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Space pebbles. Narrow "debris trails" of particles suggest a rocky composition for Comet Johnson (top) and Comet Shoemaker-Levy 3.

Comets: Solid as a Rock?

MINNEAPOLIS, MINNESOTA--New images of debris shed by comets have led an astronomer to make a startling claim: Comets may consist mainly of rock, rather than ice. The provocative study sparked interest here this week at a meeting of the American Astronomical Society, but some colleagues turned a cold shoulder.

In the classical "dirty snowball" picture, comets are made of ancient dust and bits of rock from the early solar system, embedded in a fluffy matrix of ice. Such bodies are usually cold and inert. But as a comet heads toward the sun, heat sublimates the ice. The comet then expels clouds of dust and gas that spread into dramatic tails. A warmed-up comet also constantly loses mass--far more, in fact--as larger particles sputter away into so-called "debris trails" along its orbital path.

NASA's Spitzer Space Telescope, which detects traces of infrared warmth from such particles, has now assembled the biggest survey of comet trails. Astronomer William Reach of the Spitzer Science Center in Pasadena studied 29 images of short-period comets, which loop around the sun once every few years. He found 21 debris trails, most of which were confined to extremely narrow ribbons. According to Reach's analysis, the comet expels comparatively little dust or gas, in terms of mass--even near the sun. Instead, the long-lasting tracks must consist of big grains at least a millimeter wide to remain within the linear trails. "Nearly all of the mass loss from short-period comets is in the form of pebbles," he says. "Comets are mostly rocks."

Reach's claim provoked mixed reactions. "I think it's pretty cool that he's seeing rocks," says planetary scientist Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, whose models of planet formation start with rocky ices. Other scientists countered that the debris trails must arise from the crusty outer rinds of comets, with ice preserved inside. Both ideas will face a stiff test on 4 July when NASA's Deep Impact mission plows into a comet (ScienceNOW, 12 January). If the body is mostly rocky, says Reach, the crash will make a shallow crater with a hard thud. But if there's a lot of ice beneath, the impactor might burrow deeper into the comet and trigger a geyserlike spray of material.

Related sites
Reach's home page
Spitzer Space Telescope
Deep Impact