Mercury may have shrunk 30% more than previously believed, new study says; planet’s radius may have decreased by 7.2 miles

Mercury, the closest planet to the Sun, may have contracted 10% to 30% more than previously estimated, according to a report of CNN which cited a new study. Researchers estimate that the planet’s radius may have decreased by about 7.2 miles (11.6 kilometers) as its interior cooled and contracted over billions of years.

The study points to wrinkles, cliffs and ridges across Mercury’s surface as evidence of this contraction. However, debris from asteroid and comet impacts may have obscured some of these geological features, causing previous studies to underestimate how much the planet has shrunk.

The smallest planet in the solar system, Mercury orbits the Sun at an average distance of about 36 million miles (58 million kilometers) and completes one orbit every 88 days. It formed roughly 4.5 billion years ago, when violent collisions generated enormous amounts of heat. As that heat gradually escaped into space, Mercury’s interior cooled and contracted, causing its surface to buckle.

Mercury’s shrinking surface

The contraction created geological features known as shortening structures, including long, steep cliffs called scarps. Some of these formations can rise as much as 1 mile (1.6 kilometers) and stretch for hundreds of miles.

But Mercury’s surface is also heavily scarred by impact craters from asteroids and comets. The resulting debris may have buried or obscured some of the scarps and other structures created as the planet contracted.

The new study published in 'Geophysical Research Letters, researchers used data from NASA’s MESSENGER mission to create a global map of Mercury’s surface roughness. They found that rougher regions tended to have fewer visible wrinkles than smoother areas.

This pattern led researchers to suggest that some of the missing structures may not actually be absent. Instead, they could have been concealed beneath impact debris.

The team then estimated how many shortening structures might exist if those hidden features were taken into account. Their calculations suggested that Mercury may have contracted 10% to 30% more than previous estimates indicated.

Earlier studies had estimated that Mercury’s radius had decreased by anywhere from 0.6 to 1.2 miles (1 to 2 kilometers) to as much as 4.3 miles (7 kilometers).

What Mercury’s shrinkage can reveal

Meanwhile, scientists are now hoping that this could reveal what lies beneath Mercury's surface. Dr. Paul Byrne, an associate professor at Washington University in St. Louis, told CNN said accurately determining how much Mercury has contracted could help scientists better understand its interior.

“Yet if we can accurately measure how much Mercury has contracted, we can make better estimates of its interior layering, the size and make-up of its core, its tectonic and volcanic histories, how its magnetic field is generated, and a whole lot more,” Byrne said.

Mercury’s shrinkage is particularly important because the planet has an unusually large metallic core. It is the second-densest planet in the solar system after Earth, largely because of its substantial core.

Determining how much the planet has contracted could therefore provide clues about its core and the conditions Mercury experienced as it cooled.

“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,” said lead study author Gaku Nishiyama, a planetary scientist at the German Aerospace Center’s Institute of Space Research in Berlin.

A difficult planet to study

Mercury’s proximity to the Sun makes it one of the most challenging planets to explore. Only two spacecraft had previously studied it up close: NASA’s Mariner 10, which flew past Mercury in the 1970s, and MESSENGER, which entered orbit around the planet in 2011.

Mariner 10 provided some of the earliest observations of Mercury’s contraction-related wrinkles, while MESSENGER later offered scientists a far more comprehensive view of the planet’s surface.

Now, the BepiColombo mission is expected to provide another major leap in our understanding of Mercury. After an eight-year journey, the joint European-Japanese mission has begun its arrival at Mercury and will eventually deploy two orbiters around the planet.

The mission is expected to provide new measurements that could help determine how much Mercury has actually shrunk.

“Topography data that the mission collects could enable scientists to estimate more precisely how much Mercury has contracted over time and showcase the planet’s cliffs, craters and ridges like never before,” Nishiyama said. He is also part of the BepiColombo science team.

Unlike MESSENGER, BepiColombo will make comprehensive measurements of Mercury’s topography, providing a crucial new dataset for studying the planet’s contraction.

“Those new data will be key to testing whether this and other global contraction papers are right,” Byrne said. “But I suspect we’ll find that they are, and that we still have so very much to learn about the tiny innermost planet.”