Departments News & Features Research Briefs In the News Obituary Contact Us Advertising Rates

2 July 1999 Vol. III, No. 1 Feature Article Discovery of lunar tail advances study of the moon's atmosphere By Hope Green Boston University astronomers have discovered a tail of sodium gas half a million miles long streaming from the moon. In the process, they have developed a novel observation technique that could aid future studies of the lunar atmosphere. "For the first time we have direct evidence that meteors hitting the lunar surface vaporize sodium in the rock and soil, contributing to the moon's sodium atmosphere," says Jody Wilson, a research associate in the Center for Space Physics (CSP). "We have also learned that we can observe the moon's atmosphere during the new-moon phase. No one had ever thought that would be possible." The findings of the BU research team, led by Michael Mendillo, CAS professor of astronomy, were published in the June 15 issue of the American Geophysical Union journal, Geophysical Research Letters. At the McDonald Observatory in Fort Davis, Tex., CSP research associate Steven Smith photographed what appeared as a mysterious cometlike smudge -- invisible to the unaided eye -- on three straight nights following the Leonid meteor shower last November. He and Wilson were able to rule out a number of explanations for the sighting including a comet, the impact of Leonid meteors on dust in the solar system, and dew on the lens of their sensitive, all-sky camera. Upon the pair's return to BU, Wilson suggested the possibility of a lunar sodium tail to their colleagues and performed computer modeling to test the theory. Into thin air Scientists have known for some time that the moon has a very thin, tenuous atmosphere through which gases are constantly cycling. The BU group is interested in learning what processes contribute to this atmosphere and to what extent, including the pounding action of meteors. The Leonid meteor shower consists of dust particles falling off the nucleus of Comet Tempel-Tuttle. The celestial light show occurs annually on November 17, but every 33 years Earth and its moon pass through Tempel-Tuttle's orbit, so meteor encounters occur with more dramatic frequency. Last November the three-decade cycle was at such a peak. When meteors hit Earth's atmosphere, they burn up on contact, creating a shooting-star effect. Around the moon, however, there is little to deflect the foreign debris from reaching ground. At the outset, Wilson says, the BU astronomers had hoped their observations from Fort Davis would be "a unique opportunity to see the moon really getting pummeled by meteors" and to observe whether this action released significant amounts of sodium gas from the lunar soil or from the meteorites themselves. Sighting confirmed As expected, the moon was in its new (invisible) phase on November 17, situated between Earth and the sun. The scientists knew that any sodium-liberating effects of smashing meteors would be most visible as the moon entered its first quarter several nights later. Consequently, for the time being they focused on a project Smith was conducting, which aimed to study the effect of meteors on the sodium layer in Earth's upper mesosphere. The excitement began when Smith trained his camera on the moonless sky November 18 and spotted the mysterious sodium cloud. The streak above west Texas, Smith says, grew larger and brighter on November 19, then faded slightly on November 20. Model image of the moon's sodium tail showing the Earth enveloped by the streaming sodium atoms. Model imaging by Jody Wilson After returning to Boston, Wilson modeled his lunar-tail hypothesis using computer simulation and visualization techniques. The BU team also examined some earlier all-sky photographs, taken at the Texas site on nights following the new moon of August 21. Close inspection revealed that the tail was there, though much fainter than in November. "We found 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," Wilson says. "They are pushed away from the moon by the pressure of solar radiation, and as they sweep past us, the Earth's gravity pulls on them, focusing them into a long, narrow tail." "A glowing orange cloud" The phenomenon has been observed only during the three nights before, during, and after the time of a new moon, when Earth sits directly within the 500,000-mile ribbon of sodium atoms. The viewer's perspective is down the length of the tail, and the optical effect is of a faraway cloud. Sodium is known for reflecting sunlight very efficiently and so has become a standard marker for studying atmospheres that are difficult to see. The Leonids made the tail appear brighter than it did in August, say the BU scientists, by vaporizing a fresh supply of sodium from the moon's surface and possibly adding to the sodium haze themselves. "If it were bright enough for the human eye to see, perhaps a thousand times brighter, it would be a glowing orange cloud dominating the nighttime, moonless sky," notes research team member Jeffrey Baumgardner, a CSP senior research associate. In the future, the scientists hope studying the lunar tail will help them more fully understand factors responsible for producing sodium in the moon's thin, transient atmosphere. "It's a simple system," Wilson says. "You take a camera with a fish-eye lens, put a sodium filter in, and start taking pictures of the sky. If we photograph the tail once a month, over a long period of time, we might be able to see some systemic change that corresponds to other meteor showers, or something happening on the sun. It can help us determine whether sunlight is partly responsible for creating the moon's atmosphere, or whether solar wind -- ions and electrons -- may also knock sodium atoms out of the soil."