When NASA’s Dawn space probe moves into orbit around Ceres, the largest dwarf planet in the main asteroid belt between the orbits of Mars and Jupiter, it will find that it is ejecting water vapor, according to a January 22, 2014 announcement by the space agency.
The jet of water vapor was detected by the European Space Agency’s Herschel space telescope, It has been long theorized that Ceres has a mantle of ice that, if melted, would be more fresh water than exists on the planet Earth.
“Until now, ice had been theorized to exist on Ceres but had not been detected conclusively. It took Herschel's far-infrared vision to see, finally, a clear spectral signature of the water vapor. But Herschel did not see water vapor every time it looked. While the telescope spied water vapor 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 vapor to escape in plumes at a rate of about 6 kilograms (13 pounds) per second. When Ceres is in the colder part of its orbit, no water escapes.
“The strength of the signal also varied over hours, weeks and months, because of the water vapor plumes rotating in and out of Herschel's views as the object spun on its axis. This enabled the scientists to localize the source of water to two darker spots on the surface of Ceres, previously seen by NASA's Hubble Space Telescope and ground-based telescopes. The dark spots might be more likely to outgas because dark material warms faster than light material. When the Dawn spacecraft arrives at Ceres, it will be able to investigate these features.”
Dawn had previously spent a year orbiting the asteroid Vesta between July 2011 and September 2012 before proceeding to Ceres, using an ion rocket that slowly but continuously accelerates the spacecraft. It is due to arrive at Ceres in February 2015. It was launched on September 27, 2007.