Ice crystals promote molecular eruption.

Stacking randomly placed boxes after a hasty move clears a path from one room to another. Likewise, converting a disarrayed form of frozen water into ordered crystals creates channels that permit gas molecules trapped underneath to make their way into the open, researchers have found. Further study of such structural changes should help scientists understand how molecules dissolve in liquids and provide insight into the behavior of comets.

Frozen water can assume several forms. The investigators studied an unusual one, "much more like a liquid than a solid," according to R. Scott Smith of the Pacific Northwest National Laboratory in Richland, Wash. This material, called amorphous solid water (ASW), appeals to scientists interested in liquids because the molecules in it move extremely slowly, which makes them relatively easy to examine. Smith and his colleagues describe their work in the Aug. 4 Physical Review Letters.

The investigators sprayed water vapor onto an oily substance at extremely low temperatures. Because the gas molecules stuck where they hit, they froze in random orientations. This process captured the water in its disorganized liquid form and trapped the oily layer underneath.

As the researchers raised the temperature, they initially observed no evaporation of the oily material, even after passing the point where vaporization would normally occur. As the heating continued, the gas suddenly burst through its icy restraint. The eruption occurred at exactly the temperature where ASW turns into the familiar crystalline ice.

"The water molecules begin to move around and explore as they gain energy from heat," says team member Bruce D. Kay. "As they find the most stable arrangement, they settle into it." The individual crystals are separated by spaces, which create tiny ravines that act as exit routes for the gas trapped below. The researchers have dubbed this phenomenon "the molecular volcano."

"People knew that the transition between ASW and crystalline water happened, but they did not have a good way to detect the structural changes," says physicist H. Eugene Stanley of Boston University. "This gives a clear signal."

Researchers hadn't observed eruptions of this sort in the laboratory before, but such volcanoes are thought to occur frequently in space. Although ASW exists on Earth only in laboratories, it accounts for most of the ice in the universe.

"New comets occasionally release unexpected spurts of gas," says astrochemist Louis J. Allamandola of NASA's Ames Research Center in Mountain View, Calif. "They contain pockets of gases, which want to get out when they heat up. If the ice that seals these cavities goes through this structural change as the comet warms up, it'll release the gases."