Mercury has shrunk, and by much more than the visible traces on its surface suggested.
The planet closest to the Sun has been cooling since its formation, about 4.5 billion years ago. As it loses its internal heat, its volume gradually decreases. Its rocky surface must then adapt to a smaller globe: it folds, fractures and forms long cliffs. These landforms therefore preserve evidence of Mercury’s shrinking.
View of a large escarpment formed by a fault on Mercury, attributed to the shrinking of the planet’s core.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
These features should be fairly widely distributed. Yet some regions of Mercury have far fewer of them than others. To measure this phenomenon, scientists map these folds and cliffs, then calculate the amount of terrain that has been compressed.
The researchers found that the most rugged terrains contain fewer visible traces of compression.
Craters provide a concrete explanation. When a large object strikes Mercury, the impact ejects enormous quantities of rock around the collision point. This debris falls back to the ground and creates an uneven layer. Older folds and cliffs can then become partially buried, making them difficult to identify in images and topographic maps.
The phenomenon appears notably around the Rachmaninoff crater. Features linked to shrinking become less numerous in terrains covered by impact debris. After correcting for this bias across Mercury, the team estimates that the contraction was underestimated by 10 to 30%. The decrease in radius could reach about 11.6 km, or approximately 23 km across the diameter.
This difference matters for understanding the planet’s interior. The amount of contraction depends on the heat lost over time and Mercury’s internal composition. Greater contraction also brings the observations closer to the values predicted by physical models. The figure of 11.6 km could still be lower than the actual contraction.
The data used come mainly from NASA’s MESSENGER probe, whose mission ended in 2015. They primarily allow reliable study of landforms exceeding about 5 km. Smaller structures may therefore still be missing from current maps.
New measurements should soon improve this estimate. The European and Japanese BepiColombo mission is due to study Mercury from orbit with instruments capable of examining its terrain in greater detail. Its BELA laser altimeter will be able to measure terrain irregularities on a more precise scale and search for other traces of contraction that are currently difficult to detect.