Huh. I always thought Mercury was mostly made of rock, but this picture — taken by the Messenger probe in 2011 but only released by NASA last week — clearly shows the frozen form of Han Solo trapped inside it, so it obviously must be made of carbonite. That's just science, people.
NASA themselves offered this keen insight in the release accompanying the pic, which they awesomely titled "He Will Not Be Permanently Damaged":
If there are two things you should remember, it's not to cross a Hutt, and that Mercury's surface can throw up all kinds of surprises. In this image, a portion of the terrain surrounding the northern margin of the Caloris basin hosts an elevated block in the shape of a certain carbonite-encased smuggler who can make the Kessel Run in less than twelve parsecs. This block may be part of the original surface that pre-dates the formation of Caloris, which was shaped by material ejected during the basin-forming event. The act of seeing a meaningful shape in random landforms is a form of pareidolia — and has been seen for Mercury more than a few times before...
I don't know why NASA would mention pareidolia in a picture of what is clearly Han Solo trapped in Mercury's surface, but whatever.