GRAIL Gravity Maps Reveal Moon’s History

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Gravity distributions on the Moon as revealed by GRAIL show that the Moon’s crust has been largely pulverized.

Earlier this week, I posted how LRO data showed that the Moon’s crust has been mixed by large impacts. Today I learned there was some independent confirmation of this idea already published in the literature. In an article published back in December, researchers led by Maria Zuber (MIT) explained how GRAIL’s gravity maps of the Moon show the same thing.  The reason this is back in the news is that Dr. Zuber was honored at MIT for her contributions to the GRAIL mission.

The probes, which were intentionally crashed into the Moon on December 17, spent much of 2012 mapping the variations in the Moon’s gravity field. The probes accomplished this by following similar orbits that kept them between 175 to 225km (108 to 140 miles) apart. The two probes beamed microwaves between one another, which allowed them to constantly keep tabs on exactly how far apart they were from one another. The probes also tracked on their position relative to Earth, which allowed their locations to be known with very high accuracy.

Slight gravitational differences arise on the Moon because of topography and rock type. High areas have more gravity than low areas, because there is more rock present to pull downwards. Rock types also play a major role in determining the gravitational field. Depending on their composition, some rocks are denser and heavier than others. The denser, heavier rocks also pull downwards more strongly than the lighter rocks. As the probes orbited the Moon, they were able to measure slight changes in their velocity as small as 50 nanometers a second. Those slight velocity changes are the result of small gravitational changes accelerating or decelerating the probes. This level of accuracy allowed the creation of extremely high resolution gravitational maps. Previous maps had an accuracy of approximately 10mGal but the new maps were accurate down to 0.1mGal. (The milliGal, abbreviated mGal, is the standard measurement of gravitational acceleration, and is equal to 0.01 m2/s.) The new maps are so good that the gravitational changes caused by every crater larger than 30 km (18.5 miles) were detected.

To study the interior, first the effects of topography needed to be removed. Fortunately, the LRO’s laser altimeter experiment has made over 6 billion point during its stay at the Moon, providing an extremely detailed topographic map for the authors of this study to work with. By removing the effects of topography, the authors were able to get an idea of what was going on at depth. What they saw surprised them. “It’s essentially like removing a veil to reveal the gravity due to the inside of the planet,” Zuber said. “And when we saw those maps, we were just speechless.”

Once corrected for topography (known as a Bouguer correction), gravitational anomalies almost completely disappeared from the map. The only large anomalies underneath the largest impact basins, where denser lavas had flooded the surface. Theoretically, slight regional variations in the crust had been expected. In reality, it was likely that those variations had been obliterated by subsequent events. That idea comes from another interesting finding: the Moon’s crust was not as thick as had previously been thought. Most models that simulated the formation of the Moon suggested that the crust would be between 60 and 100km (37 to 62 miles) thick. Instead, GRAIL found that the lunar crust is only 35 to 43km (22 to 26 miles) thick. In fact, many of the largest impact basins on the Moon had penetrated all the way down to the lunar mantle!

The thinner than expected crust is most easily explained by large impactors. These bodies may have been even more common than originally thought. When they slammed into the Moon, they ejected a significant portion of the original crust into space, and pulverized the rest. The end result is a thinner, homogenous crust. If this is the case (which looks more likely in light of the newer LRO data), then the inner Solar System was bombarded much more intensely than previously thought. Combined with the LRO data, we now have two independent sets of data that tell the same story.

So what does this mean for the Moon’s history? The GRAIL data set is another piece of evidence showing that the Moon’s original crust has been battered beyond recognition. This also means that modelers will also need to correct for a significant amount of missing material to calculate the Moon’s original bulk chemistry – an important stepping stone to understanding how moons and planets form. Additional understanding of the Moon’s history may be gleaned from GRAIL data, which is still being processed.

IMAGE: NASA / GSFC / Science Visualization Studio

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