The solar system’s smallest planet has suffered significant shrinkage, drawing up 30 percent more than scientists even previously estimated.
Mercury, which orbits closest to the sun, is a tiny rocky world that has continuously leaked heat into space since its sizzly formation 4.5 billion years ago. As its heavy iron core cools, the entire planet shrinks, like 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 and mountainous ridges.
A new study reveals Mercury has lost up to 14.5 miles of its diameter since birth. That might not seem like much until you realize that the planet is only 3,000 miles wide overall. For decades, researchers may have underestimated the planet’s contraction because the rough terrain was obscuring the true extent of the damage.
“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.
A barrage of meteors pock 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.
While testing Nishiyama’s hypothesis, a team of researchers discovered a clear pattern: The rugged regions contained the fewest visible wrinkles. Around major impacts, like the massive Rachmaninoff crater, tectonic cracks disappeared almost entirely under thick layers of debris.
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“We compared a global map of Mercury’s surface roughness with maps of shortening structures and contraction,” Nishiyama said. “Mercury appears to have shrunk considerably more than what the visible tectonic record alone suggested.”
After accounting for these hidden features, the researchers recalculated Mercury’s total contraction. The results were published in the Geophysical Research Letters journal this week.
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For scientists, the findings could resolve a long-standing tension. Previous estimates suggested far less shrinking than physics predicted, leaving researchers uncomfortable with their understanding of how planets cool. 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.
NASA’s Messenger spacecraft captured the Rachmaninoff crater, a double-ring basin about 180 miles wide, on Mercury in 2009.
Credit: NASA / JHUAPL / CIW
A faster shrinkage rate may mean Mercury holds a much larger metal core that contains fewer light elements, such as silicon, than previously thought. That matters because a larger iron core would rewrite what scientists know about Mercury’s birth. More metal would strongly suggest the planet survived a catastrophic collision with another body in its ancient history, an event that could have stripped away most of its rocky crust. An oversized core might also explain how such a tiny world kept its interior churning, a necessity for its global magnetic field.
Scientists won’t have to wait long to verify the team’s work with fresh data. The BepiColombo spacecraft, a joint mission of the European Space Agency and Japan’s JAXA, is on its last leg of its space journey to Mercury. It’s expected to enter orbit around the planet in November.
Equipped with advanced lasers, BepiColombo will scan Mercury’s surface to detect fine details in its surface features that earlier space missions missed. Only two missions — NASA‘s Mariner 10 and Messenger — have visited the planet before, and only Messenger has ever orbited it.
