The amount of water and molecular oxygen found in interstellar space has astronomers running hot and cold.
NASA's Submillimeter Wave Astronomy Satellite, or SWAS, has detected water vapor throughout interstellar space. However, in the very coldest reaches, where temperatures are found just 30 degrees above absolute zero, astronomers measured water vapor concentrations of only a few parts per billion.
"That's far less than predicted by most theories and presents a real puzzle to our understanding of the chemistry of interstellar clouds," said Ronald Snell, Professor of Astronomy at the University of Massachusetts at Amherst, and a member of the SWAS science team.
In warmer regions of space, though, water vapor is more plentiful. "Within gas clouds where new stars are being born, the gas can be heated to temperatures of several thousand degrees Fahrenheit; here the water concentration seems to be as much as 10 thousand times larger," said SWAS team member David Neufeld, Professor of Physics and Astronomy at Johns Hopkins University, Baltimore, MD. "We can think of these stellar nurseries as giant chemical factories that are producing water vapor at a tremendous rate. The large amounts of water vapor present in regions of star formation will help the interstellar gas to cool, perhaps eventually triggering the birth of a future generation of stars."
"Because of our belief that water is an essential ingredient for life, the search for its presence in interstellar gas clouds has always attracted particular attention and that's why these results are intriguing," said Gary Melnick of the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA. Melnick led a team of international scientists whose findings will be published Aug. 20 in a special SWAS-dedicated issue of the Astrophysical Journal Letters.
The new results are the product of 18 months of observations with SWAS, a compact radio observatory launched in 1998 to study the composition of interstellar gas clouds and their collapse to form new stars. Orbiting approximately 400 miles above Earth's surface, SWAS allows scientists to detect radiation from water and oxygen molecules that is ordinarily hidden from view by Earth's atmosphere.
SWAS is also capable of detecting oxygen molecules in interstellar space, although little has been found. Paul Goldsmith, Professor of Astronomy at Cornell University, Ithaca, NY, and Director of the National Astronomy and Ionosphere Center, also in Ithaca, said the absence of molecular oxygen is mysterious. "There must be no more than one oxygen molecule for every 10 million hydrogen molecules, otherwise SWAS would have detected a signal from molecular oxygen," said Goldsmith. "This means that most of the oxygen atoms in interstellar space remain hidden in some form that we have yet to detect."
In addition to observing distant clouds of interstellar gas, SWAS has detected water vapor closer to home in the atmospheres of Mars and the gas giants Jupiter and Saturn. "The water vapor we've detected in the gas giants is almost certainly the result of bombardment by small icy particles that come from interplanetary space and are rapidly vaporized once they hit the planetary atmosphere," said Ted Bergin of the Harvard-Smithsonian Center for Astrophysics, lead author of an article on the detection of water vapor in Jupiter and Saturn. "The water molecules within these icy particles may well have originated as water vapor in the interstellar gas cloud that formed the solar system more than four and a half billion years ago."
SWAS also measured the amount and distribution of water vapor in the atmosphere of Mars, confirming the long-held belief that the relative humidity of the atmosphere was near 100 percent. "The measurements are consistent with the atmosphere being almost completely saturated with water vapor," said Mark Gurwell of the Harvard-Smithsonian Center for Astrophysics. "However, since Mars is so cold, the total amount of atmospheric water is a several thousand times less than in the Earth's atmosphere."
SWAS is operated by NASA, with contributions from the German government and the participation of the Harvard-Smithsonian Center for Astrophysics; the University of Massachusetts at Amherst; Cornell University; the Johns Hopkins University; the University of Cologne in Germany; Ball Aerospace Corporation, Boulder, CO; and Millitech Corporation (now Telaxis Communications), South Deerfield, MA.
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