Researchers at NASA have found evidence that water flowed on Mars much more recently than previously thought -- perhaps millions of years more recently.

Scientists have previously documented an ancient era in Mars history featuring extremely wet conditions. But new observations -- collected by the Mars Reconnaissance Orbiter and analyzed by scientists at the Jet Propulsion Laboratory -- have revealed the presence of lakes and snowmelt-fed streams a billion years later.

"We discovered valleys that carried water into lake basins," researcher Sharon Wilson, a scientist with the Smithsonian Institution and the University of Virginia, said in a news release. "Several lake basins filled and overflowed, indicating there was a considerable amount of water on the landscape during this time."

Researchers have dubbed the ancient of lakes and streams the Heart Lake valley system. Analysis of the ancient valleys suggest that while the lakes were large, the streams that filled them were likely seasonal. Researchers also surmise that the climate was cold and that the stream may have been fed by snow, not rain.

"The rate at which water flowed through these valleys is consistent with runoff from melting snow," Wilson said. "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."

An analysis of craters within the ancient valleys suggests the Heart Lake system featured flowing water between two and three billion years ago. Scientists recounted their study of the valley lakes in a new paper, published this week in the Journal of Geophysical Research.

Scientists have located valley systems similar in appearance elsewhere on Mars, suggesting these freshwater systems emerged on a global scale.

Now, scientists are looking to explain how a previously frozen planet came to feature running water, and whether or not this newly identified wet era might have hosted microbial life.