Last year, Cassini captured some truly stunning images of Saturn’s “Death Star moon”, Mimas. (It’s such a cliché to call it that, so I cast around for an alternative title.) The two images show why we sometimes call it that, and a more oblique view in which the giant crater Herschel is seen in a kind of profile. Scientists believe the crater is the relic of an ancient impact that nearly shattered the moon. A crater of this size relative to Earth would be wider than the lower United States!
As others (notably Phil Plait) have pointed out, the crater in profile is rather flat-looking. But another thing that struck me in this image is how nonspherical Mimas is: you can see that very clearly if you overlay a circle on top of the image. To really see this, I rotated the image by 75º and zoomed in; my altered image can be found below the fold.
Objects like planets, stars, and moons are governed by a delicate balance between the attractive force of gravity and the repulsive force atoms exert on each other, which manifests itself in the form of pressure. If you have enough mass in one place, this balance of forces tends to make objects spherical; rotation makes objects flatten at the poles and bulge at the equator, but the shape is still mostly spherical. (For those who like fancy geometrical language, the term for this is oblate spheroid.) Mimas isn’t flattened in this way: it’s actually distorted in the opposite direction (prolate for the math buffs following at home).
I don’t know what the consensus among planetary scientists is about whether the impact that created the Herschel crater is responsible (at least in part) for the elongated shape. In this orientation, it looks like axis of elongation is perfectly aligned with the impact—but that could very well be an illusion, caused by the fact that we’re seeing a three-dimensional object in a two-dimensional image. If any planetary scientists reading this care to enlighten me, I’d be thrilled to hear what you have to say.
Whether or not the non-spherical character of Mimas is due to the impact, this is still a stunning image of a very interesting object, and I’m as always excited to see what new discoveries the Cassini mission makes in its orbit around Saturn.
(This post originally ran last year, though I have substantially rewritten it and added some new images.)