"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," said Maria Zuber, GRAIL Principal Investigator at the Massachusetts Institute of Technology (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 gravity map was created from data taken by two washing machine-sized spacecraft (GRAIL-A and GRAIL-B, or "Ebb" and "Flow") orbiting the moon. The probes transmit radio signals to each other to precisely measure the distance between them while orbiting the moon. As they pass over areas of greater or lesser gravity on the moon, the distance between them shifts slightly.
The probes also revealed that the bulk density of the moon's highland crust is "substantially" lower than predicted. The density is, however, consistent with data from the Apollo missions in the 70s. This suggests that samples of the moon brought back by astronauts are a good representation of processes on the moon. The bulk composition of the moon is also similar to that of Earth, providing evidence for models that show the moon was formed from a giant impact with Earth in the early solar system.
"We used gradients of the gravity field in order to highlight smaller and narrower structures than could be seen in previous datasets," said Jeff Andrews-Hanna, a GRAIL scientist at the Colorado School of Mines. "This data revealed a population of long, linear gravity anomalies, with lengths of hundreds of kilometers, crisscrossing the surface. 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."