Dolores Beasley

Headquarters, Washington, DC

(Phone: 202/358-1753)

Megan Watzke

Harvard-Smithsonian Center for Astrophysics, Cambridge, MA

(Phone: 617/495-7463)

RELEASE: 00-126

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

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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