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Ice and organics found on main belt asteroid
Posted: 5 May 2010

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Water-ice, accompanied by organic compounds, has been detected for the first time on the surface of a large main belt asteroid, a finding that carries important implications for the source of Earth's oceans.

The spectroscopic observations, which were made using the NASA Infrared Telescope Facility at Mauna Kea in Hawaii, were reported in last week's edition of the journal Nature by two independent teams of planetary scientists. Both studies focused on the large, 200 kilometre-wide main belt asteroid 24 Themis, and found a very definite signature of water ice and carbon-based organic materials.

Artist's conception of asteroid 24 Themis and two small fragments of this dynamical family, which resulted from a large impact more than one billion years ago. Note that one of the small fragments (top) is inert (as most asteroids are) and the other (bottom) has a comet-like tail, produced by the sublimation of water ice from its surface. Image: Gabriel Pérez, Servicio MultiMedia, Instituto de Astrofisica de Canarias, Tenerife, Spain).

"The presence of ice implies that the ice never melted in Solar System history, which is surprising," one of the paper's authors Andy Rivkin tells Astronomy Now. "We are admittedly less surprised now than we might have been 6-7 years ago, because of the existence of the Main Belt Comets, but even with that we didn't expect to see ice at the asteroid's surface."

Main Belt Comets (MBCs) are defined as objects that exhibit observable cometary activity, such as a comet-like dust tail likely driven by impact events that expose pockets of subsurface ice, but has the orbit of a main belt asteroid. There are just four confirmed MBCs in the Asteroid Belt, two of which are associated with 24 Themis. "Asteroid 24 Themis and at least some of the MBCs are members of the same dynamical family, which formed when a huge impact disrupted a ~400 kilometre-wide body a few billion years ago," says Rivkin. "The fragments were moving faster than escape velocity with respect to each other, so they now orbit the Sun in independent but recognizably related orbits."

Two MBCs 133P/Elst-Pizarro and P/2005 U1 (Read) show comet-like tails but orbit within the Asteroid Belt. More images on Henry Hsieh's website.

Rivkin's team, along with the other group of scientists led by Humberto Campins of the University of Central Florida, both arrived at the same conclusion that the signatures they were reading could be explained by the presence of an extremely thin layer of frost covering the whole asteroid, and that it is mixed with carbonaceous material. While it is not a total surprise that ice has been detected on 24 Themis – theoretical models show it could survive for billions of years if buried just a few metres to a few tens of metres below the surface – it is more surprising that it extends so widely across the asteroid. The average surface temperatures of asteroids of around 150-200 kelvin suggests that most surface ice should have sublimated away in a matter of years.

One suggestion for its longevity is that the frost layer is being continually replenished by the slow release of water vapour from inside the asteroid, which recondenses as frost when it reaches the surface. Or, just as on the Moon, 'impact gardening' of micrometeorites on the surface could turn the surface over at a rate consistent with the observed quantity of ice.

Earth's water likely originated from asteroids and comets beyond the so-called snow-line. Image: NASA/JPL-Caltech

Either way, the results have implications for the arrival of water onto the Earth, which was most likely received from impacting comets and asteroids from beyond the so-called snow line. "We see minerals that formed in the presence of water in outer belt asteroids, but not in inner belt asteroids, with the transition appearing to occur around 2.5 AU from the Sun," comments MBC expert Henry Hsieh. "This suggests that objects outside this transition point – the snow line – were able to accumulate ice as they formed, while objects inside the line were not because temperatures in the protoplanetary disc were so warm that water could only exist as a diffuse ambient vapour."

Earth and Mars both formed inside the snow line so it is unlikely that they could have accumulated significant amounts of water as they formed. "They probably required the delivery of water and other light materials like noble gases at a later time from objects from outside the snow line at a time when they had grown large enough to have enough gravity to retain atmospheres," continues Hsieh. "So, it's not so much a matter of explaining whether terrestrial planets could have retained water, rather explaining how they accumulated it in the first place."

The new data will also enable scientists to link their observations with hydrated minerals found in meteorites, some of which share similar characteristics to that of ocean water, suggesting that outer main belt asteroids are the likely providers of that water. The implication, as Hsieh highlights in Nature's accompanying News and Views article, is that "we no longer have to infer the composition of ancient asteroid water from hydrated minerals: we can study the water directly because it is still around today!"

Further investigation of ices in the Asteroid Belt will allow comparisons of the water on Earth, and thus provide further clues as to the origin of our vast oceans.