Astronomers have found a new use for the Chandra X-ray observatory: probing the surface of the moon. New observations provide direct evidence of lunar composition, scientists announced yesterday. Knowing exactly what elements make up the satellite and how they are distributed will help researchers determine just how our satellite was formed. In addition, the findings may clear up a decade-old debate about the dark regions of the moon.

So far, everything astronomers know about the moon's makeup has come from samples collected during moon landings and probes sent down to the surface. Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics (CfA) told a symposium in Huntsville, Ala, honoring four years of research with Chandra that the observatory can provide a new way of looking at the moon. "We have moon samples from the six widely-spaced Apollo landing sites, but remote sensing with Chandra can cover a much wider area," he remarks. "It's the next best thing to being there and it's very fast and cost effective." He presented data from Chandra collected in July and September 2001 that indicate the presence of oxygen, magnesium, aluminum and silicon over a large area of the lunar surface. When solar radiation hits the moon, it excites some of the elements' electrons. This energy is quickly reemitted by the elements at a characteristic wavelength in a process known as fluorescence, which Chandra detects.

The results are preliminary, Drake notes, because only a few chips were used to look for fluorescence and the angle between Chandra and the moon made measurements from some areas difficult. But they demonstrate that the process is useful. Future tests will use more chips and look at specific sites, the scientists say. Better quantification of abundance of elements in the moon's crust will help astronomers further test the giant impact theory of moon genesis, which holds that a body roughly the size of Mars hit the Earth about 4.5 billion years ago and that the moon formed from the aggregation of resulting debris.

In 1990 the German Roentgen satellite detected what astronomers thought was an X-ray signal coming from that part of the satellite. Scientists proposed that energetic electrons from the sun were striking the lunar surface and causing these so-called dark-moon X-rays. But Chandra's results reveal that this signal comes not from the moon, but from Earth's own outer atmosphere. Notes Brad Wargelin of CfA, "The observed X-ray spectrum, the intensity of the X-rays and the variation of the X-ray intensity with time can all be explained by emission from Earth's extended outer atmosphere, through which Chandra is moving."