There are many landforms on Mars that are reminiscent of dried-up river valleys – NASA’s current Mars mission, the Perseverance rover, is also supposed to land near one of these river valleys. But now a study suggests that a large part of the Martian river beds were created by subglacial meltwater rather than by free-flowing rivers. The researchers conclude from a comparison of the structure and shape of the Mars river networks with terrestrial landforms. If the results are confirmed, this could support models according to which Mars used to be too cold for long free-flowing waters.
At first glance, the traces of earlier water on Mars can hardly be overlooked: There are widely ramified networks of river beds, estuary deltas and also deeply indented gorges that are reminiscent of dried up earthly waters. According to many planetary researchers, their shape and structure indicate that these landforms were once created by water erosion. “Since the Martian valleys were discovered around 40 years ago, it has been assumed that rivers once flowed on Mars that created these valleys,” explains Anna Grau Galofre of the University of British Columbia in Vancouver. On the other hand, however, climate models for Mars reveal contradicting data as to whether the red planet was once really warm enough for a long period of flowing water. According to some models, even in the early days of the planet, the Martian atmosphere was too thin and cold to allow liquid water on the surface for a long time.
Shape and structure reveal the origin
This raises the question of how the Martian river networks could otherwise have come about – and whether one can tell from their current form which process once created them. Galofre and her team therefore took another look at 10,271 river valleys in 66 fluvial networks on Mars. They determined core parameters such as the branching angle, the width of the main rivers, the ratio of width to length of the river networks and the number of their branches and compared these with different valley shapes on earth. “If you look at the earth from a satellite, you can see many different valleys: some were created by rivers, others by glaciers or other processes – and each species has a typical shape,” explains Galofre. Because erosion from ice or subglacial meltwater can also create structures similar to riverbeds. The researchers transferred the typical features of these landforms to the Martian structures and thus drew conclusions about the most likely development process.
The evaluation showed that some river valleys on Mars could indeed have been created by rivers, but by far not all of them. “Our results demonstrate that only a small part of the valley networks show the characteristics typical of surface water erosion,” says Galofre’s colleague Mark Jellinek. “This is in marked contrast to the conventional view.” Specifically, the researchers identified 14 Martian valley networks that could probably actually have been created by rivers. They are mostly located around Arabia Terra, a highland directly north of the Martian equator. In contrast, valley networks formed by subglacial meltwater or glacial erosion are much more common and more widely spread. Galofre and her team identified at least 22 of the former and nine of the glacial valleys. “This is the first evidence of extensive subglacial erosion due to the channeled drainage of meltwater under old ice sheets on Mars,” says Jellinek.
Subglacial meltwater as a landscape shaper
According to the researchers, most of the valley networks still visible today on the Red Planet were formed by the meltwater from glaciers or even by the glaciers themselves. Rivers that were fed by groundwater or rain, on the other hand, were rather rare. According to Galofre and her team, this would also fit with the climate models for Mars, according to which at least the highlands of the planet were covered by large ice sheets very early on. “Earlier interpretations of geology required precipitation and surface runoff of rainwater to form the valley networks – which contradicted climate simulations that postulated a cool early Mars,” the researchers say. “Our picture now reconciles the climate models with the geological observations.”
Accordingly, there may have been lakes and rivers temporarily in some places on the Red Planet. A large part of its surface, especially in the higher altitudes, was characterized by ice and frost. In contrast, under the thick layers of ice of the Martian glaciers, the conditions were mild enough to allow meltwater to flow into the plains in extensive networks – similar to the meltwater channels under the ice sheets of Greenland and Antarctica today.
Source: Anna Galofre (University of British Columbia, Vancouver) et al., Nature Geoscience, doi: 10.1038 / s41561-020-0618-x