Sediment deposits at the mouth of Hypanis Valles, a river system on ancient Mars, were found to be remnants of a delta. This is a geological feature that would have formed where running water met a Martian sea.

The delta is at the planet's north-south divide, which separates the southern highlands from the northern lowlands. Whether large seas or an ocean ever existed in the northern lowlands is one of the most important unanswered questions in the study of the geological evolution of Mars.

This research, published in Earth and Planetary Science Letters, suggests that a large body of water did indeed cover the northern third of the planet.

Scientists are studying this area because it had been proposed it as a landing site for the upcoming ExoMars 2020 rover mission, which will travel the planet looking for signs of life - including signs of long-standing liquid water.

Joel Davis, a postdoctoral researcher in the Planetary Surface Group at the Museum, is a co-author of the paper. He worked with colleagues from the Open University and Imperial College.

He says, 'A Martian ocean means that Mars probably had a very Earth-like water cycle, with rivers, lakes, and now oceans, all of which probably interacted as part of a planet-wide system. We think this Earth-like hydrological cycle was active about 3.7 billion years ago, and started to shut down sometime after that.

'Our study is not definitive proof for an ocean, but these geological features are very hard to explain without one.'