Recent article in Science by Zebker et al.:

Zebker et al. Science in press, "Size and Shape of Saturn's Moon Titan". doi: 10.1126/science.1168905
(published online April 2, 2009)

Link to abstract (pay-for article): http://www.sciencemag.org/cgi/content/abstract/1168905

Article on spaceref discusses this paper: http://www.spaceref.com/news/viewpr.html?pid=27912

Figure 3 from the Science article is a global elevation map relative to barycenter.

Key points of article:

• Poles are squished - might explain why lakes are up there
• "Mountains" aren't necessarily elevated - they might've sunk down into the crust to form "a basin of their own creation".
• (quote from spaceref. article)
• Large scale features do not appear correlated with elevation
• Xanadu and Tseghi are BASINS! (ca. -600 m to barycenter)
• Adiri is higher than Xanadu (by almost 1 km)
• Dilmun is also pretty elevated (+ 400 m relative to barycenter)
• highest elevated terrain on Titan seems to be region around "Adiri junior" in the S Senkyo "basin" at ca. + 600 m above barycenter
• Shangri-La "basin" is elevated +400-800 m ABOVE Xanadu (using barycenter elevations in Fig. 3)

"Xanadu seems to be systematically lower than other parts of the equatorial belt, and not uplifted like most mountainous areas on Earth." (quote from Fig. 3 caption in article)

-Mike

Here's a cartooon showing how Xanadu could have formed:
Click to view attachment

1) A diapiric rise of warm low density (mostly due to warmth) water ice
2) reaches surface; erosion begins
3) cools and contracts; ice now denser (heavier) than surrounding crust; crustal deformation begins
3) regional depression begins as old diaipir sinks due to now-cooled higher density material

Quote from the spaceref article:

The Science article is in reference to the Barycentric center of Titan.
This could be different from the equipotential surface.

If I understand this correctly....

In a perfect world, with a uniform density gradient, the elevation and equipotential surfaces pretty much match:
Click to view attachment

A large mass concentration (high density crustal materials) can cause a localized gravity well, and cause the equipotential surface to bulge out. (More mass, more gravity, more "pull"). So you can get an elevated ocean that covers a elevated (based on barycenter distance) rise.
Click to view attachment

A mass deficit (lower density crustal materials) can cause a localized gravity deficit and cause the equipotential surface to dip in. (less mass, less gravity, less "pull"). This could cause a basin that one would think should be flooded, to be dry. The water table responds to the equipotential surface, not the elevation.
Click to view attachment

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

So now a Big Question:
Why are the poles wet, the equatorial Sand Seas dry, and Xanadu dry and sand-free?

The elevation difference can explain why the poles are wet:
Click to view attachment



But keeping lower lying downwind Xanadu free from Shangri-La's dune sands is harder to explain.

If the equipotential is depressed at Xanadu (due to a lower overall density of crustal materials), this could explain why the mobile dune sands remain in Shangri-La (which is higher barycentric elevation) and don't fill in Xanadu.
Click to view attachment

Otherwise some funky physical barriers would need to be invoked.

[The sand seas could be isolated from the north polar lakes due to many reasons: an isolating temperate airfall deposit, lack of sand reservoirs, lack of winds carrying material into the poles, and also sands getting trapped by the lakes]


This Science paper is awesome. It raises a lot of really tough questions.....

Thanks for starting off a discussion on this. One fairly safe conclusion would seem to be that for Titan a triaxial ellipsoid is a very poor fit to an equipotential. The latter must be very bumpy compared with what we're used to. But as you say there are problems everywhere with this (which likely means that it will be a very productive observation in the long run). Are Xanadu's mountains dense, or light and porous? We have observational evidence for the latter, yet here it is suggested that they are denser than surrounding terrain and thus capable of making their own basin to sit in.

A general observation (not necessarily helpful): topsy-turvy topography is less difficult to explain away if ALL the surface materials (except the liquids) are of very low density, i.e. fluffy or aerated.

Forgive my ignorance ngunn but exactly what was that observational evidence that indicates light and porous material in Xanadu?

It came from RADAR reflectance observations of the dielectric properties if I remember correctly. It was a while ago now, but it led to extensive discussion here and elsewhere about possible underground cave systems (likened to Xanadu in the poem). Emily's Planetary Society blog had at least one entry on it at the time, which may or may not have been called something like 'Caverns measureless to man'. Sorry not to be more specific - I wish I was better at keeping references.

