[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.

[edstrick]

Considering geysers vs sublimating ice on Enchiladas <grin>...

Imagine ice with a high thermal gradient... cold at the surface, warming rapidly with depth, becoming plastic when the temperatrue approaches melting, then with liquid water, perhaps in a pocket or linear body caused by an opening caused by fracture at depth.

Imagine the ice is indeed fractured, which may have been intruded by water, but is currently not open to water at the bottom, due to ice creep or whatever reason.

The cold ice at the surface doesn't sublimate. Go down into the crack.. the surfaces are facing each other more than they are sky, and can get warmer due to the heat flow from under the surface than bare sky-looking ice with the same heat flow. The warmer ice will tend to sublimate, possibly some tending to freeze on the edge of the crack where it opens on the surface and the ice is colder. Deeper in the crack the sky is less exposed, you are deeper in the regional heat gradient and possibly closer to a local heat source.... the ice is warmer and sublimates faster, vapor escaping upwards into the colder crack above...

Perhaps there is some equilibrium between heat from warm vapor and the cold ice near the surface, the crack may narrow due to icing, but not close completely. The warm ice zone at the bottom of the crack may tend to open wider and deepen ... retreating into the thermal gradient.... eventually breaching into liquid water, causing intense geysering into vaccuum until much of the water in the pocket re-freezes due to evaporative chilling and easy access of liquid water to near vaccuum is choked off.

We have very non-linear processes here with lots of feedback... I'm 100% positive what's going on in and at those tiger stripe cracks is *COMPLICATED* geometrically and thermally and geologically.

[Guest_Richard Trigaux_*]

...

We have very non-linear processes here with lots of feedback... I'm 100% positive what's going on in and at those tiger stripe cracks is *COMPLICATED* geometrically and thermally and geologically.

Yes, ery non-linear stuff, with short eruptions of water when a crack opens, and then this water freezes in the crack, keeping it more and more open, like in the oceanic ranges where basalt dykes open the cracks and repell the continents.


In the very place where this process takes place, we could see a very difficult terreain, with a kind or karst* formed by sublimating ice. With alot of holes and cracks. Worse, we can assume that most of the snow formed by evaporation of liquid water, just falls back and cover all the previous holes, turning the ground into a rover nightmare. Add to this large boulders as those already seen in other parts of Enceladus... A landing around may be much more complicated than on Europa. Perhaps a lander may have very large inflatable tyres, or crawl like a caterpilar on very large skis, to avoid to sink into snow and underlying cavities. Better: a snow bike!

In facts we really don't know how the vents look like, and it would be a good bargain if Cassini could image them precisely in the close pass at 25kms. For this it may require to tilt, in order to compensate for motion blur (at this stage of the mission, taking some risks wil be worthy)




*karst, geological forms like cracks, caves, shafts, formed by dissolution of limestone by water.