[X] My Assistant

[vexgizmo]

OK, it's time to have it out. Is Enceladus really spewing water, or are its fractures effectively sublimating warm ice like a comet?

Have a careful read of the Enceladus Science papers (specifically Porco et al vs. Spencer et al.) and you will see that the evidence for water is equivocal, and arguably circular. The prime piece of evidence for liquid water (Porco et al) is the inferred high ice/vapor ratio of the plume (top of p. 1398). This is inferred from scattering models and assumptions of plume particle sizes and argued unlikelihood of particle entrainment in sublimating gas (explained briefly in their note 30, and into p. 1399). Should we hang our conclusions, exploration strategies, and hopes for life on moels of ice/vapor ratio, particle size assumptions, and inferred difficulty of entraining particles in sublimated gas?

Instead (Spencer et al), the fractures of Enceladus may simply expose warm (T ~ 180K) ice which sublimates like a comet (p. 1405).

Show me the water.

[Guest_BruceMoomaw_*]

OK, it's time to have it out. Is Enceladus really spewing water, or are its fractures effectively sublimating warm ice like a comet?

Should we hang our conclusions, exploration strategies, and hopes for life on moels of ice/vapor ratio, particle size assumptions, and inferred difficulty of entraining particles in sublimated gas?

Does it really make any difference? Where both the possibility of life and the possibility of detecting its remains on the surface are concerned, the real key question is: does Enceladus have a subsurface ocean -- or at least a south-polar subsurface sea -- of liquid water between its ice layer and its rocky core?

If it does, then how the water gets to the surface doesn't really make any difference. If Enceladus lacks actual vents punching from the liquid-water layer all the way up through the ice layer to the surface, then what happens -- similar to Europa -- is that the top of the liquid-water layer freezes onto the bottom of the ice immediately above it, and then solid-state convection of the warm ice slowly transports the ice (and any biological remains frozen into it) up to the surface, where it sublimates into space. Fully 99% of the water expelled by Enceladus falls back onto its surface elsewhere as snow -- and the weight of that accumulating snow makes the ice underneath it slowly sink down and creep along sideways until it contacts the layer or pocket of subsurface liquid water, at which point it melts and the whole cycle begins again. Enceladus is estimated to expel 15 metric tons of water from its south pole per second -- which is enough to totally recycle its entire worldwide supply of ice every 130 million years.

And in either case, if the underground sea contains life, the frozen remains of that life can almost certainly be found in the surface ice at the venting area -- or even in the ice particles expelled from the surface, where they can perhaps be scooped up by a sample-return flyby. This is especially true since Saturn lacks Jupiter's malignant high-radiation environment which quickly destroys any biological remains that reach Europa's surface.

So the only relevant question is: how likely is it that Enceladus DOES have a subsurface sea, instead of just perpetually recirculated warm but solid convecting ice all the way down to its rocky core? Matson et all think it's virtually certain. http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2219.pdf : "Models presented so far show that it is difficult, if not impossible, to produce the observed [south polar heating] power if [tidal] dissipation occurs only in the ice." They are convinced that the south pole is heated by tidal dissipation in a big chamber of magma in the rocky core -- initially melted by a concentration of Al-26 during Enceladus' earliest days, and sustaining its heating since then through tidal friction (with an actual feedback mechanism operating, as the BBC article says, to keep the frictional heating at a high enough level to keep the magma molten). And the heat from that magma pocket, in order to drive such energetic venting, must melt the ice next to the rocky core into liquid water.

Moreover, we have the methane and nitrogen in the plume. What's maintaining their supply? Those gases don't freeze at the temperature of the Saturnian system; once they leave Enceladus they're gone forever -- and unless the venting started less than 130 million years ago (which seems unlikely), then it would already have completely purged any methane and nitrogen that had been frozen into Enceladus' ice at its formation. So an additional supply of those two gases must be coming from somewhere else -- specifically, from warm, or downright hot, geothermal vents out of Enceladus' rocky core and coming into contact with its ice/water mantle. Quoting Matson et al again: "It is interesting to note that the amount of N2 detected in the geyser (~4%) corresponds to the solubility of N2 in water for a temperature of 25 deg C and a pressure of 1 bar (~1 km depth). This is a further constraint on the mechanism responsible for the geyser."

Matson does think that the nitrogen is produced underground, by all of Enceladus' interior ammonia being broken down while it's still in the rocky core's vents -- which does raise the question of why there is some contrary evidence that Titan IS still releasing ammonia from its interior (as I noted in the "Great Ammonia Mystery" thread). But it turns out that Raul Baragiola -- whose own ideas about Enceladus' ammonia I and Frank Crary have seriously misunderstood, according to a recent E-mail from him -- has an alternative theory which may explain that puzzle completely, which I'll talk about once I get a little additional information from him.

At any rate, it seems to me that Bob is raising an unnecessary panic. If it turns out that Enceladus does just have rapidly sublimating surface ice instead of actual liquid-water geysers, it probably doesn't make any difference to either the chances of Enceladan life, or to our ability to detect it with (relatively) easy surface studies.