A new large valley has been discovered on Mercury.

Reporting their findings in the journal Geophysical Research Letters, scientists said the likeliest explanation for the existence of the "Great Valley" is that Mercury's lithosphere, which is made up of the planet's upper mantle and crust, had buckled when it contracted.

The high-resolution topographic map of the planet's southern hemisphere used by the scientists was created using stereo images from NASA's Messenger spacecraft.

"There are examples of lithospheric buckling on Earth involving ... but this may be the first evidence of lithospheric buckling on Mercury," said Thomas R. Watters, lead author of the study and a senior scientist from the Smithsonian's Center for Earth and Planetary Studies.

How Lithospheric Buckling Occurs

Earth's lithosphere is separated into numerous tectonic plates, but Mercury's is made up of just one. When Mercury's interior cools, it causes the planet's plate to bend and contract, with crustal rocks thrusting upward and valley floors sagging downward where contractional forces are strongest.

Mercury's Great Valley

The Great Valley recently uncovered on Mercury is about 250 miles wide, with floors extending up to 2 miles below its surrounding terrain. It is also 600 miles long, extending into the Rembrandt basin, which is one of the youngest but largest impact basins on the planet.

Two large fault scarps bound the Great Valley, also forming from the contraction. However, the fault scarps have grown so large that they have turned into cliffs. The valley floor is elevated well below the surrounding scarps, which hints that it was deepened by the same process that created the scarps as well.

According to Watters, even though it was expected for Mercury to contract because of its single-plate composition, it was still surprising to see that the contraction not only resulted in the formation of the Great Valley, but that it also featured the largest fault scarps on the planet and one of its biggest impact basins.

Tectonic Activity On Mercury

In September, a NASA-funded study reported that Mercury is not only tectonically active but is also still shrinking. Published in the journal Nature Geoscience, the study also used images from NASA's Messenger spacecraft and found small previously undetected fault scarps.

The fault scarps discovered were small enough that scientists initially thought they were geologically young, which also points to tectonic activity in Mercury being active.

"For years, scientists believed that Mercury's tectonic activity was in the distant past. It's exciting to consider that this small planet ... is active even today," said Jim Green, NASA Planetary Science director.

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