Evidence of water ice and organic materials has, for the first time, been discovered on an asteroid. Josh Emery, from the University of Tennessee Knoxville using NASA's Infrared Telescope Facility on Hawaii's Mauna Kea, found that the spectrum of infrared sunlight reflected by the object was consistent with frozen water. The discovery was confirmed by a team lead by Humberto Campins at the University of Central Florida. Organic materials in the form of complex, long-chained molecules were also discovered. The asteroid 24 Themis is approximately 200 kilometers wide and orbits half-way between Mars and Jupiter in the asteroid belt. As this short distance from the Sun (only about 479 million kilometres) is generally too warm for ice to exist for long, it is theorized that the asteroid may contain an abundance of ice in its interior which is outgassed to replenish the surface supply. This discovery suggests that asteroids may be responsible for bringing some of the water and organic molecules to the barren, developing Earth.
Why the water is there, or how it got there in the first place, is still unclear. Think about an asteroid for a second. Picturing a dry, rocky object in space? You aren't alone. Asteroids are widely viewed as being very dry, especially compared to, let's say, comets. Long-range comets (those coming in from the Kuiper Belt in extremely long and often elliptical orbits around the Sun) contain a lot of water but with a different isotropic signature than water found on Earth, and so they are an unlikely source of Earth's water. In 2006, small comet-like objects with tails were discovered in the asteroid belt and dubbed "main-belt comets," and its this set of objects to which 24 Themis belongs. This asteroid, or main-belt comet, belongs to a set of asteroids that is thought to have formed as a result of a large impact from a larger body, suggesting that the original object contained ice. This has implications for our theories on solar system evolution and planet formation. If this ice-on-asteroid phenomenon is relatively widespread then it is likely that ice was more prevalent in the Main Belt region of the early solar system than previously thought. If you expand this idea to other solar systems and extrasolar planets then water/ice may be more abundant than we thought. And, although that might be a stretch based on a sample size of one, it is an interesting direction to look in. Also, we can't ignore the idea that the asteroid may be from Kuiper belt and was knocked into the inner solar system or arrived there when the gas and ice giants migrated their orbits. Future observations and research will work to determine whether the composition of 24 Themis is typical of the asteroids in this region and if the water on these rocky objects is the same as that found on Earth.
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(image from www.dlr.de -- credit NASA/JPL -Caltech)