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Lunar Prospecting with Chandra

Optical and X-ray views of the Moon
The Chandra observations (right) of the bright portion of the Moon detected X-rays from oxygen, magnesium, aluminum and silicon atoms.
Despite being perhaps the most famous object in the night sky, astronomers are still unsure exactly how the Moon was formed. While several theories abound, there is still much to be learned about Earth's satellite. Knowing the elements of the lunar surface and where they are found would be an important clue in the mysterious origin of the Moon.

In the absence of sending people to pick up samples, astronomers are trying to use other methods to learn about the Moon's surface. A team recently used X-ray data collected by NASA's Chandra X-ray Observatory to see what could be revealed.

"We have Moon samples from the six widely-spaced Apollo landing sites, but remote sensing with Chandra can cover a much wider area," said Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics. "It's the next best thing to being there, and it's very fast and cost-effective."

The Chandra observations of the bright portion of the Moon detected X-rays from oxygen, magnesium, aluminum and silicon atoms. The X-rays are produced by fluorescence when solar X-rays bombard the Moon's surface.

According to the currently popular "giant impact" theory for the formation of the Moon, a body about the size of Mars collided with the Earth about 4.5 billion years ago. This impact flung molten debris from the mantle of both the Earth and the impactor into orbit around the Earth. Over the course of tens of millions of years, the debris stuck together to form the Moon. Measuring the amount and distribution of aluminum and other elements over a wide area of the Moon will help to test the giant impact theory.

Illustration of the geocorona of Earth
Illustration of Earth's geocorona

Chandra's observations have also solved a decade-long mystery about X-rays detected by ROSAT (a now-defunct X-ray telescope that flew during the 1990s) that were thought to be coming from the dark portion of the Moon.

"Our results strongly indicate that the so-called dark Moon X-rays do not come from the dark side of the Moon," said Brad Wargelin of the Harvard-Smithsonian Center for Astrophysics.

Instead, Chandra shows that the X-rays from the dark Moon can be explained by radiation from Earth's geocorona (extended outer atmosphere) through which orbiting spacecraft move. These geocoronal X-rays are caused by collisions of heavy ions of carbon, oxygen and neon in the solar wind with hydrogen atoms located tens of thousands of miles above the surface of Earth. During the collisions, the solar ions capture electrons from hydrogen atoms. The solar ions then kick out X-rays as the captured electrons drop to lower energy states.

"This idea has been kicking around among a small circle of believers for several years supported by theory and a few pieces of evidence," said Wargelin. "These new results should really clinch it."

Chandra X-ray Observatory