Subsurface map of moon reveals origin of mysterious impact crater rings

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Source:  By Paul Voosen, Science

Excerpt: Some 3.8 billion years ago... A 930-kilometer-wide impact basin perched on the moon’s visible edge, [Mare] Orientale resembles a bull’s-eye, with a smooth interior encircled by three rough rings. ...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. ...GRAIL’s two spacecraft...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....

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