Image caption Artist's impression: 24 Themis sits well beyond the orbit of Mars

Scientists have detected water-ice on the surface of an asteroid.

The first-time observation was made on 24 Themis, a huge rock that orbits almost 480 million km out from the Sun.

The researchers say that ice is not stable in such circumstances and has to be replenished by some means - perhaps from inside the object.

They tell Nature magazine the finding plays into the theory that much of the water in Earth's oceans was delivered from space.

"It's interesting that we have detected ice on an asteroid because there have been suggestions that water on Earth came from impacts with many asteroids in Earth's early history," said Professor Humberto Campins, from the University of Central Florida, Orlando, US.

"This detection of water-ice on the surface of an asteroid supports that idea," he told BBC News.

24 Themis is about 200km in diameter, making it one of the biggest rocks in the main asteroid belt. It orbits at more than one-and-a-half-times the Sun-Mars distance.

The observation that its surface is frosted was confirmed by two independent teams - one led by Professor Campins - who examined how light was reflected off the body using the US space agency's (Nasa) Infrared Telescope Facility on Mauna Kea, Hawaii.

The teams also found a signature for complex organic, or carbon-rich, compounds.

Coming up

Scientists have long since detected hydrated, or water-containing, minerals on the surfaces of asteroids - but this is a first in terms of an observation of exposed water-ice.

The researchers were drawn to make the study because smaller fragments of the rock broken off during an ancient collision look rather like comets when viewed through telescopes, and this suggested they and the larger body might harbour significant quantities of ice.

But to find it covering the surface is unexpected, say the researchers. In sunlight, and with no pressure from an atmosphere, the ice would be expected to vaporise rapidly.

This indicates the ice disappearing at the surface is constantly being replaced.

One scenario thought highly unlikely is that Themis has had a recent collision with an icy comet.

More probable explanations are that frequent impacts with smaller rocks are turning over the surface to release hidden reserves of ice, or that vapour from ice held deeper inside the asteroid is continually out-gassing and condensing briefly on the surface.

Fashionable rocks

Theorists have been concerned for some time that the Earth may have formed at too high a temperature to have started with much water, and it has become a popular theory that much of the water we see today must have come in from elsewhere.

Comets contain a lot of water and impacts could have delivered large volumes. But perhaps not enough, says Dr Andy Rivkin, from Johns Hopkins University, Laurel, US, who led the other research team.

Also, the type of atoms (isotopes) seen in Earth water do not match well a comet-only source.

"Finding ice in Themis and the Themis family opens up the possibility that you might have brought in water from asteroids as well as comets; and that potentially allows a lot more water to be brought in and it also allows the isotopic compositions to work out the way we need them to, to match the Earth," Dr Rivkin told BBC News.

Asteroids are a very fashionable topic for scientific study at the moment.

A Japanese capsule is due to return to Earth in a few weeks with a sample picked up off the surface of a space rock; the European Space Agency's Rosetta probe will flyby an asteroid later this year; and a Nasa spacecraft will go into orbit around one of these bodies in 2011.

President Barack Obama has also directed the US space agency to send astronauts to an asteroid in the mid-2020s.

"For a while we thought we knew everything there was worth knowing about asteroids, and we looked farther and farther out into the Solar System, to the moons of Jupiter and Saturn and to the Kuiper Belt," observed Dr Henry Hsieh from Queen's University Belfast, UK.

"Now there seems to be a lot going in the asteroid belt that we don't actually understand, so again these bodies are exciting."