Data from NASA’s twin Gravity Recovery and Interior Laboratory (GRAIL) probes has been used to create the highest resolution gravity map yet of any body in the Solar System. The two washing machine-sized spacecraft acted as a 225-kilometer (140 mi) long gravity detector for studying the interior composition of the Moon.
The GRAIL spacecraft were launched in September 2011 and were originally designated “A” and “B,” but were renamed “Ebb” and “Flow” in January by elementary students in Bozeman, Montana as part of a nationwide contest. They are orbiting the Moon in near-polar, near-circular orbits 50 kilometers (31 mi) above the surface. They are in precise formation to one another at a distance ranging from 175 to 225 kilometers (109 to 140 mi).
Their main mission is to map gravitational anomalies on the Moon. This is important because the Moon isn't homogeneous. There are many areas of greater or lesser density and this causes an uneven gravitational field. As the two spacecrafts orbit, they constantly measure the changing distance between them to within a few tenths of a micron per second by means of precisely timed radio signals. Flying over a gravitational anomaly alters this distance and after a series of orbits, a map can be made of these anomalies. These measurements were enhanced by laser topographical measurements taken by NASA’s Lunar Reconnaissance Orbiter, which allowed the gravitational anomalies to be mapped against topographical features.
Scientists at MIT, NASA, the Jet Propulsion Laboratory and elsewhere are using the map as a way of learning more about the interior of the Moon and the early history of the 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," said GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology. "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."
Geologically, the Moon is relatively inactive and lacks any weather, so it's very close to the state it was in billions of years ago, whereas the Earth’s tectonic plate movements and robust weather has destroyed most traces of its early days. When the Solar System formed, all the planets underwent a barrage of impacts by objects, some as big as planets themselves, and the Moon still shows signs of those times. The most dramatic evidence is the fact that the Moon’s crust is completely pulverized with cracks that may extend into the mantle, as discovered by the GRAIL gravity survey. This indicates that the early Solar System underwent a much more severe and violent origin than previously thought.
The new information also provides a much clearer picture of the nature of the lunar crust. "With our new crustal bulk density determination, we find that the average thickness of the moon's crust is between 21 and 27 miles (34 and 43 km), which is about 6 to 12 miles (10 to 20 km) thinner than previously thought." said GRAIL co-investigator Mark Wieczorek of the Institut de Physique du Globe de Paris. "With this crustal thickness, the bulk composition of the moon 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."
Another aspect of gravity mapping is that it revealed internal geological structures, such as the fact that beneath the crust, the Moon is extremely smooth and marked by linear structures formed by magma seeping into cracks. “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 guest scientist with the Colorado School of Mines in Golden. "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."
These dikes are more than just curiosities. If they were present on early earth-like planets, it would have affected how they lost heat and how fluids would move from the interior. Similarly, the degree that the Moon’s crust is pulverized tells a lot about its composition, which helps us understand the early evolution of the planets.
The GRAIL mission will continue until December 17. The first results have been published in three papers in the journal Science.
The video below is an animation of the lunar gravity map.
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