(from the European Space Agency press release, 10 April 1998)
A team of US astronomers has discovered a large concentration of water
vapour in a cloud of interstellar gas close to the Orion Nebula, using the European Space Agency's Infrared Space Observatory, ISO, which is operated with the participation of
NASA. ISO has found water vapour in many places, from the outer planets to distant galaxies, but the concentration now reported is twenty times larger than that measured previously in other interstellar gas clouds.
water vapour is in the Orion Molecular Cloud, a giant interstellar mass composed primarily of hydrogen molecules, 1500 light years from the Sun.
"The interstellar gas cloud that we observed is being
pummelled by shock waves that compress and heat the gas," says Martin Harwit of Cornell University, an ISO mission scientist and lead author on the article reporting the finding. "These
shock waves are the result of the violent early stages of starbirth, in which a young star spews out gas that slams into its surroundings at high speed. The heated water vapour that we observed is the result of that collision."
Harwit also suggests that shock waves are a cause of starbirth as well as a result. "In the future," he says, "they may also trigger
the formation of additional stars and planets as they compress the gas cloud that we observed, but only if surplus heat can be radiated away. Even though the interstellar gas is composed primarily of hydrogen molecules, water vapour is a particularly efficient radiator at far-infrared wavelengths and plays a critical role in cooling the gas and facilitating the star formation process. Because the Earth's own wet atmosphere is completely opaque at the wavelengths of interest, the observations that we are reporting today are possible only from space with the use of the ISO satellite."
The concentration of water vapour measured by the US team was roughly one part in 2000 by volume, far larger than any measured previously in interstellar space. But the strength of the water
radiation detected from Orion was in perfect agreement with theoretical predictions in the PhD thesis of team member Michael Kaufman, from NASA's Ames Research Center.
"An enhanced concentration of water is
precisely what we expected in this gas cloud," comments team member Gary Melnick of the Harvard-Smithsonian Center for Astrophysics. "We are looking at a region of interstellar space where shock waves
have made the gas abnormally warm. For the past 25 years, astrophysicists have been predicting that whenever the temperature exceeds about 100 °C, chemical reactions will convert most of the oxygen atoms in the
interstellar gas into water. And that's exactly what we've observed in Orion."
The high concentration of water measured in Orion may have also had implications for the origin of water in the Solar System
and on the Earth itself, according to team member David Neufeld of Johns Hopkins University.
"The interstellar gas cloud that we observed in Orion seems to be a huge chemical factory," Neufeld says, "generating enough water molecules in a single day to fill the Earth's oceans sixty times over. Eventually that water vapour will cool and freeze, turning into small solid particles of ice. Similar ice particles were presumably present within the gas cloud from which the Solar System originally formed. It seems quite plausible that much of the water in the Solar System was originally produced in a giant water-vapour factory like the one we have observed in Orion."