One of CallistoΓÇÖs most interesting features is Valhalla, a huge crater 600 kilometers in diameter, with concentric rings reaching out, like ripples frozen in a pond, to a distance of 3,000 kilometers. Indeed, frozen ripples may be exactly what ValhallaΓÇÖs rings are, the result, astronomers suggest, of some huge event that melted the ice on CallistoΓÇÖs surface. When the ice quickly refroze, the ΓÇ£ripplesΓÇ¥ were captured. Yet ValhallaΓÇÖs features are fairly smooth, which suggests that this event occurred after most of CallistoΓÇÖs cratering.
