The Grand Canyon in Arizona got carved by water over millions of years of slow but steady erosion. Two similarly-sized canyons on the moon got carved by flying rocks in about ten minutes.
That's because these two canyons formed in the wake of an asteroid or comet smashing into the moon, a crash that sent streaks of rocky debris flying at incredible speeds, according to a new analysis in the journal Nature Communications.
"This was a dramatic impact that was followed by a series of smaller impact events that excavated these canyons in, you know, roughly 10 minutes," says David Kring with the USRA Lunar and Planetary Institute in Houston.
"That would have been an extraordinary sight to see," he adds. "Not that I would have wanted to be on the lunar surface, but I would have liked to have been a safe distance above the surface to watch all of this unfold."
The two canyons, called Vallis Schrödinger and Vallis Planck, are located on the far side of the moon, so they can only be seen from orbit—not from Earth. Each canyon is over 165 miles long and over 1.5 miles deep.
"These are extraordinary canyons, but they're hidden, if you will, and kind of mysterious because they're on the far side in the lunar South Pole region," says Kring, who has long wondered exactly how they formed.
The canyons look like remarkably straight lines that extend outward from a circular crater that is the Schrödinger impact basin, the result of a large impact that occurred around 3.8 billion years ago.
To better understand how this impact basin and its accompanying canyons were created, Kring teamed up with Danielle Kallenborn, who is now a Ph.D. student at Imperial College London, and Gareth Collins, also at Imperial College London. They used images obtained by NASA's Lunar Reconnaissance Orbiter to carefully map the crater, the canyons, and the deposits of ejected rock that rained back down.
When the impactor hit the moon, it was moving at roughly 38,000 miles per hour, says Kring, and would have penetrated to a depth of about 15 miles. As it did, it threw up an asymmetrical pattern of debris, a curtain of rock which plummeted back down to the surface.
"These canyons were produced by two concentrated streams of rock in that curtain of debris," says Kring. "Those clusters of rock in that curtain hit the lunar surface in just a staccato series of impacts."
These impacts excavated a series of round craters that line up, which means that the canyons were "produced by a series of small impact events that form a chain," he explains.
The amount of energy needed to produce these grand canyons on the moon is 1200–2200 times larger than "the nuclear explosion energy once planned to excavate a second Panama Canal on Earth, more than 700 times larger than the total yield of US, USSR, and China's nuclear explosion tests, and about 130 times larger than the energy in the global inventory of nuclear weapons," the researchers write in their report.
Even though these particular lunar craters aren't visible from your backyard, Kring says there are similar, but smaller canyons on the near side of the moon that's visible in the night sky. These radiate out from the bright spot that is Tycho Crater.
"On a clear, clear night, Tycho, which is a little bit below the equator, has beautiful white rays," says Kring. "Those white rays are the same phenomenon, but just at a very small scale compared to Schrödinger."
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