Nigel's right, the most recent evidence comes from a Janssen article on dielectric constant (discussed here: Titan's Mid Latitudes thread, post 6). The dielectric constant in some parts of Xanadu was waaay too low to be of any normal materials, even very low dielectric organics.

Dielectric constant is modified by fluffiness (pores). Volume scattering can make the material appear to have a much lower dielectric constant. It was postulated that volume scattering was responsible for the apparent low dielectric constant.

The channel drainage pattern in W Xanadu in the T13 SAR RADAR Swath is very consistent with the Zebker elevation data.

(For E Xanadu, check out Fig 2b the freely available LPSC abstract: http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1037.pdf. For W Xanadu, check out Fig. 7 a and b in Lorenz et al. Planetary and Space Science 56 (2008) 1132-1144. "Fluvial Channels on Titan: Initial Cassini RADAR observations." doi: 10.1016/j.psss.2008.02.009

The dendritic drainage pattern across the T13 RADAR Swath pattern is to the S, towards the lowest regional barycentric elevation determined to be in SW Xanadu.

Thus, the elevation data seems to agree with the local geoid in W Xanadu.

Back to the tough question, what prevents the Shangri-La dune sands from dropping down into the SW Xanadu low elevation basin???

****

The Zebker paper also makes the airfall origin of the sand sea dunes difficult to explain. If atmospheric deposition and sintering is supposedly occuring all over, it should occur over Xanadu. The regional gradient should make it collect in the SW Xanadu basin. According to this idea, there should be a dune sea filling in the SW Xanadu depression. But there isn't. (At least no dunes are seen by RADAR, and ISS shows a bright coating rather than a dark dune sea.) There is, however, a smooth (RADAR) dark area seen with channels crossing into it, again going downgradient.

As another post noted, Xanadu has very peculiar dielectric properties - its radar echo is quite
depolarized, suggestive of porosity (see the Janssen paper)

There is also the result published by Buratti et al of the VIMS team suggesting that the near-IR
phase function of Xanadu suggests rougher material (at the microscale, IIRC)

For your first remark, we'll need to wait to see what the published gravity field is. If the 3rd order terms are, in
fact low (at one time the retrievals suggested they might not be), then in fact a triaxial ellipsoid IS a good fit
to the EQUIPOTENTIAL.

Maybe (since you are referring to the shape measured from radar) you mean that a triaxial ellipsoid isnt a good
fit to the figure ?

As far as liquids draining downhill goes, there does seem to be a consistency between lakes at high latitudes, and
the hint of a tendency (with incomplete sampling) in my fluvial paper for rivers to flow poleward, and the
difference between the topographic shape and reasonable geopotentials.

The low-latitude longitudinal contrasts (viz, low Xanadu vs the dunes) is harder to explain, as is the orientation
of the dunes themselves. I sometimes wonder about albedo-driven wind (i.e. sea-breeze type circulations -
with effectively katabatic flow away from high albedo regions).

The Zebker et al (you can call me al !) paper about the shape - like the Lorenz et al spin paper last year -
is I think just going to be the start of a long and complicated story.

On the ellipsoid vs. equipotential I'm happy to be corrected. (I can't pretend I meant something different.) Like you say, we're just at the beginning.

I'm very interested in your idea of albedo-driven winds. For some time I've been trying to shake off habitual assumptions about the importance of gravity in distributing surface materials on Titan. At the Huygens landing site for example there seems to be evidence of fairly recent fluid flow from at least two directions. To me that suggests something like a storm-driven flood, followed up by some gravitational trickling. Of course lighter materials would pay even less heed to gravitational constraint.

Even on high-g Earth with heavy silicate sand there is striking evidence that sand seas don't follow equipotentials. The Taklamakhan in western China is a good example. It is much higher at it's western end. I was lucky enough to be there last year and was amazed to see sand 'mountains' piled high on the Pamir plateau, deposited near the head of the steep valley that leads up there from the desert.

Looking at channels in the RADAR Swaths, I haven't seen any obvious places where the apparent flow direction is different from the proposed barycentric elevation differential (Fig 3 in Zebker et al.).
[BTW, the actual swath traces and their altimetry data can be seen in the figures in the text. This was really helpful.]

