[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]

I plotted the outer/inner diameter measurements of the selected large craters on Tethys, Rhea, and Iapetus on the same plot as the circular features of Titan. The selected craters are: Odysseus on Tethys, Tirawa on Rhea, and the crater located at 5N, 38W on Iapetus. The topoglogy of Odysseus and Tirawa was described in Moore et al. Icarus 171 (2004) 421-443. "Large impact features on middle-sized icy satellites". Below are the topological profiles of Odysseus and Tirawa and a plot of Titan circle features with selected craters from some of Saturn’s other moons.



Here are the measurements and plots of Titan circle features with 4 selected larger craters from Ganymede and Callisto (measured values are from the text). (Two central dome craters, an anomalous central dome crater, and Gilgamesh which may also be a central dome type crater):



According to the text in Schenk et al “Ages and Interiors: The Cratering Record of the Galilean Satellites” (Available in html-only http://66.102.1.104/scholar?hl=en&lr=&...s+and+Interiors
]here[/url]): “These domes have rounded profiles up to 1.5 km high but at high resolution are characterized by web-like networks of narrow fractures.” (They also invoke that Gilgamesh and Lofn may qualify as central dome craters). Also, “Like cental pits (craters), the dome/crater size ratio increases with increasing crater diameter.”

In contrast, “the ratio between dome and rim diameter for [anamolous dome craters] (whether observed or estimated from ejecta deposit scaling) is roughly constant at 0.4, regardless of crater size”. (I hope I got the correct diameter for the rim of the 54N, 43W Crater on Callisto: I measured 90 km, but the text stated 180 km for the rim)


Below are the measurements for a (putative) cryovolcano on Titan, Ganesa Macula. The ratio of outer diameter to central radar bright zone is about 0.11 for the 200 km pancake dome (n = 1, so who knows if this is typical.) In contrast, a 200 km bright center dark halo feature usually has a 60 km bright center, for outer/inner diameter of 0.3. Below are the images and plots.




Below are the measurements from topographical analysis of dome-like bulges in chaotic terrain on Europa. These bulges are thought to be diapirs. Below are the images and comparision a plot of Titan circle features.



As described in Shenk, Journal of Geophysical Research 100 (E9) (1995) 19,023-19,040. “The geology of Callisto”: “Bright domes fill most central pits in craters larger than 60 km (in Galilean moons), and have morphologies consistent with an intrusive origin. Dome formation could have occurred after crater formation as diapiric intrusions of soft ice, or during crater formation and collapse by the uplift of deep material, as occurs in central peak craters on Earth and the Moon.” I’m curious to know if a “web-like network of narrow fractures” would create a chaotic terrain that would then appear RADAR-bright by SAR.

At least for the Galilean satellites, “Fracture domes on these [Galilean] domes resemble those formed in a thin chilled crust over plastic material deforming under gravity (e.g., pancake domes on Venus). This suggests that uplifted dome material deformed plastically during or after emplacement, as would be expected for warm material rapidly uplifted several kilometers and left to cool to space. Dome profiles can be used to model the rheology of the uplifted material during emplacement”.



From all the graphs and plots, it seems that a cryovolcanic pancake dome, like Ganesa Macula, seems to be the worst fit with the data (but remember, n = 1).

The best fit seems to be for an impact origin, at least for most of the smaller craters (<900 km). As stated above, diapiric rise of material after impact might explain the observed morphology of the bright center, dark halo circle features.
A pure diapiric origin may fit for some of the largest circle features (those >900 km). Unfortunately, I couldn’t find any really big examples (>1000 km) of diapirs so I can’t compare morphology with Titan’s bright center dark halo circle features.


Smaller craters: impact
Bigger circle features: impacts and diapiric doming
Biggest circle features impacts and diapiric doming OR just plain old diapirs.


-Mike