Mercury has shrunk far more than scientists thought, new study finds

Mercury, the smallest and innermost planet in the solar system, has shrunk considerably more than scientists had calculated, according to a study published this week that revisits four decades of spacecraft imagery and finds that debris from ancient impacts has been masking the true scale of the planet's contraction.

The new analysis, led by planetary scientist Gaku Nishiyama of the German Aerospace Center (DLR), concludes that Mercury's diameter has shrunk by as much as 23 kilometers since the planet formed roughly 4.5 billion years ago, 10 to 30 percent more than the estimate derived from NASA's MESSENGER mission a decade ago. The paper appears in the journal Geophysical Research Letters and was announced this week by the American Geophysical Union, which publishes the journal.

The numbers behind the shrinkage

Like most rocky planets, Mercury has been cooling and contracting since it coalesced from the early solar nebula. As its interior cools, the planet's surface crumples and cracks, producing long, cliff-like ridges called lobate scarps that mark where the crust has buckled inward. Since NASA's Mariner 10 flybys in the 1970s, and more thoroughly since the MESSENGER spacecraft orbited Mercury between 2011 and 2015, scientists have used the length and height of these scarps to estimate how much the planet's radius has decreased.

Earlier work using MESSENGER data put the total contraction at somewhere between roughly 4 and 16 kilometers of diameter loss, figures widely cited as a benchmark for Mercury's thermal history. The new study argues that figure has been an undercount. By cross-referencing existing maps of contractional landforms with new measurements of surface roughness, Nishiyama's team found that heavily cratered terrain systematically shows fewer visible scarps than smoother regions nearby, not because the crust shrank less there, but because debris thrown out by impacts has buried or worn down the telltale wrinkles. Extrapolating from the less-disturbed regions, the researchers calculate that total contraction could run as much as 30 percent higher than earlier tallies, pushing the upper estimate to about 23 kilometers, or roughly 14.5 miles, of diameter loss.

How scientists arrived at the revised figure

The recalculation matters because Mercury's contraction is one of the few direct, visible records of a rocky planet's interior cooling that scientists can measure from orbit. "More shrinking means Mercury could have a larger metal core, fewer light elements like silicon mixed into the metal core, or a higher starting temperature," Nishiyama said in comments released by the American Geophysical Union, describing how the revised figure forces a rethink of assumptions about the planet's core composition and its cooling rate over billions of years.

Mercury's unusually large iron core, thought to make up roughly 85 percent of the planet's radius, has long made it an outlier among the rocky planets and a useful test case for models of how terrestrial planets, including early Earth, cool and evolve. A planet that has shrunk more than expected implies either a hotter starting point, a core with fewer light elements able to slow the cooling process, or some combination of the two. Nishiyama told Scientific American that the next step is working out what the new number implies for Mercury's overall thermal evolution, calling it "the next question that we have to tackle."

Reaction from outside researchers

The findings have drawn a favorable, if cautious, response from planetary scientists who were not involved in the work. Paul Byrne, a planetary scientist at Washington University in St. Louis who studies Mercury's tectonics, said the study adds a previously overlooked variable, the way impact debris obscures geological evidence, to the toolkit researchers use to read a planet's history from orbital images alone.

"If their results hold, and I'm confident they will, then we'll have taken a step further in investigating planets we can't yet visit in person."

That comment, made to Scientific American, reflects a broader stake in the finding: nearly all that is known about Mercury's interior has been inferred remotely, first from Earth-based telescopes, then from three flybys by Mariner 10 in the mid-1970s, and later from MESSENGER's four-year orbital survey, the first and so far only spacecraft to circle the planet. No lander or seismometer has ever touched its surface, so scarps mapped from orbit remain the closest thing scientists have to a seismic record of Mercury's interior. Byrne suggested the same debris-correction approach could eventually be applied to other airless, heavily cratered worlds, remarking that researchers should "do those worlds next" in reference to the Moon and Mars, both of which show similar contractional ridges and face the same problem of impact debris masking their true extent.

What happens next

The revised estimate arrives at a pointed moment for Mercury research. The European Space Agency and the Japan Aerospace Exploration Agency's joint BepiColombo mission, which has spent nearly eight years cruising toward the innermost planet, is scheduled to enter orbit around Mercury in November 2026, with its two orbiters separating in December and full science operations beginning in 2027. BepiColombo's cameras are expected to resolve surface features far smaller than MESSENGER could detect, filling gaps in the existing maps that are currently reliable only for features larger than about 5 kilometers across.

That resolution gap is expected to narrow once BepiColombo begins its survey, offering a way to check whether previously unresolved terrain does in fact hide the smaller scarps the new model predicts. NASA's own MESSENGER mission archive, which ended when the spacecraft was deliberately crashed into Mercury's surface in 2015, remains the primary dataset for the current study and will serve as the baseline against which BepiColombo's sharper images are compared.

For now, the paper leaves open exactly how much of Mercury's thermal history needs to be rewritten. But the underlying method, treating a lack of visible geological evidence as a data point in itself rather than an absence of information, is likely to be applied well beyond Mercury as planetary scientists reexamine other bodies where impact gardening may be hiding a fuller record of how rocky worlds have changed over billions of years.