Mercury is shrinking, its radius reduced as much as 4.3 miles over the past four billion years, scientists have confirmed, with the planet contracting as its iron core cools. The gradual reduction had been expected but unconfirmed until NASA's Messenger spacecraft beamed back photos of Mercury's surface, with previous - incomplete - footage of the closest planet to our sun showing far less shrinkage than the models predicted.
The first images came courtesy of Mariner 10, back in the mid-1970s, which spotted what are known as "lobate scarps" or deformities in the surface of the planet. However, Mariner 10 photographed less than half of Mercury's surface, and what was shown didn't tally with the scientific models.
What had been predicted was between three and six miles in reduction of Mercury's radius overall, as its crust solidified and contracted while internal temperatures of the liquid iron core fell and it began to turn solid. Mariner 10 footage, however, only indicated a 0.5 to 2 mile drop.
Messenger, however, went into orbit around Mercury back in 2011, and completed the photographic mission.
As well as the lobate scarps, lacing across the surface of the planet, the images found contraction wrinkles across Mercury's volcanic plains. Together, they allowed researchers to calculate a 3-4.3 mile reduction overall.
What makes Mercury interesting - and, indeed, different to Earth - is that its shell is one complete piece, rather than fragmented into tectonic plates. While that means Earth is unlikely to change in the same way, it could well allow us to read similar ripples in topography observed on other planetary bodies, the researchers suggest.
However the research into Mercury is nowhere near complete, and as Messenger gets closer to the surface of the planet, NASA expects to glean even more information. In fact, images at up to 10x the detail of current shots will be possible, allowing scientists to answer questions such as whether the distinctive hollows across the planet are, indeed, caused by volatile elements subliming off the surface.