Modern aeolian accumulation and erosion processes at the Martian surface revealed by the Zhurong rover of Tianwen-1

The erosion rate on Mars has been dwindling throughout its history and its signatures provide paleo-climatic information. Active aeolian abrasion supported by sands is a well-understood mechanism for modern Mars erosion. Abrasive sands undergoing redistribution, comminution, or deactivation can evolve toward depletion, leading to even weaker erosion condition. This complementary aspect of Mars surface processes has not been thoroughly investigated in-situ beyond early robotic missions.

Zhurong rover's landing on the southern Utopia Planitia, Mars, during the Tianwen-1 mission (14 May 2021, UTC, 109.925°E, 25.066°N) opened a new frontier in the understanding of a potential weakly abraded landscape. The rover, traversing southwards from its landing site, has witnessed a largely flat, apparently dust-covered landscape of Hesperian-Amazonian age bedrock dotted by cemented, bright dunes dated to 0.4-1.4 Ma. The encrusted surface of these dunes may have inhibited them as currently active abrasion sources. Orbital observations focusing on long-term surface feature retention also predicted a low erosion rate over the past 1.1 Ga (0.01-0.1 nm/yr).

Here, in the anticipated weakly abraded landscape visited by Zhurong, we report the existence of extensive, millimeter-thick coverings bearing abraded morphology over local blocks as observed by the Navigation and Terrain Cameras (NaTeCam) and Multispectral Camera (MSCam) onboard Zhurong. The coverings are unlike the micrometer-scale dust film usually seen on Mars in terms of their thickness and the abraded ventifact-like morphology differs from typical Martian ventifact in terms of its mechanical strength. Using the first Mars in-situ material physics analysis through laser-induced blast waves, facilitated by Zhurong’s Mars Surface Composition Detector (MarSCoDe) with laser-induced breakdown spectroscopy (LIBS) capability, we probed the cohesion of the covering (ranging from 0.4 to 18 kPa), which exceeds modelled electrostatic and van der Waals cohesion, suggesting a potential cementation from agents like perchlorate and sulfate salts. Our results and modellings suggest an unprecedented accumulation mode termed Cemented Aeolian Coverings (CAC): aeolian materials accumulate over local blocks by dustfall or aggregate saltation and are cemented by water-supported process and salt. They are preserved, thickened, and shaped into present ventifact-like form due to insufficient aeolian abrasion.

This example illustrates the active aeolian accumulation over the modern Martian surface under a limited erosion. The CAC is probably unique to Mars as well as its weakly abraded landscape due to its accumulation-cementation processes under a hyper-arid condition and the very limited erosion unavailable on Earth. It may be representative to a "terminal" phase of the modern Mars abrasion process where the abrasive sands have been depleted. The contemporary CAC forming scenario investigated by Zhurong, featuring an active cycle of grain adhesion, salt deliquescence, and aeolian erosion, may provide new insights into the activity of current surface processes and the formation of fine-scale, periodic geomorphology on Mars.