The angle of a river branch is determined, among other things, by how dry an area is and whether groundwater emerges from the ground. (Graphic: from Seybold 2018, Science Adv.)



The distribution of the branching angles of the river valleys on Mars are strikingly similar to those found in arid landscapes on Earth. According to lead author Seybold, this implies that there must have been sporadic heavy rainfall events on Mars over a prolonged period of time and that this rainwater may have run off quickly over the surface shaping the valley networks. This is how river valleys develop in arid regions on Earth. For example, in Arizona, researchers observed the same valley network patterns in a landscape where astronauts are training for future Mars missions. Valleys in arid regions fork at a narrow angle.

The branching angles on Mars are comparatively low. Seybold therefore rules out the influence of groundwater seepage on Mars. River networks that are strongly affected by re-emerging groundwater, as found, for example, in Florida, tend to have wider branching angles between the two tributaries and do not match the narrow angles of streams in arid areas.

Conditions such as those found in terrestrial arid landscapes today probably prevailed on Mars for only a relatively short period about 3.6 to 3.8 billion years ago. In that period, the atmosphere on Mars may have been much denser than it is today. “Recent research shows that there must have been much more water on Mars than previously assumed,” says Seybold.

Evaporation made it rain

One hypothesis suggests that the northern third of Mars was covered by an ocean at that time. Water evaporated, condensed around the volcanoes of the highlands to the south of the ocean and led to heavy rainfall there. As a result, rivers formed, which left the traces that can be observed on Mars today.

The big question is where the water has disappeared to over time. “It’s likely that most of it evaporated into space. Traces of it might still remain in the vicinity of Mars,” says the physicist, “but this is a question for a future space mission”.

