Ancient Martian ocean evidence suggests it covered half the planet

A 2026 NPJ Space Exploration study reports scarp-fronted deposits in Southeast Coprates Chasma that resemble river deltas and align at a common elevation, which the authors interpret as a coastline marking a high-water level across Mars' northern hemisphere.

A team led by Ignatius Argadestya published a study in NPJ Space Exploration describing scarp-fronted deposits (SFDs) in Southeast Coprates Chasma, a part of Valles Marineris. The researchers used high-resolution images and digital elevation models from CTX and HiRISE (MRO), CaSSIS (TGO), HRSC (Mars Express) and MOLA to map and measure the landforms. They interpret the SFDs as fan-deltas formed where rivers emptied into standing water. The authors report that these deposits sit at similar elevations and fit with previously inferred paleoshorelines across the northern hemisphere. Key findings: - Scarp-fronted deposits (SFDs) in Southeast Coprates Chasma display fan-shaped geometries and tributary channels the authors interpret as consistent with fluvial origins and delta formation. - The mapped SFDs occur at roughly the same elevation in the canyon; the authors interpret that elevation as a high-water mark or ancient coastline. - The study used combined datasets: CTX and HiRISE images, CaSSIS color imagery, HRSC stereo data, and MOLA-derived digital elevation models. - The authors place the deposits between the Late Hesperian and Early Amazonian periods, which they identify as a time of greatest surface water availability on Mars. - The paper frames these SFDs as additional evidence that a large ocean or extensive standing water reached similar levels from Valles Marineris toward the northern lowlands. Summary: The authors describe the identified SFDs as deltaic deposits that record a consistent high-water level and an implied coastline in Valles Marineris, supporting interpretations of an extensive ancient ocean on Mars. Undetermined at this time.