Mercury

Mercury is a rocky planet, but of the four in our System, it is unique. Whereas Earth, Venus and Mars have a iron core that makes up about 30% of their mass, Mercury's core makes up about 60% of hers. Scientists have puzzled for years, trying to piece together the riddle of what happened to Mercury's crust, why is her gut so heavy.

Back in 2011 we sent a probe over to the planet and have been studying her chemical make up for some time. What scientists have found (read the full article at Space.com) has led to some new theories. The latest, that the planet suffered from a massive collision back in the early days of the System's history and this blasted away most of her crust.

Interestingly the planet has lots of salt on it. Which makes me fell better, as I love salt, and I worry we might one day run out.