Numerous dry river networks and canyons on Mars show that there may have been water there once. But at least some of these valleys may have been carved out by catastrophic flash floods, not gradually being carved out by slow-flowing rivers. Researchers have found evidence of this when analyzing a good 260 former crater lakes: their edge is usually breached at one point and an unusually deeply deepened river valley emanates from this point. These formations indicate an abrupt drainage of the crater lake in a flash flood, explains the team.
In its early days, up to around 3.7 billion years ago, Mars was probably warmer and more liveable than it is today. The prevailing view was that there was enough liquid water on the Red Planet to fill numerous lakes and rivers. The presence of this water testifies to river beds and branched river networks, the shape of which has been preserved in the now dry Martian surface. How these prehistoric river beds came about has not been clearly established. “Previous studies mainly assumed that they were notched by surface water from rain or snowmelt, by groundwater leaks at the head of these valleys and / or by subglacial meltwater under large ice sheets,” explain Timothy Goudge of the University of Texas at Austin and his colleagues Colleagues. What these formation mechanisms have in common is that they require some time and are therefore likely to have emerged gradually over the course of the early days of Mars, which were rich in water.
Leaking crater lakes
According to Goudge and his team, there is another possibility: “We are looking at an alternative, catastrophic valley formation mechanism in which the river valleys were quickly notched by the strong leakage of water from lakes on the Martian surface,” the researchers say. In such events, water initially builds up in a crater, for example, until the pressure becomes too great and the crater rim breaks at one point. The result is a flash flood in which enormous amounts of water fall to the valley in a short time and with great force. “Such a crater filled with water releases a lot of stored energy,” says Goudge. Similar events on earth show that the eroding power of these water masses is very high and that they carve a new, deep bed comparatively quickly. On Mars, too, there are some craters with a river valley extending from their edge, in which planetary researchers have seen the result of such draining lakes before.
In order to find out how frequent such events were on early Mars and how much they contributed to the formation of the river valleys that can still be seen today, Goudge and his colleagues have for the first time carried out a general inventory of possible leaked lakes on Mars. On the basis of images from various Mars probes, they searched among the mapped river valleys and networks for those that begin at the edge of a crater and are not connected to other river valleys or are only connected to other river valleys during the further course of the network. The researchers found what they were looking for in 262 craters of various sizes. In all of these cases there was evidence of a flash flood caused by the leakage of a crater lake.
A quarter of river erosion from flash floods
Using the depth of the notched river beds and the size of the craters, the scientists then calculated how much these flash floods must have eroded the Martian underground. Taken together, they come to a removed volume of 14 trillion cubic meters. “The outflowing paleo lakes were therefore responsible for almost a quarter – around 24 percent – of the total material eroded in river valleys,” report Goudge and his colleagues. The larger of these primeval flash floods could have washed away so much sediment within a few weeks that it would completely fill the largest and smallest of the Great Lakes in North America. One of the valleys created by such mega flash floods is the huge Ma’adim Vallis, a gorge around 700 kilometers long and two kilometers deep that emanates from the Gusev crater. “This canyon alone contributes around 15 percent to the total eroded valley volume,” writes the team.
According to the researchers, these numbers suggest that flash floods from draining crater lakes on early Mars played a greater role than previously thought. “If we consider how much sediment was moved in the Martian landscape as a result, such lake flash floods were a globally important process,” says Goudge. From comparative studies of river bed depths, he and his colleagues also conclude that the flash floods not only shaped their own outflows, but could also have changed the course of adjacent river networks. If they cut existing beds of normal rivers, they affected their course because they were often notched deeper. “Evidence for this scenario can be seen in paleo outflows, where side valleys often hang over the bottom of the canyon or have prominent kinks,” explains the team.
Source: Timothy Goudge (The University of Texas at Austin) et al., Nature, doi: 10.1038 / s41586-021-03860-1