Interstellar Water--But Not a Drop to Earth?

Astronomers have measured large amounts of water vapor in a gas cloud near the Orion Nebula, 1500 light years away. While these measurements, to appear in the 20 April Astrophysical Journal Letters, are by far the most precise of their kind, the results come as no surprise. But they do confirm theories of how shock waves heat such clouds and form water. In a separate press release, the authors suggest that the measurements offer clues to how water collected on Earth billions of years ago--a speculation that leaves other researchers skeptical.

The water vapor in Earth's atmosphere has long frustrated attempts to gauge just how much vapor is present in space, so researchers at four institutions turned to help from the orbiting Infrared Space Observatory (ISO). Last October, ISO's Long Wavelength Spectrometer captured characteristic wavelengths of light emitted by water vapor. "We have eight different [infrared] wavelengths at which water is emitting," says David Neufeld of Johns Hopkins University. "That gives us a completely unequivocal signature that it's water we're seeing."

The instrument was pointed toward a hot, young star in a stellar nursery called Orion BN-KL. Such stars often throw off powerful winds and jets that produce shock waves when they collide with material in their path. Scientists have long assumed that heat from the shock waves forces free oxygen in interstellar clouds to combine with the much more abundant hydrogen to form water. At about 2000 degrees kelvin, the region ISO examined is well above the threshold for such reactions to take place, says Neufeld. The precise levels of water that were measured agree perfectly with the shock theory, he adds.

The interstellar water could provide a clue as to how water arises in planetary systems--and, by extension, how water cropped up in the solar system and filled Earth's oceans, the researchers say. But because the part of the cloud they observed is not actually forming a planetary system, that process would require multiple steps. Its water would first have to be frozen onto dust grains then sucked into a forming system, says Bruce Draine, an astrophysicist at Princeton University. He says that shock waves within the collapsing cloud, in the process of forming a planetary system, are a more likely source for the water.

"It's an exciting discovery," says Alex Dalgarno of the Harvard-Smithsonian Center for Astrophysics and editor of Astrophysical Journal Letters. He also casts doubt on the more speculative idea of incorporating water from such a cloud into something like a forming solar system. Dalgarno says it's possible, but wonders: "how would the water survive the formation of the solar system?"

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