 Earth's Nearest Neighbor
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Oct. 1, 2003 — Space researchers have used invisible X-rays, reflecting off the surface of the moon, to find out what our nearest solar neighbor is made of and how it was formed.
The research, done at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., found oxygen, magnesium, aluminum and silicon present over a large area of the Moon's surface.
The moonshine was analyzed using NASA's Chandra X-ray Observatory, which can pick up light in the X-ray range of wavelengths, unlike the Hubble telescope which can only see visible light.
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When solar X-rays strike the surface of the Moon, electrons in the atoms exposed to the radiation are knocked out of the inner parts of the atoms. Other electrons rush to fill the gaps, and in the process convert their energy into fluorescent X-rays. The solar X-rays that drive this process originate in the Sun's hot outer atmosphere.
So sensitive are Chandra's detectors that, based on the signature of the X-ray light coming out of atoms on the surface of the Moon, it's possible to determine the type and quantity of a range of elements on the Moon's surface.
The abundance and distribution of those elements will provide vital information on how the Moon was formed.
"We see X-rays from these elements directly," said Jeremy Drake of the Harvard-Smithsonian Center. "We have [physical] Moon samples from the six widely-spaced Apollo landing sites, but remote sensing with Chandra can cover a much wider area," he said. "It's the next best thing to being there, and it's very fast and cost-effective."
By mapping the distribution of a range of elements over a wide area of the Moon, researchers hope to test theories on how the Moon was formed. The most popular is the 'giant impact' theory, which says that when the Earth was young, around 4.5 billion years ago, a massive body the size of Mars collided with our planet. This impact flung molten debris from the mantle of both the Earth and the object which hit us into orbit around the Earth. Over the course of tens of millions of years, the debris stuck together to form the Moon.
If the giant impact theory is correct there should be a high degree of correlation between the distribution of aluminum and other elements over a wide area of the Moon and the manner in which they're distributed in the Earth's mantle. Drake and colleagues are in the process of collecting the data to test the giant impact hypothesis.
"One early result," he said, "is that there is no evidence for large amounts of calcium, so cheese is not a major constituent of the Moon."
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