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Don't stare: SHRUNKEN Mercury lost 7km, but only 'cos it's COOLING

Could a BLACK HOLE be gobbling the planet from within?*

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The planet Mercury has shrunk by up to 7km in radius over the past four billion years – much more than boffins had previously thought.

A long collection of ridges and scarps on the planet Mercury, stretching over 540km. The colours correspond to elevation - yellow-green is high and blue is low.

Ridges and scarps on Mercury, stretching over 540km. The colours correspond to elevation - yellow-green is high and blue is low. Credit: NASA/Johns Hopkins University AP Lab/Carnegie

For some reason, older images of the wrinkly surface of the planet showed that despite the fact that its interior was cooling, the rocky world had hardly shrunk at all. But the scientific models of how the planet was evolving couldn't explain why Mercury would not be getting smaller.

Researchers have used evidence gathered by NASA's MESSENGER spacecraft at the planet to build up a comprehensive map of its tectonic features that's detailed enough to show that the celestial body withered considerably as its interior rocks and metals cooled.

"With MESSENGER, we have now obtained images of the entire planet at high resolution and, crucially, at different angles to the sun that show features Mariner 10 could not in the 1970s," said Steven Hauck, a professor of planetary sciences at Case Western Reserve University and the paper's co-author.

Mariner 10 took images and data of Mercury in three flybys in 1974 and 1975, getting details for just 45 per cent of the surface. But MESSENGER, first launched in 2004 and, while getting data from the planet since 2011, has completed 2,900 orbits of Mercury.

The Earth is the only planet so far known to have a whole bunch of tectonic plates bumping up against each other. Most worlds are like Mercury, whose surface has one tectonic plate known as the lithosphere. To figure out how the world could have shrunk, the boffins looked at features on this plate called lobate scarps and wrinkle ridges, which look like long ribbons and indicate interior cooling and surface compression.

The scarps are cliffs caused by faults that have broken through the surface and can be nearly two miles high. Wrinkle ridges are faults that aren't as deep, so tend to be lower. Either can range from five to more than 550 miles long.

MESSENGER has helped scientists to map a total of 5,934 of these features, allowing them to estimate the shortening across broad sections and thereby guess at the decrease in the planet's radius, which they estimate at between 4.6 and 7km.

"These new results resolve a decades-old paradox between thermal history models and estimates of Mercury's contraction," said Paul Byrne, lead author and MESSENGER visiting investigator at Carnegie's Department of Terrestrial Magnetism. "Now the history of heat production and loss and global contraction are consistent.

"Interestingly, our findings are also reminiscent of now-obsolete models for how large-scale geological deformation occurred on Earth when the scientific community thought that the Earth only had one tectonic plate. Those models were developed to explain mountain building and tectonic activity in the nineteenth century, before plate tectonics theory." ®

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*Headlines to which the answer is no. And also this...

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