The hottest topic in the frigid outer solar system is the water of Saturn's moon Enceladus. Is it all lifeless, deep-chilled ice, or is some of it liquid and thus capable of sustaining life? Two sets of observations reported in tomorrow's issue of Nature--one made by touching ice spewing from the moon and the other made from Earth--again suggest that there could be liquid water beneath the surface of 500-kilometer-diameter Enceladus. If there is, however, it's not gushing like a geyser directly from a deep ocean.
Both groups of researchers were looking for sodium near Enceladus; one found it, the other didn't. Sodium is interesting because it indicates that deep under the ice, liquid water has been in contact with rocks, which leach salts.
Planetary scientist Frank Postberg of the University of Heidelberg in Germany and his colleagues found sodium salts in some of the icy particles carried by the water-vapor plumes jetting from four great cracks around the south pole of Enceladus, as they first reported last December at a meeting (Science, 23 January, p. 459). The particles were analyzed by the Cassini spacecraft as it flew through the plume.
This finding is "the clearest indication yet of current liquid water beneath the surface," says planetary scientist John Spencer of the Southwest Research Institute in Boulder, Colorado, a Cassini team member not involved in either study.
In the other Nature paper, astronomer Nicholas Schneider of the University of Colorado, Boulder, and his colleagues could detect no sodium vapor in the vicinity of Enceladus while searching for sodium's distinctive glow through large ground-based telescopes. Spencer says this is not evidence against liquid water, but only that the plumes are not explosively boiling off a salty ocean and escaping through the cracks. Otherwise, abundant sodium would show up as vapor.
More work is still needed to prove the current existence of liquid water, adds Spencer, because sodium-laced particles could be coming from brines formed billions of years ago and frozen ever since. So everyone is looking forward to Cassini's next flyby of Enceladus in early November--the closest to the plume source yet--in hopes of finally proving some wetness still lingers on Enceladus.