From the June 2014 issue

Web Extra: Cracking the Moon’s code

Our nearest neighbor’s murky past has come into sharper focus thanks in part to recent observations made by NASA’s GRAIL spacecraft.
By | Published: June 23, 2014 | Last updated on May 18, 2023
GRAIL illustration
An artist’s depiction of the twin spacecraft (Ebb and Flow) that comprised NASA’s Gravity Recovery And Interior Laboratory (GRAIL) mission.
Planetary scientists have known since 1959 — when the Soviet Union’s Luna 3 spacecraft returned the first images of the Moon’s farside — that our satellite’s two hemispheres bear little resemblance to each other. The quest to understand this dichotomy took a big step forward in 2012 when NASA launched its Gravity Recovery and Interior Laboratory (GRAIL) mission. The twin space probes, named Ebb and Flow, measured the Moon’s gravitational field with extraordinary accuracy. Scientists on the ground then used this data to build a high-resolution map of the Moon’s crust. The observations show that the farside crust is roughly twice as thick (about 37 miles [60 kilometers]) compared with the nearside (which ranges between 12 and 19 miles [20 and 30km]).

The first video takes viewers on a guided tour of the GRAIL mission from launch to lunar orbit and data collection. Narrated by Massachusetts Institute of Technology planetary scientist Maria Zuber, who was GRAIL’s principal investigator, the animation follows the twin spacecraft from Earth to the Moon, tracking them as they reach their mapping orbit and begin taking data. Moving in tandem, the spacecraft react to changes in lunar gravity by speeding up or slowing down. By precisely measuring the velocity of the two probes relative to each other, scientists calculated how the gravitational pull varied.

The second video presents observations made with one of GRAIL’s lesser-known instruments: the Moon Knowledge Acquired by Middle School Students (MoonKAM for short) camera. The video, taken from the Ebb probe December 14, 2012, shows the Moon’s farside surface in the vicinity of the impact crater Jackson. At the time, Ebb was orbiting about 6 miles (10km) above the surface. The first clip comprises 931 individual frames while the second squeezes in 1,498 frames. The playback runs six times faster than the probe’s true orbital motion.

Credit: NASA/JPL-Caltech/MIT

Credit: NASA/JPL-Caltech/MIT/SRS