Thousands of hills and mounds on Mars have been found to contain layers of clay minerals, which formed when running water interacted with the rocks during a period when Mars' northern reaches were flooded.
"This research shows us that Mars' climate was dramatically different in the distant past," Joe McNeil of the Natural History Museum in London said in a statement. "The mounds are rich in clay minerals, meaning liquid water must have been present at the surface in large quantities nearly four billion years ago."
