Mercury Has Wrinkles That Show It's Shrinking. Now, Scientists Say the Planet Has Shriveled Far More Than Previously Thought

Mercury, our solar system’s smallest planet and the one closest to the sun, is covered in wrinkles. They tell scientists that the teensy world has shrunk as it cooled over the past 4.5 billion years. Its birthplace was piping hot, after all.

But Mercury may have shriveled even more than previously thought, at around 10 to 30 percent, researchers report in a study published the journal Geophysical Research Letters on September 10. That’s a loss of up to 14.5 miles in width—far more than past estimates of around 2.5 to 10 miles—a fairly significant chunk, considering the planet’s current diameter of merely 3,032 miles. The findings could unlock new insights about Mercury’s interior and evolution, in turn shedding light on other rocky worlds.

Mercury is a difficult planet to study. Its proximity to the sun makes it tough for a spacecraft to reach; one trying to enter Mercury’s orbit must constantly brake to fight the gravitational pull of our star, and the planet’s surface temperatures are too hot to allow spacecraft to get very close. It’s also hard to observe from Earth, given the brightness of the sun.

Only one spacecraft has entered this little world’s orbit: The one used in NASA’s MESSENGER mission, which circled Mercury from 2011 to 2015. In the new study, researchers combined MESSENGER data with a digital model of Mercury’s terrain based on images to create a map of the planet’s surface roughness. They found that rougher areas on Mercury had fewer visible wrinkles—signs of the planet’s contraction—which led them to believe something was obstructing their view of the folds.

Perhaps rocky debris kicked up from impacts covered up the ridges formed by shrinkage. Callaway at the New York Times. Accounting for those obstructed areas helped the team to calculate the new shrinkage estimate.

Up to “30 percent is a little bit surprising, but the corrected amount of contraction actually makes sense to me,” Nishiyama says in a statement. The updated numbers allow observations of Mercury’s cooling and shriveling to align more closely with physics-based predictions. Mercury is one of two solar system planets that lack a moon.