ESA’s Rosalind Franklin rover could touch down in the remains of a vast ancient water system on Mars, after new orbital mapping showed that clay-rich deposits at its landing site extend far beyond previous estimates.
The rover is due to land at Oxia Planum, a low-lying region near the boundary between Mars’s southern highlands and northern lowlands, where clay minerals preserve evidence of a time when liquid water was active on the Red Planet. The new study suggests that these clays are not just a local feature, but part of a much larger sequence that stretches towards Mawrth Vallis, around 300 kilometres away.
That gives the landing site a much wider significance. The clay deposits may extend roughly 600 kilometres across and rise by more than a kilometre in elevation. One possible explanation is that they were shaped by an extensive body of water several kilometres deep around four billion years ago. If so, ESA says its shorelines would rank among the highest ever proposed for ancient Mars.
Another possibility is that the deposits were formed when huge volumes of water flooded across broad plains from ancient groundwater reservoirs. Either way, the result points to a large-scale watery environment rather than a small, isolated patch of altered rock.
“Because the area is so large, we are not talking about a localised occurrence, but rather a regional or global process that would have required immense amounts of water,” says Jorge Vago, ExoMars project scientist. “We are targeting the oldest deposits in the sequence, which makes the potential implications for the geology and early climate of Mars very relevant for the Rosalind Franklin mission in its search for life.”
Clay minerals are among the most important targets in the search for ancient life on Mars because they form in the presence of liquid water and can preserve chemical clues to the environment in which they were deposited. Oxia Planum was selected as Rosalind Franklin’s landing site because of its ancient, clay-bearing rocks.
This new analysis was led by Inés Torres Auré, of the University of Lyon, France, and is published in the journal Icarus. The team used mineral data from OMEGA on ESA’s Mars Express orbiter and CRISM on NASA’s Mars Reconnaissance Orbiter to compare the rock layers between Oxia Planum and Mawrth Vallis. Their results show that both sites contain similar mineral layers, suggesting that they are linked within the same broader geological story.
“We now have a new timeline: Oxia Planum’s clays formed first, about four billion years ago, predating those at Mawrth Vallis,” says Torres Auré. “By landing at Oxia Planum, we’ll uncover a large-scale process that shaped ancient clays across Mars.”
The study also identifies a palaeosurface, an ancient ground surface that was exposed long enough to be cratered before later being buried by younger deposits. Its presence suggests a pause in sedimentation, followed by a change in water chemistry and mineralogy across both Oxia Planum and Mawrth Vallis.
“We have identified a pause in deposition, which is quite puzzling because it implies a period of minimal surface activity (except for meteorite bombardment), followed by a shift in water chemistry and mineralogy in both Oxia Planum and Mawrth Vallis,” says Torres Auré.
Rosalind Franklin is designed to test these orbital interpretations from the surface. The rover carries cameras, spectrometers, ground-penetrating radar and an onboard laboratory, as well as a drill capable of collecting samples from up to two metres below the martian surface. That depth is important because any possible organic material near the surface is exposed to radiation and oxidising chemistry that can destroy delicate biological signatures over time.
“We will use the instruments on board to ground truth the discoveries made from orbit, learn about the ancient environment in which they formed, and if they preserve any evidence of martian life,” says Elliot Sefton-Nash, ExoMars deputy project scientist. “Warmth and nutrients on an early martian seabed could have provided habitats for early life.”
Although Rosalind Franklin was delayed from its intended launch by the collapse of cooperation with Russia, the extra time as allowed more detailed preparations to take place. As a result, its scientific target now looks even more compelling. Rather than simply sampling one promising clay outcrop, Rosalind Franklin may be able to investigate a surviving fragment of a much larger water-rich chapter in Mars’s early history.
Its findings could have direct implications for Mars’s climate, its habitability, and where future missions should look for the planet’s most carefully preserved traces of ancient life.
Clay continuity between Oxia Planum and Mawrth Vallis by Inés Torres Auré et al. is published in Icarus, 2 June 2026.
Read more about the search for life in the Universe in our May 2026 issue.