Mercury Has Shrunk by Several Kilometers Over Billions of Years, Scientists Report

Mercury, pointed to have been created approximately 4.5 billion years ago, is the smallest planet in the solar system. And similar to how a loaf of bread collapses as it cools, Mercury, too, has cooled; that slow contraction resulted in thrust faults all across its rocky cover. How much this shrinkage has happened has long been a bone of contention among scientists, as these cracks are what cause the planet to slowly shrink. Earlier estimates vary widely, suggesting Mercury’s radius might have contracted anywhere from 1 to 7 kilometres—about the height of a small mountain.

New Fault-Based Method Reveals Mercury Has Shrunk by Up to 5.6 Kilometers

According to a study published in AGU Advances, researchers led by Thomas R. Watters and Christian Klimczak developed a new approach to refine these estimates. Unlike older methods that relied on fault counts and boost measurements—often spitting out inconsistent numbers—the new approach measures how much the biggest fault in the dataset explains a planet’s slow, steady contraction. The effect ripples through Mercury’s global network of faults, thin cracks etched into ancient, sun-baked rock. This makes it easier to see how the planet has been becoming smaller over time.

The team ran their model on three datasets, from a hefty 6,000 recorded faults down to just 100. Scientists found that the faults by themselves had tugged the surface of Mercury in by about 2 to 3.5 kilometres—about the size of a mountain ridge—and that when they combined other changes caused by cooling, the planet’s total shrinkage came to 2.7 to 5.6 kilometres.

But tracing Mercury’s slow desiccation and warping over time, the scientists poked holes in old models and emphasised the importance of finer measurements and more creative ways to probe the planet’s geology.

Researchers emphasised that similar techniques ultimately could be employed on Mars and other rocky planets, providing new clues about their tectonic shifting and the flows of heat; it could revolutionise our perspective of the solar system.