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Lunar Sodium Tail Discovered

Boston University

Contact: Shauna LaFauci, 617/353-2399, slafauci@bu.edu

For Immediate Release: June 2, 1999

BOSTON UNIVERSITY CENTER FOR SPACE PHYSICS DISCOVERS LUNAR SODIUM TAIL

Boston, Mass. -- Boston University astronomers announced today the
discovery of an enormous tail of sodium gas stretching to great distances
from the moon. 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.

For visual information http://vega.bu.edu/moontail .


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