The smallest planet in the Solar System has become even smaller, studies by the Messenger spacecraft have shown.
Data from a flyby of Mercury in January 2008 show the planet has contracted by more than one mile (1.5km) in diameter over its history.
Scientists believe the shrinkage is due to the planet's core slowly cooling.
Studies published in the journal Science show the same process also powers the planet's magnetic field, a topic long debated by scientists.
"Cooling of the planet's core not only fuelled the magnetic dynamo, it also led to contraction of the entire planet," said Principal Investigator Sean Solomon of the Carnegie Institution of Washington, US.