Check. This. Out. What an amazing shot from Cassini!
Holy Haleakala! How gorgeous is that?
This picture really threw me for a minute. I couldn't figure it out! Obviously, you're seeing the night side of Saturn; the planet itself is an almost entirely dark disk. The bright curve you see at the edge is sunlight scattered by the upper atmosphere -- the break in the curve near the top is from the shadow of the rings!
But you can also see the moon Enceladus, too: the plumes of water geysers off the southern pole (at the bottom) are obvious. When this shot was taken, Cassini had to look past Saturn to see Enceladus; that is, the moon was farther from Cassini than the planet was.
OK, cool enough, but the problem is Enceladus looks like it's full, as if we're seeing the side completely lit by the Sun. How can that be? If we're seeing the dark part of Saturn, the Sun must be on the other side -- in other words, we're facing toward the Sun in this picture, and it's blocked by Saturn itself. But if that's true, there's no way Enceladus can be fully lit. The Sun would have to be between the planet and the moon! Draw yourself a picture if that helps. I was honestly baffled about this, until I read the caption for the picture:
Enceladus and its plumes have been brightened by a factor of two relative to the planet and rings.
Aha! The astronomers artificially brightened the part of the image with Enceladus to make it easier to see. That's why it looks so bright. And that also explains why it looks full: it's being illuminated by Saturn itself! If you were standing on Enceladus in the middle of the part we see here, it would be midnight, and you'd see a full Saturn directly overhead. It's a big, bright planet, so it would illuminate the ground just like a full Moon lights up the ground here on Earth.
Phew! I thought I was losing it for a second. The geometry of the Cassini pictures can be pretty confusing sometimes, and I thought I was totally lost with this one. Nice to know my confusion was earned honestly.