NASA’s twin GRAIL (Gravity Recovery and Interior Laboratory) spacecraft have spent the last year mapping the moon’s hidden heart.
Today at 4:28 p.m. CST they’ll be intentionally crashed into a mountaintop near the lunar north pole. You might think this a poor use of a half-billion of your tax dollars, but the mission has been an astounding success.
Now that the paired probes are nearly out of fuel, mission controllers have directed them to the far north of the moon to avoid an unintentional crash near something important like an Apollo landing site.
Nicknamed Ebb and Flow, the GRAIL probes have been orbiting in tandem around the moon for nearly a year, peeling back the moon’s internal layers by mapping variations in its gravity field. As they fly over areas of greater and lesser gravity caused by both visible features, such as mountains and craters, and masses hidden beneath the lunar surface, the distance between the two spacecraft changes slightly. By measuring these slight variations in separation and combining that with accurate time and position information, scientists created the color-coded gravity map above.
The lunar twins discovered the moon’s crust is 21 to 27 miles thick or 6-12 miles thinner than previously thought. It also contains the same amount of aluminum as found in Earth’s crust.
Both findings confirm the moon’s origin via the “giant impact hypothesis.” It’s believed the moon formed from materials launched into orbit when Earth was struck by a Mars-sized planet in the chaotic days of planet formation over 4 billion years ago.
“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,” said GRAIL co-investigator Mark Wieczorek.
GRAIL’s gravity map reveals familiar lunar features like craters, mountains and lava plains, but with its unique ability to probe beneath the surface, the probes discovered a thoroughly fractured and pulverized lunar crust riddled with subsurface “linear gravity anomalies” more than a 100 miles long.
Impacting meteoroids and asteroids common during planetary formation 4.5 billion years ago created these deep cracks in the moon’s crust that later filled with lava and solidified into dense pathways of rock.
Says Jeff Andrews-Hanna, a GRAIL guest scientist with the Colorado School of Mines: “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.”
While plate tectonics has recycled Earth’s early crust, other planets may still retain similar deep fractures attesting to the bombardment experienced during their formation long ago. Maria Zuber, the MIT geoscientist leading the GRAIL mission, thinks it possible that the cracks could have served as refuges for microscopic life on a planet like Mars when its climate changed from wet to the current cold and dry. That’s quite a wonderful thought really – that bombardment during the formation of the moons and planets created sanctuaries for life.
With the two probes now nearly out of fuel and orbiting only as high above the lunar surface as a jetliner, they’re on a collision course with an unnamed mountain near the crater Anaxagoras. The scene will be hidden in shadow at the time of impact. The first to go will be Ebb at 4:28:40 p.m. (CST) with Flow following 20 seconds later.
Slamming into rock at 3,750 mph, each probe will create a faint flash of light upon impact that might be visible through a telescope. Chances are slim, so don’t expect a big bang, but if you’re game to try, you can use the maps here to hone in on the impact location. Who knows, maybe you’ll see a tiny star-like point of light in the darkness at the appointed time. Observers on the East Coast are favored since the flashes will occur when the moon’s in a dark sky. For Duluth, Minn. and around the Midwest the sun will just be setting at that time.
NASA will provide live commentary of the scheduled lunar surface impacts beginning at 4 p.m. CST. Check out NASA TV or USTREAM to watch and listen. The Lunar Reconnaissance Orbiter (LRO) has already taken photos of the site before the crash and will photograph it soon after to record Ebb and Flow’s impact craters.
Visit the GRAIL site for more information about the mission.
UPDATE: Both craft crashed into the mountain at their appointed times. NASA plans to petition the International Astronomical Union (IAU) to name the impact site(s) in honor of Sally Ride, GRAIL team member and the first American woman in space. Ride died earlier this year.