As far as I can tell, the barycentric elevation data seem to fit the equipotential surface (geoid).

Why Xanadu is not filled in baffles me...

-Mike

Perhaps dune particles have a finite lifetime on Titan, and some areas like Xanadu are simply made of the wrong materials for producing them. I agree with your point in an earlier post that they can't be falling from the sky. It has always been awkward to explain how some areas are swept clean if that were the case.

Here is a detail of the dune pattern near the E Shangri-La/W Xanadu boundary using Amazing Dune-O-Vision (gamma-modified, contrast enhanced, negative image) for a portion of the T13 Swath. I highlighted the dune pattern orientation in some areas using yellow lines.
Click to view attachment

There seems to be a sudden turn southward near Xanadu as the dunes smack into Xanadu. For a sea-breeze circulation, I would have thought that the off-Xanadu circulation would make the dune pattern "confused" or dissappear in a marginal zone around Xanadu. Instead, while the dune orientation changes, it remains very clear right up until hitting Xanadu. (Also indicating that the Xanadu margin is not much of a topographical barrier)

And the dune sand migration seems to bend to the S when it starts to go across Xanadu.

Is this vector change sufficient to divert sand flow around the depression instead of into it? Where does the sand eventually end up?

-Mike

Correct me if I'm wrong (again), but aren't these dunes consistent with winds blowing alternately one way then the reverse? In that case you could in principle have a single longitudinal dune where the wind regime was 60% W, 40% E at one end and 60% E, 40% W at the other. That would fit with the albedo idea, I think, and it would provide geographical confinement for the sand seas.

If I understand this correctly, the dunes line up along the vector addition product of the alternate wind vectors.

So 50% winds blowing from the NW + 50% winds blowing from the SW will give an EW dune (with the mobile dune particles kinda marching to the E).

The change in wind direction could be daily, or seasonal. [http://www.tec.army.mil/research/products/...et/lslinear.htm]
(Another clue to average wind direction vector is dune bifurcation, in case of Y-junctions, the "stem" of the Y is downwind.)

The key is that to make a stable linear dune, the difference between the two alternate vectors has to be pretty large. Otherwise, the linear dune will decay into a barchan-type dune.
Check out: http://www.comphys.ethz.ch/hans/p/457.pdf

Extending this to the linear dune direction change to the SSE vector in E Shangri-La/W Xanadu, BOTH alternate wind vectors need to be shifted. So the evidenced wind regime near the margin should be (assuming a 90 degree alternate vector (theta-w) difference) WNW alternating with NNE winds.

Two different wind regimes, but with one component larger than the other, can form linear dunes with the vector addition product.

But sand movement will be the time-average of the relative strength of the two alternate regimes.

The sand movement vector can be different from the dune alignment vector.

Check out this section of a book preview here

(Note how the dune pattern changes around Australia due to interaction of winds with the seasonal anticyclonic high. This might be an example of what Ralph was mentioning above, i.e. Xanadu having a permanent pressure system.)

Mike I'm having trouble reconciling what you describe with the recently published wind vector map where the arrows seem mostly more or less aligned E-W with the dune crests. It is agreed that there is 180 degree ambiguity in the inferred wind direction at any one locality. It has also been suggested that the wind directions could reverse with the tidal cycle. If that's so then presumably material is transported both ways alternately along any particular section of dune. Even a slight preponderence of one direction over the other would be enough to produce the teardrop shapes we see around 'islands', but following the direction inferred at one point could lead you to a place where the preponderance, and therefore the net transport, is the opposite way. There need be no discontinuity in the dune pattern between the two locations, which might be at opposite sides of a sand sea.

I'm sure I'm oversimplifying . .

The maps show the linear dune orientation. The linear dune orientation is based on the averaged wind vector. The alternating winds come from two different directions. These could be daily (tidal) or seasonal.

To make a linear dune, the alternating wind vectors need to be about 90 degrees apart or more.

Looking at a linear dune, the averaged wind vector could be in either direction of the line. Looking at the forking pattern can determine which is the correct average direction.

