It's not much—just 6 kilograms per second—but there's water vapor coming off two regions of Ceres , the solar system's largest asteroid. As astronomers report online today in Nature, the Herschel Space Observatory  has discerned a watery spectral line at the far-infrared wavelength of 538 microns. Ceres revolves around the sun between the orbits of Mars and Jupiter every 4.6 years. The asteroid's path is somewhat elliptical, so its distance from the sun varies. Sunlight warms Ceres most when the asteroid comes closest, which was around the time Herschel detected the water vapor (shown with exaggerated clarity in this artist's conception). This suggests the sun does to Ceres what it does to comets: converts water ice into gas. However, there's a small chance the water vapor arises instead from geysers resembling those on Saturn's moon Enceladus . We'll find out more in February 2015, when NASA's Dawn spacecraft  goes into orbit and gives us our first close-up look at the first asteroid astronomers ever discovered, back in 1801.