[X] My Assistant

[Juramike]

I’ve put together a sequence of events that could explain the morphology of the Equatorial Sand Seas. (An example basin similar to Shangri-La is shown)

This could explain the ria-like topography [http://en.wikipedia.org/wiki/Ria] on the Eastern shore, as well as the VIMS dark blue western parts of the Sand seas, and the placement of the dark brown unit on the Eastern parts of the sand seas.

1. Basin formation.
2. Water-ice sand deposition [slowly, suddenly?] forms an ice-sand margin
3. Mobile dark brown dune sands deposit on E side, depositing inland up W facing valleys.



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The dark brown sands will blow in following the predominantly W winds and make a dust coating on low-lying terrains on the eastern margins. This will be visible by VIMS and ISS as the dark-bright margin, placed “inland” from the "real margin" and will accentuate the local topography as seen by optical instruments. This accentuation on the E margin will make the Equatorial Sand Sea visible margin look “swoopy” and windblown (in effect, it is) from the dark basin. Similarly, the W margin will have a dark blue zone that appears blown from the western bright areas.

On the Eastern shore, the RADAR images will place the smooth-dark/mottled gray boundary far to the W of the VIMS brown dark-bright margin. (RADAR should be able to penetrate a thin coating of dark sands). The features in the limbo zone have been covered by dark sands, perhaps not enough to form dune structures, but enough to cover up the ice-sand margin, the near shore terrain, and perhaps even some of the underlying bright terrain.


This makes the deposition sequence in the Equatorial Sand Seas:
1: Basin formation
2. Major water ice sand emplacement
3. Dune sands cover up low-lying downwind valleys (enough to mask visible imagery)

Other Equatorial Sand Sea basins should look very similar around Titan: Shangri-La, Belet, Senkyo, Fensal and Quivra. Local winds may play a bonus role, but the overall trend of dark sand deposition up valley should be towards the E. For example: the false-color image in Figure 6 of the Soderblom paper seems to imply a predominant wind vector in Fensal and Quivra to the ESE.

[I’m pretty sure all this has been described in pieces before, but it gave me a really great excuse to play with PowerPoint. ]

-Mike

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[alan]

Would the strength of these tides tides vary with latitude? If so I can see this as an additional reason for the lakes being concentrted at the poles. Stronger tides at low latitudes could result in liquid filled basins being enlarged by tidal motion. Eventually they could reach the point where increased evaporation or seepage caused them to dry up.

[rlorenz]

Would the strength of these tides tides vary with latitude? If so I can see this as an additional reason for the lakes being concentrted at the poles. Stronger tides at low latitudes could result in liquid filled basins being enlarged by tidal motion. Eventually they could reach the point where increased evaporation or seepage caused them to dry up.

Yes - the strength varies with latitude (and longitude) - specifically with distance
from the subsaturn point.

A brief intro to tides on Titan and their effect on lakes (from 1993 - it includes some
predicted landforms that, er, well, havent been borne out by observations so far, let's
say - but it lays out some useful fundamentals) is at

http://www.lpl.arizona.edu/~rlorenz/craterlakes.pdf

Tides are generally weaker at high latitudes. I dont think tides explains why we find
liquids where we do (but they may be responsible for the lack of wave-type features
on shorelines like sandbars - tidal displacement of the waterline is larger than typical
waveheight..)

[Juramike]

A brief intro to tides on Titan and their effect on lakes (...) is at

http://www.lpl.arizona.edu/~rlorenz/craterlakes.pdf

I attempted to relate the tide effects mentioned above and in other (papers you gotta pay for) to the Channel in W Belet.

Here are the non-free references:
Sagan and Dermott Nature 1982, 300, 731-733. "The tide in the seas of Titan".
Dermott and Sagan Nature 1995, 374, 238-240. "Tidal effects of disconnected hydrocarbon seas on Titan".
Tokano and Neubauer Icarus 2002, 158-499-515. "Tidal Winds on Titan Caused by Saturn."

The figures attached describe the tidal effects of tidally locked planet with significant orbital eccentricity and a global ocean. (e.g. Titan, but with a global ocean):



The tidal bulge is set by the gravitational attraction by the influencing body (e.g. Saturn on Titan).

