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The Moon's changing shape seen from orbit for the first time

Jenny Winder, News Writer
May 31, 2014, 15:20 UTC

Sen—Observations from two NASA missions have allowed scientists to study the Moon’s lopsided shape and how it changes under Earth's influence—a phenomenon not seen from orbit before.

Because orbiting spacecraft gathered the data, the scientists were able to take the entire Moon into account, not just the side that can be observed from Earth. The team drew on studies by NASA’s Lunar Reconnaissance Orbiter (LRO) and Gravity Recovery and Interior Laboratory, or GRAIL, mission.

"The deformation of the moon due to Earth's pull is very challenging to measure, but learning more about it gives us clues about the interior of the Moon," said Erwan Mazarico, a scientist with the Massachusetts Institute of Technology, who works at NASA’s Goddard Space Flight Center.

The mutual pulling of the two bodies is powerful enough to stretch them both. On Earth, the tension has an especially strong effect on the oceans, because water moves so freely, and is the driving force behind tides.

Earth's distortion of the Moon, called the lunar body tide, is more difficult to detect, because the Moon is solid except for its small core. Even so, there is enough force to raise a bulge about 51cm (20 inches) high on the near side of the Moon and similar one on the far side.

The bulge shifts a few inches over time. Although the same side of the Moon constantly faces Earth, the side facing Earth appears to wobble because of the tilt and shape of the Moon's orbit. From the Moon's viewpoint, Earth doesn't sit motionless but moves around within a small patch of sky and the bulge responds to Earth's movements.

"If nothing changed on the Moon—if there were no lunar body tide or if its tide were completely static—then every time scientists measured the surface height at a particular location, they would get the same value," said Mike Barker, a Sigma Space Corporation scientist based at Goddard and co-author of the new study.

Artist Concept of the Lunar Reconnaissance Orbiter with Apollo mission imagery in the background. Image credit: NASA

To search for the tide's signature, the scientists turned to data taken by LRO's Lunar Orbiter Laser Altimeter, or LOLA, which is mapping the height of features on the Moon's surface. The team chose spots that the spacecraft has passed over more than once. The researchers then calculated whether the height had risen or fallen from one satellite pass to the next; a change indicated a shift in the location of the bulge.

To reconstruct the spacecraft's orbit with sufficient accuracy, the researchers needed the detailed map of the Moon's gravity field provided by the GRAIL mission.

Artist concept of GRAIL mission. Image credit: NASA/JPL

The estimated size of the tide confirmed the previous measurement of the bulge. The other value of great interest to researchers is the overall stiffness of the Moon, known as the Love number h2, and this was also similar to prior results.

"This research shows the power of bringing together the capabilities of two missions. The extraction of the tide from the LOLA data would have been impossible without the gravity model of the Moon provided by the GRAIL mission," said David Smith, the principal investigator for LRO's LOLA instrument and the deputy principal investigator for the GRAIL mission.