Based on a study of alluvial fans on Mars, river sediment thought to have been placed at the foot of mountains, scientists have concluded that liquid water could have been flowing from as 2.5 to 3.6 billion years ago.

“We’ve known for decades that Mars had rivers and lakes around 3.5 billion years ago, but in the past few years there has been a growing body of evidence that substantial amounts of liquid water continued to erode the Martian surface for hundreds of millions of years,” said Morgan, lead author on “The global distribution and morphologic characteristics of fan-shaped sedimentary landforms on Mars” that appears in Icarus. “Water-formed landforms, such as river deltas and alluvial fans, are the most unambiguous markers of past climate. So we conducted a global survey for these features and explored patterns in their distribution and morphologic properties.”

Morgan and co-authors including PSI Senior Scientist Alan Howard found that alluvial fans are found at lower elevations than the more ancient valley networks, suggesting that stable liquid water became restricted to lower, warmer regions as Mars cooled and dried.

…What is particularly interesting about the Martian fans is that many formed much later than the valley networks, which have long been considered the strongest evidence for surface water on early Mars. Valley networks largely date to around 3.6 billion years ago, but alluvial fans date to 2.5 to 3 billion years ago.

This research merely increases the fundamental geological mystery of Mars. While the surface evidence strongly tells us that liquid water once flowed on the surface, no climate model exists that satisfactorily makes that possible. The atmosphere appears to have always been too cold and thin for liquid water.