HARVARD GAZETTE ARCHIVES
Water, Water Everywhere
Radio telescope finds water is common in universe
By Lee Simmons
Special to the Gazette
From its orbit 400 miles above the Earth, SWAS has found water in the
cold, dark interstellar clouds where new stars and planets are formed.
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The universe, it seems, is full of water. That's the message
being beamed to Earth from a new space-based radio telescope.
Launched into orbit on Dec. 5, 1998, the Submillimeter-Wave
Astronomy Satellite (SWAS) is for the first time detecting vast
amounts of water vapor hidden in the dark pockets of our galaxy.
The findings confirm what astronomers suspected, but have been
unable to prove from the ground.
"It's very gratifying," says Gary Melnick of the
Harvard-Smithsonian Center for Astrophysics (CfA), who heads the
scientific team behind the effort. "After 20 years of guessing
that nature might work this way, to finally get an instrument up into
space, turn it on, point it toward these regions and see confirmation -
- we're seeing water everywhere we look."
SWAS is designed to probe the cold, dark interstellar clouds in our
galaxy where new stars are formed. The data it is collecting will
provide crucial information about the composition and structure of
these interstellar clouds and will improve our understanding of the
early stages of star formation.
The results also offer exciting clues to the origin of the water in
Earth's oceans and suggest that the presence of potentially life-
sustaining water is not unique to our solar system.
Submillimeter Astronomy in Space
SWAS is the third scientific satellite produced by NASA's
Small Explorer Program, which was established to build small,
specialized spacecraft that are economical but scientifically powerful.
The entire satellite weighs only 625 pounds and is controlled by an
onboard computer not much different from a souped-up desktop PC.
The scientific instrument itself, however, is sophisticated.
Observing radiation at submillimeter wavelengths, a frequency band
between radio and infrared on the electromagnetic spectrum, is still
a relatively new frontier in astronomy. It is in this band that very
cold water and molecular oxygen radiate. A radio telescope must be
built and calibrated to exacting tolerances to distinguish such tiny
signals. No one had previously attempted to put such an instrument
in space.
Melnick: Looking at the big picture. Photo by Kris Snibbe.
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After 10 years of planning, construction, and waiting, the team
gathered in the flight control room at NASA's Goddard Space
Flight Center, outside Washington, D.C., for the launch this past
December. "We were all holding our breath," admits
Melnick. "The kick of a rocket launch is quite a jolt. It's
like building a fine piece of hardware and then hitting it with a
sledgehammer." To everyone's relief, the deployment
went without a hitch and the instrument survived the trauma
unscathed.
Every night, newly collected data is transmitted to the
mission's Science Operations Center at the CfA on Concord
Avenue, in Cambridge. There, astronomers analyze the data and
select the next week's targets.
Besides Melnick, the other participating scientists at the CfA are
Matthew Ashby, Ted Bergin, John Chang, Alex Dalgarno, Giovanni
Fazio, Steven Kleiner, René Plume, John Stauffer, Patrick Thaddeus,
Volker Tolls, Zhong Wang, and Yun-Fei Zhang. The project team also
includes astronomers from Cornell, Johns Hopkins, the University of
Massachusetts, the University of Cologne, and NASA.
Sifting the Stardust
The dark areas of the night sky are not mere voids, as once
believed, but rather contain enormous quantities of matter in atomic
and molecular form. This interstellar medium is concentrated in vast,
amorphous clouds of dust and gas -- the atomic rubble, presumably,
from the explosions of earlier generations of stars. It is from this
primordial material that new stars and planets are formed.
These interstellar clouds are extremely tenuous: less dense than
even the best vacuum that can be created on Earth. But a single cloud
can span hundreds of trillions of miles and contain enough mass to
create hundreds of thousands of stars like our sun.
Because the gas in these star-forming regions is so cold (typically
less than 400 degrees F), it produces no visible light. These immense
objects are generally invisible to even the most powerful optical
telescopes. They do, however, emit low-energy radiation that can be
detected by radio telescopes.
Over the past several decades, radio and infrared observations
have taught us a great deal about these stellar nurseries. The earliest
stages of star formation, however, remain poorly understood.
Something causes these diffuse, quiescent clouds to become
gravitationally unstable and begin to collapse, often fragmenting into
smaller pieces in the process. The dimensions of this collapse are
mind-boggling: analogous to something the size of Pennsylvania
being compressed to the size of a nickel.
Compression on this scale generates tremendous heat. Unless this
heat is removed, thermal pressure would eventually overwhelm the
force of gravity, halting the process of star formation. Astronomers
hypothesize that the presence of water, carbon monoxide, and
molecular oxygen helps to cool the gas, permitting compression to
continue.
Until now, however, it has been impossible to observe water and
molecular oxygen in the cold interstellar medium because its
emission is absorbed by the Earth's atmosphere. Only by lifting
the telescope above this obscuring canopy has SWAS made it possible
to measure the abundance of these molecules in interstellar space for
the first time.
The Origins of Oceans
Will the interstellar water observed by SWAS someday end up in
oceans like those on our own blue planet? "Nobody knows for
sure where oceans come from," Melnick says. "It's
believed that some portion of the Earth's water was produced
at a later stage in the evolution of our solar system. But the exact
percentage is unknown."
"It's also quite plausible," he continues,
"that some of the gaseous water from the interstellar medium
freezes onto dust grains that are carried along with the gas as clouds
collapse. Those dust grains can stick to each other, and this might
well be how comets are formed. Comets contain a lot of ice. The
collision of a big dirty snowball like this with a young planet would
certainly transport water."
SWAS is expected to observe several hundred star-forming
regions in our galaxy during its 2- to 3-year life. Melnick's goal
is not so much to collect information about individual sources, but to
observe a large enough sample of sources to be able to make broad
statements about the nature and dynamics of the interstellar
medium.
"I come in every day and get excited by what we've
learned the night before, but it's just adding another piece to
the puzzle. Once we have enough pieces in place that we can stand
back and say, 'Aha! That's the big picture -- this is how
nature works when it comes to interstellar chemistry,'
that's when I'll be willing to send up a flare and declare
this mission a roaring success."
Copyright
1999 President and Fellows of Harvard College
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