sk yandtelescope.com * AUGUST 2 018 CAS E Y R E E D THE GREAT WALLOP About 4½ billion years ago, a Mars-size body and the proto-Earth - both with atmospheres and, likely, surface oceans - crashed into each other. Already scarred by earlier, smaller impacts, the two bodies largely vaporized. The Moon and Earth coalesced out of the debris. 27http://www.skyandtelescope.com