The observations were made at the McDonald Observatory in Fort Davis, Texas, on nights following the Leonid meteor shower of November 1998. The tail of sodium gas was seen to distances of at least 500,000 miles from the moon, changing its appearances over three consecutive nights.
These results were presented on Tuesday, June 1st, at the Annual Spring Meeting of the American Geophysical Union (AGU) in Boston. Complete papers will appear in the AGU journal Geophysical Research Letters in its June 15th edition.
Since the days of NASA's Apollo Program of lunar, scientists have known that the moon has a very thin atmosphere. "It is one continuously being produced by evaporation of surface materials, and then continuously being lost by escape or impact back onto the surface," said Michael Mendillo, professor of astronomy. Such processes act daily, and so while there is always some atmosphere present, the various gases are being cycled through it. It is a "transient atmosphere" similar to the ones found in comets.
Ten years ago, groundbased telescopes revealed that sodium gas (Na) was in the lunar atmosphere, an element that can be used to trace the shape and behavior of such a thin atmosphere. Sodium reflects sunlight very efficiently and so has become a standard way for space scientists to study gases that are otherwise difficult to see.
"There are less than 50 atoms of sodium per cubic centimeter in the atmosphere just above the surface of the moon," says Jeffrey Baumgardner, Senior Research Associate in the University's Center for Space Physics.
"But the most modern camera systems built in our lab can photograph such a thin environment out to distances that are several times the radius of the moon," Baumgardner added. In contrast to the tenuousness the moon's atmosphere -- only 50 atoms per cubic centimeter -- there are 10**19 molecules per cubic centimeters in earth's atmosphere at the surface.
During the November observations, the BU team pointed their sensitive camera in the opposite direction from the moon and recorded, just by chance, images of the tail in an otherwise moonless sky.
"At the time of the Leonid meteor shower on November 17, 1998, the moon was in 'new' phase, impossible to see at its location between the Earth and the Sun," described Dr. Steven Smith, research associate in the center for space physics.
"Our team was operating on the nightside of the earth, essentially looking away from the sun and moon, searching for meteor effects in our atmosphere." After one night of uneventful observations, on November 18th our imaging system detected a small patch of sodium emission in the dark skies above west Texas. "It grew to be larger and brighter on November 19th, and then faded slightly on November 20th," Smith said.
The BU team considered several theories that could explain these unusual features, ruling out a comet, the impact of Leonid meteors upon dust in the solar system, and even possible instrumentation problems.
Dr. Jody Wilson, research associate in the BU space physics group 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 explained.
"The pieces of the puzzle fit together rather well," Mendillo added. "While some of the Leonid meteors burned up in their streaks through the earth's atmosphere on the night of November 17th -- producing spectacular showers in some locations -- others crashed into the moon's dusty soil liberating sodium gas. These atoms, speeding away from the earth-moon system, were then captured in photographs from our instrument in Texas several days later, looking down the length of the tail."
"If it were bright enough for the human eye to see, perhaps a thousand times brighter," Baumgardner added, "it would be a glowing orange cloud dominating the nighttime, moonless sky."
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 BU team examined some earlier data taken at their site in Texas.
During the previous August, similar observations were made, fortuitously on the nights following the new moon of August 21, 1998. "It was there," Dr. Smith said, "several times fainter, but with the same shapes over the same three nights spanning the new moon, just as occurred in November."
Taken together, the August observations without meteors and the November observations with meteors 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.
"What we do not know yet is whether the entire atmosphere of the moon is produced by meteors, or just the small component of fast sodium atoms that can escape from it," Mendillo said.