Show me the water, Water geysers vs. ice sublimation Options

[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.

[ugordan]

To be fair, isn't the evidence of water on Europa circumstantial as well? Granted, it's very strong evidence, but one could still also say about Europa: "Show me the water".

[The Messenger]

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Circumstantial, and a little curious: Shallow craters. Assuming they were originally much deeper, and backfilled with liquid water, why do they contain the central peaks characteristic of 'solid' crater floors? I've never seen ice bunch up at the center of a frozen lake.

I'm also curious about the assertion that water vapor sublimes away from comets. Haven't we observe rather discreet jets on both Wild 2 and Tempel 1? What am I missing?

[djellison]

Look at the phase diagram of water

http://images.encarta.msn.com/xrefmedia/ae...ha/T073590A.gif

At deep space pressures ( i.e. very near zero - and indeed anything below 6mb ) - water doesnt exist at any temperature.


Also - what's wrong with a central peak in a crater that is then backfilled with water?

Doug

[vexgizmo]

At deep space pressures ( i.e. very near zero - and indeed anything below 6mb ) - water doesnt exist at any temperature.

It is H2O gas that sublimates away--that is what is intended and implied. Cometary jets are presumed to be places where ice is exposed at the surface, rather than mantled by non-ice debris.

[Bob Shaw]

Circumstantial, and a little curious: Shallow craters. Assuming they were originally much deeper, and backfilled with liquid water, why do they contain the central peaks characteristic of 'solid' crater floors? I've never seen ice bunch up at the center of a frozen lake.

The impact event happens, the rebound of the central peak happens, then the crater floods with meltwater, which freezes over quickly enough not to sublimate away.

Bingo!

Bob Shaw

[The Messenger]

Look at the phase diagram of water

http://images.encarta.msn.com/xrefmedia/ae...ha/T073590A.gif

At deep space pressures ( i.e. very near zero - and indeed anything below 6mb ) - water doesnt exist at any temperature.
Also - what's wrong with a central peak in a crater that is then backfilled with water?

Doug

That works well, if the original central peak was above the backfill level - which I assume is what you are telling me. If the original crater was a factor of 10 deeper, and the top ~70% was water, I would think that the original central peak would be small, since all of the water on the surface at the time of the impact would have vaporized. This is what I am trying to understand: How deep do they estimate the crater was before the backflow, and how high should the central peak be?

[volcanopele]

Getting back to Enceladus, the case for liquid water is based on the amount of solid particulates in the plume compared to the amount of water vapor in the plume as seen by UVIS, the lack of ammonia within the plume as measured by INMS, and the appearance of the plume (collumated jets, extended plume shape) in ISS images. The calculated abundance of water ice particles in the plume is based on the assumption that the effective size of the particles is 1 micron. Where that number came from, I am not sure. The particle size distribution was calculated such that the number of particles above 2 microns would match the abundance found by CDA.

Now, there are a few problems I can see, admittedly, but there are ways Cassini can plug those holes. First, the effective particle size is assumed to be 1 micron. I am not sure where that number derives from. However, using multispectral observations of the plumes, using both VIMS and ISS, I think that the effective particle size can be determined as the mission runs along. Maybe VIMS already has that from their Rev18 data. According to http://photojournal.jpl.nasa.gov/catalog/PIA06443 , the average particle size is 10 microns...

The second problem that I see is the use of data from different time periods. All the CDA, INMS, and UVIS data was taken during the Rev11 flyby of Enceladus while the ISS data is from Rev18 in late November. The MIMI instrument (Jones et al. 2006) determined that there is likely significant variability in the amount of outgassed material. UVIS observed indirect evidence for a major outburst in early 2004. So perhaps the data from Rev18 and Rev11 can not be directly compared. Combined observations more directly linked in time may be needed to resolve this issue, perhaps during Rev61. The Rev61 encounter is a much closer encounter with Enceladus (currently slated for a 23 km close approach distance). Observations by CDA can be used to determine particle abundance at 2 microns. ISS possible could image "glowing" plumes when Enceladus is in eclipse (much like Io).

