Distant comets: driven by carbon monoxide?

When a comet nears the sun's warming rays, some of the frozen water on its surface turns to gas, whooshing into the vacuum of space. This rushing gas drags with it dust from the comet, creating two familiar features -- a comet's tail and its dusty shroud, or coma. But farther from the sun, where temperatures are too low to convert frozen water directly into vapor (a change of phase known as sublimation), some comets still manage to flaunt a tail or expel a jet of gas and dust.

What fuels such activity so far from the sun? New observations suggest that the jetlike release of carbon monoxide, which sublimes at temperatures much lower than water, provides the oomph.

In their study, Matthew C. Senay and David Jewitt of the University of Hawaii in Honolulu used two short-wavelength radio telescopes to detect gas emissions from Comet Schwassmann-Wachmann 1, which never ventures nearer to the sun than Jupiter. At the comet's closest approach, water on its surface remains frozen.

But observations with the James Clerk Maxwell Telescope atop Hawaii's Mauna Kea reveal that the comet spews a startling amount of carbon monoxide, a molecule that sublimes at a chilly 25 kelvins. The detection marks the first time that astronomers have found emission of neutral carbon monoxide molecules from a comet so distant, Senay says. He and Jewitt report their work in the Sept. 15 Nature.

The astronomers note that the carbon monoxide has about the same velocity as the comet but that the expelled gas moves in the general direction of the sun. This suggests, they say, that the gas originates from the part of the comet's frigid surface that received the strongest illumination from the sun during their observations. The sunlight sublimates the carbon monoxide, producing a jet of gas and dust headed toward the sun, Senay and Jewitt assert.

Using a higher-resolution radio telescope, the Caltech Submillimeter Observatory on Mauna Kea, the astronomers examined the gas emissions in greater detail. They deduced that the comet expelled about 2,000 kilograms of carbon monoxide each second, roughly the rate that Comet Halley attained in 1986 during its closest approach to the sun.

Models suggest that each gram of gas that a comet expels takes with it about 2 grams of dust. Thus, the release of carbon monoxide can easily generate and replenish the dusty coma that Comet Schwassmann-Wachmann 1 sports for much of its 14.8-year orbit, Senay says. "These results provide the first direct evidence that the sublimation of volatiles [such as carbon monoxide, can drive the activity of distant comets," the astronomers write.

However, they cite other sources that might also activate faraway comets: Molecular nitrogen, for example, also sublimes at low temperatures. Michael F. A'Hearn of the University of Maryland at College Park says that to assess fully the role of carbon monoxide, researchers should observe whether its release rate increases when a distant comet undergoes an outburst and declines when it loses its coma.

A'Hearn adds that although carbon monoxide seems a plausible source of activity for distant comets that visit the inner solar system periodically, other molecules may power first-time visitors, known as "new" comets. The sublimation of a group of molecules known as radicals, thought to be created when cosmic rays bombard comets in the outer solar system, may power new comets, A'Hearn says.