Finally astronomers have the photographic proof they've been looking for — ice volcanoes actively erupting on Enceladus. In January the Cassini spacecraft looked back toward Saturn's icy moon and caught its profile backlit by sunlight. In those images, several discrete plumes tower above the 500-kilometer-wide satellite. The spray fills Saturn's E ring with icy particles that ultimately coat many of the planet's other satellites.

Courtesy NASA/JPL/Space Science Institute.

In this week's issue of the journal Science, astronomers working with Cassini orbiter data publish a suite of papers detailing the story behind the active volcanism on Saturn's small moon Enceladus. The moon was long suspected to be the source of the water-ice particles forming Saturn's tenuous E ring. The latest work supports the idea that Enceladus has reservoirs of liquid water near the surface, which provide the source for active geysers in the southern hemisphere.

As reported in the June 2005 (page 16), November 2005 (page 16), and March 2006 (page 40) issues of Sky & Telescope, the hopes for discovering evidence for liquid water near the icy moon's surface grew more promising through 2005 as the intrepid orbiter completed a series of close flybys of Enceladus. First, observations by the craft's magnetometer hinted of a possible atmosphere of primarily water-ice crystals around the moon. Then thermal measurements of the southern hemisphere located a "hot" spot where liquid water could exist just below the surface. Stellar-occultation measurements conclusively identified a tenuous atmosphere above the hot spot. Cassini images released in November 2005 sealed the deal. As seen above, geysers from the southern hot spots are spewing ice particles hundreds of kilometers into space. Scientists think these particles might be coming from pools of subsurface liquid water. This would give Enceladus something in common with the Jovian moons Europa and Ganymede, which also appear to have underground water.

"Other moons in the solar system have liquid-water oceans covered by kilometers of icy crust," says imaging team member Andrew Ingersoll (Caltech). "What's different here is that pockets of liquid water may be no more than tens of meters below the surface."

"If we are right, we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms," adds Cassini imaging team leader Carolyn Porco (Space Science Institute).


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