Ice confirmed on an asteroid

Frozen water leaves its signature over the entire surface of the asteroid 24 Themis

FAJARDO, Puerto Rico — Space rocks may be dead as doornails but some contain ingredients that could have given life on Earth a foothold.

Planetary scientists reported October 7 that they have, for the first time, confirmed that an asteroid contains frozen water on its surface. Evidence of water-ice, along with organic compounds, on the surface of the asteroid 24 Themis supports the theory that asteroids brought both water and organic compounds to the early Earth, helping lay the foundation for life on the planet.

Humberto Campins of the University of Central Florida in Orlando and his colleagues recorded spectra of the asteroid 24 Themis over a seven-hour period, corresponding to 84 percent of the rotational period of the spinning rock. The spectra, taken with NASA’s Infrared Telescope Facility on Hawaii’s Mauna Kea, revealed the consistent presence of frozen water as different parts of the asteroid’s surface came into view, Campins reported at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences.

The finding corroborates earlier observations (SN Online: 7/18/08) of the same asteroid by Andrew S. Rivkin of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and Joshua Emery of the University of Tennessee in Knoxville, who also used the Infrared Telescope Facility. Over several years, Rivkin and Emery had found evidence of frozen water in single spots on 24 Themis but had not studied the asteroid as it made one entire rotation. Together, the two teams’ findings reveal that the asteroid’s entire surface is coated with frozen water, Campins says.

The analyses of the sunlight reflected off the asteroid also show that organic compounds are widespread on the surface, he added, including polycyclic aromatic hydrocarbons, CH2 and CH3.

At the asteroid’s average distance from the sun — 3.2 times Earth’s distance to the sun — frozen water on the surface would readily vaporize, noted Campins. That means the ice must be continually replenished, possibly by a reservoir of frozen water within the rock, he speculates.

One possibility is that ice lies buried several meters below the surface of the roughly 160-kilometer-wide asteroid and makes its way to the top when the asteroid is pummeled by space debris. Norbert Schörghofer of the University of Hawaii at Manoa proposed last year that ice can persist for billions of years just below the surface of a dusty space rock if the asteroid’s surface temperature is less than about 145 kelvins. The temperature of 24 Themis lies in that range.

The presence of frozen water on 24 Themis also suggests that some asteroids resemble comets, becoming active suddenly and venting material into space when pockets of ice vaporize, Campins noted.

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