Aram Dorsum is one of three candidate landing sites for the ExoMars rover, due to launch in 2020 with the main objective of searching for traces of past and present life. We were interested in investigating the surrounding region to provide some context for any potential future landing. Sure enough, we soon realised that Aram wasn’t alone—many such inverted channels were present nearby and in the wider Arabia region.

Using Context Camera (CTX) images from NASA’s MRO spacecraft, which enables features down to about 20-30 metres across to be identified, we identified and mapped over 17,000 kilometres of inverted channels in Arabia Terra. The main reason that many of these features had not been previously identified was their size—the valley networks are perhaps up to tens of kilometres in width, whereas the inverted channels were only tens to hundreds of metres in width; inverted channels simply aren’t visible in lower data resolution. The inverted channels are also fairly discontinuous and patchy in their distribution, only occurring in segments up to a hundred or so kilometres in length, which is consistent with Arabia Terra’s history of widespread, intense erosion.

Unlike the valley networks, which appear to mostly be cutting into basaltic lava plains and impact crater ejecta, the inverted channels seem to be surrounded by material similar in composition to the channels themselves. This, and that we observed only small changes in elevation in the downstream direction of the inverted channels, may mean that much of Arabia Terra was once a giant and relatively flat flood plain, through which rivers once meandered. Arabia Terra appears to once have been very wet indeed!

Many of the inverted channels interconnect with the valley networks to the south, meaning that much of the water and sediment transported from further upstream in the highlands may have passed through Arabia Terra, and that much of it will have been deposited here within the Arabia Terra flood plain. Aram and the other inverted channels also appear to have been buried, not long after their formation in the Noachian, by hundreds of metres of sedimentary rocks, including those that make up Meridiani Planum.

A flood plain setting and rapid burial means that Aram fits nicely with the goals of the ExoMars mission—potential organics and traces of life could have been sourced from a wide area, deposited in the flood plains around the channel, before being buried and preserved for billions of years, which would have protected organics from harmful radiation. The other two candidate sites—Oxia Planum and Mawrth Vallis—are also found within Arabia Terra, near the northern boundary of the region. Both these sites contain thick stacks of clay minerals, which can form due to interaction with water, and are also good at preserving organic material.