UH comet discovery points to Earth origin
University of Hawaii astronomers have discovered a new class of comets that they say might eventually reveal how life began on Earth. The "main-belt comets," as they are called, are a new type of comet from a totally different region from other comets, Institute for Astronomy professor David Jewitt said in an interview.
The region, in the main asteroid belt between Mars and Jupiter, is one of the leading locations from which Earth might have obtained water from impacts of icy objects, he said.
Jewitt and graduate student Henry Hsieh reported their discovery in yesterday's edition of Science Express and in an April edition of Science.
Comets normally come from the outer part of the solar system -- a large spherical cloud called Oort surrounding the sun and the Kuiper belt beyond the orbit of Neptune, Jew- itt said.
The Kuiper belt had been a theory until Jewitt and then postdoctoral fellow Jane Luu discovered the vast region in 1992 with millions of cometlike remnants of the early solar system.
The three icy comets in the new class appear to have formed in the inner solar system, inside Jupiter's orbit, rather than outer parts of the solar system beyond Neptune, Jewitt and Hsieh reported.
"That's interesting because they're very far away from the source regions for other types of comets," Jewitt commented.
He said the new type of comets are much closer to the sun than any place the astronomers thought they would find ice. "The interest is that ice could survive 4.6 billion years so close to the sun for so long."
He said the ice probably has been able to survive because most of it is buried and protected by a layer of dirt from the sun's heating. But the impact of a small asteroid or boulder on the surface could blow a hole in the ice layer, he said.
Earth is believed to have been hot and dry when it formed with water supplied by comets or asteroids after the planet cooled, the astronomers said.
Comets were the prime candidates for showering Earth with water for many years, but "they have properties that might rule them out as a dominant source," Jewitt said.
Analysis of comet water has shown it is "significantly different" from Earth's typical ocean water, the astronomers said.
"If we just took comets and melted them, they would not look like oceans," Jewitt said. "Whether that means instead that oceans came from the main-belt comets impacting Earth in the past, we don't know, but we have a chance to find out now."
Jewitt and Hsieh plan to continue looking for main-belt comets with the Pan-STARRS observatory, which is expected to be working later this year, searching the sky for dangerous asteroids and other objects.
Hsieh, in a paper on main-belt asteroids, said they were thought of as "inert, rocky bodies which may have contained ice in the past but have long since been boiled dry by 4.6 billion years of close solar heating."
The Jewitt-Hsieh discovery changes that.
Hsieh said he set out to test a hypothesis for his doctoral thesis that a main-belt asteroid named 133P/Elst-Pizarro with comet activity actually is an icy asteroid that has lived in the main asteroid belt since the solar system began.
"Based on that, and since we only looked at a couple hundred asteroids and there are many thousands of them, many more have ice in them as a possible source of water for Earth," Hsieh said.
They are a lot closer than the source of classical comets and are more likely to have hit Earth in its early stages, he said.
Main-belt comets have flat, circular, asteroidlike orbits rather than the elongated orbits of normal comets, but with their cometary appearance, they also do not fit into the asteroid category, Hsieh said.
Classification systems break down with main-belt comets, Jewitt said. "They're asteroids by orbits but comets by physical nature. They're both asteroids and comets."