Mercury has likely shrunk 10 to 30 percent more than previously estimated, according to a study published on Thursday in Geophysical Research Letters. Researchers at the German Aerospace Center found that surface roughness from impact craters obscured numerous tectonic wrinkles, revealing a total radial contraction of up to 14 miles.
Planetary scientists have long understood that the solar system’s smallest and innermost planet has been shrinking ever since its formation 4.5 billion years ago, contracting as it cools from its fiery origins. Its hot iron core—oversized for such a small world—cools and contracts along with the mantle and crust, tightening the surface like a girdle to fit its new slimmer self. Yet accounting for billions of years of impact bombardment changes the scale of that aging process significantly. When researchers mapped the planet’s surface roughness alongside its tectonic shortening structures, they discovered that rough terrain had hidden many of the telltale ridges that indicate planetary shrinkage.
How Impact Craters Obscured Mercury’s Shrinkage
The new analysis, reported by the Associated Press, demonstrates that Mercury’s shrinkage may be 30 percent greater than previously believed. This represents a loss in diameter of as much as 14 miles, or 23 kilometers, on a world barely 3,000 miles, or 4,900 kilometers, across. Previous estimates relied heavily on visible tectonic scarps and faults mapped by spacecraft, but rough patches created by debris hurled from impact craters effectively concealed the true extent of the compression.
Nishiyama and his team compared maps showing faults and other geologic signs of contraction at Mercury with newer maps highlighting surface roughness. They discovered that the roughest patches exhibited fewer shrinkage wrinkles, concluding that the missing wrinkles could be buried under impact debris.
Back in 2013, Paul Byrne presented findings at a conference indicating that Mercury had shrunk by about 11 kilometers. The latest research suggests that the contraction reached up to 23 kilometers, or about 14 miles.
“In this work, the authors have thought about an additional angle to figuring out Mercury’s geological history from afar, which is the effect of surface roughness from billions of years of impact bombardment,” says Paul Byrne, a planetary scientist at Washington University in St. Louis, who was not involved in the study. “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.”
BepiColombo Mission Prepares for Orbital Survey
The timing of the study coincides with a crucial phase for deep-space exploration. Just one week prior to the findings, a pair of European and Japanese spacecraft shed its cruising platform and advanced toward Mercury. Known as BepiColombo, the linked craft are expected to enter orbit around Mercury in November before splitting up for a fuller survey.
Gaku Nishiyama, lead author of the study and a planetary scientist at the German Aerospace Center’s Institute of Space Research who is taking part in the space mission and is also affiliated with Hokkaido University in Japan, noted that BepiColombo’s laser instrument should confirm how much Mercury is withering as a result of internal cooling. Past space missions provided foundational data, starting with NASA’s Mariner 10 in the 1970s—the only other spacecraft besides Messenger to ever visit Mercury—followed by measurements from NASA’s Messenger spacecraft in the 2010s. Mercury is particularly mysterious because it is so close to the sun, making it difficult to send spacecraft there and hard to observe from Earth due to bright starlight.
Implications for Thermal Evolution and Other Rocky Worlds
Understanding the exact rate and extent of Mercury’s thermal contraction provides vital clues about its evolution. Retracing the history of Mercury enables scientists to better understand what’s going on underneath its rocky surface, and the new estimates on Mercury’s radial contraction would tell a different scenario of Mercury’s thermal evolution.
“We are quite excited,” Nishiyama said in an email, adding that it feels like “we are approaching the reality of Mercury’s evolution.”
Researchers suggest that the lessons learned from Mercury’s concealed wrinkles could eventually apply across the cosmos. Byrne is positive that other celestial bodies such as the moon and Mars may show similar shrinkage, noting about those worlds: “Do those worlds next!”
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