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The Southern California Public Radio news item What Cassini saw on Titan: 'Dunes of the Arabian desert' but made of water chips, not sand explains:

"Little chips of water ice come off (the mountains) and get washed downstream, and then they get pushed up onto the beaches," Ray said. "They get dried out and then they get picked up by the wind and blown into the deserts. And around the entire equator of Titan is nothing but dunes as far as the eye can see -- just like the dunes of the Arabian desert. But they're made out of little chips of water covered in hydrocarbon goo."

According to the article, Trina Ray is the Titan orbiter science co-chair at JPL.

Question: What is the evidence that the dunes of Titan are made of "water chips"?

below: Dune patterns on Titan, pia15225 from NASA JPL. Click for full size. Despite the names, these represent various imaging techniques by Cassini of dunes on Titan. There is a much longer description in the pia15225 link.

enter image description here

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  • $\begingroup$ We know dunes of water crystals on earth called snow banks, but we also know glaciers build from snow over time. Is it possible that the environmental conditions of Titan prevent the transformation of water chips into solid ice? $\endgroup$
    – Uwe
    Commented Jul 26, 2017 at 9:21
  • $\begingroup$ @Uwe that sounds like a separate question, and a very interesting one! On titan the ice does not fall as precipitation, so it does not build up. Also with only 14% of Earth's surface gravity, there's less compaction. $\endgroup$
    – uhoh
    Commented Jul 26, 2017 at 9:32
  • $\begingroup$ But there should be no compaction at all if a dune on Titan should exist as a dune of water crystals for a very long time. $\endgroup$
    – Uwe
    Commented Jul 26, 2017 at 10:04

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It is modeled that the dunes must form from chips of Titan's bedrock. Whether those chips are primarily hydrocarbons or primarily water ice, depends on what Titan's crust is presumed to be composed of. There are varying models, depending on what is known of Titan (such as the moment of inertia), and one model with a reasonable level of support involves a substantial water ice crust.

A.D. Fortes explains in "Titan’s internal structure and the evolutionary consequences" that Titan's moment of inertia was estimated from measurements taken by Cassini of Titan's quadrupole gravity field. From these observations, an internal structure with a shell of water ice can be derived.

So, to answer your question, there is no direct evidence that the dunes are made of water chips, rather what the dunes are made of is assumed based on what model of Titan's composition is considered most likely. Further, modeling the geological effects, depending on the assumed composition, may help to refine our understanding of Titan's dunes.

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    $\begingroup$ OK I understand; excellent answer, thanks! I noticed that your linked paper is open access, just click Download PDF, yay! Also, i.sstatic.net/tVAKH.png "Fig. 4. Scale illustration of one possible fully differentiated internal structure model for Titan, in which the core consists entirely of hydrous silicates and an ocean of liquid water exists in the upper part of the ice mantle." $\endgroup$
    – uhoh
    Commented Jul 26, 2017 at 14:37
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    $\begingroup$ Somewhat related and very interesting question Any proposed missions to explore the black liquid on Titan? Technical challenges? $\endgroup$
    – uhoh
    Commented Jul 26, 2017 at 15:05

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