A Lunar Tail

The moon already had a face; now it also has a tail. Astronomers from Boston University's Center for Space Physics discovered the faint orange wisp of neutral sodium atoms, perhaps over half a million kilometers long, during the Leonid meteor shower last November. Their findings will be published in the 15 June issue of Geophysical Research Letters.

Since the days of the Apollo moon landings, scientists have known that the moon has something akin to an atmosphere: a thin mix of gases generated as surface material evaporates under the constant bombardment of energetic particles and photons from the sun. But the moon's gravity is too weak to hold on to these gases, and they are steadily blown away by the pressure of sunlight, resulting in a tail. During the new moon, when the moon is between Earth and the sun, the tenuous tail sweeps across Earth. But nobody knew how long or pronounced it was.

Steven Smith, Jody Wilson, Michael Mendillo, and Jeffrey Baumgardner noticed the tail by accident on 18 November 1998, when they were using a sensitive camera at McDonald Observatory in Fort Davis, Texas, to study wavelike patterns in a sodium-rich layer in Earth's mesosphere. After ruling out several alternative sources, they concluded that the vague orange light must come from a long tail of sodium atoms stretching across the sky from the moon.

The team says the tail measures well over 400,000 kilometers and could be as long as 800,000 kilometers. When they searched images made 3 months earlier, they also noticed the faint glow, but much weaker; the astronomers conclude that the impacts of the Leonid meteors on the moon's surface released a pulse of sodium which made the tail two to three times brighter.

David Lynch of the Aerospace Corp. in Los Angeles, who studied the effects of Leonid dust on spacecraft, says it's very surprising that the meteor shower would have such a strong effect. During the shower, "every square meter of the moon is hit by one tiny particle," he says. "So only a very small percentage of the surface is influenced by the Leonids." Mendillo, however, thinks Leonid dust could be responsible for brightening the tail. "It's not much mass," he says, "but these particles have a very high speed."