Mercury is shrinking up to 30 percent more than previously believed, losing nearly 15 miles of its diameter since its formation, according to a study published in Geophysical Research Letters. Researchers say impact debris and rough terrain on the solar system’s smallest planet hid the true extent of these surface wrinkles.
The solar system’s innermost world is quietly withering. For 4.5 billion years, Mercury has continuously leaked heat into space following its sizzly formation. As its heavy iron core cools, the entire planet shrinks like a grape transforming into a raisin under the sun. Its outer crust buckles and cracks around it, creating gigantic cliffs and mountainous ridges known as lobate scarps. New research reveals that this contraction is far more severe than historical models suggested.
Uncovering Hidden Wrinkles Beneath Planetary Debris
For decades, scientists wrestled with a persistent discrepancy. Previous estimates suggested far less shrinking than physics predicted, leaving researchers uncomfortable with their understanding of how planets cool. The missing piece of the puzzle turned out to be Mercury’s aggressively battered terrain. 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.
Researchers discovered this hiding mechanism by comparing global maps of Mercury’s surface roughness with maps of shortening structures and contraction. The rugged regions contained the fewest visible wrinkles. Around major impacts, like the massive Rachmaninoff crater—a double-ring basin about 180 miles wide captured by NASA’s Messenger spacecraft—tectonic cracks disappeared almost entirely under thick layers of debris. After accounting for these obscured regions, scientists recalculated the planet’s total contraction, discovering a loss in diameter of as much as 14.5 miles—roughly 30 percent higher than previously estimated.
Thirty percent is a little bit surprising, but the corrected amount of contraction actually makes sense to me.
By correcting the math, astronomers may be able to unlock clues about Mercury’s original temperature and how other shrinking rocky worlds, like our own moon, evolve over time.
Cracked Toast and Recent Mercury-Quakes
The continuous shrinkage is not just an ancient relic of planetary cooling. Up-close imagery captured by NASA’s Messenger spacecraft, which orbited Mercury for over a decade from 2004 to 2015, revealed small geological cracks known as grabens running parallel to the lobate scarps. These features form from the fault lines trying to bend a piece of stiff rock. As study co-author David Rothery, a geologist at The Open University in the U.K., explained in an article for The Conversation, If you try to bend a piece of toast, it may crack in a similar way.
The team found 48 confirmed grabens, and a whopping 244 more features likely to be grabens. By studying impact gardening—how quickly the dust generated from meteor impacts blurs out features on Mercury’s surface—researchers dated these rocks and calculated they’re about 300 million years old based on how blurry the grabens appeared in images. This timeline proves that many scarps have continued to move in geologically recent times, even if they were initiated billions of years ago. Just as the moon experiences moonquakes as it shrinks and wrinkles, the shifting scarps generate equivalent “Mercury-quakes” on Mercury.
Rewriting Planetary Birth and Upcoming Space Missions
Nishiyama, who is affiliated with Hokkaido University in Japan, noted in an email that he and his team are quite excited, feeling like we are approaching the reality of Mercury's evolution.
Only two other spacecraft have ever visited Mercury: NASA’s Mariner 10 in the 1970s and NASA’s Messenger in the 2010s.
Scientists will soon have an opportunity to test these models with fresh data. A linked pair of European and Japanese spacecraft known as BepiColombo shed their cruising platform and advanced toward Mercury, with the linked craft expected to enter orbit around Mercury in November before splitting up for a fuller survey. Equipped with a laser instrument, BepiColombo should confirm how much Mercury is withering as a result of internal cooling, potentially revealing boulder tracks that could be additional evidence of recent activity.
