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GRAIL Mission Tells New Story About The Moon’s Face

November 11, 2013
Image Caption: Using a precision formation-flying technique, the twin GRAIL spacecraft mapped the moon's gravity field, as depicted in this artist's rendering. Credit: NASA/JPL-Caltech

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redOrbit Staff & Wire Reports – Your Universe Online

Scientists have long believed that the moon’s trademark craters were created from the impact of massive asteroids, but new research appearing in the journal Science suggests that smaller planetoids might actually have been responsible for pummeling the lunar surface.

The lunar craters were believed to have been created approximately four billion years ago, during an event known as the Late Heavy Bombardment (LHB). During the LHB, a storm of giant asteroids collided not only with the moon, but also with Mercury, Mars, Venus and the Earth. However, in the new study, researchers from the Massachusetts Institute of Technology (MIT) and their colleagues claim that this might not be the case after all.

The study authors analyzed data obtained from NASA’s twin GRAIL probes, which orbited the moon last year collecting gravitational data. Maria Zuber, a geophysics professor and the GRAIL principal investigator at MIT, and her associates used the satellites’ information to craft a detailed map of the moon’s crust, showing regions where the surface becomes thicker or thinner.

They discovered that, in general, the moon’s far side has a thicker crust than the near side. Zuber called the results “very interesting,” noting that scientists believed that they knew the approximate sizes of the impacting asteroids that created the larger craters on the near side. She added that what her team’s research indicates is that “the flux of large impacting bodies during the Late Heavy Bombardment has been overestimated.”

According to NASA, the majority of the largest near-side impact basins that could have been used for determining their dimensions have been filled with lava flows, masking clues about the shape of the land that could have been useful for discerning their dimensions. However, the GRAIL data has allowed scientists to redefine the sizes of those craters, and the new impact simulations conducted by Zuber’s team suggests that impacts into the thinner crust regions would have created basins up to twice the diameter of similar impacts into thicker crust.

Those simulations, which were generated by University of Paris post-doctoral researcher Katarina Milijkovic, included crustal thickness data obtained from GRAIL and estimates of the moon’s early internal temperatures based on thermal modeling and analysis of lunar lava deposits. The researchers found that more volcanic activity took place on the near side, meaning that the far side had lower internal temperatures.

With crustal thickness and temperature data incorporated into the model, Milijkovic then simulated the effects, on both the moon’s near and far sides, of impacts by asteroids of the same size and velocity. She found that identical asteroids would have had very different impacts on the two sides, creating basins on the near side that were as much as twice as large as those on the far side – a result that matches the size distribution of structures seen today.

“Zuber says the near side’s thinner crust and higher temperatures may have made the surface more deformable than the thicker, cooler crust of the moon’s far side. These results, she says, suggest that the LHB may have involved less massive asteroids than scientists have thought,” the institute added. “Why the near side should have larger impact basins than the far side is puzzling, since previous work has shown that the impact flux on both sides should be about the same. According to the team’s results, the far side of the moon may better reflect the size distribution of asteroids that pummeled the early inner solar system.”

Originally launched in September 2011, the probes originally known as GRAIL A and GRAIL B (later renamed Ebb and Flow) traveled in a nearly circular orbit near the lunar poles, according to NASA. They traveled as an altitude of approximately 34 miles, with the distance between the two vehicles becoming slightly altered as they passed over regions of greater or lesser gravity. The GRAIL mission officially ended last December.


Source: redOrbit Staff & Wire Reports - Your Universe Online



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