[X] My Assistant

[Juramike]

Question: So how come Earth still has oceans after at least 3,5E9 years of rain and river erosion?

Answer: The silt is carried back and scraped off on the continental edges by plate tectonics.

Corollary: If the equatorial sand-seas are silted-up oceans, then Titan doesn't have plate tectonics.

Bingo.

Or the Sand Sea material may not subduct very well. Lower density materials may not get pulled down into the "trenches". Who knows???

Or maybe the silting-in rate >> subduction rate.

-Mike

[tty]

Or the Sand Sea material may not subduct very well. Lower density materials may not get pulled down into the "trenches". Who knows???

Actually that is what happens with a lot of the sediment on subducting plates, it accretes at the continental edges. Many geologists thinks that is how the continents have formed in the first place. Continental rocks are significantly lighter than oceanic, which is why continents are higher.

[nprev]

Nice logic, TTY, and that would certainly set some interesting constraints on interior models of Titan as well as the interaction of surface solvents with the crust. I wonder if magnetic fields & plate tectonics are strictly phenomena of silicate/water planets (omitting gas giants for the former, since a big ball o' metallic hydrogen's likely gonna have a field...)

Juramike: ! Exactly how many PhDs do you hold? Brilliant, just brilliant is all...

[volcanopele]

Nice work Juramike. I don't know if you answered Ralph's question, so I will just go ahead and re-ask it. Why are you arguing that the equatorial dune seas hold significant amounts of liquid when it appears that these particles are forming large sand dunes, suggesting the region is bone dry (okay, as per the Huygens GCMS data, maybe not bone dry, but only mildly damp)? Besides, the lakes we are finding up in the polar regions appear to be more than enough to supply the atmospheric methane.

[belleraphon1]

Agreed... Juramike does good stuff. Wish my brain could wrap around all the details.

volcanopele, I think Juramike concerns may be related to the juxtaposition of these vast dunes fields amidst "young" channel networks. Huygens definitely detected methane evaporation out of the ground not far from dune fields.

Wish I could remember the reference, but Dr. Lorenz himself mentioned going to a conference somewhere in the nordic countries and standing on dune fields right up against the northern sea. This was not "desert" territory. I often walk mature dunes on the coast of Lake Erie in Ohio where the sand and the lake interact in a VERY moist environment. But the sand still dances in the summer while it is locked in the embrace of every winter. VERY dynamic environment.....

So "bone dry" is not necessary......

Sure hope we can seee the monsson season, if it happens.

Craig

[belleraphon1]

All...

the dynamic environment on the headlands of Lake Erie... sand, wind and water at work. An analogue for the seasons on Titan?

I took these images over the course of several weeks. Pressure ice changed to open Lake in just a week.

http://www.flickr.com/photos/7323592@N07/s...001960002/show/

Point is that LARGE features can come and go in short time frames. The dry hydrocarbon/ice sands ot Titan could even get submerged briefly in the spring, only to reappear to a mature phase in the course of a Titan season. Nature is not adverse to moving tons of material in geologically short time spans.

So, frankly, would not surprise me if the Titan dunes hold moisture at there base.

Craig

[volcanopele]

Yes, but the places where these types of dunes form on Earth, like in Namibia and Libya, are quite arid. Now that doesn't mean that no rain falls, obviously, but I am not quite understanding how longitudinal dunes and moist sand can co-exist. That makes no sense to me.

[Matt]

Doesn't Titan dry up pretty quickly at those latitudes anyway, due to high evaporation rates?

[hendric]

Why are you arguing that the equatorial dune seas hold significant amounts of liquid when it appears that these particles are forming large sand dunes, suggesting the region is bone dry (okay, as per the Huygens GCMS data, maybe not bone dry, but only mildly damp)?

Well, the solid is not silica, and the liquid is not water. Hydrocarbons have a lower surface tension than water, and adhere to other things much less than water, being nonpolar. A sand-filled basin with water in the bottom would rapidly get the sand wet through capillary action.

