The observation that water plumes erupt from cracks on Saturn's moon Enceladus has fired speculation about a possible subsurface ocean. The latest searches for sodium salts point to the existence of such an ocean.
Do the spectacular plumes of water vapour and ice particles seen on Saturn's icy moon Enceladus come from liquid water just below its frigid surface? That is the fascinating question addressed by Postberg et al.1 (page 1098 of this issue) using data from the Saturn-orbiting Cassini spacecraft, and by Schneider et al.2 (page 1102) using ground-based telescopes. Despite their very different techniques, the two teams use the same key element, sodium, in their search for Enceladan water.
Of all the icy moons orbiting the giant planets of our Solar System, Enceladus, just 500 kilometres in diameter, is the only one (so far) where we can watch, in detail, geological processes as they happen. Four relatively warm 120-km-long fractures, informally called tiger stripes, slice across Enceladus's south pole and eject supersonic plumes of water vapour and ice particles thousands of kilometres into space3,4,5,6,7 (Fig. 1). The ice particles populate a dust ring around Saturn — the E ring, which is much larger and fainter than the planet's better-known main rings — and the vapour populates a torus of atoms and molecules encircling the planet. The chemistry of the plumes is of intense interest not only because it provides a unique opportunity to sample the interior of an icy moon directly, but also because the interior of this particular moon provides a potential habitat for extraterrestrial life.
Life requires at least three ingredients: a source of energy, which in Enceladus is provided — at least in part — by tidal heating caused by the varying gravitational pull of its parent planet as Enceladus travels around its slightly eccentric orbit; a suitable mix of chemical elements, which seem to be present based on Cassini's analysis of the plume gases4; and liquid water. So the question of whether Enceladus's internal heat can provide that water, by melting a portion of the ice shell that comprises much of the moon's bulk, is one of the hot issues in planetary science today.
Sodium is a valuable tracer of possible liquid water for two reasons. First, it is highly soluble in water, and so any Enceladan water that has had prolonged contact with the moon's silicate core should be rich in sodium salts, like Earth's oceans. Second, when dispersed in its atomic form, sodium scatters sunlight with extreme efficiency at its resonant wavelength of 589 nanometres (the familiar orange–yellow colour of sodium street lights), and is thus easy to detect even in minute quantities.
Schneider et al.2 use spectrographs on ground-based telescopes to search for sodium emission in Enceladus's gas plumes and Saturn's neutral torus. They find no sodium there to high precision, in striking contrast to the bright sodium emission seen in the output from Jupiter's volcanic moon Io, and even in the ultra-thin atmospheres of comets, Mercury and our own Moon. If there is salty water in Enceladus, some process must be very efficient in preventing most of the sodium from escaping into space.
Postberg et al.1 focus instead on the ice grains from the plumes, using the Cosmic Dust Analyzer instrument aboard Cassini to determine their chemical composition directly as Cassini flies through the E ring. They find that, although all the grains are dominated by water ice, about 6% of them are quite salty, containing roughly 1.5% of a mixture of sodium chloride, sodium carbonate and sodium bicarbonate. The authors note that the grain composition is similar to the expected composition of an Enceladan ocean8, and therefore suggest that these particles are derived from direct freezing of salty water from that ocean at the plume source. They propose that the rest of the ice grains, which have very little salt, may come from direct condensation of the plume vapour.
So the question that naturally arises is: are Schneider and colleagues' no-sodium-emission observations in conflict with Postberg and colleagues' findings? The short answer is no. Although the ice grains in the E ring eventually vaporize and liberate their contents into the neutral torus, their total cargo of sodium, when diluted by all the sodium-poor grains and the sodium-free gas, would be too small to be detected by Schneider and colleagues' ground-based observations.
The simplest, and perhaps most likely, inference from Postberg and colleagues' particle-composition data is that the plumes are directly derived from salty liquid water. But it's always possible that we are being fooled: for instance, the plume sources might currently be too cold for liquid water to exist, and the plume might be excavating the sodium-rich ice grains from some long-frozen salt pockets in Enceladus's crust. We also can't be sure that, if there is water at the plume source, it is connected to a salty ocean — Schneider and colleagues point out that the water might originally be salt-poor, only becoming salty by preferential evaporation of the more-volatile water vapour. But in any case, those salty grains provide our current best smoking (or steaming) gun pointing to present-day liquid water near the surface of Enceladus.
The salty water cannot, however, be boiling explosively straight into the vacuum of space, otherwise the sodium would be carried along and would have been easily detectable by Schneider and colleagues. Instead, the plume production process must leave most of the sodium behind. Distillation of fresh water vapour from salty water happens over Earth's oceans, of course, and could happen on Enceladus too if the evaporation proceeded slowly in pressurized chambers, for example, rather than in a vacuum. The plumes might then be supplied by leakage from the chambers along narrow fractures leading to the surface. Postberg et al.1 point out that water evaporation must occur slowly for another reason: to prevent the water from freezing due to rapid loss of latent heat during the evaporation process (see online Supplementary Information to ref. 1).
Cassini will make four more close fly-bys of Enceladus before mid-2010, with the chance of an additional twelve fly-bys up to 2015, if NASA approves the 7-year extended mission currently in the planning stages. Many more discoveries are therefore likely, and with each one Enceladus becomes more exotic. Our picture of its subsurface must now be expanded to include the possibility of misty ice caverns floored with pools and channels of salty water, lurking beneath the tiger stripes. What else may lurk in those salty pools, if they exist, remains to be seen.