[X] My Assistant

[nprev]

Drifting back into small lake morphology here...When I lived in Alaska a few years ago, I noticed that the south-central region was peppered with lakes. What's interesting is that many of them (esp. in wooded areas) are "punch-outs" with little if any slope on the bottom near the shore--just a steep drop-off-- and no feeds from streams or rivers...basically, the swampy terrain just gave way & made a lake.

One reason for this might be that so much of the soil consists of volcanic ash (small, glassy, weakly cohesive particles). Perhaps Titan's small lakes indicate something important about local soil composition if this analogy is valid.

[ustrax]

I had bought Lifting Titan's Veil as soon as it came out, and loved it...

You will love to know that a sequel is on its way...
"I talk in more depth about my experiences throughout the project in my book Lifting Titan’s Veil (Cambridge, 2002) and its sequel Titan Unveiled which will be published at the end of this year by Princeton."
Back at spacEurope.

Thanks for that rlorenz!

EDITED: + space.com article on Titan craters.

[ngunn]

ustrax thanks for those links. The idea that proto-life could get going in a temporary lake formed by a meteorite impact is the most wonderful instance I know of serious scientists flexing their minds in the -WHAT IF?- zone that planetary exploration lays open. Lorenz and Lunine have excelled in this, and I just love all their papers.
(Oops, I said it..... 'Titan'... 'planetary'..)

nprev I think you're right to think about the consistency of the 'soil'. I think there is quite a good chance that much of the surface material on Titan is 'aerated' and of rather low density. This includes the ice, which I expect to be full of methanated nitrogen bubbles, and the smust, which I expect to be fluffy like soot. That leaves any liquid present, and liquid residues, as by far the densest things around. The liquid would head for the interior pretty smartly if it could, forming an alkanifer. Whilst on the surface it could easily transport buoyant pebbles of ice-pumice. Anything dense on the surface - either liquid or liquid residue- would quite likely compress the material below, perhaps creating significant topography. Some of the channels may have deepened themselves in this way, rather than by erosion as we understand it. And the lakes . . . ?

[The Messenger]

nprev I think you're right to think about the consistency of the 'soil'. I think there is quite a good chance that much of the surface material on Titan is 'aerated' and of rather low density. This includes the ice, which I expect to be full of methanated nitrogen bubbles, and the smust, which I expect to be fluffy like soot. That leaves any liquid present, and liquid residues, as by far the densest things around. The liquid would head for the interior pretty smartly if it could, forming an alkanifer. Whilst on the surface it could easily transport buoyant pebbles of ice-pumice. Anything dense on the surface - either liquid or liquid residue- would quite likely compress the material below, perhaps creating significant topography. Some of the channels may have deepened themselves in this way, rather than by erosion as we understand it. And the lakes . . . ?

Ah, we have already landed one object on Titan, and while the terrain it landed on may not broadly represent the surface as a whole, it does not resemble what you have described. Likewise, the dunes are unlikely to be fluffy - fluffy materials do not form dunes...do they? Anyone ever seen a dune of fluffy stuff? My physical models tell me fluffy stuff would be ground round in the dune forming process.

We already have one made-up word used to describe Titan surface materials: Tholins. What we need is materials we know exist chemically that have the spectral and physical properties we are observing. It took a long time to recognize the sulfuric acid in the Venus atmosphere, mostly because we had prior expectations that clouded interpretations and critical analysis. Rocks and sand are what we are seeing. What chemicals are consistent with the spectra and refractive index we have observed?

[ngunn]

fluffy materials do not form dunes...do they? Anyone ever seen a dune of fluffy stuff?

I remember some discussion of linear snow features in Antarctica similar to Titan's dunes (though flatter). Smust as discussed in the literature is thought to have lots of pore spaces in its structure. This could lead to extremely different properties when wet and dry - more extreme than the differences between wet and dry sand. But I agree it's all guesswork at this stage.

[stevesliva]

I remember some discussion of linear snow features in Antarctica similar to Titan's dunes (though flatter).

http://en.wikipedia.org/wiki/Sastruga

Linear parallel to the prevailing winds, rather than perpendicular.

[Juramike]

Should I change my T-shirt to "Ski Titan" rather than "Surf Titan"?


But seriously though, wouldn't a a liquid cutting through a fluffy material make a different type of drainage pattern?

Do streams and rivers in the polar area hava a different aspect or form from those in equatorial areas?

-Mike

[ngunn]

But seriously though, wouldn't a a liquid cutting through a fluffy material make a different type of drainage pattern?
-Mike

We've only seen the (possibly fluffy) dunes from a distance and a thoroughly flooded (and therefore consolidated) outwash plain close up. If the dunes are fluffy then yes there would probably be interesting detailed morpholology on ones that were partly eroded by rain and partly intact but we haven't yet got any imaging that would reveal it.

I'm not suggesting the light 'highlands' are fluffy, just aerated, probably mainly a closed cell foam structure. There is a paper somewhere that says the radar properties of parts of Xanadu are consistent with a very low density material so it's not just one of my wild ideas. Either type of low density material would have rather a low load-bearing strength. Tall relief does not exclude this if that tall relief is itself made of very light stuff and the gravity is weak.

Skiing in Titan's dunes could be risky - you might sink in over your head!

[Juramike]

I'm wondering if the polar regions are made up of a fluffy material, as nprev had suggested. With the surface lakes pressing down into the material. Or maybe the material is dissolved out from underneath after hitting a harder layer below and pooling up. So that the lakes may actually be sinkholes. (So: karst, calderas, or pressed in lakes?)

It did appear that many of the polar streams were very short. Has anyone found indications of a stream going underground and making a resurgence? (I haven't looked at the images in that much detail yet, but I'll bet someone has!)

