New theories about Mercury's movements around Sun
16 February 2013 Last updated at 09:15 GMT
Scientists working on the Nasa mission to Mercury have shown off the most detailed picture ever taken of the planet - and say it may have moved closer to the Sun since it formed.
The Messenger probe has found high levels of sulphur and potassium, volatile elements which should not really be present on the rocky little planet if it formed so close to the Sun.
Pallab Ghosh reports from Boston.