While hydrated minerals thought to indicate the presence of water have previously been detected on Ceres, the Herschel space observatory has made the first direct observation of water vapor, NASA's Jet Propulsion Laboratory in Pasadena, Calif., reported Wednesday.
Ceres, around 590 miles in diameter, is classified as a dwarf planet, larger than an asteroid and smaller than a planet.
Herschel, a European Space Agency mission with important NASA contributions, used its far-infrared vision to see, finally, a clear spectral signature of the water vapor.
"This is the first time water vapor has been unequivocally 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 Kuppers of ESA in Spain, lead author of a paper on the findings published in the journal Nature.
Scientists say they 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 has its own mission, dubbed Dawn, now headed toward Ceres after spending more than a year orbiting the large asteroid Vesta.
It will arrive in Ceres in the spring of 2015, NASA scientists said.
"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, JPL deputy principal investigator for the Dawn mission. "Dawn will map the geology and chemistry of the surface in high resolution, revealing the processes that drive the outgassing activity."