Comets: life in an imperfect icebox.

Comets: Life in an imperfect icebox

Comets often are described as examples of the most pristinematerial in the solar system, preserved in the permanent deep-freeze of space since the system's earliest days. Yet this does not necessarily mean that they are absolutely perfect relics, surviving unaltered since their formation.

Though comets are widely believed to reside largely at thesystem's outermost fringes in a region known as the Oort cloud --except for those few that are sent in toward the sun by the gravitational influences of passing stars--some researchers have proposed in recent years that there may be factors causing them to change from their original state. High-energy radiation, for example, as well as impacts by solid grains of interstellar material, could be chipping away at cometary surfaces, while penetration by the charged particles of galactic cosmic rays could be having an effect farther in.

Now two additional possibilities have been suggested byAlan Stern of the University of Colorado in Boulder. These factors, he says, could make significant changes in a comet's just-born appearance in as little as a billion years, about one-fifth of the estimated age of the solar system.

One such possibility is the heat of passing stars, which couldbe evaporating volatile materials such as carbon dioxide and nitrogen with even a tiny increase in warmth. "We think we're talking about a perfect icebox,' says Stern, "but someone comes along and leaves the door open from time to time.' Though passage through the Oort cloud by numerous stars with luminosity like our sun's would have "processed' only "an insignificant fraction' of the cloud's comets, according to Stern, the far smaller number of stars that are 1,000 to 1,000,000 times brighter could have far-reaching effects. Based on the relative populations of different stellar luminosity classes, Stern finds it "statistically likely that all comets in the Oort cloud have been [heated] to 27 kelvins [-246|C] at least once, and that 20 to 40 percent of all Oort comets have experienced at least one episode of surface heating to 50 K [-223|C].' As few as 50 to 100 stars with 10,000 times the sun's luminosity may ever have been through the cloud, he says. But he notes that such a star can evaporate methane, a presumed cometary constituent, from as far away as 20,000 astronomical units, about 40 percent of what some researchers estimate to be the cloud's diameter. Other possible components, such as nitrogen, carbon monoxide, argon and neon, could be vaporized even more easily. "This finding is important to understanding the behavior of new comets approaching the sun,' says Stern, "and is in accordance with the lack of observed methane and nitrogen volatiles in long-period comet spectra.'

Another factor that may have been at work changing thefaces of comets since the day they were born is collisions, not so much the rare ones between comets but those between comets and the much larger number of small chunks and chips that may reside in the Oort cloud. Calculating the mean time between collisions of different-sized chunks (and assuming a population of 10(14) and 10(12) objects, respectively, in the cloud's inner and outer portions), Stern finds in his model that there are no pristine objects at all that are smaller than about 10 meters across. In addition, a "large fraction' of the surface area of the larger, surviving objects has been heavily "gardened,' or scarred, by impacts from the littler bits. Even those collisions are relatively rare, he notes. But he adds, although "an impact per thousand years may not sound like much, over the age of the solar system there could have been lots of them.'

Stern's next question, which he is investigating together withcolleague Michael Shull, is that of how such "evolution' may actually be affecting the comets themselves, as well as the interstellar medium--the not-quite-empty space between them.