Another spectacular planetary science image, this time from Messenger orbiting Mercury. This close-up image of the hollows of Mercury only illustrates their mystery. The insert shows the context of the close-up image. These irregular sinks are here found on the mountain top ridge of an inner crater rim. Also, some but not all of the hollows have bright interiors.
Scientists have proposed that some form of impact melt process caused these hollows. At impact, the ground literally rippled like water when you toss a stone into a pool. Here, however, the molten ripples quickly froze, creating the inner and outer crater rim rings. To my untrained eye, the hollows look like collapse features where the surface hardened first, then collapsed when the molten inner material drained away as it became solid.
Why some hollows are bright, however, is not yet understood.