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.
Vesta and Ceres, the two largest objects in the asteroid belt, are not only an asteroid and a dwarf planet. They are protoplanets: infant bodies that formed in the earliest times of the solar system, but whose processes of development were halted. Where other protoplanets continued to grow by drawing in more and more material and "snowballing" into full-fledged planethood, the growth of objects like Vesta and Ceres was stunted—but this turned out to be an advantage for us.
Similar to how fossils can tell us about life forms and environments that existed on Earth long ago, leftover protoplanets like Vesta and Ceres are time-capsules of information about conditions in the earliest times of the solar system. The presence of water on Ceres may also inform us of the role that water played in the early formation of planets like Earth.
Ceres has been an object of mystery and revelation as long as we've known about it. An astronomical chameleon, it has defied definitive classification not once, not twice, but perhaps three times. Discovered in 1801 orbiting between Mars and Jupiter, Ceres was originally thought to be another planet, but later downgraded to the status of the newly-created classification of "asteroid" when other, smaller objects were found orbiting the sun in its realm—sound familiar, Pluto?
Then in 2006 Ceres was reclassified again as a dwarf planet, along with former planet Pluto, Ceres' kindred spirit in the Kuiper Belt. And now, with the discovery of water vapor and a possible thin atmosphere, Ceres is subtly--and unofficially--elevated to the likeness of a world, a quality earned by a celestial object when we learn of the existence of air and water.
In 2015, we'll have a much clearer view of what a dwarf planet actually looks like. Not only will Dawn arrive at Ceres, NASA's New Horizons spacecraft, after an almost ten year journey, will reach Pluto.