Smallest planet Mercury 'is getting smaller'
17 March 2014 Last updated at 06:20 GMT
The planet Mercury is about 7km smaller today than when its crust first solidified over four billion years ago, a Nasa mission has revealed.
The solar system's innermost world has shrunk as it has cooled over time, and the planet's surface has been cracking and wrinkling in the process.
Pallab Ghosh reports.