Department of Earth Sciences

The new HRSC image shows the ancient highlands of Arabia Terra. This region lies between two major outflow channels – the meandering Mawrth Vallis to the north and Ares Vallis to the southwest (see context map). Mawrth Vallis is particularly known for its diverse mineralogy, including hydrated sulfates and clays that formed in the presence of water. Looking at the image, one feature immediately stands out: craters, craters, and more craters. It is indeed a heavily cratered landscape. The most prominent feature is the approximately 130-kilometre-wide impact crater on the right-hand side, known as Trouvelot Crater, of which the image shows the western half (see annotated image). In addition to Trouvelot, the scene includes three other large impact craters and countless smaller ones.

The abundance of craters in this region is no coincidence – it reflects the great age of Arabia Terra, which dates back to the Noachian period, around 3.7 to 4.1 billion years ago. That ancient age has also left its mark on the craters themselves. Trouvelot Crater and the one next to it have highly degraded crater rims that appear crumpled and wrinkled due to erosion over time. The rim of the crater next to Trouvelot is barely visible, suggesting that it is the oldest of the four large craters – definitely older than Trouvelot, which formed on top of it. Trouvelot Crater also has a terraced wall, a characteristic feature of complex craters (see annotated image). These terraces form when the crater walls collapse under their own weight, creating step-like structures.

All large craters in the image show dark deposits either on their floors or along their walls (see annotated image). A closer look at Trouvelot Crater reveals that this dark material has accumulated into dunes formed by wind-driven (aeolian) processes – specifically, a type known as barchan dunes (see annotated image). Regions on Mars covered by dark material are known to be rich in mafic minerals such as pyroxene and olivine. These dark intra-crater deposits may have originated from a thick subsurface layer of volcanic sediments rich in mafic minerals. Large impacts, like those that formed the craters seen in the HRSC image, could have cut through this buried layer, exposing the dark material along the crater floors and walls. Once exposed, the material was redistributed by wind and gravity-driven transport down the crater walls, forming the dark sheets and dunes visible today. When zooming out from Trouvelot Crater, it becomes evident that most large craters in the surrounding area of Arabia Terra also reveal these dark deposits – indicating that they are not merely a local feature, but part of a more widespread phenomenon on Mars.

Another intriguing feature is the light-toned mound in Trouvelot Crater (see annotated image). When zooming in, it shows a ridged and grooved surface. The mound, approximately 20 kilometers long, is partly embedded by the dark material and the dark material also overlies the mound in some parts. Similar light-toned rock outcrops are found elsewhere on Mars. These features are often associated with water-altered minerals and are generally characterized by a higher albedo relative to their surroundings. While multiple formation processes and deposition scenarios are possible, most involve the presence of water – either in a lacustrine environment (like seas) or through groundwater upwelling. A comparable, though larger and higher, light-layered deposit can be found in Becquerel Crater, located just a few kilometers northeast of Trouvelot Crater. The press release from 2013 focuses on Becquerel Crater: A 'radiant' beauty – sulphurous sediments in Becquerel Crater. The formation of these light-toned mounds on Mars remains a complex and ongoing subject of research.