June 7, 1999
Web posted at: 4:56 p.m. EDT (2056 GMT)

Model of enhanced atmosphere due to Leonid meteor shower. Reload browser to repeat animation.

(CNN) -- Boston University astronomers on Monday announced the discovery of a new feature of the Earth's moon -- a sodium-gas "tail" that stretches at least a half-million miles.

The observation was made by chance in November when a Boston University team pointed a sensitive camera in the opposite direction from the moon, attempting to photograph the Leonid meteor storm.

Instead, they recorded a patch of sodium emission in an otherwise moonless sky.

"It grew to be larger and brighter on November 19, and then faded slightly on November 20," said Steven Smith, a research associate in the university's center for space physics.

The team considered several theories that could explain the unusual feature, ruling out a comet, the impact of Leonid meteors upon dust in the solar system, and even possible instrumentation problems.

Researcher Jody Wilson suggested that the mysterious sodium gas might come from the moon, and set out to model it using computer simulation and visualization techniques.

"We found out that when the moon is new, it takes two days or so for sodium atoms leaving the surface to reach the vicinity of the Earth. They are pushed away from the moon by the pressure of sunlight and, as they sweep past us, the Earth's gravity pulls on them, focusing them into a long narrow tail," Wilson said.

In trying to determine if this comet-like appearance of the moon occurred only on nights following a strong meteor shower, as happened with the Leonids, the team examined some earlier data taken at their site in Texas.

During the previous August, similar observations were made, following the new moon of August 21, 1998.

They found the phenomenon was there, but several times fainter, with the same shapes over the same three nights spanning the new moon.

Taken together, the August and the November observations imply that the daily flux of micrometeors that strikes the moon's surface creates an extended tail at all times; it was just so enhanced during the strong Leonid storm that it was observed rather easily, the astronomers said.

The Boston team's findings were presented last week at the Annual Spring Meeting of the American Geophysical Union (AGU) in Boston.

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