Water is a critical molecule for human life, and, because it is abundant in space, it also plays an important role in the life of molecular clouds and the stars and planets that form in them. In 1998, a NASA team led by Harvard-Smithsonian Center for Astrophysics astronomers launched a mission to study water in space–the Submillimeter Wave Astronomy Satellite. SWAS found water nearly everywhere it looked, but also uncovered a puzzle: there was less water (in relation to other molecules) in space than astronomers had expected. In a series of papers SWAS scientists concluded that considerable amounts of water are frozen out onto the surfaces of cold grains of dust.
Image right: The Orion Nebula
Writing in this month’s Astrophysical Journal, Center for Astrophysics astronomers and SWAS team leaders Gary Melnick and Volker Tolls, along with two previous members of the Center for Astrophysics and SWAS community and four colleagues, published summary conclusions from a study of water over a very large area of the sky. They mapped water vapor along a ridge of warm material in the Orion nebula nearly 18 light-years in length. The Orion ridge is associated with the closest region of massive star formation to the Earth, a complex that has long been a key site for astronomers investigating how stars form, which physical processes are at work, and what chemistry takes place in the cloud.
The astronomers report in their study that most water vapor originates near the surface of the cloud, and does not extend into the cloud by more than about one hundredth of a light-year, probably because it turns into ice. This result is in conspicuous disagreement with theoretical predictions from the last decade, but is consistent with the team’s earlier conclusions. The study explains why the total abundance estimates of water had been too low and also helps to resolve what happens when radiation from hot stars impinges on the surface of a molecular cloud.