GRAIL Puts a New Face on the Moon
“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 in Cambridge. “We know the dark splotches are large, lava-filled, impact basins that were created by asteroid impacts about four 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.”
Understanding lunar impact basins has been hampered by the simple fact that there is a lack of consensus on their size. Most of the largest impact basins on the near side of the Moon (the Moon’s face) have been filled with lava flows, which hide important clues about the shape of the land that could be used for determining their dimensions. The GRAIL mission measured the internal structure of the Moon in unprecedented detail for nine months in 2012. With the data, GRAIL scientists have redefined the sizes of massive impact basins on the Moon.
Maps of crustal thickness generated by GRAIL revealed more large impact basins on the near-side hemisphere of the Moon than on the far side. How could this be if both hemispheres were, as widely believed, on the receiving end of the same number of impacts?
Scientists have long known that the temperatures of the near-side hemisphere of the Moon were higher than those on the far side: the abundances of the heat producing elements uranium and thorium are higher on the near side than the far side, and 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,” notes lead author Katarina Miljkovic of the Institut de Physique du Globe de Paris.
Launched as GRAIL A and GRAIL B in September 2011, the probes, renamed Ebb and Flow by schoolchildren in Montana, operated in a nearly circular orbit near the poles of the Moon at an altitude of about 34 miles (55 kilometers) until their mission ended in December 2012. The distance between the twin probes changed slightly as they flew over areas of greater and lesser gravity caused by visible features, such as mountains and craters, and by masses hidden beneath the lunar surface.
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