Titan's topography, strange.... Options

[ngunn]

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.

[Juramike]

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.

[Juramike]

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.)

[ngunn]

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 . .

[Juramike]

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.

[ngunn]

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

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.

[Juramike]

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?)

[ngunn]

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.

[Juramike]

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.

[Juramike]

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.

If Xanadu is a hypothetical large mountain, a wind pattern converging in Xanadu that entirely disagrees with the dune observations is predicted. If instead Xanadu is a large basin, the wind arcs clockwise north of Xanadu and anti-clockwise west and southwest of Xanadu, in agreement with the dune orientations in the vicinity of Xanadu.

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

[ngunn]

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

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.

[ngunn]

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.

[ngunn]

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.

[Juramike]

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.

[ngunn]

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