For well over a year now, two washer-machine sized twin spacecraft have been orbiting the moon, carrying out NASA’s Gravity Recovery and Interior Laboratory mission, also known as GRAIL. The twin spacecraft, named Ebb and Flow, have since created a high-resolution gravity field map of the moon, revealing to scientists its internal structure and composition in unprecedented detail.
The map is the highest resolution gravity field map of any celestial object in space, and it will help researchers better understand how Earth, Mars, Mercury, and other rocky planets formed and evolved 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,” said GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology in Cambridge. “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.”
VIDEO CREDIT: NASA/JPL-Caltech/MIT/GSFC
The map reveals tectonic structures, craters, volcanic landforms, and other features that have never been observed in such detail. Ebb and Flow have shown that the moon has been pulverized by the leftover materials which built the solar system much harder than previously thought. Its gravity field is unique from any other terrestrial planet in the solar system. The bulk density of the moon’s highland crust is much lower than scientists had previously believed, which agrees with data obtained from samples returned to Earth by Apollo astronauts over 40 years ago. The lunar gravity field preserves a record of the moon’s violent history of impacts endured throughout its existence, and the map reveals fractures in the moon’s interior that extend into its deep crust, possibly even into its mantle.
“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 kilometers), which is about 6 to 12 miles (10 to 20 kilometers) 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.”
The twin GRAIL spacecraft, launched in September 2011 from Cape Canaveral Air Force Station in Florida, created the map by communicating with each other using radio signals to precisely define the distance between them as they orbit 34 miles above the lunar surface. They can measure the slightest change in distance from one another, down to a few microns—about the diameter of a red blood cell. Flying in formation at 3,600 mph in a near-polar, near-circular orbit, Ebb and Flow travel through an uneven gravity field, which causes slight changes in their speed. The change in speed by one spacecraft as a result of gravity can then be measured as a change in distance between the two vehicles by the other spacecraft. This technique is a first for any mission beyond Earth’s orbit. The Gravity Recovery and Climate Experiment (GRACE) mission has been using the same technique to map Earth’s gravity since 2002.
“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.”
The primary phase of the mission is now complete, and the extended mission will continue until December 17, with the two spacecraft continuing to collect gravity science about our moon as they gradually lower their orbital altitude.
“Next time you look up and see the moon, you might want to take a second and think about our two little spacecraft flying formation, zooming from pole to pole at 3,600 mph,” says David Lehman, GRAIL project manager at NASA’s Jet Propulsion Laboratory. ”They’re up there, working together, flying together, getting the data our scientists need. As far as I’m concerned, they’re putting on quite a show.”