Mercury is still shrinking; new study reveals the planet has contracted by up to 11 kilometers since formation

Mercury, the smallest planet in the solar system, is still slowly contracting as it cools. Now, new research offers the clearest picture yet of how much the planet has shrunk since its fiery birth 4.5 billion years ago.

According to findings published in AGU Advances, Mercury’s radius has decreased by 2.7 to 5.6 kilometers over its lifetime, narrowing down earlier estimates that ranged from 1 to 7 kilometers.

Like a baked good that shrinks as it cools, Mercury’s rocky shell has been forced to adjust as the planet’s interior lost heat. This process created massive thrust faults, cliff-like scarps where sections of the crust have been pushed upward.


Scientists studied the fault system and then measured the degree of contraction the planet has undergone. However, earlier approaches that calculated shrinkage based on the length and height of landforms often produced inconsistent results.

A new way of measuring shrinkage

To solve this, researchers Stephan R. Loveless and Christian Klimczak applied a fresh method. Instead of tallying up every fault, they focused on how much the largest fault in each dataset could account for contraction and then extrapolated that across the planet.

They tested this approach on three different datasets: nearly 6,000 faults, another with 653, and a smaller set of 100. Remarkably, all pointed to the same result, about 2 to 3.5 kilometers of shrinkage from faulting alone. When combined with additional cooling-driven processes, the total contraction reaches up to 5.6 kilometers.

Why is mercury shrinking in the first place?

Mercury is shrinking because its interior has been cooling since the planet first formed about 4.5 billion years ago. When a planet loses heat, its volume contracts, much like metal shrinking as it cools. Mercury, with its unusually large iron core making up most of its volume, sheds heat more quickly than larger, rockier planets like Earth. As the core and mantle contract, the planet’s crust is forced to adjust to the smaller volume beneath it.

Altogether, this process has reduced Mercury’s radius by about 2.7 to 5.6 kilometers, meaning its diameter has shrunk by as much as 11 kilometers since its formation.

What it means beyond mercury

The study not only sharpens our understanding of Mercury’s thermal history but also highlights a new tool for planetary science. The same methodology could help investigate tectonics on other rocky worlds, including Mars, where vast fault systems also mark the surface.