Frosty Asteroid Leaks Water Into Space

At least one asteroid in the belt between Mars and Jupiter is unexpectedly covered in water ice and organic compounds, a pair of studies report (Nature 2010, 464, 1320 and 1322). Because the water on Earth was likely delivered by asteroids or comets, the finding raises the possibility of being able to study asteroid composition to pinpoint biogenic building blocks. In separate but complementary data collected at NASA’s Infrared Telescope Facility on Mauna Kea, Hawaii, two sets of researchers observing the asteroid 24 Themis found an absorption feature at 3.1 μm that is indicative of water ice, plus features at longer wavelengths that correlate with organic compounds. One team was led by Humberto Campins of the University of Central Florida and the other team consisted of Andrew S. Rivkin of Johns Hopkins University Applied Physics Laboratory and Joshua P. Emery of the University of Tennessee, Knoxville. Although researchers had previously observed hydrated minerals in asteroids, finding actual water on an asteroid surface was surprising—at temperatures of 150–200 K, surface ice should have sublimed away long ago. The researchers propose that there is a reservoir of ice inside the asteroid that is constantly sublimating. As the material diffuses up and out of the core, some of it recondenses as frost on the surface.