NASA’s twin Grail probes have revealed the surface of the moon in unprecedented detail, showing unbelievably deep cracks, craters and tectonic structures.
They’ve also shown that the moon’s gravity field is unlike that of any terrestrial planet in our solar system.
“What this map tells us is that more than any other celestial body we know of, the moon wears its gravity field on its sleeve,” says Grail principal investigator Maria Zuber of MIT.
“When we see a notable change in the gravity field, we can sync up this change with surface topography features such as craters, rilles or mountains.”
The moon’s gravity field reflects the bombardment it once received from comets and other bodies – which seems to have cracked it all the way down to the deep crust and possibly even the mantle.
The moon’s highland crust now seems to be substantially less dense than thought, showing that its bulk composition is similar to that of Earth.
“This supports models where the moon is derived from Earth materials that were ejected during a giant impact event early in solar system history,” says Grail co-investigator Mark Wieczorek of the Institut de Physique du Globe de Paris.
The two probes, named Ebb and Flow, transmit radio signals to determine precisely the distance between them as they orbit the moon in formation. As they fly over areas of greater and lesser gravity, the distance between the two changes slightly.
“This data revealed a population of long, linear, gravity anomalies, with lengths of hundreds of kilometers, crisscrossing the surface,” says Jeff Andrews-Hanna, a Grail guest scientist with the Colorado School of Mines in Golden.
“These linear gravity anomalies indicate the presence of dikes, or long, thin, vertical bodies of solidified magma in the subsurface. The dikes are among the oldest features on the moon, and understanding them will tell us about its early history.”
The probes have been in a near-polar, near-circular orbit for nearly a year. On December 17, they will move to a lower orbit.
The lunar gravity map is here.