Joly sez,
These print-resolution stills were created for the
cover of the February 8, 2013 issue of Science. They show the
free-air gravity map developed by the Gravity Recovery and Interior
Laboratory (GRAIL) mission.If the Moon were a perfectly smooth sphere of uniform density, the
gravity map would be a single, featureless color, indicating that
the force of gravity at a given elevation was the same everywhere.
But like other rocky bodies in the solar system, including Earth,
the Moon has both a bumpy surface and a lumpy interior. Spacecraft
in orbit around the Moon experience slight variations in gravity
caused by both of these irregularities.The free-air gravity map shows deviations from the mean, the
gravity that a cueball Moon would have. The deviations are measured
in milliGals, a unit of acceleration. On the map, dark purple is at
the low end of the range, at around -400 mGals, and red is at the
high end near +400 mGals. Yellow denotes the mean.These views show a part of the Moon’s surface that’s never visible
from Earth.
GRAIL Free-Air Gravity Map for the Cover of Science