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SYFY WIRE Bad Astronomy

An unusual view of the Death Star moon

By Phil Plait

If you showed me this picture with no preamble, I'd know it was from Cassini, and showed a moon of Saturn, but I'd be baffled as to which one it is:

[Click to ensithenate.]

If you told me it was Mimas, I'd be surprised... and I was when I saw it! But it's true. Mimas is a 400 km ball of mostly ice (and some rock) orbiting Saturn about 180,000 km out. From this angle, Cassini was looking down at the north pole from a steep angle, and that's not how we usually see it.

This is how we usually see it! The giant crater Herschel dominates the face of the moon, giving it as definite Vaderesque feel. But in the big image the crater isn't visible, so the landscape looks markedly different. This image was processed by Ian Regan, who notes that the blue band you can see around the edge of the moon is real. It's a bluer region that wraps around the equatorial regions of Mimas, the origin of which is still something of a mystery. However, it does match the very odd thermal pattern seen by cameras on board Cassini, a pattern that makes Mimas look like a giant PacMan in the sky.

Cassini takes so many pictures it's ironically not surprising that some will be surprising. Still, when it comes to astronomy, surprises are fun. They're also a chance to learn something -- as I did since I didn't know about the blue band. And I also learned that even a familiar place can look very different if you get a different angle on it.

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