Picture a plum. When it is ripe, the surface is smooth, but when it starts to age and shrink, wrinkles can appear along the surface. On a planetary scale, when a celestial body starts to contract, those wrinkles can form deep valleys, as is the case on Mercury. Using images from NASA's MESSENGER spacecraft, researchers have just spotted a valley there that could measure as much as 2 mi (3 km) deep and 250 mi (400 km) wide which, they say, came about as the planet cooled and contracted.

All planets cool as they age and, as they do so, their crust and upper mantles – known as the lithosphere – can shift and buckle. Because Earth consists of multiple tectonic plates, as it cools, the individual plates can shrink independently of each other and form valleys like the Great Rift Valley in Africa. Mercury, however, has only one plate, so as it cools, it buckles, sending some parts of its crust upwards and others downward.

Using stereo images from MESSENGER, the researchers were able to construct a high-resolution topographic map of an area of Mercury's southern hemisphere where they found the valley, which has been named the "Great Valley." It sits in one of the planet's largest impact basins, known as the Rembrandt Basin.

The buckling forced the crust up sharply on both sides of the valley, forming what are known as large scarps, or cliffs. One scarp is known as Enterprise Rupes and is the largest fault scarp on the planet. The other is called Belgica Rupes. Such scarps have also been found on our Moon indicating it too is likely shrinking.

The MESSENGER spacecraft was launched in August of 2003 and it spent about 12 years observing and photographing Mercury before crashing into the planet's surface in 2015. It's name comes from the work it did: MErcury Surface, Space ENvironment, GEochemistry, and Ranging. Earlier this year, MESSENGER data was also used to point to the fact that Mercury might still be shrinking by pinpointing smaller scarps that were formed relatively recently.

"Even though you might expect lithospheric buckling on a one-plate planet that is contracting, it is still a surprise when you find that it's formed a great valley that includes the largest fault scarp and one of the largest impact basins on Mercury," said Watters.

The finding has been reported in Geophysical Research Letters, a journal of the American Geophysical Union. The following video from NASA shows the way in which MESSENGER's images were converted to a topographic map of the area in which the Great Valley is found.

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