Titan Dunes, CHARM presentation by Lorenz Options

[Phil Stooke]

Regarding the 'bright dunes' - what about them makes you think the dark spaces between the bright lines are not the dunes? - or, let's say, that they are not fractures? I see those features and think of Maxwell Montes.

Phil

[Juramike]

It is possible that these are fractures, crevasses or Maxwell Montes type ridges.

But…(comparing/contrasting with initial literature describing Titan Equatorial dunes)

* The examples noted so far are only at higher latitudes. They are not found at lower latitudes below 30 N. (Equatorial dunes are present in areas from 30N to 30 S.) {A Maxwell Montes type ridge shouldn’t show a latitude preference unless something really funky is going on.}
* The “high-latitude linear features” all seem to be associated with regionally brighter RADAR materials (the wind streaky looking stuff). (Equatorial dunes overlay several types of terrain, the extensive sand seas are in VIMS dark brown unit material).
* These features all seem to have similar spacing (1 km), length and a parallel orientation. (Similar to Equatorial dunes, except the feature spacing seems to be about 50% that of Equatorial dune spacing. This actually makes it more similar to terrestrial dunes. See Fig. in Lancaster, 2006)
* They appear predominantly straight. (Unlike Equatorial dunes which deviate a bit).
* There are no apparent tuning fork shapes (although the arrow in the T30 image above might be indicating an example).
[Equatorial dunes have apparent tuning fork joins as they join and split].
* They appear to have alternating RADAR-bright/RADAR dark parts (brighter part facing RADAR look direction). Bright topographic glints seem to be observed. (Similar to Equatorial Dunes)
* They are slightly modified around topographic areas in a way consistent with wind. (Equatorial Dunes have clear divergence and reconvergence around topographic obstacles. To be fair, topographic obstacles do not appear to be placed in the center of any high latitude dune feature areas.)

Back when Titan dunes were first characterized in the T3 RADAR Swath publication Elachi et al. compared the RADAR appearance of dark Titan Equatoirial dunes to Antarctic dunes: “Remarkably similar images of snow dunes in Antarctica show alternating light and dark bands with narrow, low-backscatter bands alternating to wider, high-backscatter ones, which imply variation in grain size; these are also reported to have little topographic expression" (Elachi et al, 2006).

Looking at the initial articles describing Titan dunes (Ralph can give more insight), it appears that the Titan Equatorial dunes had most of the same characteristics as these high-latitude linear features. The biggest difference, aside from RADAR reflectivity and brightness temperature, is that the high latitude dunes appear to be associated with one terrain type (the RADAR brighter grey windstreaky stuff) and are in fairily limited regions. To provide further evidence, the topographic expression of these features needs to be measured and compared to Equatorial dunes.

Happily, the “high-latitude dunefield” S of the Arrowhead feature was observed by both Ta and T23 RADAR. I think the look angle was similar, but it still might be possible (?) to derive topographic height or slope of these features. That would at least indicate if they are windstreaks or dunes.

-Mike

References:
1) Elachi C. et al, Nature 441 (2006) 709-713. “Titan Radar Mapper observations from Cassini’s T3 fly-by”. doi: 10.1038/nature04786.
2) Lorenz R. et al. Science 312 (2006) 724-727. “The Sand Seas of Titan: Cassini RADAR Observations of Longitudinal Dunes”. doi: 10.1126/science.1123257
3) Lancaster, N. Science 312 (2006) 702-703. (Perspective) “Linear Dunes on Titan” (Check out graph of dune height with dune spacing.)
4) Radebaugh,J. et al. LPSC 38 (2007) Abstract 1412. “Dunes on Titan from Cassini RADAR.” (Fig 4 shows latitude distribution of the Equatorial dunes.) (Freely available here)