Planetary Science: Mercury may have shrunk less than previously thought

The planetary contraction of Mercury as it cooled after the Late Heavy Bombardment may have reduced the planet’s radius by no more than 1 to 2 km, according to a paper published in Communications Earth & Environment. This is much less than the previously estimated 7 km contraction, and suggests that the planet may have retained more of its primordial interior heat.

Mercury’s crust is formed of a single plate, unlike the interlocking tectonic plates of Earth. After a period of heightened asteroid collisions with the inner planets of the Solar System around 3.9 billion years ago, known as the Late Heavy Bombardment, the planet began to cool and contract in on itself, forming ridges and faults across the surface.

Thomas Watters used orbital images from NASA’s MESSENGER mission and global topographic data to re-map tectonic landforms and estimate the overall reduction in the planet’s radius. Watters observed a higher concentration of faults and other tectonic landforms in the southern hemisphere relative to the northern hemisphere. The smaller reduction in the planet’s radius than previously estimated is due to more definitive identification of Mercury's tectonic landforms.