Mean Radius of Mercury

For a slowly rotating rocky body a single mean radius is the honest figure: Mercury's 59-day spin raises no measurable equatorial bulge, so its equatorial and polar radii agree to within a kilometre and the volumetric mean — the radius of a sphere of equal volume — describes it well. The modern value comes from MESSENGER's laser altimeter, which ranged the northern hemisphere from orbit between 2011 and 2015, and from radio occultations for the south.

The radius is not quite constant. Mercury's enormous iron core has been cooling and solidifying for four billion years, and as it contracts the crust wrinkles over it like the skin of a drying apple. The evidence is written across the surface as lobate scarps — cliffs hundreds of kilometres long where one block of crust has thrust over another — and adding them up implies the whole planet has lost some 7 km of radius since its surface formed.