17 Nov 2016

A newly discovered giant valley on the planet Mercury makes the Grand Canyon look tiny by comparison. Located by scientists at the University of Maryland, the Smithsonian Institution, the German Institute of Planetary Research and Moscow State University, the expansive valley holds an important key to the geologic history of the innermost planet in our Solar System.
Discovered using stereo images from NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, the "great valley" lies in the planet's southern hemisphere and overlaps the Rembrandt Basin - a large crater formed by a relatively recent impact from an asteroid or other such body. But the "great valley" formed in a much different way, according to a research paper published online November 16, 2016 in the journal Geophysical Research Letters.

Unlike Earth, which has a crust and upper mantle (collectively known as the lithosphere) divided into multiple tectonic plates, Mercury has a single, solid lithosphere that covers the entire planet. As the planet cooled and shrank early in its history, roughly 3-4 billion years ago, Mercury's lithosphere buckled and folded to form the valley, much like the skin of a grape folds as it dries to become a raisin.
The valley is about 400 km wide and 1000 km long, with steep sides that dip as much as 3 km below the surrounding terrain. To put this in perspective: if Mercury's "great valley" existed on Earth, it would be almost twice as deep as the Grand Canyon.

More notable than its size, according to the researchers, is how the valley most likely formed and what that reveals about Mercury's geologic history.
The valley's walls appear to be two large, parallel fault scarps, step-like structures where one side of a fault moved vertically with respect to the other. Both scarps plunge steeply to the flat valley floor below. According to the paper’s authors, the best explanation is that Mercury's interior cooled rapidly, forming a strong, thick lithosphere. The entire floor of the newly discovered valley is one giant piece of this lithosphere that dropped between the two faults on either side.

This would make sense if, like most planets, Mercury has been steadily cooling since its formation. But researchers note that there are several clues to suggest that Mercury went through a more recent period of warming. This analysis, if true, would upend some time-tested assumptions about Mercury's geologic past.