It is unknown, however, to what extent the planet is still shrinking today—and, if so, for how long that is likely to continue. Now our new paper, published in Nature Geoscience, offers fresh insight.

Because Mercury's interior is shrinking, its surface (crust) has progressively less area to cover. It responds to this by developing "thrust faults"—where one tract of terrain gets pushed over the adjacent terrain (see image below). This is like the wrinkles that form on an apple as it ages, except that an apple shrinks because it is drying out whereas Mercury shrinks because of thermal contraction of its interior.

The first evidence of Mercury's shrinkage came in 1974 when the Mariner 10 mission transmitted pictures of kilometers-high scarps (ramp-like slopes) snaking their way for hundreds of kilometers across the terrain. Messenger, which orbited Mercury from 2011 to 2015, showed many more "lobate scarps" (as they had become known) in all parts of the globe.

From such observations, it was possible to deduce that gently dipping geological faults, known as thrusts, approach the surface below each scarp and are a response to Mercury having shrunk in radius by a total of about 7km.

But when did this happen? The accepted way to work out the age of Mercury's surface is to count the density of impact craters. The older the surface, the more craters. But this method is tricky, because the rate of impacts that produce craters was much greater in the deep past.