The planet Mercury has shrunk in size by as much as seven kilometres across its radius over the past four billion years, according to a study published online in Nature Geoscience. The findings suggest that Mercury has contracted much more than previously estimated.

Paul Byrne and colleagues mapped ridges and faults across the surface of Mercury using images acquired by the MESSENGER spacecraft. These geologic structures are thought to have formed through the buckling and fracturing of the planet’s crust, due to compression as the initially hot planet cooled and contracted. The researchers added up the total amount of crust displaced by the ridges and faults, and found that the total reduction of Mercury’s surface is equivalent to a loss of up to seven kilometres of the planet’s radius.

Intriguingly, the observed deformation of Mercury’s surface is consistent with a nineteenth-century theory for a shrinking Earth. This theory has long been obsolete for our planet, as Earth’s surface is broken into plates, but Mercury is enclosed by a single plate and cannot lose its heat through plate tectonics processes.

In an accompanying News and Views article, William McKinnon writes that, “Mercury provides an example of what may really happen to a planet that is shrinking.”