The New York Times > Science > Astronomers Contemplate Icy Volcanoes in Far Places

Astronomers Contemplate Icy Volcanoes in Far Places

By KENNETH CHANG

Published: December 9, 2004

undreds of tiny worlds circling beyond Neptune have been thought of as frozen in time, almost unchanged since they formed at the birth of the solar system five billion years ago. A discovery now has some astronomers entertaining visions of icy volcanoes roiling on some of them, at least at one time.

David J. Stevenson, a professor of planetary science at the California Institute of Technology, speculates that the decay of radioactive elements within some of these worlds, known as Kuiper Belt Objects, would produce enough warmth to melt a mixture of water ice and ammonia, an antifreeze. The liquid could then rise up through cracks and erupt onto the surface.

"This would be in exact analogy with what happens inside the Earth with volcanic processes," he said. Dr. Stevenson described these speculations in today's issue of the journal Nature.

The discovery that spurred Dr. Stevenson's imaginings is more modest, at least on the surface. In an article also appearing in today's Nature, David Jewitt of the University of Hawaii and Jane Luu of the Lincoln Laboratory at the Massachusetts Institute of Technology, who together discovered the first Kuiper Belt Object in 1992, report the presence of crystalline water ice on the largest known Kuiper Belt Object, Quaoar (pronounced KWAH-war). They also report signs of ammonia hydrate.

Ice cubes and snow on Earth all develop in crystalline form, with the water molecules stacking in a hexagonal pattern. (The hexagonal structure produces the symmetric shapes of snowflakes.)

But what is common on Earth was a surprise for Quaoar, located about four billion miles from Earth and about one billion miles farther out than Pluto. In ultrafrigid conditions like those found on Quaoar - the surface temperature dips below minus-360 degrees Fahrenheit - ice generally takes an amorphous form more like glass, where the water molecules stack haphazardly. Laboratory experiments indicate that crystalline ice forms only at temperatures warmer than minus-260 degrees. It is not known whether Quaoar's ice consists of hexagonal crystals or an alternative cubic structure.

Furthermore, bombardment by ultraviolet light and cosmic rays turns crystalline ice into amorphous ice, within about 10 million years. That means the crystalline ice seen by Dr. Jewitt and Dr. Luu must have formed or been exposed relatively recently for something that has been circling at the outer reaches of the solar system for the last four billion years or so.

The questions then are, how did the crystalline ice form and why is it still there?

A mixture of ice and ammonia melts at minus-145 degrees, and Dr. Stevenson thinks temperatures deep within Quaoar still exceed that today. However, because erupted ice would not be likely to return to the core, any volcanism is likely to have occurred during Quaoar's youth, Dr. Stevenson said. "I think it makes most sense if it was in the far past, like three billion years ago," he said.

In this theory, the crystalline ice spotted by Dr. Jewitt and Dr. Luu would have formed underground, but was exposed within the last few million years by a collision with a smaller Kuiper Belt Object.

Crystalline ice has also been spotted on Pluto's moon, Charon, and on the moons of Saturn and Uranus. "It's everywhere in the outer solar system," said Michael E. Brown, a colleague of Dr. Stevenson's at Caltech.

Measurements by Dr. Brown, one of the discoverers of the 780-mile-wide Quaoar two years ago, agree on the presence of water ice, but suggest the presence of methane instead of ammonia hydrate.

Dr. Brown proposes a less exotic idea - the impacts of micrometeorites - to explain how the surface was heated enough to produce crystalline ice. But, he added: "The story has to be more complicated than just that. There are other things that we see on the surfaces of these bodies that couldn't survive this heating."

Another possibility is some unknown process that created the crystalline ice at the current cold temperatures. "These processes that happen over millions and millions of years," Dr. Brown said, "are hard to simulate in the lab."