A giant impact 3.8 billion years ago sent a curtain of rock flying away from a point near the moon’s south pole. When that curtain fell, its rocks plunged up to 3.5 kilometers into the lunar surface with energies 130 times greater than the global inventory of nuclear weapons, new calculations show.
And that’s how a hailstorm of boulders carved out two gargantuan canyons on the moon in less than 10 minutes.
“They landed in a staccato fashion, bang-bang-bang-bang-bang,” says planetary geologist David Kring of the Lunar and Planetary Institute in Houston, who reports the finding February 4 in Nature Communications.
The two channels, Vallis Schrödinger and Vallis Planck, extend in straight lines from the 320-kilometer-wide Schrödinger basin marking the initial impact. Until now, the circumstances of the canyons’ formation have been a mystery. The canyons are 270 and 280 kilometers long and up to 2.7 and 3.5 kilometers deep, respectively.
“The landscape of the south polar region of the moon is so dramatic,” Kring says. “If it occurred on Earth, it would be a national or international park.” The Grand Canyon, for example winds for a sinuous 446 kilometers and is only 1.9 kilometers deep at its deepest point.
A tale of two canyons
A comparison of the width and depth of the Grand Canyon along the Bright Angel hiking trail (top) and Vallis Planck on the moon’s south pole (bottom). The colors each represent 500 meters of elevation.
The south pole also contains some of the oldest rocks on the moon, perhaps dating back to its formation about 4 billion years ago. Collecting samples from there would allow scientists to test some of the biggest mysteries about the moon’s history.
But there’s a potential problem. The rim of the Schrödinger basin is about 125 kilometers from the anticipated landing site of NASA’s Artemis astronauts. If the impact that formed the basin splashed rock in all directions, those tantalizing older rocks could have been buried.
So Kring, together with geologists Danielle Kallenborn and Gareth Collins of Imperial College London, analyzed spacecraft images of the Schrödinger basin and its canyons to deduce the physics of their formations. In addition to finding that the canyons’ origin was swift and explosive, the team found that the straight lines converge toward the southern edge of Schrödinger basin, not the middle. That convergence suggests the impacting object came in toward the moon at an angle, and splashed material preferentially northward, away from the Artemis exploration zone.
“That means that very little of the Schrödinger material is going to be burying this very old terrain,” Kring says. “We have an opportunity to peer deeper into lunar history and better understand the earliest epoch of the Earth-moon system.”
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