I bet that the lower surface tension of hydrocarbons, and their lower adhesion, might allow dunes on top of a liquid ethane reservoir, or much "wetter" dunes than we are normally used to on Earth. There would be less capillary action, and the hydrocarbon-wet particles wouldn't stick nearly as much as water-wet sand.

So Ralph, here's a question for you: How well would, say, liquid ethane with some dissolved methane adhere to "smust"? Or, for that matter, water-ice dust?

Reference on surface tension:
http://chemed.chem.purdue.edu/genchem/topi...rty.php#tension

Here's the first page of what (grr) looks to have been a very interesting article:
http://pubs.acs.org/cgi-bin/abstract.cgi/j...je60066a001.pdf

Another, should be free I think
http://www.sciencemag.org/cgi/reprint/261/5124/1018.pdf
but doesn't go down the methane/ethane, but does show the change in surface tension for large chain hydrocarbons as they cool

Reference on surface tension difference between water and Gasoline:
http://www.newton.dep.anl.gov/askasci/mats05/mats05037.htm
Surface tension for Ethane is likely to be lower than Gasoline; since it is a smaller molecule there is less room for Van der Waals forces to work.

[ngunn]

How well would, say, liquid ethane with some dissolved methane adhere to "smust"? Or, for that matter, water-ice dust?

Hope Ralph does share his thoughts on that, but in the meantime I thought I'd jump in.

My impression is that the methane adherence would be very different in the two cases, and would involve two completely different mechanisms. For the water ice the individual grains would be effectively impermeable and a surface tension based model may be appropriate. In the case of smust, however, we may have to consider that significant quantities of methane could actually lodge within the loose molecular structure of individual particles. If that does happen then only after as much methane as possible had been adsorbed in that way, and then still more was added, would the particles become externally 'wet'. They could increase in density considerably before getting to the wet stage. I think we urgently need to make some of this smust and do some experiments.

A thought experiment (cheaper!): Imagine a large pile of dry sponges, blowing around in the wind. Now add some rain. The sponges can still blow around a bit to start with. They only stop moving when they're completely waterlogged.

[Juramike]

I imagine the Equatorial Sand Seas as a wet slurry of hydrocarbon solvent at lower levels, with drier material blowing around on top of a moist layer.

[For the chemists among us, picture slurry-packing a chromatographic column with hydrocarbon and silica, and blowing the top few inches dry (oopsie, it happens). This is what the Sand Sea looks like in cross section].

I’m imagining that a cross section of the Equatorial Sand Sea would have a wet slurry at the bottom, then a moister zone of “sand” then a much drier zone on top, then finally dry loose stuff blowing around on the surface forming the dunes.

This is similar to a beach at the seashore. If you get to the dunes and dig down, you will eventually hit wet sand, then finally water will percolate in and fill the lowest part of the hole.

An Earth analog might be the dunes in Death Valley, which I believe are over a salt pan which then has water underneath. (Is there an “organic” pan that partially seals off the moister layer? Who knows?)

That far down, you may have an insulating effect of the all those particles against rapid evaporation of hydrocarbon solvents. [On California highway 4? highway workers uncovered blue ice that had been buried during a rockfall in the last Ice Age. They covered it back up and resited the highway. There is a highway marker near the summit on the W side – nice picnic area, too.]

In fact, the sand particles may act as theoretical distillation plates and may allow preferred fractionation of the more volatile solvents (i.e. methane) over the higher boiling solvents. I’d be really interested to see if this was observed from the Huygens MS release. (Would you even see preferred release of lighter isotopes of methane compared to the normal abundance mix?)

While the lakes might rapidly evaporate methane/ethane into the atmosphere at a set ratio, due to fractionation, the Equatorial Sand Seas might only preferentially slowly release methane.

The sand itself may be water-ice that has been coated in an organic polymer. I like the idea of polystyrene beads in heptane as a model.

