JOINT NEWS RELEASE - 1 June 1999 BOSTON UNIVERSITY CENTER FOR SPACE PHYSICS and AMERICAN GEOPHYSICAL UNION 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 appearance 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. It has been known since the days of NASA's Apollo Program of lunar exploration 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 re-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. What was unusual about the most recent observations is that 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, a streak of sodium emission was seen 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. ---------------------------------------------------------------- Illustrations: 1. Color Image: [Available at http://vega.bu.edu/moontail/] (top): A composite CCD all-sky image of the night sky taken through a sodium filter on the night of 19 November 1998. North is at the top and east is to the left. The sodium "spot" is highlighted in a box to the right. In the all-sky image, a meteor streak is seen to the northeast and the constellation Orion below it to the east. (bottom): Computer simulation results of the comet-like atmosphere of the Moon. On the left, sodium atoms ejected by meteor impact leave the Moon with speeds close to the required escape speed of 2.3 km/s. Light from the Sun (off to the left, not shown) exerts a pressure on the Na atoms, pushing them to higher speeds, and eventually to escape. They travel in approximately straight line paths until they reach the Earth, where gravity focuses them into a stream of atoms moving past the planet. On the right, a computer generated image is shown to depict the appearance of the "sodium spot" caused by looking down the long axis of the tail from the nightside Earth. [Note: Black and White versions of each panel available upon request.] 2. A sketch of the mechanism responsible for the extended lunar tail of sodium atoms. On the left, and not to actual scale, the Moon is located between the Sun and the Earth, in its "new" phase. Meteors strike the lunar regolith (surface soil), liberating various gases, one of them being Na atoms. The high speed impact of meteors gives the ejected atoms velocities close to that needed to escape from the Moon. The extra push required is provided by solar energy. Sunlight striking individual sodium atoms impart a small amount of momentum, causing the atom to accelerate away from the Moon. These high speed gases reach the vicinity of the Earth in about two days. The pull of the Earth's gravity causes the atoms to be focused into a stream that passes over and past the Earth. [Also at Web site quoted above] 3. A movie of observations and an animation of the model results also appear on the web site. 4. Science writers and science public information officers (only) may obtain a copy of the Smith et al. observation paper, "Discovery of the Distant Lunar Sodium Tail and its Enhancement Following the Leonid Meteor Shower of 1998," and the Wilson et al. modeling paper, "Modeling an Enhancement of the Lunar Sodium Tail During the Leonid Meteor Shower of 1998," from Daryl Tate at the American Geophysical Union . ------------------------------------------------------------------------- Additional Information and Terms: 1. The Leonid meteor shower occurs each November 17th as the Earth passes through the debris left by the passage of comet Tempel-Tuttle near the Earth's orbit around the Sun. Approximately every 33 years, the period of the comet's passage through the inner solar system, a fresh supply of meteoritic debris is left near the Earth. This occurred last in 1966, when spectacular meteor storms were seen in the Midwest USA. And so 1998 and 1999 are the current times to expect such enhanced Leonid meteor events. For example, typical Leonid events have 15 meteors/hour visible to the naked eye. On November 17th, this increased to approximately 250 meteors/hour in some locations. 2. The daily influx of so-called micro-meteors amounts to several tons of material over a 24 hour period. A meteor storm may contain an amount smaller or larger than this, but it is concentrated into a shorter time span, and hence the dramatic displays and interest of space scientists. 3. Lunar Regolith. The surface material on the Moon is a fine dust that results from the constant bombardment and disintegration of rocks by meteors, large and small. The chemical composition of the regolith is well known since Apollo Astronauts brought back ample samples for analysis. 4. Other Sources of the Moon's Atmosphere: In addition to meteors, sunlight is capable of evaporating gases from the lunar regolith. Also the stream of electrically charged particles from the Sun, called the solar wind, impacts the regolith and can cause gases to be ejected. The exact mix of these possible sources is not known, and the study of each is an active area of planetary science research. 5. CCD Imaging: The detector system used to study the faint sodium atmosphere of the Moon is a so-called CCD (charge-coupled-device) chip. This detector or CCD camera is the final component of a modern telescope system. Depending on the field of view needed to capture the target under study, a CCD camera may be on very large or very small telescopes. In the case reported on here, where the widest possible field of view was used, the optical components were an "all-sky" lens system, one popularly called a "fish-eye" lens. In this way, the entire sky can be photographed in a single image. Of course, the object under study must be large in angular extent to appear in such an image. The "sodium spot" found here spanned nearly 10 degrees of sky, comparable in size to 20 full Moons. 6. Names and Addresses: Michael Mendillo, Professor of Astronomy Jeffrey Baumgardner, Senior Research Associate Jody Wilson, Research Associate Steven Smith, Research Associate All at the Center for Space Physics, Boston University, 725 Commonwealth Ave., Boston, MA 02215 Tel:(617)353-5990 Fax:(617)353-6463 E-mail: mendillo@bu.edu jeff@spica.bu.edu jkwilson@bu.edu smsm@bu.edu Boston University Public Relations Office: Mr. Kevin Carelton, Director 25 Buik Street Boston, MA 02215 Tel: (617)353-3669, Fax: (617)353-4048, e-mail: carleton@bu.edu Ms. Shauna LaFauci Tel: (617)353-2399, Fax: (617)353-4048, e-mail: slafauci@bu.edu American Geophysical Union Public Affairs Office: Mr. Harvey Leifert Public Information Manager Tel: 202-939-3212 e-mail: hleifert@agu.org