A New Class of Comets Emerges Nearby
Friday, March 24, 2006
Astronomers have identified a new class of comet in the asteroid belt, offering fresh support for the view that the large volumes of water that transformed the barren early Earth into today's fertile planet may have come from a much closer source than the icy bodies that periodically swoop in from deep space.
Researchers at the University of Hawaii late last year discovered a supposed "asteroid" ejecting dust like a comet, and conducted separate observations of two other objects that had been seen spewing dust tails. All three were between 293.4 million and 297.1 million miles from the sun -- in the "main belt" of asteroids between Mars and Jupiter.
"Together these three objects form a new class of comets," authors Henry Hsieh and David Jewitt wrote in today's edition of the journal Science. The two researchers called them "main-belt comets."
The new research gives formal recognition to a phenomenon that some astronomers had long suspected -- that not all comets originated in the Kuiper Belt or the Oort cloud far beyond Neptune.
But by adding a third class of comets, Hsieh and Jewitt revived debate not only about the origin and behavior of comets, but also about the early evolution of the solar system and of Earth itself.
"We've known about one of them [main-belt comets] for quite a while, and we thought of it as a strange duck, an outlier," said comet expert Donald K. Yeomans, a senior research scientist at NASA's Jet Propulsion Laboratory. "Now we have three, and we are going to have to deal with the interesting suggestion that they formed within the asteroid belt."
Asteroids are small, stable, irregularly shaped bodies composed of stone and, sometimes, metal -- principally iron and nickel. Comets, by contrast, are frequently described as loosely packed "dirty ice balls" that erupt with geyserlike jets of dust when exposed ice sublimates, or vaporizes, as the comet nears the sun.
The origin of asteroids is unclear, but scientists generally regard comets as deep space remnants of the spiraling gas and dust disk that formed the solar system 4.5 billion years ago. Astronomers study comets as time capsules, pristine samples of the solar system's original building blocks, preserved in a cosmic icebox since the beginning of time.
Most comets remain "dormant," in deep space, but occasionally the gravity of a passing star or of some other cosmic phenomenon disturbs a comet's orbit, sending it into the inner solar system on an elliptical path.
For most of the ensuing journey, the comet remains unchanged, but once it passes the "snow line" inside Jupiter, the sun's heat becomes strong enough to vaporize the ice, creating the characteristic tail and "coma" known to astronomers.
Earth, 93 million miles from the sun and well inside the snow line, began as a hot, waterless wasteland but began to cool about 3.9 billion years ago, while volcanic activity created a carbon dioxide atmosphere that stabilized the climate at temperatures that allowed ice to melt without vaporizing.
Some astronomers say the ice was provided by incoming comets large enough to reach Earth's surface intact. The water filled the oceans and provided Earth with a weather cycle, key events in the subsequent evolution of life.
One difficulty with this interpretation is that the ratio of hydrogen to "heavy" hydrogen, or deuterium, atoms in ocean water does not match that of any deep space comet whose ice has been analyzed.
Jewitt suggested that Earth's ice may have come from the asteroid belt: "We have not measured d-h [deuterium-hydrogen] ratios in these guys [the main-belt comets], but they are not that far away," Jewitt said. "They are probably good targets for future spacecraft."
This view, however, has its own difficulties. Much of the asteroid belt appears to be inside the snow line, so there is no easy reason why main-belt comets should still have ice on them unless they migrated in from deep space with the ice insulated beneath a blanketing layer of soil.
"Could they be captured comets? Perhaps," Jewitt said. "But nobody has yet figured out how that might have happened." Instead, he suggested that the comets formed in the asteroid belt itself and crusted over at a time when the snow line may have been closer to the sun than it is today.
"I'm inclined to accept what they're saying -- that these things did originate in the region," said Brian Marsden, director of the Minor Planets Center at the Harvard-Smithsonian Center for Astrophysics. "There are three of these objects now, and it would be difficult to explain all three any other way."
Jewitt said the comets, while dormant, looked "just like asteroids" but were probably activated when an impact from a meteorite or other object punched through the insulating crust to the ice below. He and Hsieh estimated in their paper that there could be between 15 and 150 comets in the asteroid belt. "They're not very bright, but you can find them if you look hard," Jewitt said.