The sand may be pretty close to the isopectic point (I don’t think this is the right term or spelling) when suspended in hydrocarbon solvent. This is the point where the material may sit perfectly distributed in hydrocarbon solvent, neither floating or sinking. This could make for a really fluid (and self-leveling) slurry. [From a past life the isopectic point for polystyrene is 2:1 CH2Cl2: THF]. When all packed down, the hydrocarbon solvent fills the spaces between the sand grains.

The question I’m trying to puzzling out is, after the big monsoon rain and runoff into the Equatorial Sand Sea, where does the methane go? It may just all evaporate off, OR it may percolate through the sand until it makes a significant reservoir deep under the Sand Sea.

How do we check?

-Mike

[tty]

All this talk of methane in ice dunes makes me wonder if methane hydrates might occur on Titan. It is probably too cold for them to form on the surface, but they might form at depth and be carried to the surface by cryovolcanism or tectonics as a sort of Titanian metamorphic rocks. However I'm uncertain whether they would be stable on the surface, but I think they probablyy would.
The significant thing is that methane hydrate is a hard "dry", white substance, but can still contain a remarkably large amount of methane (about 15 % by weight).

If there is methane hydrates there a retro-rocket landing might be dramatic since one litre of methane hydrate can liberate about 200 litres of methane gas on heating,

[JRehling]

On the wet sand / dry sand point, I'll add my conjecture on what happens on Mars, something that may have an analogue on Titan:

That is, I think that Mars's dusty surface is obviously usually quite dry, but capable of exceeding the melting point of salty water for certain minutes out of the martian year. So what I think happens there is that dry dust (with or without water ice in it) settles atop a drift, and remains there, dry and free to the wind, until the next summer afternoon that melts the water, allowing it to combine with salt, very briefly wetting the dune surfaces. Before 2pm local time, the water is either ice or vapor, leaving the salt as a mortar cementing the surface of the dune. A crusty dune could thus grow over time, perhaps trapping a fair amount of water ice under a shallow burial as it goes.

I don't have the chops to suggest what could play those same roles on Titan (eg, what is the "salt" on Titan?), but that sort of a model could allow for a methane reservoir in the sand seas.

[The Messenger]

One observation worth mentioning:

There is nothing akin to 'alkali blum' in the limited Huygens landing scene. Alkali blum occurs in heavily salted lake beds, such as Lake Powell and Lake Mead. As the water is drawn by capillary action through cracks in the mud, the mineral salts are deposited in frost-like structure that follows the cracks. Experienced 4-wheelers know to avoid driving on blum because it means the water table is less than a foot below the surface.

This implies that the 'water table' where Huygens landed is not very near the surface - which is exactly opposite the measured increase in atmospheric methane after the landing. So one interpretation would be that there is not a large quantity of heavier hydrocarbons in the 'soil' where the Huygens probe landed.

Admittedly this is iffy, but the presence of 'hydrocarbon blum' would have clearly indicated the surface of Titan where Huygens landed is rich in methane-soluble hydrocarbons.

[rlorenz]

Lots of good questions and thoughts....

contact angle (i.e. wetting - adhesion vs cohesion) of methane on ice etc. isnt known - Brian Jackson
and Catherine Neish at LPL were hoping to measure it....

As for occasional precipitation on top of typical evaporation over equatorial regions of Titan
giving widespread dunes, with the occasional river channel, some people are actively working
on that topic...
http://www.cosis.net/abstracts/EGU2007/115...007-J-11529.pdf

I guess precipitation at low latitudes is most likely when there is upwelling at low latitudes, viz
around equinox - stay tuned for 2009 !

The dampness of the Huygens site could be in part due to ethane (which would not evaporate
quickly, and would retard methane evaporation too, much as sugar in syrup slows its
evaporation). So the fact that it was damp doesnt mean necessarily that it had rained in
the last year or anything, although in geological terms I guess it must have been recent.

On the other hand, the dunes 20km away must have been formed under dry conditions, but
that doesnt make them dry now...