Recent observations of the dwarf planet Ceres by the European Herschel Space Observatory have revealed for the first time the presence of water vapor on this object in the Main Asteroid Belt.
This is a tantalizing discovery. Although the presence of water ice on the rocky objects of the asteroid belt has long been theorized, this is the first definitive detection and the first detection of a possible atmosphere on a Main Belt object. Further, the amount of water ice on Ceres may be greater than all the water on Earth.
The Herschel Space Observatory, operated by the European Space Agency until its planned shutdown last April, was the largest infrared telescope ever launched into space. Herschel's cool-gas-and-dust-sensing infrared vision has been applied to a number of observing programs, one of which was to analyze the chemical composition of the surfaces and atmospheres of objects in the solar system, including planets, moons, comets, asteroids…and the dwarf planet Ceres.
Herschel made detections of water vapor on Ceres on several occasions, but interestingly not in all observations. The timings of the detections suggest that the water vapor is outgassing in periodic bursts (probably powered by heating when Ceres swings closer to the sun), and the source of outgassing may be localized to specific regions on its surface.
Herschel's discovery comes at an opportune moment as NASA's Dawn spacecraft, en route from its last port of call, the asteroid Vesta, gets ready to rendezvous with Ceres. Dawn is scheduled to arrive at Ceres in the spring of 2015, so we don't have to wait long to get a detailed, up close look at the source of the water vapor and the nature of Ceres' probable atmosphere.