AAS DPS 2008: The moon's thick backside
Sho Sakada, of Japan's national astronomical observatory, was on hand yesterday to show off the latest results from Kaguya / SELENE, the lunar orbiter launched last year by the Japanese space agency JAXA.
SELENE doesn't have the power that Maria Zuber hopes her GRAIL mission will have. But Sakada showed some of the first high-resolution gravity anomaly maps of the lunar far side, made from radio tracking maps of the surface, as SELENE is tugged ever so slightly up and down by differential gravity. A gravity anomaly map shows where the force of gravity strays from the mean. If there's more mass under you, you weigh more. So standing above a topographic high, or on top of a really dense lode of rock, you'd be slightly heavier: a high gravity anomaly.
What Sakada found was expected: the far side of the moon is full of gravitational lows. Scientists have long thought that the near side cooled more slowly, and that the moon's warm mantle and core were tugged closer to the Earth. This dense rock is much closer to the surface on the near side, as evidenced by the mare basalt flows in all of the basins. The far side has a much thicker crust.
And so Sakada's maps show gravity highs in nearside impact basins, where the underlying dense rock was allowed to well up to the surface. Impacts on the farside, however, only partially excavate the thick, less dense crust, creating a ring-like basin that is a topographic, and gravitational, low. Hopefully that's enough explanation to help you figure out which picture here came from which side of the moon.