The GCM for Titan predicted an overall westerly course for the average winds on Titan (so average wind vector from the E). The observations show an average westerly vector for the surface winds. This is in contrast to the predicted GCM model (by Tokano et al.).

Check out: http://www.astronomy.com/asy/default.aspx?c=a&id=7972


And you are correct, a typical sand particle will have a zig-zag path due to the alternating winds, but will run downcrest. If I understand it correctly, a strong wind from one of the alternating components could cause it the particle to "shift" it's path to the next dune set during it's zig-zag path (it zigs more than it zags). Thus the net sand flow could be off-angle to the dune crests.

Hope I'm not trying your patience, but this is the bit I don't quite get. What you say implies winds that blow, in general, at angles of 45 degrees or more to the equator. But this is not what Huygens observed, which was roughly E-W movement (parallel to the pair of nearby dunes), with reversals. It also seems at odds with the way we see clouds behaving, forming more or less latitudinal streaks.

If I remember correctly, the GCM wind field changes as Titan goes around Saturn (there is a tidal component). The average vectors predominate from one direction, then alternate with another that is roughly orthogonal.

I'm not sure how this all fits with the Huygens descent data (which also changed direction with altitude):

Did the probe descend during a "typical" predominate wind day?
Are the winds that Huygens encountered typical of the surface winds?
What kind of winds do the banded clouds indicate? (And how do they relate to surface winds?)

I'm just reading the transcript of the Titan spin CHARM now and it includes an explanation of the dune formation consistent with yours, that is: two wind directions separated by 120-odd degrees. But it also mentions the Tokano et al climate model which has winds switching between W-E and E-W with the seasons (not with the tides - I got that wrong before). So which is it? Do the dune particles move inexorably eastward, albeit by north-south zig-zags, or do they move alternately eastward and westward? The implications of the two are very different.

From looking at RADAR images, it looks like the W sides of most of the sand seas are lacking in dunes, while the E sides have dunes.

I'd take this as observational evidence that net flow is eastward.

Bingo!

Tokano, T. Icarus 194 (2008) 243-262. "Dune forming winds on the surface of Titan and the influence of topography." doi:10.1016/j.icarus.2007.10.007

Pay-for article, abstract available here.



Here is an additional explanation of the GCM with pretty diagrams: http://www.esa.int/SPECIALS/Cassini-Huygens/SEMF9F9RR1F_0.html ://http://www.esa.int/SPECIALS/Cassini...9F9RR1F_0.html

So, four possibilities:
1/ There is a sink of dune particles on the west facing 'coasts'.
2/ On reaching a 'continent' the dune particles skip right over without sticking and start again on the other side.
3/ There is a net eastward flow over much of the sand seas but the net flow reduces to zero at their eastern margins.
4/ Come back in half a million years to see the Xanadu sand sea and the Shangri-la mountains.

Nice that the Tokano paper predicted the Xanadu basin - thanks for posting that.

A further thought on albedo-controlled winds. Since the dunes themselves are dark, winds that systematically blow from light to dark would constitute a positive feedback mechanism tending to accentuate the separation between light and dark areas. This would be akin to the situation on Iapetus, albeit involving a totally different physical process. All you would need to start it off would be a slight initial tonal dichotomy or a slightly uneven supply of dune forming material.

Let's take this even further into the realm of guesswork. If a computer simulation of this positive feedback system were run it would not surprise me if the light and dark areas evolved towards a state where their EW extent became roughly commensurate with their NS extent. Since the feedback system is confined approximately between 30N and 30S that would lead to 6 alternating light and dark areas ranged around the equator, i.e. 3 major sand seas separated by 3 'continents'. I'd love to see a frequency spectrum of the actual longitude distribution of Titan's sand seas.

It also means that Xanadu, and processes that operate there, could be a potential source of dune sands.

Since any sands generated in this region would get "swept" away.

Possibly, but it also restores - or rather preserves - the credibility of the current 'official' hypothesis that the dune particles form in the atmosphere.

Yup. And if you invoke the sands being able to bounce easily across bright terrain (your item #2), it supports the atmospheric deposition hypothesis. Any atmospheric fallout that hits bright terrain could be blown off. But I'm not sure how this would explain the relatively dune-free W margins of the dune seas.


A Xanadu depression makes some of my earlier speculations go bye-bye: it is hard to invoke an earlier sand-sea filling nitrogen ocean without filling in Xanadu as well.

