I know I just posted a MESSENGER photo of craters, but this one is different and spectacular enough that I figure, why not? I love a big, splashy, wide-angle shot of a rayed crater! So here's the lovely, 80-km wide impact crater Debussy on the surface of Mercury:
[Click to haphaestenate.]
Craters make rays when the ejected material blasted out forms long plumes which fall across the surface. On airless worlds, those trajectories are ballistic, heading straight out from the center of the impact. Deeper material tends to be a lighter shade than surface material, so the interior of the crater and the rays are lighter than surrounding surface stuff. You can also see what's called the apron, the layer of material that falls immediately around the crater, surrounding it (that's more clear in an earlier image of the crater looking more straight down on it).
Rayed craters are common (even on our Moon; take a look at Tycho!), and usually indicate the impact was recent (geologically), since the rays eventually get eroded by the solar wind, cosmic rays, and subsequent meteorite impacts. Debussy is therefore one of the younger features on Mercury. It still has that youthful shine.
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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