Titan's Equatorial Sand Seas Options

[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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[Juramike]

There are a few examples of both RADAR dark and RADAR bright channels lying on top of (presumably) VIMS dark blue deposits in the Equatorial Sand Seas.

These are shown here (channels indicated by yellow arrows):



I was not able to find any examples of a dune field clearly cut through by a channel (either bright or dark). The area near western Xanadu in the T13 RADAR swath shows a large delta complex that feeds into the Shangri-La. A “channel” may be barely visible, but it does not cut through a dune area located to the W. The dark smooth area may be where a broad outflow fan of ice sands was deposited, then overlain by dune seas. This is shown below (delta indicated inside yellow box, potential channel/outflow fan with green arrow):




So it appears that RADAR-dark channels and RADAR-bright channels overlay dark blue ice sands, but not dark brown dune sands.

So deposition sequence (first to last):

1) Dark blue ice sands
2) Channels (RADAR-bright/RADAR-dark)
3) Dark brown dune sands

This also implies that the dark blue ice sands are not always infinitely permeable to liquid. They can support a stream, at least in some locations. This is very similar to wet terrestrial sand. Perhaps the dark blue ice sands are almost saturated with liquid, or perhaps they have a slight barrier to the liquid (semi-permeable crust). Perhaps in most locations, a small decent-sized flow can soak in, but a massive flood event will scour across the surface for a distance. In some areas, permeability is locally reduced, and even normal amounts of flow will course across the surface.

Since a massive feeder network of channels and ponds is not observed, rain must be able to soak into the sands to some extent. (Impermeable dirt would quickly create an eroded out network.)

At the Huygens landing area, there are large and small channels visible in the straights off the island. There are also streams that flow off the island into the dark area. But I could not discern a connecting channel between the channels in the straight, and the outflow of the streams coming off the island. The outflow from the island streams seems absorbed by the dark blue ice sands. Perhaps the channels were carved during major flood events, presumably from the flow coming down the straight (SW to NE) and not from the island streams.

From the available data so far, RADAR-bright channels streaking across the ice-sands are not common. This implies that the equatorial sand seas are relatively flat. Irregular rough (RADAR bright) boulders cannot be supported by gradual slopes. The fact that we do observe channels does imply a slight change in elevation, so the Sand Seas are not totally flat. Further analysis should be able to determine flow direction (thus topography).

-Mike

[ngunn]

So deposition sequence (first to last):

1) Dark blue ice sands
2) Channels (RADAR-bright/RADAR-dark)
3) Dark brown dune sands


-Mike

Nice work, Mike. However I would propose an alternative caption to your 1 2 3 list as follows:

"Titan surface features in decreasing order of formation timescale:"

My reasoning is that the active processes involved are probably all concurrent over the long term. All that's needed to produce the apparent sequence is for the massive outwash floods to be very infrequent compared with 'ordinary' channel-forming rains and dune movement. I'm not even sure about the dunes coming third. Have you found any dunes that have drifted across (and blocked) a channel?

[Juramike]

Nice work, Mike. However I would propose an alternative caption to your 1 2 3 list as follows:

"Titan surface features in decreasing order of formation timescale:"

Ah, perfect: that the key phrase I was trying to come up with. Thanks!

All these process are occuring either simultaneously or episodicaly. From any given point, I'm asking "What is the last event that happened here to make it look this way?"

For example: a particular area may have had decades of stream deposition with dunes covering them at the same time, but then a massive flood came along and erased evidence of both of these events.


As for dunes covering over channels, I have not yet found any that clearly cut across a dark channel. (Anyone else seen one?)

-Mike