Sufficient quantities of water flowed on Mars’ surface to create large “debris flows” at certain spots when there was a high axial tilt. Debris flows are slurries of water, mud and rock which surge downhill – on Earth they can be very hazardous and are triggered by heavy rain or volcanically melted ice or snow (also called lahars).
The team investigated deposits of these debris flows within an impact crater on Mars called “Istok”. Because meteorites are always falling onto Mars (approximately 30 10m impact craters form per year), they estimated the age of a surface by counting the number of impact craters – the more there are, the older the surface.
Using this standard technique they estimated that Istok crater formed around one million years ago. Because the debris-flow deposits are found within the crate, they deduced that they must have happened after Istok crater formed. Since there are hundreds of overlapping deposits the team also concluded there must have been repeated episodes of water saturating the surface to trigger the debris flows.