Using images from cameras on Mars orbiters, an international research team has discovered structures on Mars that are very similar to classic river deltas on Earth. These are traces of rivers that have deposited their sediments into an ocean. This shows that Mars was a “blue planet” around 3 billion years ago.

The existence of water on Mars is a central topic in planetary research. Previous studies have already provided evidence of oceans and rivers on Mars, indicating a once humid and possibly habitable environment. Evidence of former water and a possible ocean have also been discovered for the Valles Marineris—the largest canyon system on Mars, which stretches along its equator. These come, among other things, from discoveries of minerals that have been altered by water.

A research team from the University of Bern, in collaboration with the INAF—Osservatorio Astronomico di Padova, has now gained new insights into the geological past of Valles Marineris: Using high-resolution images from various Mars cameras, the researchers have found geomorphologic structures near the canyon system that resemble river deltas on Earth. These structures represent the mouth of a river into an ocean. The new study thus provides clear evidence of a coastline and consequently of an earlier ocean on Mars.