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See Inside July 2010

Damp Rocks from Space

Icy discovery bolsters view that asteroids delivered water to Earth

An asteroid circling the sun between Mars and Jupiter harbors water ice and organic compounds on its surface—the first time such components have been discovered on asteroids. Those traits had been associated with comets, which spring from colder, more distant reservoirs in the solar system. The finding supports the notion that asteroids could have provided early Earth with water for its oceans as well as some of the prebiotic compounds that allowed life to develop.

Two teams reported complementary observations of the 200-kilometer-wide asteroid, known as 24 Themis, in the April 29 Nature. (Scientific American is part of Nature Publishing Group.) Both groups saw infrared absorption features indicating a thin coating of frost, along with unidentified organic compounds. “They have found something that a lot of people, including myself, have been chasing in the solar system for a long time,” says Dale Cruikshank, a planetary scientist at the nasa Ames Research Center.

The asteroid is intriguing in part because it occupies a similar orbit to so-called main-belt comets—and likely stems from the same parent body. Main-belt comets reside in the asteroid belt but feature cometlike tails thought to arise from sublimating ice. These newly discovered main-belt comets, and now Themis, “are very interesting objects and potentially one of the sources of Earth’s oceans,” Cruikshank says.

University of Central Florida astronomer Humberto Campins, a co-author of one of the studies, says other asteroids may harbor ice as well. “Or it could be unique to Themis,” Campins says. “We don’t know.”

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