Lakes and snowmelt-fed streams on Mars formed much later than previously thought possible, according to new findings using data from NASA's Mars Reconnaissance Orbiter.
The recently discovered lakes and streams appeared roughly a billion years after a well-documented, earlier era of wet conditions on ancient Mars, NASA said.
These results provide insight into the climate history of the red planet and suggest the surface conditions at this later time may also have been suitable for microbial life.
"We discovered valleys that carried water into lake basins," said Sharon Wilson of the Smithsonian Institution and the University of Virginia in the US.
"Several lake basins filled and overflowed, indicating there was a considerable amount of water on the landscape during this time," said Wilson.
Wilson and colleagues found evidence of these features in Mars' northern Arabia Terra region by analysing images from the Context Camera and High Resolution Imaging Science Experiment camera on the Mars Reconnaissance Orbiter and additional data from NASA's Mars Global Surveyor and the European Space Agency's Mars Express.
"One of the lakes in this region was comparable in volume to Lake Tahoe," Wilson said, referring to a California-Nevada lake that holds about 188 cubic kilometres of water.
"This particular Martian lake was fed by an inlet valley on its southern edge and overflowed along its northern margin, carrying water downstream into a very large, water-filled basin we nicknamed 'Heart Lake,'" Wilson said.
The chain of lakes and valleys that are part of the Heart Lake valley system extends about 150 kilometres.
Wilson and colleagues mapped the extent of stream-flow in "fresh shallow valleys" and their associated former lakes. They suggest that the runoff that formed the valleys may have been seasonal.
To bracket the time period when the fresh shallow valleys in Arabia Terra formed, scientists started with age estimates for 22 impact craters in the area.
They assessed whether or not the valleys carved into the blankets of surrounding debris ejected from the craters, as an indicator of whether the valleys are older or younger than the craters.
They concluded that this fairly wet period on Mars likely occurred between two and three billion years ago, long after it is generally thought that most of Mars' original atmosphere had been lost and most of the remaining water on the planet had frozen.
The characteristics of the valleys support the interpretation that the climate was cold.
"The rate at which water flowed through these valleys is consistent with runoff from melting snow," Wilson said.
Researchers note that similar valleys occur elsewhere on Mars between about 35 and 42 degrees latitude, both north and south of the equator.
The research was published in the Journal of Geophysical Research, Planets.
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