Geomorphology and Albedo distribution on the Anti-Mars hemisphere of Deimos observed with Asteroid Framing Cameras (AFC) on the Hera Spacecraft

ESA’s Hera mission performed a Mars flyby on March 12, 2025, en route to the binary asteroid system Didymos–Dimorphos, which has been greatly influenced by NASA’s DART spacecraft impact (Michel et al. 2022). This flyby event presented a unique opportunity to observe extended light sources using Hera’s scientific instruments, which is crucial for their calibration. Additionally, the flyby offered a rare chance to observe the anti-Mars hemisphere of Deimos, as Mars orbiters, such as NASA’s Mars Reconnaissance Orbiter and ESA’s Mars Express, can only capture the sub-Mars hemisphere from their orbits.

Thanks to the great efforts of Hera’s flight dynamics team in optimizing the flyby timing to coincide with Deimos approaching Hera’s trajectory, along with precise spacecraft attitude control to track both Deimos and Mars in the field of views of cameras, Hera’s three complementary camera systems, Asteroid Framing Cameras (AFC), the HyperScout-H (HS-H) instrument, and JAXA’s Thermal Infrared Imager (TIRI), were able to observe both Mars and Deimos simultaneously.

The AFC captured over 230 images of Mars and more than 50 of Deimos. Although most Deimos images were small (≤ a few pixels across), they were taken at low solar phase angles (2–4°), which are valuable for constraining the phase function of Deimos’s anti-Mars side. The largest Deimos image (~140 pixels in diameter) was taken from a distance of 917 km. In total, the AFC acquired four images of this size range at phase angles between 13–17°, allowing for the identification of surface features.

The AFC images revealed a number of important geomorphologic features and albedo distribution on anti-Mars hemisphere. Notably, the anti-Mars hemisphere exhibits “streamers”, which are long, high albedo features similar to those seen on the sub-Mars hemisphere (P. Thomas et al. 1996, N. Thomas et al. 2011). The albedo contrast between the streamers and the surrounding terrain is comparable to that observed on the sub-Mars hemisphere, with variations reaching tens of percent. The albedo decreases monotonically from topographic highs to lows over several kilometers.

The albedo histogram of Deimos shows a highly asymmetric profile with a triangular shape: a sharp increase in frequency on the lower albedo side and a gradual decrease on the higher albedo side. This asymmetric histogram is similar to both the space weathering color index histogram (Koga et al. 2018) and the albedo histogram (Nakahara et al. 2023) observed on the S-type asteroid Itokawa. Although further analysis is needed to determine the exact cause, model calculations by Nakahara et al. (2023) suggest that the observation is consistent with a thin layer of regolith over a differently colored substrate. This could indicate space weathering, a mixture of two color components, or a combination of these processes. Regardless, the surface likely undergoes vertical mixing and diffusive mixing driven by the random motion of small regolith particles.

Understanding these surface characteristics will help constrain the nature of Deimos’ surface materials and optimize the observation plans of JAXA’s MMX mission, which will collect samples exclusively from Phobos. Consequently, proximity observations of Deimos will be crucial for comparing the similarities and differences between the two Martian moons (Kuramoto et al. 2022).

References:

Koga, S. et al. 2018. Spectral decomposition of asteroid Itokawa based on principal component analysis, Icarus, 299, 386-395.

Kuramoto, K., et al.., 2022. Martian moons exploration MMX: sample return mission to Phobos elucidating formation processes of habitable planets. Earth, Planets and Space 74, 12.

Michel, P., et al., 2022. The ESA Hera Mission: Detailed Characterization of the DART Impact Outcome and of the Binary Asteroid (65803) Didymos. Planet. Sci. J. 3, 160.

Nakahara, S. et al. 2023. Space weathering distribution and resurfacing on asteroids, JpGU, # MZZ40-02.

Thomas, N. et al. 2011. Spectral heterogeneity on Phobos and Deimos: HiRISE observations and comparisons to Mars Pathfinder results, Planet. Sp. Sci., 59, 1281-1292.

Thomas, P. et al. 1996. The Surface of Deimos: Contribution of Materials and Processes to Its Unique Appearance. Icarus, 123, 536-556.