.065 ± 0.004, respectively. We found a very high porosity of 86% at the top-layer surface (~10 μm), consistent with the tentative presence of complex-shaped grains or fractal aggregates. Such a high porosity would likely also indicate the presence of a thick dust layer. We did not observe significant variations of the opposition surge across the surface. We observed a blue unit on Deimos in a similar way to Phobos, located on the streamers, which themselves are on the equatorial ridge. The Deimos blue unit exhibits variations relative to its average surface that are similar to those of the blue unit on Phobos, characterized by an average reflectance increase of about 35% (and up to 58%) and a spectral slope decrease of 50%. This blue unit may be due to a different texture of the surface between the two units, with a finer grain and/or a higher porosity. In contrast to the “blue unit” photometric behavior exhibited by Phobos on several crater rims, no such behavior has been observed for Deimos.

Conclusions. The Deimos photometric properties, including the SSA, opposition surge, and phase integral, are very similar to Phobos. The presence of a blue unit on Deimos reinforces the idea that the Martian moons have a common origin. The capture of two different bodies with similar spectroscopic and photometric properties appears very unlikely.