An asteroid circling the sun between the orbits of Mars and Jupiter has for the first time been shown to harbor water ice and organic compounds. Those traits had been associated with comets, which spring from colder, more distant reservoirs in the outer solar system, but not their asteroidal cousins. 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 of researchers report complementary observations of the 200-kilometer-wide asteroid, known as 24 Themis, in the April 29 issue of Nature. (Scientific American is part of Nature Publishing Group.) Both analyses are based on spectroscopic observations from the NASA Infrared Telescope Facility atop Mauna Kea in Hawaii, which show absorption features that indicate the presence of water and unidentified organic compounds. The ice appears to coat the entire asteroid as a thin layer of frost. The evidence for water on 24 Themis had been presented at conferences by the two groups in 2008 and 2009 but is only now appearing in a peer-reviewed journal.

"They have found something that a lot of people, including myself, have been chasing in the solar system for a long time, and that is water and organic material," says Dale Cruikshank, a planetary scientist at the NASA Ames Research Center in Moffett Field, Calif.

The asteroid was a promising target in part because it shares a similar orbit with a few so-called main-belt comets, which are objects in the asteroid belt that feature comet-like tails thought to be provided by the sublimation of ice to water vapor. Because 24 Themis likely came from the same parent body, it seemed plausible that it could harbor ice as well.

Cruikshank notes that some meteorites bear the signature of water and organic compounds, and that researchers have long been looking for the source of those meteorites. Now it appears that 24 Themis could fit the bill. "These newly discovered [main-belt comets], and now Themis, are very interesting objects and potentially one of the sources of Earth's oceans."

The two studies give a fairly comprehensive view of the asteroid; one sampled 24 Themis at various points through its orbit, in brief intervals spanning several years, whereas the other followed the asteroid for several hours in one sitting to spot any changes as the body rotated on its axis.

"I thought, 'There has to be something in that family that is making these small objects behave like comets'," says Humberto Campins, an astronomy professor at the University of Central Florida who co-authored the study that was based on a seven-hour observation of 24 Themis in 2008. There was indirect evidence to suggest that some asteroids had not been baked dry by the sun, Campins says, but no proof.

The authors of the other study observed the asteroid seven times between 2002 and 2008 before they were convinced. "At first, we didn't necessarily believe it," says Andrew Rivkin, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory who collaborated with Joshua Emery, a planetary scientist at the University of Tennessee, Knoxville. "It took a few tries, including one of those years where we were able to see it on back-to-back nights, before we could say, 'Now we really do believe this is for real'." The stability of ice on an airless body in space, Rivkin explains, is very sensitive to temperature, and 24 Themis appeared to be right on the cusp of plausibility. "If it were a few degrees warmer, you'd say no way," he says.

As astronomers look farther out in the solar system with constantly improving telescopes and instrumentation, Campins says, they may find ice on more asteroids. "We may need to look more carefully," he adds. "Or it could be unique to Themis. We don't know."

Cruikshank says it is somewhat surprising to find water ice on an airless body so close to the sun, but he notes that such revelations are becoming the norm—just look at the recent demonstrations of widespread water ice on the moon. "We're quite accustomed to being surprised," he says.