Valles Marineris

This week’s guess the planet comes from Mars, and is an image from the HiRISE camera on the Mars Reconnaissance Orbiter. Credit goes to NASA/JPL/University of Arizona. The HiRISE team have a great write up about the geology of this site, which explains what is going on in the image. In essence it shows an impact crater on the very edge of a large canyon. This is the edge of one of the largest and most recognisable features on the planet. The Valles Marineris. This immense canyon system is 4000 km long and seven kilometres deep. The valley system is obvious from space, as this mosaic of images from the Viking spacecraft shows. More information about this immense valley system can be found at this NASA link.

They compare its dimensions to those of the Grand Canyon on Earth, which is dwarfed by the martian valley system. The two features are often compared, however they actually have very little in common. The Grand Canyon is tiny because it was carved by the action of water. It formed over the course of several million years, as the Colorado river cut through the soft rock of the Arizona desert. This is very different to its martian counterpart. The modern consensus among planetary scientists is that Valles Marineris formed as a tensional fracture in the martian crust. It is likely that erosion by water caused later changes to the valley, but was not the dominant force in its formation.

This means that a better analogue for its formation are the rift valleys of Earth. These continent spanning structures are formed by tectonic processes. The East African rift valley is pictured here. It spans thousands of kilometres and formed as the tectonic plates pulled apart, causing new crust to form in between through volcanism.

There is just one problem. Mars does not appear to exhibit plate tectonics. On Earth, we see lots of evidence of the movement of the plates, from the interlocking shapes of continents to chains of volcanoes, and the earthquakes caused by the movement of faults. On Mars this is does not seem to be the case. We’ve discussed in a previous blog http://outer-reaches.blogspot.co.uk/2017/01/the-tallest-mountains.html that the largest martian volcanoes tower above their terrestrial counterparts, suggesting that the crust on which they sit isn’t moving over a volcanic hotspot. We don’t know for certain yet whether marsquakes occur.

This means that while Valles Marineris formed through rifting it wasn’t as a result of two tectonic plates pulling apart. Instead it was probably due to the presence of those massive volcanoes. Olympus Mons is just one of several huge shield volcanoes which occupy a region of Mars called Tharsis. This can be seen just to the north west of Valles Marineris on the image above, where two large, circular features are visible on the edge of the view. These are Ascreus and Pavonis Mons, two of the largest volcanoes on the planet. Along with Olympus and Arsia Mons (which are just off the edge of this image) they form a massive triangle of volcanic activity, and a substantial bulge in the surface of Mars. The thickening of the martian crust in this area would have produced substantial tensional forces, and it is believed that this is what caused the rifting along the Valles Marineris.

Image credits

HiRISE image of the edge of Valles Marineris; NASA/JPL/Universityfo Arizona

Viking Mosaic of the Valles Marineris Hemisphere of Mars; NASA/JPL-Caltech

Satellite image of the east African rift; NASA