So, I understand where vex is coming from. The case for liquid water is not a done deal, but based on our current data, it is better than the alternative, sublimation model. It isn't enough to drop Europa in favor of Enceladus, yet, but data gathered over the next few years, I think, can more solidly make the case.

[scalbers]

Or would you want to more "liquidly" make the case?

Actually the cometary jet question is a good one in terms of whether comets can make discreet jets without liquid water.

I wonder also how a glowing effect might be produced in the event of an eclipse?

And, would a hypothetical 10 micron mean particle size (vs 1 micron) imply a greater ice abundance, and better changes of liquid water in the geysers?

[hendric]

How much of a difference in plume velocity would there be between warm ice sublimation and liquid water evaporation? Enceladus is much larger than the average comet, but the 212 m/s escape velocity isn't that high.

Would warm ice sublimation (WIS) tend to be a self-limiting effect, as dust etc covers the top of the ice, a la comets or glaciers? With liquid, dust etc could get blown off easier or end up sinking in the water.

Could warm ice continually sublimate for long periods? IE, how fast would the warm ice have to be moving towards the surface to keep sublimating at the same rate, given an estimated crack length and width? It would be easier for a reservoir to feed liquid vs solid.

[Guest_Richard Trigaux_*]

Even if it is "only" ice sublimating like on comets, it is still very interesting. It is well known that comets release a quantity of dust, even if it is mainly from sublimation of ice (dust particules would be released when the surrounding ice disappears).

That makes that, even in the hypothesis of sublimation, if there are biological molecules or particules, it is still possible to catch them with the equivalent of the stardust mission to Saturn passing through the plumes.


The original idea by James Oberg


my thread on the subject with my proposition of an effective trajectory to catch Enceladus dust and bring it back to Earth.

The above idea would be much cheaper than a complex orbiter/lander project like an Europa lander, so that it does not come at the expense of such a large mission. At at least it would give us hard evidence in only some years.

(Please if you want to switch the discussion on this idea, go to the appropriate thread. This one is about liquid water or not)

[The Messenger]

Spawling occurs in nozzle throats when there is differential heating within the carbon cloth / phenolic resin matrix. It is thought to happen directly beneath resin-rich pockets in-between the plies. Since carbon cloth conduits heat much better than the resin, the area under a resin rich region will be super-heated from the sides inward, rather than directly above. In addition, resin rich regions are less porous, therefore any volatiles generated are easily trapped and build up pressure.

I mention this seemingly unrelated topic because of the observational results: High speed video of a spawling nozzle shows round, ice cream scoop-like pockets of nozzle throat material blasting into the gas stream, and leaving crater-like pockets. The video resembles what was observed in the jetting on Wild 2.

I can see a strong analogy with what might be happening on and in Enceladus: Water underneath the ice is differentially heated from the bottom up, leading to explosive bumping of water vapor, cracking the ice and propelling the water vapor/particles to escape velocity. What I am suggesting is that the venting proceeds like superheating: Once a critical point is reached, there is a cascading effect that propagates outward and upward in a conic circle, directing the plume upward.

[tty]

Could warm ice continually sublimate for long periods? IE, how fast would the warm ice have to be moving towards the surface to keep sublimating at the same rate, given an estimated crack length and width? It would be easier for a reservoir to feed liquid vs solid.

Well lets see now... Warm active glaciers move up to a meter per hour, but that is mostly basal sliding, which requires liquid water. Cold-based glaciers which move by internal creep are much slower, say from a centimeter to a meter per day. For ice at 180 K which would be quite stiff I think a centimeter per day is high, but let's assume it.

The amount of H2O emitted has been quoted as 150-450 kg/s which comes to about 1.3 - 3.9 x 10^10 cm^3 per day, i e 1 cm/day over 1.3 - 3.9 x 10^10 cm^2 = 1 to 4 square kilometers, or a 200 to 800 km long and five-meter wide crack.

It sounds possible, but perhaps a bit on the high side.


tty

[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.