Astronomers have once again found water in an unexpected place, this time on an asteroid in the main belt between Mars and Jupiter.

This artist's conception shows asteroid 24 Themis and two small fragments, lit by the sun. The artist has depicted one of the small fragments as inert, as most asteroids are, and the other as having a comet-like tail, produced as water ice vapourizes from its surface. ((Gabriel Prez, Servicio MultiMedia, Instituto de Astrofisica de Canarias, Tenerife, Spain))

The asteroid, called 24 Themis, appears to be covered with a thin layer of frost. The researchers also found evidence of molecules containing carbon.

This is the first time that water or organic compounds has been found on asteroids.

The astronomers used telescopes at NASA's Infrared Telescope Facility on Hawaii's Mauna Kea mountain to analyze light reflected by the asteroid. The spectrum of the light was consistent with the presence of water and carbon-containing molecules.

Josh Emery of the University of Tennessee, Knoxville, co-author of one of two studies on the find appearing this week in Nature, said the find supports the idea that an asteroid could be the source of both water and carbon molecules on Earth.

"The organics we detected appear to be complex, long-chained molecules. Raining down on a barren Earth in meteorites, these could have given a big kick-start to the development of life," Emery said in a statement.

The water was surprising to find, the researchers said, because at the temperature of the asteroid belt, about –120 to –70 C, water ice vapourizes into space.

The asteroids are billions of years old, so the ice on the surface must be replenished somehow. The astronomers said the most likely source is inside the asteroids.

"This implies that ice is quite abundant in the interior of 24 Themis and perhaps many other asteroids. This ice on asteroids may be the answer to the puzzle of where Earth's water came from," said Emery.

Because asteroids are usually thought of as dry, rocky bodies, the discovery of water on 24 Themis is blurring the line between what's considered a comet and what's an asteroid.