ESA has released new images captured by its Mars Express space probe’s High Resolution Stereo Camera (HRSC) showing Nirgal Vallis on the planet Mars. It’s a river valley that extends for over 700 kilometers, so vast that it crosses the Coprates and Margaritifer Sinus quadrangles. A few billion years ago, when the rivers were filled with liquid water, they probably filled Holden Crater, making it a lake with a diameter of about 150 kilometers and a depth of up to about 250 meters.
Just south of the Mars equator there’s an area in which both the flow of water and the impacts of meteorites left such deep geological traces that they’re visible after more than three billion years. The characteristics of the various craters of Nirgal Vallis and neighboring areas led scientists to estimate that the valley system is between 3.5 and 4 billion years old. This confirms that it dates back to the time when the Martian environment was similar to the Earth’s, and in that area there were flowing liquid water rivers.
In this case, the scientists studied in particular the wester edge of the Nirgal Vallis river system, a particularly branched section about 75 kilometers long which is dissolving. The easter edge is much less branched and is more clearly defined as a single valley that opens into what’s known as Uzboi Vallis, where there could be an ancient lake, which has also been dry for a very long time.
The remains of the ancient river system are those most visible in Nirgal Vallis but the traces of the valley are interesting. Like other valleys in that area, they’re at least partially in areas that were also affected by volcanic activity so there are also traces of lava flows.
The shape of Nirgal Vallis reminds of an amphitheater and has some equivalents on Earth, such as in the Atacama desert in Chile. On Mars there are other valleys of that type as well such as Nanedi Valles and Echus Chasma. These are formations that are interesting from a geological point of view because they indicate an erosion caused by water.
It’s long been established that water flowed on Mars, the difficulty lies in reconstructing what happened on the planet. For example, on Earth, traces like steep walls and flat bottom of the valley are more typical of glacial erosion. According to scientists, the river probably didn’t flow constantly but sometimes carried large amounts of water.
On Mars there are many areas that show very ancient traces left by the water that flowed over 3 billion years ago but some have particularly interesting features. In Nirgal Vallis there are traces of various geological activities that were normal on Mars when the planet was young and are now useful to reconstruct its history.