A moon with atmosphere.

Saturn's tiny moon Enceladus has an atmosphere containing water vapor, observations by the Cassini spacecraft reveal. The source of the atmosphere could be icy volcanic eruptions, geysers, or gases escaping from the frigid moon's surface, NASA announced March 16.

Evidence for the atmosphere, which is too tenuous to be imaged, comes from measurements of Saturn's extensive magnetic field taken by Cassini as it flew past the moon on Feb. 17 and March 9. Near the moon, the craft detected oscillations at the exact frequency expected from ionized water molecules gyrating along magnetic field lines. The ions are presumably created when sunlight strikes water vapor emanating from the moon.

From the strength of the oscillations, Chris Russell of the University of California, Los Angeles calculates that the moon loses 125 kilograms of water each second. The moon is too tiny to retain an atmosphere for long, so it must be continuously replenishing the ions with new material.

The flybys gathered additional evidence for atmospheric ions. Cassini found that Saturn's magnetic field is bent near Enceladus, an indication that ions from the moon are being swept up and accelerated by the field.

One of the possible sources of water vapor, icy volcanic eruptions from Enceladus, could explain why the moon has one of the most reflective surfaces of any object in the solar system.--R.C.