That wasn't meant to be a serious suggestion. I take it as read that corrugated terrain would be harder for sand to cross than a flat plain. Neither was option 1. (What are we to imagine - critters lining the beaches and eating the stuff?) Option 3 was the serious one, though on reflection you could have a blend of 3 and 4, with a bright albedo obstacle like Xanadu temporarily arresting an inexorable eastward migration of the sands, only to be overwhelmed in a comparatively swift event when eventually the piling up of sand to the west of it becomes just too great and everything moves to it's next quasi-stable position.

I have also been thinking about disposal of the atmospheric fallout that lands on Xanadu. In corrugated terrain perhaps not all of it blows clean away. Some could be trapped in pockets or fissures too small for current imaging to resolve and too small also to affect the overall albedo of the region.

This thread has gone very quiet! In the meantime I have been trying to put some thoughts into words, and I hope I may be allowed the indulgence of a longer than usual post, even if some of it repeats what's already been said. Here goes:

- - -

Ever since we found out that the low latitude dark markings on Titan consist of drifting sands the map of Titan has been giving me a dull headache. It takes sand movement to form and maintain dunes and that means we cannot be looking at a static system. A steady state dynamic system is also ruled out by the presence of a single complete gap in the pattern. Xanadu acts in effect like a giant capacitor blocking the DC flow of sand. The next possibility is AC sand flow - an oscillating system. However, the asymmetries that we see in the sand seas must have taken ages to form, implying a very long time period for the oscillations. There is no obvious driver for such a process. It seems hard to avoid the conclusion that we are looking at a pattern that is somehow being maintained in a non-equilibrium state, hence the headache.

Now for the first time that I am aware of we have, in albedo-driven winds (and wind-driven albedos), a process proposed which would involve positive feedback. Positive feedback is exactly what's required to maintain a system in a non-equilibrium state. Positive feedback can give even a feeble or inefficient process the leverage to transform worlds, creating emergent order that makes no sense in other terms. That's why I find this idea so exciting. It raises a host of new questions and the pleasant anticipation that some of them may soon be answered.

There is a nice symmetry of scales to this explanation too. Sand dunes and the regular patterns they form are themselves emergent non-equilibrium structures maintained by a different positive feedback system involving wind and another partner, in that case local-scale topography.

We are still a long way from knowing that we have the final answer, but that does not worry me at all. I think that at least we can see clearly now what kind of answer we are seeking: a positive feedback mechanism of some sort which can render Titan's sand seas self-confining and its 'continents' self-cleaning. We have in view at last a domain of possible explanations that does not defy reason. The headache is cured.

What about the asymmetry of the sand seas and its strong implication of DC sand flow? No problem. We can have eastward drift and positive feedback operating together. This would likely produce a sand flow regime that is continuous at some longitudes and episodic at others where some underlying property of the substrate - very plausibly albedo - dams the flow. The large scale pattern of the sand seas may indeed migrate eastward over long time scales, with Xanadu simply happening to be the most effective dam operating in the particular configuration that prevails in our epoch.

- - -

I hope Ralph will pay us another visit here and share more of his insights, but even if he doesn't I don't think we've heard the last of this by a long way.

A poem:

- - -
The Walrus and the Carpenter
Were walking close at hand;
They wept like anything to see
Such quantities of sand:
"If this were only cleared away,"
They said, "it would be grand!"

"If seven maids with seven mops
Swept it for half a year.
Do you suppose," the Walrus said,
"That they could get it clear?"
"I doubt it," said the Carpenter,
And shed a bitter tear.

- - -

And a piece of music:

- - -
http://images.google.com/imgres?imgurl=htt...sa%3DN%26um%3D1

The dune particles do not form in the atmosphere. The atmospheric haze is about 1 micron across, while the sand particles are like 300 microns across. The stuff that falls out of the atmosphere is 300 million times too small (by mass) to form sand-sized grains.

The haze must be being reprocessed somehow to get the sand grains. We still don't know how that process works, yet.

- Jason

Check that -- lost an order of magnitude there. Should be 30 million times.

- Jason

What's an order of magnitude among friends?

