NASA's Cassini spacecraft took some surprising pictures of one of the most mysterious moons of Saturn. The pictures show that this small moon, which is almost 7 times smaller than Earth's Moon, has active geysers. Scientists already knew that Enceladus was made mostly from ice so when the plumes were observed they wondered whether it was possible that somehow the ice could be converted into gas. However this hypothesis was eventually ruled out and another more exciting idea took its place: the jets might be erupting from near-surface pockets of liquid water.

"Other moons in the solar system have liquid-water oceans covered by kilometers of icy crust," said Dr. Andrew Ingersoll, imaging team member and atmospheric scientist at the California Institute of Technology. "What's different here is that pockets of liquid water may be no more than tens of meters below the surface."

"We realize that this is a radical conclusion - that we may have evidence for liquid water within a body so small and so cold," said Dr. Carolyn Porco, Cassini imaging team leader at Space Science Institute. "However, if we are right, we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms."

Dr. John Spencer, another Cassini scientist from Southwest Research Institute also said: "We previously knew of at most three places where active volcanism exists: Jupiter's moon Io, Earth, and possibly Neptune's moon Triton. Cassini changed all that, making Enceladus the latest member of this very exclusive club, and one of the most exciting places in the solar system."

The mechanism of the jets is laid out in the image below. What happens is that Saturn's large gravity produces such a tidal force on Enceladus that there is sufficient energy to melt some of the ice and produce liquid water below the surface. This water is under high pressure and sometimes this pressure is sufficient to produce a crack in the surface and to expel the water into space.

This helps explain one of the enigmas of the Saturn's system: "As Cassini approached Saturn, we discovered that the Saturnian system is filled with oxygen atoms. At the time we had no idea where the oxygen was coming from," said Dr. Candy Hansen, Cassini scientist at NASA's Jet Propulsion Laboratory in Pasadena. "Now we know that Enceladus is spewing out water molecules, which break down into oxygen and hydrogen."

The type of evidence for liquid water found on Enceladus is different from what was found in case of Jupiter's moon, Europa. The crack's found on Europa's surface indicate there is an internal ocean. However, the evidence for Enceladus is more direct and it points to a different picture of the internal structure of this moon. The water vapor venting from sources close to the surface indicate that only pockets of melted ice are formed under the surface. These might be only temporary.

Cassini will fly by Enceladus again in the spring of 2008, at a distance of around 350 kilometers.

"There's no question that, along with the moon Titan, Enceladus should be a very high priority for us. Saturn has given us two exciting worlds to explore," said Dr. Jonathan Lunine, Cassini interdisciplinary scientist, University of Arizona.

Photo credits: NASA/JPL/Space Science Institute