Comets: icy studies probe sunny behavior.

When a comet gets too close to the sun's warming rays, some of its ice turns to vapor. The sudden transformation spews jets of vapor that drag dust out along with them, rendering the comet visible millions of kilometers from Earth.

Though a familiar scenario, one detail of this picture still puzzles scientists. Volatile gases trapped in cometary ice at low temperatures shouldn't remain at higher temperatures. Well before water ice becomes warm enough to vaporize, it changes its structure to a more ordered, crystalline form that expels such trapped molecules. So why are gases such as carbon monoxide and carbon dioxide found in the jets of water vapor released by comets? Researchers now suggest that the answer may lie in the nature of a newly discovered form of water ice.

Peter Jenniskens and David F. Blake of NASA's Ames Research Center in Mountain View, Calif., identified the new form while monitoring structural changes in water ice as it warmed over a wide range of temperatures in the laboratory. Using beams of electrons to probe the frozen water, they confirmed the existence of two preiously observed forms of ice--a colder, high-density form and a warmer, low-density form--both of which lack the orderly structure of a crystal.

More significantly, the researchers discovered a third type of amorphous ice. Found at higher temperatures--148 to 188 kelvins -- this form coexists with one of the two crystalline forms of frozen water, Jenniskens and Blake report in the Aug. 5 SCIENCE. Because it coexists, this amorphous type may allow comets to retain at surprisingly high temperatures--greater than 150 kelvins -- some of the trapped gases that crystalline water ice would normally expel, they note.

In addition, new data about the two amorphous types identified earlier may help distinguish between the proposed sources of comets -- the Kuiper belt, thought to lie well beyond Pluto, and the more distant Oort cloud. Astronomers believe that despite its chillier location, the Oort cloud formed in a warmer region than the Kuiper belt, probably between Uranus and Neptune.

Because Oort cloud comets may preserve the ice structure created in their warmer birthplace, the researchers believe that these comets are most likely to have the warmer, low-density form of water ice previously found. In contrast, Jenniskens and Blake calculate that the Kuiper belt members are most likely to have the high-density form. This structural difference may lead to noticeable differences in activity when comets from either group first visit the inner solar system, they suggest.