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Cassini failed to detect any waves in the seas/lakes it observed, despite winds of 72 km/h (45 mph) being present on Titan which are able to create "sand" dunes from frozen particles, and with milimeter precision nothing turned up. They appear to be perfectly smooth like glass. One explanation put forward is that there is a film of tar or thin layer of methane ice that blocks the formation of waves, but isn't the simplest explanation that the lakes are actually entirely frozen sheets of hydrocarbon ice much like the frozen nitrogen "heart" on Pluto?

One reason I've heard scientists are sure that it's liquid is because we've observed clouds passing over an area and then transient lakes appearing. The thing is, how do we know that these clouds weren't dropping a rigid snow/hail/ice that then collected into sheets? Then there are the rivers that appear to have been carved out, but couldn't this easily be the product of methane glacial activity slowly carving out a valley?

I've never heard an explanation of why we are sure they are liquid hydrocarbons and not frozen, other than knowing that based on its distance from the sun, and the predicted greenhouse/antigreenhouse effects, Titan should be at the triple point of methane. Is there some difference between solid methane/ethane/etc and their liquid forms that would make it VERY clear? Radar revealed the depths of some of these lakes, so I guess that would show it was liquid, unless frozen hydrocarbons are similarly see through as liquid hydrocarbons.

What's the answer here? Why do we have confidence that we are dealing with liquid rather than ice?

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    $\begingroup$ Related: astronomy.stackexchange.com/questions/36632/… .. astronomy.stackexchange.com/questions/48975/… $\endgroup$
    – Nilay Ghosh
    Commented Jul 16, 2022 at 4:43
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    $\begingroup$ different but related: How did Arecibo detect methane lakes on Titan, and image Saturn's rings? I think the answer will be related to some radio/microwave measurement and probably involve angles (liquid surfaces can be quite flat and specular) $\endgroup$
    – uhoh
    Commented Jul 16, 2022 at 6:28
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    $\begingroup$ I still don't understand why sending a probe to Titan isn't an absolute top priority for NASA. Can you imagine how profound it would be to see images of those lakes? $\endgroup$
    – White Prime
    Commented Jul 16, 2022 at 11:46
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    $\begingroup$ @WhitePrime - NASA's top priority is not to provide images. Those can help gain public support, and hence budget, but are usually not the main goal. And Titan is but one of a massive number of interesting objects for research. $\endgroup$
    – Rory Alsop
    Commented Jul 16, 2022 at 11:58
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    $\begingroup$ @WhitePrime - Titan exploration is a top priority: a nuclear-powered flying drone mission to Titan, Dragonfly, is being developed at JPL and will be launched in 2026. $\endgroup$
    – antlersoft
    Commented Jul 17, 2022 at 19:21

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There are waves on the lakes on Titan.

SAILING THE LAKES OF TITAN? PREPARE FOR ROUGH SEAS.

If you look at the paper they refer to a number of properties of these mare that point to them being liquid: Wind driven waves on the surface, tides, currents in tributaries. These properties are not consistent with a glassy ice plane, nor slush, but with liquid hydrocarbons.

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  • $\begingroup$ That's really interesting. I wonder why they failed to spot these waves other times. They found unusually smooth seas that required theories about wave inhibition, but there actually are spots where the seas are rough? That's weird. $\endgroup$
    – Axion
    Commented Jul 22, 2022 at 17:36
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I found an excellent article on the optical properties of ice, snow, and liquid that can be applied to your question, since radar was used to study Titan.

This from the abstract:

The interactions of electromagnetic radiation with ice, and with ice-containing media such as snow and clouds, are determined by the refractive index and absorption coefficient (the ‘optical constants’) of pure ice as functions of wavelength. Bulk reflectance, absorptance and transmittance are further influenced by grain size (for snow), bubbles (for glacier ice and lake ice) and brine inclusions (for sea ice). Radiative transfer models for clouds can also be applied to snow; the important differences in their radiative properties are that clouds are optically thinner and contain smaller ice crystals than snow. Absorption of visible and near-ultraviolet radiation by ice is so weak that absorption of sunlight at these wavelengths in natural snow is dominated by trace amounts of light-absorbing impurities such as dust and soot. In the thermal infrared, ice is moderately absorptive, so snow is nearly a blackbody, with emissivity 98–99%. The absorption spectrum of liquid water resembles that of ice from the ultraviolet to the mid-infrared. At longer wavelengths they diverge, so microwave emission can be used to detect snowmelt on ice sheets, and to discriminate between sea ice and open water, by remote sensing. Snow and ice are transparent to radio waves, so radar can be used to infer ice-sheet thickness.

Floating "icebergs" are unlikely because most materials would not float in liquid hydrocarbons. And other similar logic can be applied in theory to the application.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6501920/

Cheers.

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    $\begingroup$ In your answer, could you please include a weblink to the article you mentioned. $\endgroup$
    – Fred
    Commented Jul 18, 2022 at 12:56
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    $\begingroup$ My bad! I meant to and accidentally shut the window. Thanks. $\endgroup$
    – ChairmanMeow
    Commented Jul 18, 2022 at 20:37

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