Phil

If Xanadu is "self-clearing" then the dune particles must be going around the southern edge of Xanadu. (The dune pattern in W Xanadu indicates a southern flow)

So you'd expect to see an ISS-dark and RADAR-dark dune sea somewhere along the southern margin...but you don't.

One possibility is if there is a temperate/polar bright deposit that forms faster than the dune seas move. So instead of dark dune seas forming, there are bright smooth deposits that form in the temperate regions in south-central Xanadu.
In effect the dune seas get "tamped down" and covered up by an ISS-bright atmospheric deposit. So limited "equatorial dunes" in the temperate regions, the deposit seals up the sand supply.

So in the big picture of timing of features on Titan, that would imply:
Temperate/polar atmospheric deposits>dune seas>other process (save cryovolcanic outlflows)

This seems to fit not only Xanadu but the rest of the temperate regions in general.

I'll bet that the normalized brightness of Mezzoramia is still not as dark as that of Shangri-La.

(Another piece of evidence, look at craterforms: not much in polar/temperate regions, some in dune seas, but pretty much everything exposed in Xanadu)

Not necessarily. There may be no flow across or round Xanadu at this particular epoch but rather a very gradual encroachment at its western margin. When eventually enough of it is covered a lot of sand may cross in a relatively short period.

I do think though that there must be some net leakage of sand to temperate latitudes where maybe it becomes damp and less mobile. A lot of dark streaks seem to be diverging from the tropics.

RE "The dune particles do not form in the atmosphere"

EDITED LAST PARA:
I expressed myself sloppily there, and thanks Jason for pointing that out. On the respective particle sizes of haze and sand, I'm not forgetting that there must be a processing step or five to get from the former to the latter, if that is indeed what happens. The point as it relates to this discussion, though, is whether all parts of Titan receive equal doses of the raw material for making sand or whether it has a more localised origin. I was merely pointing out that the albedo winds idea is consistent with virtually any origin for the sand particles, including an atmospheric source for the material, whereas some other conceivable explanations for their uneven distribution might rely on localised sources and sinks.

I've just peformed an experiment which crudely simulates what I think happens to the sandflow at Xanadu. It only took a few minutes to set up. No pics unfortunately but here's what I did.

I set up an airtrack at a slight gradient. Above the airtrack I positioned a hairdryer angled slightly downwards but facing the airtrack's uphill direction. Before starting the airflow to the track I placed three pucks well spaced out along it and turned on the hairdryer.

On starting the airflow to the track the pucks began moving downhill, remaining evenly spaced. As the first one approached the hairdryer it stopped and reversed back up until the second puck ran into it. Now joined together (blu-tack) the first two pucks came to a halt in front of the hairdryer. When the third puck caught up with the first two they collectively overcame the headwind from the hairdryer and all moved through together.

Very nice work there ngunn - I was struggling to follow the process until you described your own experiment. Very nice demonstration - even if there were no pics.

Glad you liked it. Did you listen to the Takemitsu?

An interesting idea. If this were true, then what we might see would be Barchan dunes racing around Xanadu to the North and South, carrying these spillover sands faster than the longitudinal dunes. I haven't seen this in the VIMS data, but we might reasonably not be expected to given small Barchans and our outrageously crappy spatial resolution near 90W. Does RADAR see anything?

My personal best guess right now (subject to change when more data arrive!) is that there is no net sandflow through Xanadu, and that in general W-to-E flux of sand is rather low (but nonzero!).

- VIMS Jason

Well, if it's raining 1um grains, and the dunes are 300um, perhaps the 1um stuff blows around on short timescales and corns up into bigger clumps through freeze-thaw cycles. Or something. Snowdrifts definitely don't have the same flake size after just a few days.

The fast-moving mid-latitude barchans is an interesting idea too, but I have a query about your last point. As ever we are bedevilled by our ignorance of timescales for Titan's active processes. Are you saying that unidirectional flow could produce the observed sand sea features yet still be so low that even after hundreds of millions of years (maybe!) the sand would not all have piled up on western Xanadu?

For the record what my experiment was attempting to simulate was the periodic overwhelming of Xanadu by a whole sand sea in one big flush, after which (being bright) it would dust itself off quite quickly and remain sand free for perhaps another million years, damming the flow completely as we seem to see it doing now.

After sleeping on it I'll now try to be more explicit about why I have difficulty with Jason's quasi-static scenario above.

