Oh, for the good old days, when asteroids were asteroids and comets were comets! In the simplest model of the solar system, which most planetary scientists had accepted for decades, asteroids are rocky, geologically dead bodies and comets are icy objects that flaunt majestic dust tails when they near the sun.

asteriods
©Hsieh, Jewitt
Although all three of these objects lie in the asteroid belt, they flaunt comet-like dust tails and are known as main-belt comets. New observations and models are further blurring the line between comets and asteroids.


That definitional dividing line began to blur a few years ago when several astronomers, including David Jewitt of the University of Hawaii at Manoa in Honolulu and Henry Hsieh, now at Queen's University Belfast, noticed that three known asteroids had sprouted dusty tails, just like comets.

Now, detailed simulations of the environment and evolution of these three asteroids - now considered hybrid bodies - and new observations of the composition of another asteroid are further eroding the differences between asteroids and comets.

Researchers presented the findings on July 16 at the Asteroids, Comets, Meteors meeting in Baltimore.

All three of the objects, dubbed main-belt comets or icy asteroids, have circular orbits that lie in the same plane in which the planets orbit the sun, just as ordinary asteroids do. That's an indication that the objects formed where they now reside, in the asteroid belt, the band of rocky material between the orbits of Mars and Jupiter, rather than emigrating from either of two frozen reservoirs of comets that lie at the fringes of the solar system, says theorist Nader Haghighipour of the University of Hawaii at Manoa.

"It is probable that the main-belt comets represent a new comet class in the solar system, one located unexpectedly close to the sun and revealing a previously unsuspected reservoir of ice nearby," says Jewitt.

As a comet approaches the sun, the ice on its surface suddenly converts from solid to gas (a process called sublimation). This change drags out dust as the ice vents into space, supplying the force that drives comets. However, most of the asteroid belt is now too warm for ice to remain stable on the surface of any of its denizens.

Haghighipour and Jewitt suggest that early in the solar system, the asteroid belt was cooler, and they note that the sun didn't produce quite as much heat when it was very young. More importantly, some of the sun's radiation was absorbed by the much larger population of dusty debris floating around the belt during those formative years, which would have given icy objects a chance to form there. As the asteroid belt heated up, surface ice would have melted, but pockets of ice just beneath and insulated by a cover of dirt or rock could have endured.

New simulations by Haghighipour show that collisions between these ice-laden asteroids and smaller bodies common in the asteroid belt can readily knock off the surface dirt and expose the hidden ice. Then, the next time one of these icy asteroids nears the sun: Voilà! It turns into a comet, replete with a dusty tail.

Two of the three main-belt comets lie within the Themis family of asteroids, and the third resides just outside that group. At the meeting, Andy Rivkin of the Johns Hopkins University's Applied Physics Laboratory in Laurel, Md., reported the first evidence that another family member, 24 Themis, not known to have a tail, has water ice on its surface. The new observations confirm that "sublimation of ice is the only real possibility for driving cometary activity in these Themis family objects," says Rivkin.