The moon beams in extreme ultraviolet.

Scanning the cosmos for emissions in an elusive part of the electromagnetic spectrum, a U.S. spacecraft has captured the first images of the moon aglow in the extreme ultraviolet. Astronomers presented the images this week at the annual Lunar and Planetary Science Conference in Houston.

The Extreme Ultraviolet Explorer (EUVE), launched last June, detects this band of radiation, which can't penetrate Earth's atmosphere and is intermediate in energy between the near ultraviolet and X-rays (SN: 5/23/92, p.344). While the craft devotes most of its time to studying the atmospheres of stars many tens of light-years beyond the solar system, it has cast its eye on an object closer to home.

In August, the EUVE recorded extreme-ultraviolet light reflected from the first-quarter moon (see left image above). And in December, the Earth-orbiting craft recorded extreme-ultraviolet light reflected from the full moon (right). In both of these false-color images, yellow denotes the highest intensity, blue the lowest. In the full-moon image, the brightest areas coincide roughly with lunar highlands, while the dark areas align with lava-covered plains known as maria.

Randy Gladstone, a member of the EUVE research team at the University of California, Berkeley, notes that the extreme-ultraviolet glow stems from solar radiation striking the lunar surface.

The glow consists primarily of extreme-ultraviolet photons from the sun that bounce off the moon, he says. But some of the emissions might result f rom solar X-rays that are absorbed by atoms on the moon's surface, causing them to fluoresce at the lower energies associated with specific wavelengths in the extreme ultraviolet.

Gladstone says that spectra of the moon's ultraviolet emissions, already taken by the EUVE but not yet analyzed, should indicate how much of the moonglow comes from reflected light and how much from fluorescence. If fluorescence contributes significantly to the moon's ultraviolet emissions, then such images would provide a new tool for uncovering the relative abundance of elements on the lunar surface, Gladstone says.