I have no knowledge of how far sand particles must travel to build a dune sea or the streamlined teardrop shapes around islands, but we can see how far it must have travelled to clear Xanadu. To do that, any sand that formed in the middle of it must have drifted through at least one Xanadu radius. If the global eastward drift were much less than that we would see positive accumulations of sand on [i]both[/ sides of Xanadu, not just the western side. Instead we see a depletion of sand on the eastern side. I think that implies that the global eastward drift at least equals, and may exceed, the flow rate associated with the albedo cleaning process. In other words each sand particle must have drifted, over the history of the sand seas, a distance at least comparable with the dimensions of the sand seas themselves, and possibly much further. That in turn suggests that if Xanadu did not occasionally flush there should by now be a very much bigger pile of sand at its western margin.

No -- they tend to agglomerate into ice sheets. At least they do on my driveway if I don't shovel it in time!

Bob Brown at U of Arizona (VIMS Team Leader) suggested that maybe the grains are sintering together like hot metals, and I talk about that briefly in my sand dunes paper. Bottom line is that it's possible, but it would require the right compositional properties and resistance to both erosion and to sintering past the saltation size.

And they can't turn into a solid block under the weight of 100 meters of the stuff

- VIMS Jason

Both the ice and metal analogies are crystalline. I imagine that this goop wouldn't be. Always have to throw in a "or sumthin'" qualifier because I have no idea what Titan goop really does. Perhaps LN-cold wax is a better analogy. Give it a little heat over a long time period and it'll start sticking together, right?

"No disrespect," to quote Jon Stewart, but this is why I don't like the application of goo, gloop, etc to Titan. Wax IS crystalline; you can check out some fascinating papers on the crystallography of candle wax by Dorset in Acta Cryst. (e.g., http://dx.doi.org/10.1107/S0108768195005465).
I have every expectation that the ices, hydrates, clathrates, AND the organic solids will be crystalline. I cover this in slightly more detail in my paper in press - http://dx.doi.org/10.1016/j.asr.2008.11.024

Link for Dorset, Acta Crystallographica B51 (1995) 1021-1028. "The crystal structure of wax."
(Pay for article; no abstract): http://scripts.iucr.org/cgi-bin/paper?S0108768195005465

The authors showed that mixed paraffin waxes after evaporation from light peteroleum ether (mostly pentanes, probably the best room-temperature terrestrial analog for liquid methane) oriented themselves into laminar sheets.

When viewed from the side, the position of the carbon atoms is in a hexagonal arrangement (like a sheet of benzenes). The kinks of the chain are evidently 180 degrees out-of-phase with the chain below it. So the distance of 2 adjacent carbons is close, then far, then close as one moves along the chain.

The atoms at the center of the chain are pretty locked in, but at the edges the location is more random. (Probably due to voids or other defects since these are mixed component crystals)

(The structures in the article were not placed in the Cambridge Crystallographic Database).

However, the crystal structure of the more complex beeswax, was much less resolved. Since one would expect multiple functionalities on Titan surface deposit materials (tholin NMR analysis indicates multiple functional groups in the mix), I'd expect that beeswax might be a better analog.

[Although some of the functional groups might help orient the molecules in a crystal lattice]


Another article mentions that large long chain alkanes with varying lengths compounds can orient themselves in different ways. One form which can cause large voids in the structure.

http://scripts.iucr.org/cgi-bin/paper?S0108768195005465

But again, these are long straight-chain boring hydrocarbons.

AFAIK, the tholin formation/deposition experiment results have been goopy smears.


****

Titan's surface organics are presumably caused either by atmospheric fallout or by chemical reactions from stuff released at the surface (declathrate/reaction) or processing of stuff in hydrocarbon or aqueous solution in the subsurface.

Stuff falling out of the atmosphere should be in the amorphous phase (and fluffy).
It would need to be reprocessed by either solution (aqueous/organic), pressure or melting to dissolve/reorder/crystallize or reorder/recrystallize the materials.

I'm not quite sure I understand the mechanism by which non-volatile dry atmospheric fallout could recrystallize/reorder at 1.5 atm and 95 K.

(However, under pressure, in partial-solution, or at higher temperature the grains could agglomerate and recrystallize)