New evidence from NASA's Mars Reconnaissance Orbiter has revealed that entire networks of lakes and streams fed by melting snow existed on Mars way later than anyone had thought possible.
Based on new images taken of Mars' ancient northern region, researchers have suggested that there was a "considerable amount of water" roughly a billion years after we thought the red planet's 'wet era' had come to an end. And that means it might have been suitable for microbial life much longer than we've assumed.
"We discovered valleys that carried water into lake basins," said one of the team, Sharon Wilson of the Smithsonian Institution. "Several lake basins filled and overflowed, indicating there was a considerable amount of water on the landscape during this time."
Wilson and her team have been analysing new images snapped of Mars' northern Arabia Terra region - a vast, densely cratered, and heavily eroded area thought to be one of the oldest terrains on the planet.
Matching up the evidence from these images to data collected by NASA's Mars Global Surveyor and the European Space Agency's (ESA) Mars Express, they were able to identify signs of a long-evaporated water system that would have resembled some of North America's greatest lakes.
"One of the lakes in this region was comparable in volume to Lake Tahoe," Wilson said.
For reference, Tahoe holds an average of 37 trillion gallons (140 trillion litres) of water - enough to cover the whole of California in 14 inches (35 cm) of water.
According to the US Department of Agriculture, if you decided to drain Tahoe, it would take around 700 years to fill back up again. That's how much water is estimated to have been in just one of these Mars lakes.
Wilson explains that this particular Martian lake was fed by a valley to the south, and overflowed along its northern edge into a very large, water-filled basin they've now nicknamed Heart Lake.
Heart Lake appears to have been part of an entire system of lakes and valleys that ran for about 90 miles (150 kilometres) along Mars' north face. The team has estimated that Heart Lake held about 670 cubic miles of water (2,790 cubic kilometres) - more than in Lake Ontario, one of North America's Great Lakes.
Based on information from 22 craters in the area, the team dated the existence of these lakes to between 2 and 3 billion years ago - long after scientists had assumed that most of the planet's original atmosphere had been lost and its water had been frozen.
Instead, at this time, it appeared that snow had been melting seasonably to feed all these vast bodies of water.
"The rate at which water flowed through these valleys is consistent with runoff from melting snow," Wilson says. "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."
The researchers say they've also found evidence of similar valleys and lakes elsewhere on Mars, both north and south of the equator, suggesting that these wet regions were widespread, and not just a regional anomaly. And that's good news for the possibility of finding remnants of ancient life.
"Evidence for a snowmelt based hydrology and considerable depths of water on the landscape in Arabia supports a cold, wet, and possibly habitable environment late in Martian history," they conclude.
The research has been published in the Journal of Geophysical Research, Planets.