Great lakes on Mars: Rewriting the Red Planet’s history

Liquid water existed on the surface of Mars more recently than previously thought, according to a new study that found several lakes, some as large as the Great Lakes of North America, that were formed between 2 billion and 3 billion years ago.

The study, which is set to be published in the Journal of Geophysical Research, Planets, was conducted through NASA's Jet Propulsion Laboratory. The new discovery not only suggests that water remained on Mars once it lost its magnetic field it started losing its atmosphere about 4 billion years ago, but also that the Red Planet was capable of sustaining microbial life much longer – and more recently – than previously thought.

"We discovered valleys that carried water into lake basins,” Sharon Wilson of the Smithsonian Institution, Washington, and the University of Virginia, Charlottesville, said in a statement from NASA. "Several lake basins filled and overflowed, indicating there was a considerable amount of water on the landscape during this time."

"One of the lakes in this region was comparable in volume to Lake Tahoe," said Dr. Wilson said, while one nicknamed "Heart Lake" held more water than Lake Ontario.

Wilson and her colleagues found evidence for valleys and crater lakes in photos of Mars’s Arabia Terra region taken by three orbiting spacecraft – NASA’s Mars Global Surveyor, Europe’s Mars Express, and NASA's Mars Reconnaissance Orbiter.

By counting the number of craters on different sections of the landscape, the team formulated a timeline and determined that this wet period occurred 2 billion to 3 billion years ago, long after many scientists believe Mars’s "warm, wet" period had ended.

At first scientists thought that water on Mars had existed only very briefly, and primarily underground. But since Mars Global Surveyor arrived at Mars almost 20 years ago, a growing chorus of scientists is arguing that Mars had large volumes of water above-ground and for long periods of time.

"What we thought we knew about water on Mars is constantly being put to the test,” Michael Meyer, lead scientist for NASA’s Mars Exploration Program at NASA Headquarters in Washington, said in 2015, when researchers discovered other evidence for lakes on Mars.

The new study is consistent with previous findings, and adds new evidence.

"This paper presents evidence for episodes of water modifying the surface on early Mars for possibly several hundred million years later than previously thought, with some implication that the water was emplaced by snow, not rain," said MRO project scientist Rich Zurek, of NASA's Jet Propulsion Laboratory, in a statement.

The study suggests that the water came from spring snowmelt, although the researchers have not yet determined what could have warmed snow enough to melt. One theory is an extreme change in the planet's tilt.

"The rate at which water flowed through these valleys is consistent with runoff from melting snow," Wilson said in a statement. "These weren't rushing rivers. They have simple drainage patterns and did not form deep or complex systems like the ancient valley networks from early Mars."

Perhaps most importantly, the discovery means that microbial life could have existed on Mars more recently than previously thought, which could influence future Mars research and missions. 

"A key goal for Mars exploration is to understand when and where liquid water was present in sufficient volume to alter the Martian surface and perhaps provide habitable environments," said Dr. Zurek.