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

TL;DR

This study characterizes the photometric properties of Deimos using two decades of Mars Express data (HRSC and SRC) and Hapke-based modeling, including a first absolute SRC calibration anchored by Jupiter and stars. It finds a strong shadow-hiding opposition effect with negligible coherent backscattering, a high top-layer porosity of about 86%, and an SSA around 6.8–7.5%, similar to Phobos. Disk-resolved analyses reveal regional variations linked to the equatorial ridge, with a blue unit exhibiting up to 58% higher reflectance and about a 50% flatter spectral slope, suggesting texture-driven differences in fine-grained, highly porous material. The overall photometric behavior of Deimos closely matches Phobos, reinforcing a common origin hypothesis for the Martian moons and providing critical inputs for MMX mission planning and surface interpretation.

Abstract

The goal of this study is to analyze the photometric properties of Deimos using Mars Express (MEX) observations, to improve the photometric properties and provide new insights into the texture and composition of the surface of Deimos, in preparation for the MMX mission. We analyzed the data obtained by the HRSC and the SRC cameras onboard MEX. The HRSC data, obtained through the use of four filters (blue, green, red, IR), provides 390 to 800 m/px resolution, while the SRC data reach 85 to 300 m/px and cover a wide phase angle range (0.06-138°). We performed the disk-integrated and disk-resolved photometric analysis using the Hapke model. The Deimos surface is dark and predominantly backscattering, with a single-scattering albedo (SSA) value (6.8%-7.5%) comparable to Phobos. The Deimos phase curve shows a strong opposition effect due to shadow-hiding, with negligible coherent backscattering. 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 high porosity of 86% at the top-layer surface, consistent with complex-shaped grains or fractal aggregates, suggesting a thick dust layer. We did not observe significant variations of the opposition surge across the surface. A blue unit on Deimos, located on streamers of the equatorial ridge, shows reflectance increases up to 58%, and a spectral slope decrease of 50% in comparison with the average surface. This blue unit may be due to a different texture of the surface between the two units, with finer grain and/or higher porosity. Deimos photometric properties, including 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, making the capture of two different bodies with such similar properties unlikely.

Figures (24)

• Figure 1: Deimos opposition effect observed by the SRC during orbit K803 on 2020-06-15. The flux is in ADU.s$^{-1}$; no other normalization was applied here (the heliocentric distance was the same for a given orbit).
• Figure 2: Example of SRC mutual events observations -- from the Martian orbit -- of different types of objects: stars, Saturn, and the Moon. The shape of the stars is not symmetric because of the astigmatism observed in the SRC point-spread function (PSF, Oberst_2008). The $\gamma$ Ori and $\alpha$ Phe images are examples of saturated stars. Saturated images were discarded for the calibration.
• Figure 3: An example of Jupiter SRC observation (2022-07-12T18:59:16) and the associated method to determine Jupiter's flux. Left: Image of Jupiter obtained by the SRC instrument. The bands of the atmosphere can be seen pretty well. Right: the same Jupiter image but clamped to 200 DN to show the background. The circle (blue) and the aperture annulus (red) are also shown. Note that the Io moon is present at the top of Jupiter. It is important to avoid the satellite for the aperture photometry method.
• Figure 4: Synthetic filter used for the absolute calibration of the SRC.
• Figure 5: Observed vs. expected signal for each object/method. The black squares represent Jupiter data. The diamond shapes correspond to the stars used in this study, with the different colors representing the different spectral types. The black solid line represents an ideal camera with a ratio observed/expected equal to 1.