FWD: [UASR] Lunar Sodium Tail Discovered
- Posted by : "[UASR]> Perry van den Brink" <owner-uasr@...>
Contact: Shauna LaFauci, 617/353-2399, slafauci@...
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
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
"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
"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 .