Water detected on dwarf planet

Scientists at the Herschel space observatory recently observed something unexpected - signs of water on a large asteroid.

Ceres, on whose surface vapours have been observed, is the largest and roundest object in the asteroid belt and is classified as a dwarf planet - a solar system body bigger than an asteroid and smaller than a planet.

Plumes of water vapour are thought to shoot up periodically from Ceres when portions of its icy surface warm slightly.

"This is the first time water vapour has been detected on Ceres or any other object in the asteroid belt and provides proof that Ceres has an icy surface and an atmosphere," said Michael Küppers of the European Space Agency (ESA) in Spain, lead author of a paper in the journal Nature.

In the last century, Ceres was known as the largest asteroid in our solar system. But in 2006, the International Astronomical Union, reclassified Ceres as a dwarf planet because of its large size. It is roughly 950 km in diameter.

Scientists believe Ceres contains rock in its interior with a thick mantle of ice that, if melted, would amount to more fresh water than is present on all of Earth.

NASA's Dawn mission is already on its way to Ceres. Dawn is scheduled to arrive at Ceres in early 2015, when it will take the closest look ever at its surface.

"We've got a spacecraft on the way to Ceres, so we don't have to wait long before getting more context on this intriguing result, right from the source itself," said Carol Raymond from NASA's Jet Propulsion Laboratory in California.

But Herschel did not see water vapour every time it looked. While the telescope spied water vapour at four different times, on one occasion there was no signature.

Here is what scientists think is happening: when Ceres swings through the part of its orbit that is closer to the Sun, a portion of its icy surface becomes warm enough to cause water vapour to escape in plumes at a rate of about 6 kg per second. When Ceres is in the colder part of its orbit, no water escapes.

The results are somewhat unexpected. "The lines are becoming more and more blurred between comets and asteroids," said Seungwon Lee of JPL. "We knew before about main belt asteroids that show comet-like activity, but this is the first detection of water vapour in an asteroid-like object."

Herschel is an ESA mission with important NASA contributions.