Lunar craters
Ernest Wright, NASA/GSFC Scientific Visualization Studio

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Subsurface map of moon reveals origin of mysterious impact crater rings

Some 3.8 billion years ago, Mare Orientale got started with a bang. A 930-kilometer-wide impact basin perched on the moon’s visible edge, Orientale resembles a bull’s-eye, with a smooth interior encircled by three rough rings. For decades, scientists have debated the significance of these rings, which are found around the largest impact basins on the moon, Mars, and Earth. Do any of the rings match the original crater rim left by the striking asteroid or comet? Now, a new subsurface moon map from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, published today in Science, suggests that the answer is no. Four years ago, as GRAIL’s two spacecraft neared the end of a 1-year orbital mission with a planned crash into the lunar surface, they measured Orientale from a scant altitude of 2 kilometers. At such close range, the spacecraft were exceptionally sensitive to tiny changes in the moon’s gravity caused by buried rocks of different density–giving the GRAIL team a picture of the subsurface, and a better idea of how the impact actually went down. They found that the Orientale strike hollowed out a crater some 320 to 460 kilometers wide—smaller than any of the rings. Within an hour, the crater’s steep walls toppled inward. Hot mantle rocks, rebounding in the void like the splash of a stone in water, rose up into a central tower as high as 140 kilometers. A stiffer crust, riding on top of the mantle rocks, cracked and settled to form the two outer rings. The central tower’s subsequent collapse piled up into the innermost ring. Earlier this year in the Gulf of Mexico, scientists drilled into the dinosaur-killing Chicxulub impact crater to show that a similar process occurred on Earth. Beyond settling the mystery of the rings, the GRAIL researchers expect that their map will help refine the models that connect the speed and size of impactors to the scars they leave across the solar system.