-Mike

[Mongo]

Would fluffy, aerated material be stable in that form over geological time, though? 'Fluffy' material would also have very low mechanical strength. The closest analogous Earth material would be, I assume, snow. But snow, left for lengthy periods of time, self-compresses into ice sheets.

I might be wrong here, but I find 'fluffy' dunes (presumably millions of years old) to be a bit implausible.

Bill

[ngunn]

Well, in fact fluffy dunes are only a tentative suggestion on my part - Ive got nothing staked on it - but I can't help pointing out the obvious objection to one part of that objection. Water ice, even in Antarctica, isn't that far above it's melting point. A moderate amount of pressure between particles is enough to surface-melt it, fusing the crystals together. A little more puts the solid mass through a variety of phase changes that render it more or less plastic. There is no plausible analogy here with Titan dune material, whatever it may turn out to be. I only mentioned snow to counter another more fundamental objection - that fluffy material can't form dunes at all.

The point that smust particles would be too fragile to remain fluffy over many cycles of dune movement extending through geological time is more convincing. Add to that the effects of precipitation in the form of occasional downpours or slow drizzle of both, even at long intervals, and the picture looks more complicated. Nevertheless I don't have any difficulty imagining lightweight dunes that have considerably more pore space than Earth's sandy ones - the materials to hand are intrinsically less dense and gravity much weaker.

[The Messenger]

Well, in fact fluffy dunes are only a tentative suggestion on my part - Ive got nothing staked on it - but I can't help pointing out the obvious objection to one part of that objection. Water ice, even in Antarctica, isn't that far above it's melting point. A moderate amount of pressure between particles is enough to surface-melt it, fusing the crystals together. A little more puts the solid mass through a variety of phase changes that render it more or less plastic. There is no plausible analogy here with Titan dune material, whatever it may turn out to be. I only mentioned snow to counter another more fundamental objection - that fluffy material can't form dunes at all.

There is a plausible analog: Grainy materials stack asymmetrically, creating point loading at tremendous pressures. If they are windblown, micro stress, friction and pressure constantly wear (or fuse, if they have low melting points), grainy materials. Low melting points in this case, being less than ~400C - think of the action in a rock tumbler. This is why persistant dunes on earth are primarily refractive minerals, not soapstone or pumice. The dunes on Titan are very high, and subject to the same types of stress loading. The melting point of the dune making materials is likely much higher than the surface temperature of Titan.

[nprev]

Interesting. Does this then imply that the "soil" in the north polar regions is markedly different than the equatorial regions? If true, I've been assuming that it's a function of grain size rather than chemical composition.

Juramike: Yeah, I definitely favor sinkholes; the abrupt shorelines (600m cliffs?! ) combined with the lakes' apparently fractal geometry seem to imply dissolution from beneath rather than erosion from above.

One other wild thought: It may be that dune formation is restricted in some areas due to a combination of locally milder climate (?) and perhaps polymer formation in some circumstances...i.e., it gets stickier.

[Juramike]

The point that smust particles would be too fragile to remain fluffy over many cycles of dune movement extending through geological time is more convincing. [...] Nevertheless I don't have any difficulty imagining lightweight dunes that have considerably more pore space than Earth's sandy ones - the materials to hand are intrinsically less dense and gravity much weaker.

"Fluffy" was a bad adjective choice my part. A better word would be "extremely low density".

Snow usually fits the bill of extremely low density material on Earth, but a better example on Titan might be organic polymeric material.

Two examples that pop to mind of extremely low density, high melting, organic materials are polystyrene beads and styrofoam beads. These are pretty mechanically resistant and yes, you could make dunes of them. (And they would be really fun to play in). Polystyrene beads are pretty frictionless as well and can flow like water across the floor making a mess.

I could easily imagine a little ethane (ethylene?) ice grain grain rolling across a floor of acrylonitrile which then cross-links itself up and makes cute little hollow plastic beads when the ethane evaporates away. This is following the thought of an ethane (ethylene?) drizzle coming out of the stratosphere, maybe it makes it to the ground for a short while before evaporation, like a dust-coated hailstone.

[Another possibility is water-ice rolling around which then coats itself up with polymer, but this result in higher density beads unless you could get the water out of the encapsulation.]

Both of these examples have a better mechanical resistance than snow crystals. Styrofoam can support some weight until the cell structure breaks down. And polystyrene beads can be quite resistant to mechanical breakdown, depending on the polymer, cross-linking%, and physical structure. So while they have a "good" mechanical resistance, it is not infinite - could the weight of a lake be enough to press itself into the terrain?

Styrofoam dissolves really easily in organic solvents (dribble paint thinner or acetone on a stryofoam block and watch what happens!). But polystyrene beads are pretty much impervious to chemical solvents (they are used in synthesis as a solid support for many types of chemical transformations). This gives yet another possibility for the dissolution of material for the karst hypothesis - there could be multiple polymer types, some more dissolvable than others.

For the dunes, the key is to have something of low enough density so that it blows around easily in the wind. (Electrostatics is also something to consider, the buildup of charges could make the stuff really fly around easily). For the lakes, you need to have something easily dissolved, or easily pressed into.

I do not think that either stryofoam or polystryene are the actual materials on Titan's surface. But they might make better models for the properties of the funky organic polymers that are on the Titanian surface.

-Mike

[ngunn]

Juramike, that post exactly fits with my current thinking on all this! I very nearly posted yesterday a thought experiment on filling a depth-graded swimming pool lined with expanded polystyrene. In the deep end the lining would compress. Some bits might break off, float up and form a 'beach'.