Hidden clues show Mercury shrank more than previously thought - USA Today

Here's what to know about Mercury, and what the latest research says about how much smaller it has gotten since it first formed.

- New research suggests Mercury has shrunk more drastically than previously believed.

- The planet's cooling interior causes its surface to wrinkle and contract over time.

- Debris from asteroid and comet impacts has obscured evidence of the planet's shrinkage.

- A joint European and Japanese mission, BepiColombo, will arrive at Mercury in 2026 to study it further.

Mercury may be the smallest planet in Earth's solar system, but evidence suggests it was not only much larger, but has shrunken far more drastically than scientists realized.

Conspicuous wrinkles on Mercury's surface have long indicated that the tiny world has gotten smaller and smaller over the eons.

Formed in the chaos of our solar system's early days, Mercury was shaped by violent forces that generated massive amounts of heat. As the years went by and that heat slowly escaped, Mercury has steadily been shrinking like a balloon in the cold.

That much astronomers have long known, thanks to the scars stretching across Mercury's surface that serve as evidence of how it has contracted over time.

But now, new research has found asteroids and comets that have bombarded Mercury have created debris obscuring just how much Mercury has shrunk.

What to know about Mercury, closest planet to the sun

Slightly larger than Earth's moon, Mercury is the smallest planet in the solar system. It's also the closest to the sun and the fastest – completing one circle of our star once every 88 days, according to NASA.

Named for the swiftest of the ancient Roman gods, Mercury formed about 4.5 billion years ago from swirling gas and dust. Similar to other rocky, terrestrial planets, Mercury has a central metallic core, a rocky mangle and a solid crust.

On the surface, impact craters from colliding space rocks have scarred the planet in a similar fashion as Earth's moon. Energy from these impacts generated massive amounts of heat, which Mercury has been losing ever since, according to astronomers.

As a result, Mercury's interior has been shrinking as it cools, with its outermost rocky layers crumpling and cracking as the planet changes size.

Why is Mercury shrinking? Smallest planet in our solar system was once bigger

While scientists have been aware that Mercury has steadily been getting smaller for ages, new research suggests the planet has contracted much more than thought.

In fact, Mercury may have shrunk by up to 30% more than previously believed, according to a study published Thursday, Sept. 10, in the journal Geophysical Research Letters. That means Mercury may have lost up to 14.5 miles of diameter since its formation, rather than between 2.5 and 10 miles as previously estimated.

In theory, the cooling Mercury has experienced should shrink the planet uniformly, leading to the creation of wrinkle-like features stretching across its surface. But depressions and debris littering Mercury's landscape from new impact craters have covered up such wrinkles, thus obscuring just how much the planet has shrunk, the researchers said in a press release announcing their findings.

To reach their conclusions, the team of researchers combined previous maps of Mercury's geology with newer maps accounting for its roughened surface for more accurate estimates.

Understanding Mercury’s shrink rate can help scientists learn more about the planet's enigmatic interior and how it evolved.

“More shrinking means Mercury could have a larger metal core, less light elements like silicon mixed into the metal core, or a higher starting temperature,” lead study author Gaku Nishiyama, a planetary scientist at the German Aerospace Center’s Institute of Space Research in Berlin, said in a statement.

Two orbiters to arrive around Mercury in 2026

But Nishiyama acknowledged that the new understanding of Mercury's shrink rate could still be a significant underestimate.

What's more, existing Mercury data from NASA’s MESSENGER mission, which ended in 2015, can only be used to measure much smaller features.

Fortunately, an unprecedented mission sending two orbiters to Mercury later this year could help answer lingering questions.