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Researchers have found evidence that suggests Mars once had an ancient ocean and a water cycle similar to Earth's.
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.'
So far, planetary scientists have not been able to reliably identify clear evidence for a large body of open water on Mars, like fine-grained oceanic deposits or clear shorelines, so they need alternative evidence. That could include the presence of large sediment fans, like the delta in this study.
River deltas are formed when a river meets a slower-moving or still body of water and slows down. Any small sediments that the river is carrying settle on the ground and form a geological feature over time.
Rivers deltas have been found on Mars before, but only in craters, where the water flowed into a lake. This is the first time that researchers have been able to say with any certainty that the deltas suggest a pool of water big enough to be an ocean. The Hypanis delta is the largest known delta identified on Mars so far.
The study also shows how the Mars ocean retreated, as the climate became colder and drier. Water levels dropped by nearly 500 metres, forcing the Hypanis delta to grow outwards.
About 3.6 billion years ago, the water system dried up and disappeared, possibly because of a change in the ancient climate.
Now, freezing surface temperatures on the planet mean there is no liquid water on the surface of Mars, although an underground lake has recently been found.
Lead author of the paper, Dr Peter Fawdon of the Open University, says, 'The research has significantly contributed to our understanding of the climate on early Mars, which we now know went from having a water cycle similar to that of Earth to being a cold, desert-like landscape in a relatively short period.
'We would like to gain a better understanding of how many of these fluvial deltas exist on Mars so that we can determine the position and size of its ancient seas.'
Both the ExoMars 2020 and NASA 2020 rovers are landing near to the proposed ocean shoreline, and the existence of an ocean could increase their chances of finding life.