Mercury shrinking more than scientists previously thought

By Stephen Beech

Mercury is shrinking by up to 30% more than previously thought, according to new research.

The rough, impact-debris-covered surface of the nearest planet to the Sun has buried the signs of a shrinking world, say scientists, leading them to underestimate contractions.

They explained that Mercury's surface is riddled with wrinkles - evidence of a once-larger planet that has shrunk over time.

The new study, published in the journal Geophysical Research Letters, suggests that Mercury may have contracted 10% to 30% more than previously believed.

That equates to a loss of nearly 12 miles (19 km) of total diameter since the planet formed - due to debris from impact craters obscuring the signs of the planet's shrinking.

Scientists say Mercury formed through a flurry of violent collisions between rocks and asteroids orbiting the Sun around 4.5 billion years ago.

They explained that energy from those impacts generated huge amounts of heat, which Mercury has been losing ever since.

But, like a balloon left out in the cold, Mercury's interior shrank as it cooled, and the outermost rocky layers compensated for the planet's changing size by crumpling and cracking to form tectonic features such as scarps and ridges.

The degree of shrinking is important for helping researchers infer key features of Mercury's interior and evolution.

Study lead author Gaku Nishiyama said: "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."

He explained that cooling should shrink a planet roughly uniformly, making "wrinkle" features, known as shortening structures, similarly common everywhere.

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But the formation of new impact craters creates depressions and covers the landscape in debris, making the surface rougher and rendering the signs of shrinkage harder to spot among billions of years of geologic features.

Nishiyama, a planetary scientist at the German Aerospace Center (DLR) Institute of Space Research, worked with his team to combine previous maps of geologic evidence for contraction with new maps of surface roughness - a measure of how lumpy a landscape is - for the entire planet's surface.

They showed, for the first time, that the roughest areas on Mercury have fewer visible wrinkles.

Nishiyama said: "It made us think that there's a process obscuring shortening structures."

He believes impact debris in rough areas could be covering up the shrinkage wrinkles, like freshly added gravel hiding ruts in a road.

Nishiyama and his team used the contraction required to form shrinkage ridges and scarps in less disrupted areas to estimate how much contraction likely occurred planet wide, including under rough patches.

They found that missing features in rough areas could amount to 10% to 30% more shrinkage over Mercury's lifetime - a total change of up to 14.5 miles (23 km) in the planet's diameter rather than the currently estimated 2.5 to 10 miles.

Nishiyama added: "30% is a little bit surprising, but the corrected amount of contraction actually makes sense to me."

He believes the updated figures could still be an underestimate.

Existing Mercury data from NASA's Messenger mission, which ended in 2015, can only be reliably used to measure features larger than about three miles across.

But in November this year BepiColombo, only the third-ever mission to Mercury, will begin collecting higher-resolution scans of the planet's surface.

Nishiyama, who is part of the mission science team, hopes the new data will reveal scarps, ridges and impact craters in more detail than ever before.