Titan's topography, strange.... Options

[Juramike]

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.

[ngunn]

sands being able to bounce easily across bright terrain (your item #2)

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.

[ngunn]

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.

[ngunn]

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

[Jason W Barnes]

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

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

[Jason W Barnes]

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.

- Jason

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

- Jason

[Phil Stooke]

What's an order of magnitude among friends?

Phil

[Juramike]

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)

[ngunn]

If Xanadu is "self-clearing" then the dune particles must be going around the southern edge of 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.

[ngunn]

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.

[helvick]

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.

[ngunn]

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

[Jason W Barnes]

I've just peformed an experiment which crudely simulates what I think happens to the sandflow at Xanadu.

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

[stevesliva]

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!).

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.

[ngunn]

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!).

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.