Mercury Shrank Up to 30% More Than Scientists Thought, Study Finds

Mercury may have shrunk by as much as 11.6 kilometers, roughly 7.2 miles, more than the 8.3 kilometers scientists previously calculated. A new study suggests that debris from ancient asteroid impacts, along with other effects tied to rough terrain, may be hiding some of the evidence researchers use to measure how much the small, rocky planet has contracted, meaning earlier estimates may have undercounted its shrinkage by as much as 30 percent.

Mercury cools from the inside out, and as it cools, its interior shrinks. That shrinking squeezes the outer crust, which wrinkles and cracks the way a grape wrinkles into a raisin. These wrinkles show up as long cliffs and ridges on Mercury’s surface, and for decades, scientists have measured them to figure out how much the planet’s radius has shrunk over time. Earlier estimates ranged from as little as one to two kilometers up to seven kilometers, depending on how researchers interpreted smaller features in the tectonic record.

Those numbers have always raised a nagging problem. If Mercury shrank evenly all over, its cliffs and ridges should be spread out fairly evenly across the whole planet. Instead, some regions are packed with them while other areas appear almost blank, a pattern that never quite added up. Researchers reporting in the journal Geophysical Research Letters wondered whether the missing structures weren’t actually missing. Perhaps they had simply been buried or scrubbed away by crater debris, meaning the true amount of shrinking has been underestimated all along.

How Scientists Measure Mercury’s Shrinking Surface

Mercury’s cliffs and ridges, sometimes called shortening structures, form when the planet’s crust buckles under pressure as the interior cools and contracts. Mapping these features has long been the main way scientists estimate how much Mercury’s radius has decreased since it formed. Getting that number right matters because it feeds directly into models of the planet’s internal heat, its core, and its geological history.

Researchers on this project took a different approach to the puzzle of the missing cliffs. Instead of just mapping fault lines, they built a global map of surface roughness, a measure of how bumpy or smooth different patches of Mercury’s terrain are. For this analysis, roughness served as a clue to how recently a patch of ground had been disturbed. Impacts leave behind rough ejecta and secondary craters, while later landslides, triggered by shaking from distant impacts, gradually soften that relief.

To build the roughness map, the team combined laser measurements collected by an instrument aboard NASA’s MESSENGER spacecraft with an image-based terrain model of the whole planet. Because the laser data mostly covered Mercury’s north polar region, the image-based model filled in gaps across the equator and southern hemisphere, giving the researchers a complete, planet-wide picture for the first time. They focused on roughness measured over 10-kilometer stretches of ground, a scale known to be sensitive to fresh crater debris. They then overlaid that roughness map on an existing catalog of Mercury’s mapped cliffs and ridges, letting the team see, region by region, whether rougher terrain had fewer tectonic features than smoother terrain.

Where Mercury’s Wrinkles Go Missing

Results showed a clear pattern. Cliffs and ridges were consistently less common in rough regions and more common in smoother ones, a relationship that held up across the whole planet. Around large, relatively young craters such as Rachmaninoff, rough debris rings had few mapped tectonic features, while the smoother surroundings had plenty. Statistical tests confirmed the pattern was not random noise.

Researchers laid out three possible explanations. Crater debris might physically bury older cliffs and ridges. High background bumpiness might simply make it harder for scientists studying images to spot a subtle ridge in chaotic, broken terrain. Or, in especially battered ground, the crust itself might behave differently, soaking up the shrinking quietly through internal compaction rather than forming large, visible faults. Observations near some large craters lined up best with debris burial, but the authors stopped short of naming a single cause for the planet-wide pattern.

Correcting Mercury’s Shrinkage Number

Using the relationship between roughness and mapped contraction, researchers built a correction. They divided the planet’s roughness values into groups and looked at how much contraction was measured in each one, separately for smooth plains and heavily cratered plains. Below a certain roughness level, more bumpiness actually came with more measured contraction, since the ridges themselves add some texture to the ground. Beyond that point, however, rising roughness came with a steep drop in measured contraction, suggesting real tectonic features were being hidden rather than absent.

Applying that threshold as a correction, the team estimated that earlier calculations of Mercury’s shrinkage had been underestimated by as much as 3.3 kilometers (about 2 miles). A previous estimate of 8.3 kilometers (roughly 5.2 miles) of shrinkage rose to 11.6 kilometers (about 7.2 miles) after correcting for roughness bias. Even under a more cautious version of the analysis that only counted the most obvious ridges, the underestimate still came out to 0.6 kilometers. Researchers concluded that roughness effects have skewed global contraction estimates downward by about 10% to 30%, meaning Mercury’s true shrinkage is likely larger than 6.9 kilometers.

Why Mercury’s Shrinking Matters Beyond Mercury

A revised, larger contraction number carries weight for models of Mercury’s interior, including how much heat-producing material it holds, how thick its outer rubble layer is, and how much sulfur sits in its core, all of which shape how fast the planet cools and how its core has changed over time. The study’s authors note their correction favors a thinner surface rubble layer and lower sulfur content in the core than some earlier models suggested.

This roughness bias isn’t unique to Mercury. Earth’s Moon has even bumpier terrain than Mercury’s cratered plains, and lunar cliffs have previously been used to estimate less than a kilometer of lunar shrinkage, a number that has never quite matched what heat models predict. Researchers behind this study suggest that hidden, roughness-obscured cliffs could help explain that mismatch on the Moon too, and that Mars might face similar issues, though wind and water erosion there complicate the picture. Upcoming high-resolution observations from the joint European-Japanese BepiColombo mission could help researchers refine these estimates and fill in more of Mercury’s tectonic record.

Mercury’s wrinkled, shrunken face has always told a story about its cooling interior, but this study makes clear that story has been incomplete. Crater debris and rough terrain appear to do more than decorate the surface; they may also cover up some of the very clues scientists depend on. Getting an honest count of how much smaller Mercury has become is more than bookkeeping. It reshapes how researchers understand the deep, ongoing cooling of a planet that has been shrinking since it formed billions of years ago.