Three icy comets may hold clues to the origin of Earth’s oceans, researchers say.

The objects, in essence, look like comets but act like asteroids. They’re members of what some astronomers are calling a newfound class of comets.

Dubbed “main-belt comets” by University of Hawaii graduate student Henry Hsieh and Professor David Jewitt, their existence suggests asteroids and comets are much more closely related than previously thought, the researchers claimed.

The discovery, they added, suggests icy objects from the region between Mars and Jupiter could be a major source of Earth’s water. The findings appeared March 23 in the early online edition of the research journal Science.

Traditionally, asteroids and comets are considered quite distinct.

Both are objects a few to a few hundred miles across that orbit throughout our solar system. By conventional definitions, comets are essentially balls of icy dirt, often followed by a tail of particles and gas. They’re believed to originate in the cold outer solar system. Asteroids are rocky and dry, and thought to have typically formed much closer to the Sun in an “asteroid belt” between Mars and Jupiter.

The newly described objects, Hsieh and Jewitt reported, look like comets, but orbit the sun along paths similar to those of asteroids. And they seem to have formed in the warm inner solar system in the main asteroid belt.

The researchers based their findings on observations made on November 26, 2005, using the Gemini North Telescope on Mauna Kea, Hawaii.

They found that an object designated as Asteroid 118401 was ejecting dust like a comet. Together with a mysterious comet known for almost a decade but still poorly understood, and another comet discovered just a month earlier, it forms a new class of comets, they concluded.

The other two are designated 133P/Elst-Pizarro and P/2005 U1, respectively.

“The main-belt comets are unique in that they have flat, circular, asteroid-like orbits, and not the elongated, often tilted orbits characteristic of all other comets,” said Hsieh. “At the same time, their cometary appearance makes them unlike all other previously observed asteroids. They do not fit neatly in either category.”

Our planet is thought to have formed hot and dry, so its water must have been delivered after it cooled, the researchers said. Some scientists believe comets or asteroids might have supplied the water. Comets were leading candidates for many years because of their ice content, but recent analysis of comet water has found that it’s quite different from ocean water.

Asteroidal ice may give a better match to Earth’s water, some scientists propose.

But until now, researchers believed the relatively nearby Sun would have baked away water from the asteroids long ago, making it inaccessible for study, according to Hsieh and Jewitt. The discovery of main-belt comets would mean this ice is still accessible, right on the surfaces of at least some objects.

Spacecraft missions to these objects could provide new information on their ice—providing insight into the origin of our water, and ultimately life, they added.


- Courtesy University of Hawaii at Manoa
and World Science staff

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