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Moon Hit With a Double Whammy of Impacts

16 September 2010 5:12 pm
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The moon may be only 384,000 kilometers away, but that doesn't mean it isn't full of mystery. Scientists studying the first year's worth of data from NASA's Lunar Reconnaissance Orbiter (LRO) have found unexpected mineral deposits, the possibility that our natural satellite was bombarded in two waves in its early history, and virtually no trace of a pristine lunar surface. If scientists can confirm these findings, they could take big steps toward understanding not only the moon's distant past but also Earth's and the rest of the solar system's as well.

In a news briefing held online today, in conjunction with three papers published in Science, researchers presented a tantalizing preliminary portrait of what the moon must have endured during its early history, ending around 3.5 billion years ago. LRO's Lunar Orbiter Laser Altimeter (LOLA), which can determine the height of a small patch of lunar soil to within 10 centimeters, has enabled scientists to compile a detailed topographical map of 5185 lunar craters, all more than 20 kilometers in diameter.

Planetary geologist James Head III of Brown University told reporters that the LOLA data on the moon's impact history support an emerging idea about early bombardment in the solar system. By analyzing the topographical map, the researchers could determine when objects struck existing craters, and they could measure the size of the objects. The data suggest that larger objects tended to hit the moon earlier in its history than did smaller objects. There seemed to be two stages of impacts, he said, and they were "distinctly different." That's an important clue, he added, about what was going on in the early solar system—including on Earth—because "the same population [of objects] that was hitting the moon certainly was hitting the Earth." Why the moon took such big hits early on isn’t clear.

Planetary scientist Benjamin Greenhagen of NASA's Jet Propulsion Laboratory in Pasadena, California, described a study he co-authored of lunar surface chemistry by LRO's Diviner Lunar Radiometer Experiment (DLRE), which analyzes chemical composition by reading the lunar soil's infrared emissions—its internal heat. He said this method gives a much more detailed picture than using the heat from reflected sunlight, the method used by previous studies, because each mineral emits a distinctive radiation signature. The Diviner survey has provided the first chemical map of the entire lunar surface, he added.

Based on the Diviner data, Greenhagen said the lunar orbiter has located five previously unobserved sites in the moon's highlands that are surprisingly rich in oxygen-bearing silicate minerals. As of now, the reason for the deposits is unknown, but the presence of oxygen in the lunar rock would make those sites attractive for future moon missions, even for human colonies, which could use the oxygen for breathing and as a rocket-fuel component.

Planetary scientist Timothy Glotch of Stony Brook University in New York said the DLRE data also revealed concentrations of what he called highly silicic minerals, such as quartz and feldspar. These minerals, which likely originated deep below the lunar surface, are different from anything found on the lunar surface before, so they raise the possibility of an as-yet-unknown geologic process at work.

Among the mysteries unearthed by Diviner is the apparent complete lack of pristine lunar mantle on the surface. The scientists had hoped to find spots on the moon containing material going back to its formation. But the instrument's surveys turned up no such evidence, even at surface locations considered to be the moon's oldest. Glotch said all of the new mineral discoveries should make for "some new, really juicy targets for future [lunar] exploration."

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