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Week of 9 October 1998 Vol. II, No. 9 Feature Article Viewing the invisible For this astronomer, discovery follows the dictum: let there be no light By Eric McHenry Michael Mendillo couldn't have asked for a better visual aid. A spectacular full moon suspended itself over the Tsai Center on October 5, the evening of the 54th University Lecture. Mendillo, CAS professor of astronomy and ENG professor of electrical and computer engineering, was speaking on a topic that required extensive illustration -- Astronomy Through a Glass, Darkly: Searching for Extended Atmospheres of Planets, Moons, and Comets. The sky complied beautifully. The University Lecture The University Lecture was established at Boston University in 1950 as a semiannual event honoring members of the faculty engaged in outstanding research. The lecture, given only once a year since 1958, provides an opportunity for all members of the University community -- as well as the general public -- to hear a distinguished scholar and learn about a topic of recognized excellence. Each spring, all members of the faculty are invited to make nominations for the subsequent fall's lecturer. The University lecturers from the previous five years act as the nominating committee. Past University lecturers include Lynn Margulis, Phyllis Curtin, and Glenn Loury. With numerous screen projections and an occasional reference to the striking moon visible "over Kenmore Square and the darkened Fenway Park," Mendillo discussed the difficulties astronomers face when studying dim areas around bright celestial bodies. In successfully confronting such challenges, Mendillo and colleagues have made many significant contributions to the study of lunar and planetary atmospheres. "To take a picture of a weak atmosphere around the moon is a formidable task," said Mendillo. "The moon is very bright, and very bright signals do very bad things to sensitive detectors. Consider that the Hubble Space Telescope is essentially forbidden by NASA canon law to look at anything bright; there are no Hubble pictures of Mercury, Venus, or even the moon." When bright objects further obscure what is already faint, astronomers must be resourceful, Mendillo said. Through considerable trial and error, he and his colleagues in the department of astronomy have developed several effective ways of viewing the nearly invisible. Larger telescope lenses take in more light, he said. Traditionally, astronomers have sought to affix the most effective detectors to telescopes with the biggest available lenses. "Jeff Baumgardner [CAS senior research associate] and I wondered why an otherwise normal evolutionary step had been passed over," said Mendillo. "No one, it seemed, had ever put the world's best detector behind the world's smallest telescope. And so that is what we decided to do." Baumgardner built an instrument with a lens only four inches in diameter, which he and Mendillo installed at the Boston University Station at the McDonald Observatory in southwest Texas. They further worked to eliminate undesired light by blocking the bright object at the center of their viewing field -- whether Jupiter, the earth's moon, or the Hale-Bopp comet -- with a black dot of appropriate size, and by using a filter that allows only the light of sodium atoms to reach the detector. In the case of the moon, they took a few additional steps, such as observing the area around it during four full lunar eclipses and "painting everything inside the telescope black as sin." "This," said Michael Mendillo to the 1998 University Lecture audience, "is the first picture ever taken of the moon's atmosphere." Mendillo and colleagues have discovered extensive sodium atmospheres surrounding the moon, Jupiter, and the Hale-Bopp comet. Photo by Vernon Doucette "It may seem odd that to find very faint things, we throw away all the light in the spectrum that comes from that object, except for one wavelength, or color," said Mendillo. "This filtered imaging is actually the key to success, in that the darkness of one color provides the contrast not possible with full, or white, light." With filtered imaging, Mendillo and colleagues have made unprecedented observations of extended sodium atmospheres around Jupiter, the Earth's moon, and the Hale-Bopp comet. Each, he noted, is "a transient atmosphere." "If it is always there, and it is always escaping, then it is always being formed," said Mendillo. "There is a dynamical equilibrium: the nebula is continuously created and lost, and some of it is always there to see." A projection illustrating this phenomenon included a pair of analogous images -- water being poured into a punctured bucket, and money entering and leaving a personal checking account. Revealing the presence of these atmospheres, Mendillo said, can help guide investigators to areas of productive research. Sodium, he said, "is not a particularly important element in the cosmos. Doctors tell you that a low-sodium diet is good, and that is probably true. It is important for us because it is easy to detect and therefore can serve as a tracer of other species that may be more abundant but less easily detected." Often, Mendillo added, the sodium itself is in some way telling. The Hubble telescope recently found both oxygen and sodium in the atmosphere of Europa, one of Jupiter's moons. The sodium's source could be the large nebula Mendillo and colleagues observed emanating from another of Jupiter's moons, Io. Or it could be coming from frozen salt water on Europa itself. If that's the case, he said, then Europa is intriguingly earthlike in a couple of respects -- it has oxygen in its atmosphere and salt water on its surface. "And so, one of the great questions of humanity, the origin of life in our solar system, may be answered in part by a careful analysis of the size and shape of Europa's sodium atmosphere," Mendillo said. "At the very least, we have another opportunity to take a picture of something you cannot see."