Comets give clues to solar system's infancy

For millenniums, humans have looked in wonder - and often dread - as comets stretched wispy tails of luminous dust across the night sky. When Danish sky-mapper Tycho Brahe decided to measure the distance to a comet in 1577, however, it put comets on the watch list of astronomers.

Today, the study of comets is entering something of a golden age, giving scientists fresh insights into the life and times of the solar system's infancy 4.6 billion years ago.

"We've been stuck looking through a glass darkly, but now we're beginning to see face to face," says Michael Mumma, in the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center in Greenbelt, Md.

An unprecedented array of hardware is in place to monitor passing comets. Automated telescopes are scanning the skies for asteroids and comets, spotting them with enough lead time to allow researchers worldwide to coordinate observations. And a decade-long space-borne assault on comets is under way, beginning with the National Aeronautics and Space Administration's Deep Space 1 encounter with comet 19P/Borrelly this summer.

Studies in today's edition of the journal Science may illustrate the point. In six separate reports, astronomers from eight countries on three continents chronicle the breakup of comet C/1999 S4 LINEAR, which gently disintegrated last summer, 10 months after it first appeared.

The breakup, researchers say, has opened a unique window on the structure of comets, provided insights into early planet formation, and appears to bolster the case for comets being the delivery trucks for substances critical for the emergence of life on Earth.

Comets are believed to have formed from material in the cloud of dust, ice, and gas surrounding the young sun, according to Hermann Boehnhardt, an astronomer at the European Southern Observatory's Very Large Telescope in Chile. Their nursery ranged from the orbit of Jupiter to beyond Neptune's, where sunlight was so weak that grains of carbon dioxide and ice could remain frozen to form comet cores. As the solar system took shape, microscopic-sized particles merged gently into mountain-sized planetesimals. At that point, growth became a "runaway" affair as these embryonic planets collided and merged. Gravity from the resulting planets then swept much of the remaining material away.

Comet-sized leftovers that formed beyond Neptune took up residence in what has become known as the Kuiper Belt. Those that formed closer to Jupiter, however, yielded to the planet's gravitational tug and were flung like Jovian lightning bolts nearly a billion miles from the sun. There, as many as a trillion comets form a tenuous cocoon called the Oort Cloud.

Scientists theorize that C/LINEAR's pass into the solar system was its first - and perhaps only. But it was a revealing pass.

During its demise, C/LINEAR's half-mile nucleus shed more than 116 million cubic feet of water - enough to supply the United States' freshwater needs for 2-1/2 days. The water's composition - in particular, its ratio of deuterium to hydrogen - closely matched that of sea water, lending credence to the idea that comets provided Earth with its initial water supply.

A cloud had been thrown over that idea in earlier studies of comets such as Hale-Bopp, Hyakutake, and Halley, which carried water with deuterium/hydrogen ratios that failed to match sea water's. In addition, C/LINEAR's nucleus was dominated by dust rather than ice - unlike Hale-Bopp, Hyakutake, and Halley. This suggests to researchers that it formed close to Jupiter.

There, temperatures would have been warmer than beyond Neptune, reducing the amount of ice available. Since asteroids formed in this region were rich in carbon, it's likely that C/LINEAR carried such material too, notes Harold Weaver, who led one of the comet-observing teams.

Some "4.6 billion years ago, you had trillions of these flying around," he says. Many of them would have passed through the inner solar system, he adds, colliding with the inner planets and depositing their cargoes.

During its relatively slow disintegration, the comet shed material in a range of sizes, throwing its creation into reverse and confirming ideas about how early planetesimals evolved.

As researchers pour over the data from C/LINEAR, they are eagerly awaiting the coming decade's five US and European missions to comets. Beyond Deep Space 1, NASA's Stardust mission - currently en route to comet 81P/Wild 2 - is designed to return dust samples taken from near the comet. By 2013, the European Space Agency hopes to place a spacecraft in orbit around a comet and place a lander on its surface.

(c) Copyright 2001. The Christian Science Monitor