Astronomers are pretty sure that most of the water on Earth and the water that probably flowed billions of years ago on the surfaces of Mars, and maybe Venus as well came from icy comets, raining down on the newborn planets. As for where the comets came from, it's long been assumed that they formed directly out of clouds of interstellar gas and dust. That's why scientists are so interested in them: by studying comets, they figured they could get a handle on material that's been unchanged for billions of years.
But a new study suggests it's not so simple. Published in the Aug. 30 issue of Nature, the paper shows that the creation of a comet is probably a much more complicated process than anyone thought. The evidence comes from the Spitzer Space Telescope, which uses infrared-sensitive cameras to peer through the shroud of dust that surrounds newly forming planetary systems to see what's going on inside. In one such system, known as IRAS 4B, about 1,000 light-years from Earth, astronomers from the University of Rochester have detected a disk-shaped knot of material that will one day emerge as a suite of planets orbiting a young star.
And onto that disk, a rain of ice crystals is falling, slamming into the disk at supersonic speeds and vaporizing to form a cloud containing five times the water in Earth's oceans. What the astronomers are convinced will happen next is that the water will re-freeze into ice particles and eventually form comets which in turn will crash down onto whatever planets they find, forming oceans that the future scientists of these worlds will someday be scratching their heads over.
If the Rochester astronomers are right, the idea that comets are pristine remnants of the material from which our solar system originally formed isn't going to hold up. Indeed, last year, comet-dust particles brought back by NASA's Stardust mission showed that the particles had been heated to high temperatures sometime in their lives, which implied that at least the dust in comets might not be primordial. And now it looks like the ice in them isn't either.