Our atmospheric moon.

New observations reveal that the moon's tenuous atmosphere extends twice as far above the lunar surface as previous observations had shown. The new images, taken during a lunar eclipse, also suggest that sunlight striking the moon probably generates its thin atmosphere.

Michael Mendillo and Jeffrey Baumgardner of Boston University reported their findings in the Oct. 5 Nature and provided further details later that month, at the annual meeting of the American Astronomical Society's Division for Planetary Sciences in Kohala Coast, Hawaii.

During a total lunar eclipse, Earth blocks sunlight from reaching the lunar disk, eliminating the glare that usually arises as moonlight scatters in Earth's atmosphere. Mendillo and Baumgardner detected the faint glow of sodium atoms in the moon's atmosphere beyond the shadow cast by Earth.

"We were surprised to find that this glow extended to over nine times the radius of the moon, to a height of about 14,000 kilometers," says Mendillo. "This is nearly twice the size of the lunar atmosphere as recorded in earlier observations that we had conducted at quarter-moon phase." The researchers observed the moon with a small telescope at McDonald Observatory in Fort Davis, Texas.

Astronomers have suggested three sources of energy that might loosen sodium atoms from the moon's surface and generate the atmosphere: micrometeorites, charged particles of the solar wind, or sunlight.

A uniform distribution of micrometeorites would generate a highly uniform atmosphere, not the irregular geometry that's observed, Mendillo notes. If the solar wind were the source, he adds, the atmosphere shouldn't have been visible when the moon was full and was shielded from the sun's stream of charged particles by Earth's magnetic field. These arguments leave sunlight as the most likely explanation. The researchers note that sunlight may also account for the thin atmosphere observed around Mercury and some asteroids.