Star's icy disk may suggest water origin
By Amanda Jermyn



We take it for granted that water is essential to life as we know it, yet the early Earthís surface is thought to have been hot, rocky and dry. So where in the universe did our water come from? We know that some had its origins as steam escaping the primordial Earthís cooling crust, and as gases released during volcanic eruptions. Once the Earth cooled enough to sustain an atmosphere of sufficient pressure, this escaping water vapor would have remained within the atmosphere and begun to form liquid water. However, the large quantities of water in our oceans, rivers, lakes, ice caps and atmosphere could never have been produced by these methods alone.

The main candidates for the source of our water today are icy comets and asteroids from the outer solar system colliding with the early Earth. These bombardments were certainly much more frequent than they are today, but did the outer reaches of our solar system harbor enough water to account for the formation of our vast oceans? The recent discovery of massive quantities of water in a planet-forming gas disk around a young star seems to indicate yes. This research, published in the October 21st issue of the journal Science, involved a team of astronomers pointing the Herschel Space Observatory at a young star called TW Hydrae, about 175 light years away. This star, which is only about 10 million years old, is surrounded by a disk of gas similar to that of the young Sun 4.6 billion years ago. Using the Herschelís Heterodyne Instrument for the Far Infrared, the team found water vapor, something never previously detected in the outer regions of such a disk. The vapor is most likely formed when ultraviolet light from the central star and others nearby bombards large quantities of ice in the disk.

The researchers estimate that the disk holds several thousand oceansí worth of water ice, an indication that the outer part of the gas disk that formed our solar system could have held vast quantities of ice as well. This would have been crucial to the formation of Earthís oceans. Comets, which spend most of their time in these outer regions, are comprised of rock and large quantities of ice. The collisions, far more frequent than today, of several million of these comets and other icy space debris with the early Earth would have transported enough water to form our planetís oceans.

Doubt was previously cast on comets as the source of our water when analysis of the composition of ice in certain comets showed chemical signatures different from those in our oceans. However, it was recently established that the chemical signature in water ice from comet Hartley 2 is similar to that of our oceans. The earlier measurements came from comets in the Oort cloud, a collection of icy bodies surrounding the entire solar system, whereas Hartley 2 inhabits the Kuiper Belt, a collection of objects at the edge of the solar system. So comets from the Kuiper Belt are a more likely source of Earthís water.

The discovery of such vast quantities of water in a planet-forming disk around a young star suggests that Earth may not be the only planet in the Universe with abundant water. And where there is water there may be life.

While there is no public STARS Club meeting in December, please watch out for next yearís exciting line-up of lectures and events.