The other day I posted a great picture of Saturn and its rings taken by Cassini. While digging around in my archives looking for other posts about the rings, I found one from earlier this year that had a picture of the icy moon Enceladus with the rings in the background. When I saw the picture, I got a jolt: there was a crater chain on the surface that looks just like the one on the asteroid Vesta! Here's a side-by-side comparison:
Enceladus is on the left, Vesta on the right (click those links for higher-res shots). Pretty cool, huh? You can see both have two big overlapping craters of roughly the same size, and a smaller third one roughly aligned on top. The set on Vesta is nicknamed -- for obvious reasons -- "Snowman". Craters like this form when the impacting object is not a single body; for example, many asteroids are known to be binaries, with both objects about the same size. Getting hit by that would leave two craters either very close together or overlapping, depending on the sizes, distances, and velocities of the impacting bodies. Sometimes, too, there are long chains of many craters, sometimes dozens. We see those on the Moon and Mercury, for example, and they may be from comets that have disintegrated into many pieces before they hit, like the comet Shoemaker-Levy 9 did before it whacked Jupiter over and over again in 1994. The impacts on Vesta and Enceladus look remarkably similar. But I wonder. The two big craters on Vesta both have lots of shared characteristics: size, sharp rims, and so on. They're the two biggest craters on Vesta, so it would be very unlikely to get them so close together unless they were from the same event. But the third crater has a softer rim (implying greater age due to erosive forces like the solar wind and smaller impacts over eons), is smaller, and doesn't quite line up with the other two. There are also several craters that size on the surface. It's possible it's unrelated to the other two, and coincidentally nearby. Enceladus, though, looks like all three are related. Even though one is smaller, it lines up pretty well with the other two and has similar features. Maybe this really was a triple-system that hit. The asteroid Kleopatra, for example, has two moons (though I must note Kleopatra is big, which helps it hold on to two moons; an impact from something like that would come close to shattering a moon like Enceldaus). I have no real scientific conclusion to draw here, except that multiple-body asteroids and comets are certainly more common than we might have thought 20 years ago. It's amazing that the evidence for their existence was literally carved into the surfaces of other big bodies out there. With all of this new and marvelous imagery we're getting from our robots plying the solar system, I wonder what other things we'll learn as we build up this huge database of pictures?