GRAIL creates most accurate Moon gravity map (w/ video)

December 5, 2012, NASA
These maps of the moon show the "Bouguer" gravity anomalies as measured by NASA's GRAIL mission. Bouguer gravity is what remains from the gravity field when the attraction of surface topography is removed, and therefore represents mass anomalies inside the moon due to either variations in crustal thickness or crust or mantle density. Red areas have stronger gravity, while blue areas have weaker gravity. Image credit: NASA/JPL-Caltech/CSM

(—Twin NASA probes orbiting Earth's moon have generated the highest resolution gravity field map of any celestial body.

The new map, created by the Gravity Recovery and Interior Laboratory (GRAIL) mission, is allowing scientists to learn about the moon's internal structure and composition in unprecedented detail. Data from the two washing machine-sized spacecraft also will provide a better understanding of how Earth and other in the solar system formed and evolved.

The gravity field map reveals an abundance of features never before seen in detail, such as tectonic structures, volcanic landforms, basin rings, crater central peaks and numerous simple, bowl-shaped craters. Data also show the moon's gravity field is unlike that of any in our solar system.

These are the first scientific results from the prime phase of the mission, and they are published in three papers in the journal Science.

"What this map tells us is that more than any other 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 features such as craters, rilles or mountains."

This movie shows the variations in the lunar gravity field as measured by NASA's Gravity Recovery and Interior Laboratory (GRAIL) during the primary mapping mission from March to May 2012.

According to Zuber, the moon's gravity field preserves the record of impact bombardment that characterized all terrestrial planetary bodies and reveals evidence for fracturing of the interior extending to the deep crust and possibly the mantle. This impact record is preserved, and now precisely measured, on the moon.

The probes revealed the bulk density of the moon's highland crust is substantially lower than generally assumed. This low-bulk crustal density agrees well with data obtained during the final Apollo lunar missions in the early 1970s, indicating that local samples returned by astronauts are indicative of global processes.

These maps of the near and far side of the moon show gravity gradients as measured by NASA's GRAIL mission. Red and blue areas indicate stronger gradients due to underlying mass anomalies. Image credit: NASA/JPL-Caltech/CSM

"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 Mark Wieczorek, GRAIL co-investigator at 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 map was created by the spacecraft transmitting radio signals to define precisely the distance between them as they orbit the moon in formation. As they fly over areas of greater and lesser gravity caused by visible features, such as mountains and craters, and masses hidden beneath the lunar surface, the distance between the two spacecraft will change slightly.

A 300-mile-long (500 kilometer-long) linear gravity anomaly on the far side of the moon has been revealed by gravity gradients measured by NASA's GRAIL mission. GRAIL data are shown on the left, with red and blue corresponding to stronger gravity gradients. Topography data from NASA's Lunar Reconnaissance Orbiter's Lunar Orbiter Laser Altimeter for the same region show a surface saturated with craters (red is higher terrain and blue is lower). The lack of any topographic signature over the gravity anomaly indicates that it is older than the craters, and thus is one of the oldest features on the moon. The units of the gravity gradients are Eötvös, and of the topography are kilometers. Image credit: NASA/JPL-Caltech/CSM

"We used gradients of the 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."

While results from the primary science mission are just beginning to be released, the collection of gravity science by the lunar twins continues. GRAIL's extended mission science phase began Aug. 30 and will conclude Dec. 17. As the end of mission nears, the spacecraft will operate at lower orbital altitudes above the .

A profile across one of the linear gravity anomalies found by NASA's GRAIL mission shows that it has higher gravity than the surroundings (the anomaly is the red line in graph at bottom left; its location is shown as a blue line in the center of the GRAIL gravity gradient map at top left). Models show that this gravity pattern indicates the presence of giant dikes beneath the surface of the moon (white line in graph at bottom left). A dike is a solidified magma filled crack that forms beneath the surface. Dikes on Earth are sometimes exposed by erosion so that they are visible on the surface, as seen at Ship Rock, New Mexico, pictured here at right. The lunar dikes identified by GRAIL are 50 times longer and 1,000 times wider than the dike seen here. Image credits, Left: NASA/JPL-Caltech/CSM, Right: Photo Copyright © Louis Maher

When launched in September 2011, the probes were named GRAIL A and B. They were renamed Ebb and Flow in January by elementary students in Bozeman, Mont., in a nationwide contest. Ebb and Flow were placed in a near-polar, near-circular orbit at an altitude of approximately 34 miles (55 kilometers) on Dec. 31, 2011, and Jan. 1, 2012, respectively.

NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the mission for NASA's Science Mission Directorate in Washington. GRAIL is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Ala. Lockheed Martin Space Systems of Denver built the spacecraft.

Explore further: NASA's GRAIL moon twins begin extended mission science

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2.4 / 5 (5) Dec 05, 2012
"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."

Not so fast.

The Moon shouldn't have much of a Mantle at all, and should have almost all "crust" since the Mantle should have hardened.

With a lower amount of mass than the Earth, the Moon should have run out of both radioactive material and accretion related heat many times faster than the Earth. This means the crust should have cooled far faster; The Moon should cool six or seven times faster than the Earth. This in turn means the Moon's crust should be much thicker than the Earth's crust*, not thinner, if it had actually come from an Earth impact, since both would have been molten at the same time had they actually been formed from an impact.

*obviously this is limited by the radius of the smaller object.
5 / 5 (1) Dec 05, 2012
"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."

Not so fast.

The Moon shouldn't have much of a Mantle at all, and should have almost all "crust" since the Mantle should have hardened.

Apparently, there was sufficient time after the Moon's formation for the differentiation of mantle and crust --which are characterized mineralogically, as opposed to stratigraphically.

What I noticed was the markedly more intense gravity strength of many of the craters. Does this indicate impact by high-density(Ni-Fe) asteroids, as opposed to more chondritic or cometary bodies?

5 / 5 (2) Dec 06, 2012
Lurker: Are you trying to say that the radioactive elements in the moon should have decayed faster? Last time I checked, decay rates didn't depend on gravitational field strength.

The moon may have lost its heat of accretion faster, but there is still the heat generated from residual decay. Uranium and thorium have enourmously long half lives. Al-26 may have generated a lot more heat during and just after initial formation, but even on earth, that heat has been dissipated.
1 / 5 (6) Dec 06, 2012
The crater and linear gravity anomalies have a simple explanation in an 'Electric Universe', but to consider such heresy will subject you to the ridicule of those who believe that gravity is the only force of consequence (beyond molecules) in the Universe.

not rated yet Dec 06, 2012
Does this indicate impact by high-density(Ni-Fe) asteroids, as opposed to more chondritic or cometary bodies?

Not necessarily. Most of these high-gravity craters are on the side facing away from Earth. The crust in this parts is composed of mafic rock types like andesites or anorthosites with densities lower than for example basalt. We know that many large crater are filled with basaltic lavas which, together with upwelling of deeper mantle plumes, may show up on as areas with higher gravimetric readouts.

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