Last week, the MESSENGER spacecraft passed the solar system's smallest planet for the third and final time; when they next meet it won't be some quick fling, it'll be for a long term relationship.
Several gorgeous images were returned from the spacecraft, but this one is my favorite so far:
This is a large impact basin about 260 km (160 miles) across. It's never been seen before! Only one other spacecraft has visited Mercury before -- Mariner 10 in 1974 -- and its orbit was such that it never did see many parts of the planet. MESSENGER was in the right place at the right time to snap this picture.
Note that it's a double ringed crater. It's not quite clear why these features form. It may be due to the forces generated upon impact, when a shock wave travels through the rock and rebounds inside the crater, or it may be from subsequent volcanic flows. Double rings are only seen in large impact events, so that must have something to do with it. You can also see concentric troughs or cracks in the crater middle. Those are due to the stretching of the crater floor after the impact.
Other images of Mercury from this third pass are just as cool: a bright splash around a double crater (seen here; most likely lighter material under the surface blasted out on impact), a crater with an elongated pit in its floor that makes it a pretty good smiley face, and a lovely shot of the northern limb of the planet spattered with craters.
I imagine they'll release a handful more images over the next few days, but that'll be it for the most part until March 2011, when MESSENGER meets up with Mercury one last time, settling into orbit... and then we'll see lots more images. Lots and lots more... and they'll be even higher resolution than these. What wonders will we see then?