Scientists prove Mercury is slowly shrinking

They say the world is getting smaller, and in Mercury’s case it’s true. Already the tiniest planet in the solar system, Mercury has signs of contraction clearly seen in distortions of the planet’s searing surface, scientists say.

The findings, published in Nature Geoscience, solve a decades-old mystery about the evolution of the little planet’s interior and provide scientists a window into changes that affect worlds without Earth-like plate tectonics.

As the solar system’s innermost planet, Mercury sits less than 36 million miles from the sun, less than two-fifths of the Earth-to-sun distance. It’s mostly made up of its heavy iron core, which has about a 1,255-mile radius and leaves a thin rind of just 261 miles for its crust and mantle. Even though it’s unbearably hot, the planet also hosts permanently shadowed regions inside craters that are among the coldest spots in the solar system.

Researchers have long thought that Mercury must be shrinking, because as the planet cools, and the liquid iron core turns solid over time, it contracts.

When NASA’s Mariner 10 spacecraft flew by the planet in 1974 and 1975, it discovered snaking “lobate scarps” on its surface. Those scarps were the signs that the planet had shrunk, causing its rocky skin to deform.

Mariner 10 imaged only 45 percent of the planet, and scientists could account for only about 0.5 to 2 miles of shrinkage in the radius. The models said that Mercury’s radius should have shrunk roughly 3 to 6 miles over the last 4 billion years, since its crust solidified.

NASA’s Messenger spacecraft, which flew by the planet in 2008 and 2009 and entered Mercury’s orbit in 2011, mapped the 55 percent of the planet Mariner 10 missed. Scientists found that lobate scarps covered the whole globe randomly. The scientists also found wrinkle ridges all over Mercury’s volcanic plains, and though not as high as the lobate scarps, they’re another sign Mercury has been contracting.

Researchers found Mercury’s radius had probably shrunk about 3 to 4.3 miles since its crust solidified, within range of the theoretical predictions.