Mars: Young rivers surprise researchers

Some Martian rivers were water rich and surprisingly large just a billion years ago

False-color image of riverbeds on Mars - some of these valleys seem surprisingly young, as a survey suggests. © NASA / JPL, Univ. Arizona / uchicago
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Water on the desert planet: The rivers of Mars apparently remained much longer than previously thought. Two billion years after the end of the mild, humid early climatic period, a surprising amount of water flowed through some river systems. Some of these Mars rivers were even twice as wide as their earthly counterparts, according to researchers in the journal Science Advances. Where they got their water, however, is highly puzzling.

Mars used to have lakes, rivers, and maybe even an ocean - that's what argued for clay deposits, wide-rutted and notched river valleys, and even river pebbles swept along by the water on the Red Planet. But already around 500 million years after the formation of Mars, the initially mild and humid climate changed. The Martian atmosphere was thinning and it was getting drier and colder. When the Hesperian Age ended about three billion years ago, Mars was cold and dead - that's the common scenario.

River valleys as "time machines"

But how long did the rivers of Mars last? Did your water already boil in the early Hesperian or were there still late refuges? To find out, Edwin Kite of the University of Chicago and his team have evaluated orbital images of well over 200 Martian river valleys from the Late Period of the wet period and used them to create digital terrain models.

The sense behind it: From the breadth and depth of riverbeds and the size of their catchment area, one can estimate how much water once flowed in these river nets even if the waters are long since dried up. Also, the width of the M ander loops in rivers in shallower terrain gives hints. Just how violent the flow once was in different parts of the river can be deduced from the amount and size of river pebbles.

Twice as wide as earthly rivers

The surprising result: Contrary to expectations, even until a billion years ago and even after that, water was still pouring through some of the great river networks of Mars. "The dimensions of the Martian rivers provide evidence of an intense flow long after the transition from wet to dry climate, " the researchers report. display

From the length of the marshes and the relief of the riverbeds, the scientists conclude that the late Martian rivers must have been unusually wide. "With an equally large catchment area, these rivers were about twice as wide as terrestrial rivers, " Kite and his colleagues report. The youngest rivers of Mars were located in the region between the 40 southern and the 52nd northern latitude and concentrated mainly in low-lying, flat regions.

Geographical distribution of river networks and possible rainy seasons in the

Surprisingly large amounts of water

Also unexpected: The amount of water in the late rivers of Mars apparently reached similar levels at times as in the damp early days of the planet. "One would expect that the amount of water gradually decreases with increasing dryness, " says Kite. "But that's not what we see here." Instead, the river nets did get shorter during the late season they were only a few hundred kilometers long instead of a few thousand, but they still did almost as much water.

According to the researchers, the surface runoff at that time was between three and 20 kilograms of water per square meter per day. "These observations make an already difficult problem even more difficult, " says Kite. Because even for the time around three billion years ago, it is difficult to explain the presence of liquid water on Mars at all with previous models. The now determined discharge rates, even for much younger Martian rivers, seem virtually impossible.

Hothouse pests through evaporating polar ice?

According to popular theory, Mars was already a cold desert planet about three billion years ago. Rain was almost non-existent, because the atmosphere was too long for thick rain clouds. But the riverboats from this time paint a completely different picture. Accordingly, there were still periods with strong, short rainfalls in between, despite the frosty W stenklimas. Such torrential rain had already been postulated by other researchers.

According to the researchers, there must have been climate processes on Mars that produced at least short-term precipitation. "The combination of data indicates strong positive feedbacks in the early Mars climate system, " said Kite and colleagues. A possible trigger could be a gradual melting and evaporation of the polar ice deposits. Carbon dioxide and water vapor were released, causing a kind of greenhouse effect for a short time.

"Our study raises some exciting questions - and increases the challenges for the climate modelers, " says Kite. Because they have to look now, where the late rivers of Mars could have got their water. (Science Advances, 2019; doi: 10.1126 / sciadv.aav7710)

Source: University of Chicago

- Nadja Podbregar