You Might Drink Some Comet Today

Really. Icy rocks from space might have provided the Earth with large portion of its water.

The Earth is believed to have formed hot and dry, meaning that much of its current water content must have been delivered after the planet cooled.

Possible candidates for supplying this water are comets. Because of their large ice content, comets from the outer solar system were leading candidates for many years, but then analysis threw doubt on the idea. The water in these comets was shown to differ chemically from ocean water; it has a higher incidence of deuterium--water with an extra hydrogen atom.

But then, recently, a closer band of comets has been discovered which have the "right stuff." In 1999 a member of this band shattered upon its approach to the Sun allowing astronomers to analyze its composition. Unlike its brethren which circle much farther out in the solar system, this closer family of comets has the same kind of water that makes up the oceans, breathing new life into the idea that the comets provided much of the water on earth.

PB