Mercury
Mercury's Great Valley is the dark blue stripe across the center of the imageNASA/JHUAPL/Carnegie Institution of Washington/DLR/Smithsonian Institution

Astronomers just discovered a valley on Mercury that dwarfs the Grand Canyon. At about 250 miles wide and 600 miles long, Mercury's "great valley"would stretch between New York City, Washington DC and Detroit and cut into the Earth twice as deep as does the Grand Canyon.

"The scale of this valley is enormous," University of Maryland assistant professor Laurent Montesi told Digital Trends. "We do not have anything comparable on Earth."

The vast scar on the surface of the planet closest to the sun is the largest valley on the planet, according to a press release from the American Geophysical Union.

The canyon even dwarfs Earth's largest valley, the 277-mile long Grand Canyon (however, 1,860-mile-long Valles Marineris on Mars is still the Solar System's grandest canyon in terms of length).

The Great Valley was discovered using images captured by NASA's MESSENGER spacecraft, which orbited Mercury between March of 2011 and April of 2015 before (intentionally) crashing into the surface of the planet. Tom Watters, a senior scientist at the Smithsonian National Air and Space Museum, and his team found the valley while analyzing data from that mission. Their research is detailed in the journal Geophysical Research Letters.

While the size of the Valley is certainly impressive, what it reveals about the composition and history of Mercury is even more significant.

Unlike the Grand Canyon, which was created by the flow of water, or East Africa's Great Rift, which is caused by two tectonic plates pulling apart, the Great Valley appears to have been created from another process, Watters says.

According to the Smithsonian, on Earth, tectonic plates constantly pull apart and smash into one another. But Mercury has one single plate, called a lithosphere, that acts as a shell around the planet. As the planet's core cools, the surface contracts and buckles. One of the result are "fault scarps" like Enterprise Rupes and Belgica Rupes, the two giant cliffs bordering the Great Valley.

"This is why we explore," NASA Planetary Science Director Jim Green says in a press release. "For years, scientists believed that Mercury's tectonic activity was in the distant past. It's exciting to consider that this small planet—not much larger than Earth's moon—is active even today."

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