[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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[Juramike]

From the “circular features” EXCEL table above, I attempted to plot the morphometry of the Titan circle features using a plot similar to that found in Jones et al. Icarus, 164 (2003) 197-212 “Morphology and origin of palimpsests on Ganymede based on Galileo observations.” (Pay article: Abstract here) . The data in the referenced paper can be found in Table 2 and the plot is in Figure 12 (p. 209 in the reference).

Taking the data for the “circular features on Titan” in the EXCEL table, I plotted the outer diameter (long axis) as the x component and the inner diameter (long axis) as the y component in a logarithmic graph. I also included (red circles) the unity line for [outer diameter (x), outer diameter (y)]. Only circle features that have two measured diameters (outer and inner) will lie off the unity line.



Yellow diamonds are bright center dark halo circle features [dark halo diameter (x), bright center (y)].
Purple dots are bright rim only circle features [bright rim diameter (x), bright rim diameter (y)). These are buried on the unity line.
Green squares are dark center bright apron impact craters [pedestal deposit = apron [x], crater rim [y]]
Orange squares are multi-ring impact basins [outermost rim diameter (x), innermost rim diameter (y)]. If the inner rim could not be measured, the point will be buried on the unity line as [outer (x), outer (y)].
Deep purple crosses are dark halo circle features [outermost rim diameter (x), outermost rim diameter (y)] and the points are buried on the unity line.


It is interesting to note that the putative impact craters (green squares) and the bright center dark halo circle features (orange squares) all pretty much lie on the same line (at least as far as a log plot goes).

This shows that the ratio of crater rim diameter/pedestal deposit diameter for putative impact craters on Titan is similar to the ratio of the bright center diameter to dark halo diameter when plotted on a log scale.
Where multi-ring craters had two rings measured, these points also fit in the same log-scale relationship.

The Jones et al. article defined the correspondence of morphological units between craters, palimpsests, and penepalimpsests on Ganymede and Europa.

So: (palimpsests, craters)

Outer deposits = continuous ejecta
Concentric massif facies = pedestal deposit
Unoriented massif facies = crater rim
Central plains = central pit

Here are pictures of a crater (Melkart), a palimpsest (Buto Facula), and a penepalimsest (dome crater) (Nieth) showing the defining units. (All are on Ganymede).



In the paper, several impact craters, ring structures and palimpsests on Europa and Ganymede were plotted in the same type of graph. (Palimpsests and ring structures used in the text: Callanish, Tyre, Buto Facula, Zakar, Memphis Facula, Epigeus; Craters: Achelous, Sebek, Isis, Melkart, Osiris, and Enkidu).


Adapting the derived lines from the Jones et al paper onto the graph of circle features of Titan we get this plot:
(By definition in the paper, the crater rim or unoriented massif facies is the unity line, so it should be no surprise that it lines up with the Titan circle feature unity line (red dots) as well.)
It is interesting that the central plains/central pit to unoriented massif facies/crater rim points line up almost with the yellow diamond, green squares and orange squares in the Titan plot.





Adapting the derived lines from the Jones et al paper onto the graph of circle features of Titan, but this time shifting the reference diameter to “concentric massif facies/pedestal deposit” we get this plot.
(So by the new definition, the pedestal deposit/concentric massif facies is the unity line, so it should be no surprise that it lines up with th eTitan circle feature unity line (red dots) as well.)
Again, it is interesting that the central plains/central pit to unoriented massif facies/crater rim points line up almost with the yellow diamond, green squares and orange squares in the Titan plot.



Neither plot proves that the circle features of Titan have an impact origin. But they do show that the “ratio of corresponding morphological facies with those of craters, palimpsests, and ring structures of the Jovian moons (well, at least Ganymede and Europa) are similar, at least at the level of a log plot.”

A better plot would show Saturn system inner dome craters and their relationship to Titan circle features…

-Mike