Mercury, that hot, rocky planet closest to the Sun not that much bigger than Earth’s Moon has always been something of mystery. We’ve only ever sent two probes to the planet, and the first of them, Mariner 10, was back in 1975.


Fortunately, NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft (MESSENGER for short) has been busy collecting data since it arrived some 30 years after its predecessor, and the discovery of an enormous 600-mile long and 250-mile wide canyon will go some way to helping us unlock the secrets of the planet’s geologic history – even suggesting Mercury is getting smaller.

Unlike Earth, with tectonic plates that slowly float around on the surface of a molten mantle, the surface of Mercury is like one giant shell called a lithosphere. By studying MESSENGER data from stereo images used to create high-resolution images of the planet’s surface, the discovery of the new valley suggests that when the planet began to cool and shrink some 3-4 billion years ago, this outer layer began to get sucked in and buckled into valleys, much like a shrivelled grape would when it dries into a raisin.

“Unlike Earth’s Great Rift Valley, Mercury’s great valley is not caused by the pulling apart of lithospheric plates due to plate tectonics,” says Tom Watters, senior scientist at the Smithsonian National Air and Space Museum, “it is the result of the global contraction of a shrinking one-plate planet.”

A 3-D perspective view of Mercury’s great valley, as seen in an image mosaic obtained by NASA’s MESSENGER spacecraft © NASA/JHUAPL/Carnegie Institution of Washington/DLR

The new valley, found in the planet’s southern hemisphere, has steep two-mile high walls and a large floor made entirely from one piece of lithosphere, suggesting that instead of tectonic pressures pushing the sides up, rapid cooling sucked the valley’s floor down.

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"This is a huge valley. There is no evidence of any geological formation on Earth that matches this scale," says Laurent Montesi, co-author of the research paper. "Mercury experienced a very different type of deformation than anything we have seen on Earth. This is the first evidence of large-scale buckling of a planet."

The study was published in Geophysical Research Letters.


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Alexander McNamaraOnline Editor, BBC Science Focus

Alexander is the former Online Editor at BBC Science Focus.