Enceladus, a small satellite of Saturn, has captivated planetary scientists for years with its watery polar geysers and ridgelike surface features known as "tiger stripes." Now it has a new layer of intrigue. The gas and ice escaping from Enceladus and shooting out from the moon's south pole in towering jets, which fill Saturn's diffuse E ring, also seem to rain down on Saturn itself, providing water vapor to the giant planet's upper atmosphere.

Paul Hartogh of the Max Planck Institute for Solar System Research in Lindau, Germany, and his colleagues used the European Space Agency (ESA) Herschel space telescope to identify a doughnut-shaped cloud of water around Saturn. The doughnut is only about one Saturn radius (58,000 kilometers) in thickness but is more than 10 Saturn radii across, according to an ESA press release. The cloud comes from Enceladus spewing out material through its south polar jets, which were first spotted by NASA's Cassini probe in 2005 but whose ultimate origin is still somewhat uncertain.

Given the outflow rates from Enceladus, Hartogh and his colleagues estimate that the jet-filled water cloud should suffice to provide the water vapor that has been detected in Saturn's atmosphere. Hence, they reported in the August issue of Astronomy & Astrophysics, "Enceladus is thus the likely source of Saturn's external water, though an additional confirmation could be provided by the latitudinal distribution of H2O on Saturn."

Interestingly, Enceladus seems not to be the source of water vapor detected on Titan, Saturn's largest moon. Hartogh and his co-authors calculate that Enceladus's geysers provide far too little water vapor to account for the observations of Titan. For now, they wrote, "the origin of Titan's water remains a puzzle."

Photo of Enceladus's jets: NASA/JPL/Space Science Institute