Still Hope For Salty Water On Enceladus

The results of two searches for sodium in the gas and ice grains spewing from geysers on the surface of Saturn's moon Enceladus keep alive the tantalizing hypothesis that a salty ocean may lie beneath the moon's icy surface. If true, this kind of environment might have the right conditions to support microbial life. Ice grains from the Enceladus plumes feed the faint, outermost "E" ring of Saturn, and the gaseous portion of the plume encircles Saturn in a doughnut-shaped torus of atoms. Frank Postberg of the Max Planck Institute for Nuclear Physics, in Heidelberg, Germany, and colleagues report that the Cassini spacecraft used its onboard mass spectrometer to analyze ice grains in the outer ring and found that about 6–7% of them contained large amounts of salts. This correlates well with hypotheses that Enceladus harbors a salty reservoir of liquid water (Nature 2009, 459, 1098). However, planetary scientist Nicholas M. Schneider of the University of Colorado, Boulder, and colleagues found no sodium in ground-based observations of the torus, which contains gas from the geysers (Nature 2009, 459, 1102). It would seem that the results from the two papers are contradictory, but, as John Spencer writes in an essay accompanying the papers, "The short answer is no." Given the amounts of sodium salts found in the ice grains, the sodium atoms in the vapor portion would be too dilute—below the detection threshold—for ground-based observations to spot.