The European Mars Express probe continues to amaze. Check out the image above: it's one piece of an excruciatingly large image (warning: huge download of a 5500x2500 pixel image!) of Apollinaris Patera, a volcano on Mars. In the above picture, all you're seeing is a smallish section of the caldera-- maybe 22x18 kilometers (roughly 14x11 miles).
The obvious thing to see is the double crater. What could cause such a thing? My first thought was, unlikely as it might be even on the heavily-battered surface of Mars, one crater was already there, and another impact caused the second crater. I even figured the crater on the upper right was older: it has more smaller craters in it, while the lower left crater is more pristine. Older stuff has more time to accumulate craters inside.
But this seemed unlikely to me, since both craters are about the same size, and they don't really overlap: where they meet, the rims are straight.
The problem here is that we're too close; we're too zoomed in. Take a step back, as it were, and check out the overview:
Now take a look at the double crater in context. See the raised rim in a circle all the way around both craters? You get features like that when an impact melts ice under the surface, causing mud flows (volcanoes can make this pattern as well, but those two guys are pretty clearly impact craters). So these craters formed at the same time. It looks to me like this was a binary asteroid or a comet, an object that had either split in two or was two objects in orbit around each other. They would have hit almost simultaneously, blowing out those two pits and causing that apron of outflow material. The presence of smaller craters inside the upper right one may just be coincidence.
Or, I might be totally wrong. If this were a supernova remnant I'd be on more solid ground (ironically, I suppose), but areology isn't my forte. Still, the point is there is a lot to see on Mars, and with the new probes going there taking incredible high-resolution shots (this one is at 11 meters per pixel!) it sometimes helps to step back a little and get -- literally -- a bigger picture.
P.S. For those with broadband, and I mean broadband, there is a 14 Mb 3D anaglyph of this image as well that is way cool.'