Composition Of Enceladus Jets Still Unclear

Researchers are looking forward to two more close flybys of Saturn's strange moon Enceladus by NASA's Cassini Saturn orbiter in November to resolve continuing questions about the source of spectacular water jets emerging from cracks at the moon's south pole.

Separate new studies using large ground-based telescopes and an instrument on Cassini have drawn seemingly conflicting conclusions about Enceladus. Direct measurements of Saturn's outermost ring, made up of water-ice particles from the moon's jets, with Cassini's cosmic dust analyzer have found concentrations of salt that researchers believe are so high they could only have been dissolved in liquid water.

Yet spectral analysis of the jets using the 10-meter Keck 1 and the 4-meter Anglo-Australian telescopes revealed almost no sodium in the water vapor, according to a team headed by the University of Colorado at Boulder. The results suggest that the jets originate not from a subsurface liquid ocean, but perhaps from smaller caverns or from ice melting in the heat of tidal friction.

"These are all hypotheses, but we can't verify any one with the results so far," says Colorado's Nicholas Schneider, who led the ground-based study. The results of both studies appear in the journal Nature.

Discovered in 2005 by Cassini, the jets send water vapor and ice deep into space from a series of cracks across the south pole of Enceladus. Data from the cosmic dust analyzer also turned up carbonates in them, and a slightly alkaline pH that could permit the formation of "life precursors" in liquid water with heat measured at the south pole. The plumes also have been found to contain organic compounds.

"Finding salt in the plume gives evidence for liquid water below the surface," says Sascha Kempf of Germany's Max Planck Institute for Nuclear Physics, which supplied the dust analyzer. "The lack of detection of sodium vapor in the plume gives hints about what the water reservoir might look like."

Cassini will fly by Enceladus twice in November as part of its two-year mission extension. On Nov. 2 it will make its deepest plunge into the plumes, passing only 99 kilometers (60 miles) over the surface for more direct measurements of the jets' composition and density. On Nov. 21 it will pass over the south pole at a distance of 1,603 kilometers to study the "Tiger Stripe" cracks where the jets emerge.

Artist's concept of Cassini: NASA