The ancient rivers of Arabia Terra on Mars

Map of the ancient rivers system of Arabia Terra on Mars (Image courtesy J.M. Davis et al.)
Map of the ancient rivers system of Arabia Terra on Mars (Image courtesy J.M. Davis et al.)

An article published in the journal “Geology” describes the study of the remains of a system of ancient riverbeds in the region of the planet Mars called Arabia Terra. A team of scientists led by Joel Davis of the University College London analyzed high-resolution photographs taken by the Mars Reconnaissance Orbiter (MRO) space probe to find traces of rivers that flowed nearly 4 billion years ago. This is another confirmation of the fact that at that time the climate on Mars was warm and wet, similar to the Earth.

In recent decades, scientists have been accumulating evidence of the existence of water flows back to the Noachian period on Mars, between 3.9 and 3.7 billion years ago. They left geological traces that various space probes have found with valleys and channels excavated and eroded by water flows and also by rains. Those structures have been found in many regions of the red planet but not in the one called Arabia Terra, where instead traces of ancient supervolcanoes were found.

The team led by Joel Davis examined images at a much higher resolution than was possible in the past, which covered an area of a size comparable to that of Brazil. The result was the discovery of a few valleys but of a very large system of fossil riverbeds for a total of over 17,000 kilometers (almost 10,600 miles).

Today these ancient riverbeds are visible as inverted channels, meaning channels that were filled with sand and gravel transported by a river. The consequence is that when the river got dry the channels ended up having a greater height than the surrounding terrain formed by materials that undergo a greater erosion.

On Earth there are inverted channels as well, which typically are formed in environments that have become dry desert with rivers that have dried up. In most environments, however the channels get eroded before they can become inverted. When the weather once warm and wet Mars turned into today’s dry planet, over time the channels got inverted.

The traces found by Joel Davis’ team include remains of giant rivers as there are inverted channels with a height of around 30 meters (100 feet) and a width of 1 to 2 kilometers (0.6 to 1.2 miles). These rivers were active in the Noachian period dried up and were filled quickly, so any traces of organic materials may have been protected, like fossils on Earth.

This research provides another piece of the jigsaw puzzle formed by the history of the planet Mars. One of the inverted channels of Arabia Terra, called Aram Dorsum, was selected among the candidates to be the landing place of ESA and Roscosmos’ ExoMars Rover mission, which will be launched in 2020. For the moment, the researchers will keep on studying the region through the pictures taken by the MRO space probe.

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