A giant asteroid that struck the moon long ago, creating what is possibly the largest and oldest impact crater in the entire solar system, may explain the surprisingly strong magnetic fields that emanate from the lunar crust today, researchers say in the 8 March issue of Science.

On a quest to locate water and other volatile minerals in the Moon’s soil, the LCROSS experiment—Lunar Crater Observation and Sensing Satellite—hurtled a spent Centaur rocket into a dark crater at the lunar South Pole last year. The crater, known as Cabeus, is one of the permanently shadowed regions of the Moon, and researchers believe it is also one of the coldest.