Valhalla Multi Ring Impact Structure

This week’s image comes from Callisto, one of the moons of Jupiter. It shows the Valhalla impact structure, a large crater with several concentric rings. Valhalla is the largest multi ring crater in the solar system. The entire structure is about 3800 km in diameter. This image was captured by the Voyager One spacecraft, when it flew past this small moon in 1979. The crater is located in the Jupiter facing quadrant of the moon’s northern hemisphere. Several smaller impact craters are superimposed on the larger basin, suggesting that it is fairly old, and that other impacts have occurred since. Valhalla is not the only multi ring structure on Callisto. Another four are located on the moon, including the nearby Asgard and Utgard impacts. Asgard is the larger of the two, and the Utgard basin is superimposed upon it, indicating that it must have formed later. Craters are the main surface feature on Callisto, and the presence of numerous multi ring basins suggests that numerous very large impacts have occurred.  

The multi ring basins also result from the properties of Callisto’s lithosphere, the solid outer layer of the moon. This is likely to be quite thin and brittle, with a large proportion of ice within its composition. When a giant impact hit, the lithosphere is punctured. This imparts stress to the softer layer below, and causes concentric fractures to form around the impact site. These have the form of heavily degraded “Grabens”. These are rift valleys which generally form through tectonic processes. Graben are named after the German word for trench, as they consist of troughs where a block of a planet’s crust has been forced downwards relative to the surrounding blocks. They are bordered by two normal faults, running parallel to one another, in this case around the impact structure. These faults form steep scarps, discontinuous cliffs and ridges that give the structure its distinctive form. The result looks like a series of ripples, and this is a reasonable analogy. The stress caused by the impact has propagated outwards from the impact site in all directions. This stress has caused a series of failures of the brittle crust, resulting in concentric faults and scarps. Although the process that formed the outer rings is tectonic, it was triggered by the force of the massive impact. 

Image credit: NASA/ JPL https://en.wikipedia.org/wiki/Valhalla_(crater)#/media/File:Valhalla_crater_on_Callisto.jpg