Kuiper belt object shows hint of activity.

Locked in the deep freeze of the outer solar system, residents of a reservoir of comets are thought to have changed little since the birth of the solar system 4.5 billion years ago.

New observations may overturn that view. Studying four of the brightest members of this reservoir, called the Kuiper belt, researchers have found tentative evidence that one of the objects has vented enough gas and dust to generate an atmosphere, or coma.

Last year, Alan Fitzsimmons of Queen's University of Belfast, Northern Ireland, and his colleagues used the Hubble Space Telescope to take images of the Kuiper belt object 1994TB, which currently lies some 30 times farther from the sun than Earth does. Edel Fletcher of Queen's University showed the images last month at a workshop at the European Southern Observatory in Garching, Germany.

Researchers need to do further analysis to verify that the slight brightening above part of the comet's surface is truly a coma and not an artifact, Fitzsimmons emphasizes. If the finding holds up, it would mark the first time that astronomers have witnessed activity in an object so far from the sun.

"It's awfully hard to determine whether you can really see a coma" around such a distant object, notes Brian G. Marsden, who studies comets at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Although the evidence so far is inconclusive, Marsden and other scientists say they're intrigued by the possibility that an object residing in a region where the temperature is only about 30 kelvins could vent gas and dust. Astronomers have observed material venting from comets at about two-thirds of 1994TB's distance from the sun. Those comets were traveling outward from the inner solar system, and residual heat from the sun could have powered their distant outbursts. In contrast, 1994TB ventures no closer to the sun than its current distance.

Tobias C. Owen of the University of Hawaii in Honolulu notes that highly volatile gases, such as molecular nitrogen and methane, can become trapped within the ice grains that formed the Kuiper belt objects. Even a small amount of heat, generated perhaps by collisions between objects in the belt, could unleash the gases, blowing out dust and generating a coma.

The behavior of 1994TB may broadly resemble that of Pluto, which has a nitrogen-rich atmosphere and is considered by some scientists to be a large member of the Kuiper belt. The Hubble images, says Owen, "open new vistas, inviting verification [of the coma] and testing on other objects."