Titan's Equatorial Sand Seas |
Titan's Equatorial Sand Seas |
May 7 2007, 03:53 PM
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
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. :attachment] 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 -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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Jun 5 2007, 10:07 PM
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
Following the calculations in the Sagan, Dermott, and Lorenz papers, I repeated their calculations for the putative W Belet channel to see if tidal resonance could furnish exteme tides (like in Bay of Fundy)
From the Sagan, Dermot and Lorenz papers, the formula for a tidal resonance is: Tg = 2L/[N(gD)^0.5] where N is the mode number (we’ll use 1), g is the surface gravity (g = 1.34 ms-2), D is the depth of the basin, and L is the length of the basin along the direction of wave propogation. “Resonance arises when one of the periods of the normal modes equals the tidal period, or in the case of Titan, the orbital period of the satellite, T (T = 5.4E6 s). For the channel in W Belet/E Senkyo, the basin to the NW (from the ISS image) has a EW length of 500 km. (Happily, the E Senkyo side to the SW is about the same size.) Assuming a depth of 1-200 m from the above post’s estimation we then get: (gD)^0.5 = 1.2 – 16.3 ms-1 [similar values as the articles above indicated] Applying this to the W Belet basin: Resonant period is 1E6 m/1.2 ms-1 = 8.3E5 s. Which is still shorter than the orbital period of 5.4E6 s. (Bummer). However, if we consider a larger basin with a large EW fetch, such as Shangri-La (2000 km EW basin size), we get a resonant period of: 4E6 m/1.2 ms-1 = 3.3E6. Which is approx half of the orbital period (so it’s close, but no cigar (Bummer)). For there to be tidal resonance, the seas would need to be much larger, but still shallow. However, just like on Earth, local tide effects on Titan could be strongly controlled by bottom topography, currents and complicated gyres in the sea basins. These would need to be invoked for above average tidal currents, since resonance effects do not seem a likely player. (Happily, I have absolutely no idea how to predict or calculate these). -Mike P.S. Assuming a bottom slope of 0.1% due to downwarp, the maximum tide range of a 9 m tide bulge differential would give an intertidal distance of 10 km. So really impressive beaches are possible in large tidal basins where the tidal height range is largest, near the Anti-Saturn or Sub-Saturn points (W Shangri-La basin or E Aztlan) Then E Fensal (where we have altimetry data) area should have been a large intertidal zone and will be interesting to examine by ISS and upcoming RADAR passes. (Sadly, the Fens of E Fensal are probably covered by dune sands now – no more Titan oyster beds!) -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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