Mercury has shrunk by up to 30 percent more than scientists previously estimated, according to a study published in September 2026 in the journal Geophysical Research Letters. Impact debris and cratering have buried the planet’s tectonic shrinkage wrinkles, leading researchers to recalculate its total diameter loss to nearly 14.5 miles.
Our solar system’s smallest planet is hiding a dramatic contraction.
Slightly larger than Earth’s moon, Mercury is the closest planet to the sun and the fastest—completing one circle of our star once every 88 days, according to NASA. Formed about 4.5 billion years ago from swirling gas and dust in the chaos of our solar system’s early days, the tiny rocky world was shaped by violent forces that generated massive amounts of heat. Similar to other rocky, terrestrial planets, Mercury has a central metallic core, a rocky mangle and a solid crust. As its heavy iron core cools, the entire planet shrinks like a balloon left out in the cold, or a grape transforming into a raisin in the sun. Its outer crust buckles and cracks around it, but on a planet scale, those surface wrinkles translate into huge cliffs, mountainous ridges, and tectonic features such as scarps.
How Impact Debris Hides Mercury’s Tectonic Wrinkles
In theory, a cooling planet should contract uniformly, making “wrinkle” features, known as shortening structures, similarly common everywhere. Instead, researchers discovered a clear pattern while mapping the planet’s geology: the rugged regions contained the fewest visible wrinkles.
A barrage of meteors pocks Mercury’s surface, cratering the land and hurling giant shattered rock across the planet. The rubble seems to create a fresh gravel spread, concealing the planet’s wrinkles. By comparing a global map of Mercury’s surface roughness with maps of shortening structures and contraction, scientists noticed that around major impacts—like the massive Rachmaninoff crater—tectonic cracks disappeared almost entirely under thick layers of debris.
The rubble acts much like debris obscuring the true extent of the damage. To account for these obscured regions, researchers provided a fresh take on Mercury’s evolution, publishing their findings in the journal Geophysical Research Letters.
Rewriting the Numbers on Planetary Contraction
Previous estimates for decades may have underestimated the planet’s contraction because the rough terrain was obscuring the true extent of the damage. The updated research suggests that Mercury may have contracted 10% to 30% more than previously believed.
This equates to a loss of nearly 12 miles (19 kilometers)—or up to 14 miles or 14.5 miles—of total diameter since the planet formed, which is significant for a planet barely 3,000 miles across overall. Gaku Nishiyama, a planetary scientist at the German Aerospace Center Institute of Space Research and lead author of the study, noted in a statement that the corrected figures resolve long-standing questions about planetary cooling physics.
“Thirty percent is a little bit surprising, but the corrected amount of contraction actually makes sense to me,” said Gaku Nishiyama, a planetary scientist at the German Aerospace Center Institute of Space Research and lead author on the study, in a statement.
Mercury appears to have shrunk considerably more than what the visible tectonic record alone suggested,
Nishiyama said. Nishiyama and his team noted that the shrinkage could be even more than his team is estimating based on measurements from NASA’s Messenger spacecraft in the 2010s, building upon historical data from only one other spacecraft that has ever visited Mercury, NASA’s Mariner 10 in the 1970s.
Implications for Mercury’s Massive Metal Core
Understanding the degree of shrinking is important for helping researchers infer key features of Mercury’s interior and evolution. Study lead author Gaku Nishiyama stated that More shrinking means Mercury could have a larger metal core, less light elements like silicon mixed into the metal core, or a higher starting temperature.
What BepiColombo Will Reveal Next in November
Scientists will soon have an opportunity to test these revised calculations using fresh observational data. The news comes one week after a joint European and Japanese mission—known as BepiColombo, where a pair of European and Japanese spacecraft shed its cruising platform and advanced toward Mercury—is expected to enter orbit around Mercury in November before splitting up for a fuller survey.
BepiColombo’s laser instrument should confirm how much Mercury is withering as a result of internal cooling, said Gaku Nishiyama, the study’s lead author, who is taking part in the space mission.
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