The Gravity Recovery and Interior Laboratory mission has yielded clues to the asymmetric distribution of lunar impact basins, NASA's Jet Propulsion Laboratory in Pasadena, Calif., reported Thursday.
"Since time immemorial, humanity has looked up and wondered what made the 'man in the moon,'" said Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology. "We know the dark splotches are large, lava-filled, impact basins that were created by asteroid impacts about 4 billion years ago. GRAIL data indicate that both the near side and the far side of the moon were bombarded by similarly large impactors, but they reacted to them much differently."
GRAIL revealed more large impact basins on the near-side hemisphere of the moon than on the far side, a puzzling finding since scientists assumed both hemispheres were on the receiving end of the same number of impacts.
However, scientists have long known the temperatures of the near-side hemisphere of the moon were higher than those on the far side, caused by an abundance of the heat-producing elements uranium and thorium; as a consequence, the vast majority of volcanic eruptions occurred on the moon's near-side hemisphere.
"Impact simulations indicate that impacts into a hot, thin crust representative of the early moon's near-side hemisphere would have produced basins with as much as twice the diameter as similar impacts into cooler crust, which is indicative of early conditions on the moon's far-side hemisphere," said study lead author Katarina Miljkovic of the Institut de Physique du Globe de Paris.