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An illustration shows asteroid 24 Themis.

The asteroid 24 Themis and a smaller "family member" appear in an artist's conception.

Illustration courtesy Gabriel Pérez, Instituto de Astrofisica de Canarias

Brian Handwerk

for National Geographic News

Published April 28, 2010

Water has been spotted on an orbiting asteroid for the first time, according to a new study of a space rock that appears to be coated with frost.

What's more, the frost seems to be mixed with carbon-bearing material, according to results from two independent teams studying the asteroid, which is known as 24 Themis.

"We report the first detection of water ice and of organic molecules on an asteroid, and they are both on the same asteroid," said Humberto Campins of the University of Central Florida, leader of one of the teams.

Both teams used NASA's Infrared Telescope Facility in Hawaii to watch how sunlight reflected off the asteroid at different wavelengths, revealing the watery signature.

A closer look at similar asteroids, perhaps during a proposed NASA mission, could help reveal whether the water we drink—and maybe even the building blocks of life—were delivered to Earth by impacting space rocks. (See "Comet Swarm Delivered Earth's Oceans?")

Asteroid Water Ice Exposed by "Impact Gardening"?

Asteroids are believed to be the leftovers of planet formation, with compositions that have remained almost pristine for 4.6 billion years.

The asteroid 24 Themis orbits about x million miles (480 million kilometers) from the sun. It's one of the largest asteroids in the main asteroid belt, which lies between the orbits of Mars and Jupiter. (Explore an interactive solar system).

Unlike comets, which originate from beyond the orbit of Neptune, asteroids are thought to be relatively dry, since they orbit much closer to the sun.

But previous theories have suggested that ice could still exist in the main asteroid belt if it's buried below the surfaces of the space rocks. (See asteroid and comet pictures.)

In fact, two nearby "relatives" of 24 Themis are members of a strange group called main-belt comets, which are known to leave dust tails that may be fueled by water ice sublimating, or turning directly from a solid into a gas. (Related: "Strange 'Comet' May Be Asteroid Collision Debris.")

The discovery of frost on 24 Themis means that the asteroid "represents a 'living' window into the early solar system in the form of ice that, according to conventional wisdom, should have been long gone," astrophysicist Henry Hsieh, of Queen's University Belfast, wrote in a commentary on the new study.

On 24 Themis, the frost might be coming from subsurface ice that sublimates when sunlight warms up the asteroid, sending vapor to the surface. The gas then recondenses on the surface when darkness falls and temperatures dip, study author Campins said.

Or micrometeorites may practice "impact gardening," churning the surface enough to gradually uncover a layer of preserved subsurface ice.

Or the answer may be something else entirely, Campins admits: "One of the fun things about having so little information is how much one can speculate," he said.

Asteroids May Have Brought Water to Earth

More data on asteroid composition could arrive as early as June, when the Japanese spacecraft Hayabusa returns to Earth potentially carrying the first sample taken directly from an asteroid. (Read about NASA's plans to send astronauts to an asteroid.)

Hayabusa landed on the asteroid Itokawa in 2005. Although the craft failed to collect any formal samples, scientists think some dust from the space rock's surface might have accidentally gotten into the sampling chamber.

Meanwhile, a proposed NASA mission dubbed the Origins Spectral Interpretation Resource Identification Security-Regolith Explorer, or OSIRIS-REx, could bring back a sample from an asteroid called RQ36.

"It so happens that this main target of OSIRIS-REx is an asteroid of the same classification as 24 Themis," Campins said, which means a sample from RQ36 might contain water like that seen on 24 Themis.

By some accounts, Earth should have been too hot in its early days to have retained any of its original water. This has led some scientists to suspect our oceans were delivered by a barrage of asteroids or comets once the planet had cooled.

Matching the chemical qualities of the asteroid's water with Earthly H2O could solve the mystery. And if the OSIRIS-REx sample also has organics, it could bolster theories that space rocks delivered the starting materials for life.

"It could be that the composition of this nearby object is also like 24 Themis," Campins said of RQ36. "So we might be able to sample the primitive organic molecules that could have been the building blocks that [enabled] life to arrive on Earth by a near-Earth asteroid."

The asteroid-ice discovery is described in this week's issue of the journal Nature.

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