Wrinkled Mercury's shrinking history

Planet Mercury is about 7km smaller today than when its crust first solidified over four billion years ago, according to BBC Sunday.

The innermost world has shrunk as it has cooled over time, its surface cracking and wrinkling in the process.

Scientists first recognized the phenomenon when the Mariner 10 probe whizzed by the planet in the mid-1970s.

But the latest images from the U.S. space agency's Messenger satellite have enabled researchers to refine their estimate for the amount of contraction.

And, as they report in the journal Nature Geoscience, it is significantly more than previously realized.

Mariner made two passes of Mercury, in 1974 and 1975, photographing about 45 percent of the planet's surface.

Evident in those pictures were long scars where rock had been thrust upwards as the body shortened.