At the sub-Saturn point, Saturn pulls on the hydrocarbon ocean since the ocean is closer to Saturn than the core of Titan. (Think of it as Saturn trying to accelerate the ocean away from Titan).
At the anti-Saturn point, Saturn pulls on the core of Titan more than the hydrocarbon carbon ocean which is further from Saturn. (Think of it as Saturn trying to accelerate Titan away from the ocean.)
At the poles and hemisphere edges, the ocean is at the same relative distance from Saturn and the core of Titan, so there is no bulge at these locations.

(For more info, check out: http://en.wikipedia.org/wiki/Tidal_bulge
http://home.hiwaay.net/~krcool/Astro/moon/moontides/ )

Since Titan is tidally locked, and the same point (more or less) faces Saturn, the tidal bulge is pretty much frozen in place. From Sagan et al. the tidal bulge of a theoretical global hydrocarbon ocean on Titan would be 120 m higher at the sub-Saturn point and at the anti-Saturn point.

The tide is set by the relative height differential of the tidal bulge. If Titan’s orbit were perfectly circular, there would be no difference and the sub-Saturn point tidal bulge would always be at 120 m. So no differential, thus no tide.

But Titan has an eccentric orbit. (e = 0.0289).

When Saturn is closer to Titan, the tidal bulges are more accentuated than when it is farther away. This is by 9% (3e = 3 x 0.0289), or 9 m higher at the sub-Saturn point.
When Saturn is farther away, the bulge is lower by 9 m. But since the liquid volume is fixed, the excess mass gets distributed to the remainder of the ocean. So the ocean height at the hemisphere edge increases (slightly – all that 9 m of bulge differential is distributing over a much, much larger area). This increase would be the local high tide at the periphery of the hemisphere and poles.
There will also be a point of inflection that would not experience any tidal differential.

In addition, there is a slight effect due to orbital libration. This causes a drift along the equator of 150 km of the tidal bulge of the sub-Saturn (and thus anti-Saturn) points. This would result from the rotation of Titan being fixed but the orbital speed is variable. At it reaches periapsis, Titan is zipping along and the rotation will be dragging behind. As it whips around the periapsis point, the rotation is still slower so the Tidal bulge will appear to move forward. The net effect will be to “ratchet” the Tidal Bulge forward (eastward) a small amount. There would be a similar drift westward at the far end of the orbit for the same reason, but the effect will be much smaller (since the bulge will be smaller at apoapsis). This will also smear the Tidal Bulge along the EW direction.



Combining these effects we get a theoretical picture of the global ocean height at periapsis (closest to Saturn) and apoapsis (farthest from Saturn). The sub-Saturn and anti-Saturn point will have the largest effect (9 m!). The poles and hemisphere edges will have a much smaller effect (<9 m) and there will be zone of inflection with no differential is shown in green.

This model is assuming a global ocean. On Titan, it appears that there were multiple shallow sea basins that were flooded during the last major inundation. That means that any relaxation of tidal volume from a bulge at one end of a sea would need to be taken up at the other end of the sea. For example, the Anti-Saturn point is at the W end of Shangri-La basin; as Titan’s orbit swings farther from Saturn, the liquid under the tidal bulge would be somewhat redistributed to the farther parts of the Shangri-La basin (and would also try to redistribute to other connected basins, such as Ching-Tu and Belet).

From the logic above, the best places to look for tidal channels would be nearest the sub-Saturn and anti-Saturn points in the narrow constrictions between basins or archipeligos.




In the Anti-Saturn hemisphere, the dark areas between Dilmun and Adiri (the channel between Shangri-La and Belet) should be a really good place to look for tidal channels (sliced carrot feature?). Another place would be the passages connecting Shangri-La and Ching Tu, as well as Ching-Tu and Belet. (Is the Huygens channel a former tidal channel? It’s location fits nicely with predicted tidal differentials!) .

In the Sub-Saturn hemisphere, narrow channels between Aztlan and Aaru, narrow channels between Aztlan and Fensal (extreme E Quivra) and Senkyo/Fensal and Aaru/Senkyo would be interesting to check out. The areas between Senkyo and Belet may be also of some interest. The possible Tidal channel in W Belet is located in an area that may not have the largest possible tidal differential. (But heck, the potential Earth analog, Deception Pass, gets an impressive flush from only a 1.5 m tide height differential.)

The poles should have almost zero tides, especially if they are/were disconnected from any bodies of liquid involved in the SubSaturn or AntiSaturn Tidal Bulges.

Other effects that may influence tidal flushes include both sea bottom topography and tidal resonances.

-Mike