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Update II: Emily has a lengthy discussion of this image, with some typically fine insight.
Whoa. Just: whoa.
This is the highest res image yet returned from the MESSENGER spacecraft of Mercury, taken when it was 27,000 kilometers from the planet (after the encounter).
Mercury rotates three times on its axis (a "day") for every two times it goes around the Sun (a "year"), so there is all sorts of weird phasing that goes on (something like the Moon always showing the same face to Earth, but a bit more complicated). Not only that, but its orbit is highly elliptical, ranging from 46 to 70 million km from the Sun. All this together means that the same half of Mercury is facing the Sun every other* time it reaches perihelion (closest point in its orbit to the Sun).
Do you see that circular feature to the upper right, slightly lighter in color than the surrounding plains? That's Caloris Basin, the Basin of Heat. It's called that because when Mercury reaches perihelion, that spot is close to being aimed right at the Sun. It's one of the worsts spot on Mercury to live, if you're considering a winter home there. Caloris Basin (formed in some ancient and very large impact) is over 1300 kilometers across -- roughly as big as Texas. The mountains ringing it are 2 km high.
Because of the weird spin/orbit coupling, when Mariner 10 passed Mercury in 1974, it never was able to see the western rim of Caloris Basin. But there it is, right there, seen by human eyes for the very first time.
The cratering is lovely. Mercury is literally saturated with craters; a new impact is likely to wipe out more craters than it would form. There are rays visible, too, streaks of material ejected after an impact. We see those on the Moon as well. It looks like there are more craters near the left edge, but I think that's an illusion; to them the Sun is low on the horizon and rim shadows are longer. That exaggerates the crateriness of the craters there, while ones under the Sun have short shadows, making them hard to see.
All in all, wow. Wowee wow wow.
And this is just the first close up image! We'll be getting more in a few days, and then more again in the next flyby, and then, in 2011, we'll get as many images of this tiny, hot, battered, dense and neglected planet as we can handle.
Look what we did! Man, I love this stuff.
* Well, nuts. Commenter Solomon pointed out I messed up this description; I originally left the word "other" out. Oops. I put it in, and thank Solomon for finding this!