point out that some lakes on Titan have been measured to be quite deep, 100 meters in some cases.

Both articles point to the April 15 Nature Astronomy article Deep and methane-rich lakes on Titan but it's paywalled, but enquiring minds want to know!

From the JPL news article:

On its final flyby of Saturn's largest moon in 2017, NASA's Cassini spacecraft gathered radar data revealing that the small liquid lakes in Titan's northern hemisphere are surprisingly deep, perched atop hills and filled with methane.

The new findings, published April 15 in Nature Astronomy, are the first confirmation of just how deep some of Titan's lakes are (more than 300 feet, or 100 meters) and of their composition. They provide new information about the way liquid methane rains on, evaporates from and seeps into Titan - the only planetary body in our solar system other than Earth known to have stable liquid on its surface.

Question: How do they know some lakes on Titan are 100 meters deep?

Answer(s) to Why are Titan's lakes “black” in radar images rather than transparent? point out that at these radar frequencies the hydrocarbons are fairly opaque to radar.

From this answer:

This article suggests that the radar can penetrate the lakes and reports them to be hundreds of meters deep. The space.com article referenced seems to be sourced from a Geophysical Research Letters article from 2008 (not paywalled) which gives the radar wavelength (2.2cm) and claims that it would be absorbed in 2-20m of clean hydrocarbon, but also give other less direct ways of estimating lake depth which give higher values.

A final, and completely independent, measure is radiometric. The darkest parts of some lakes, generally the largest ones, are ‘black holes’, offering no measurable radar return down to the instrument noise floor of ∼ −26 dB [Stofan et al., 2007]. This requires not only that the surface reflection be very small (consistent with a smooth surface of a low dielectric constant material, such as a liquid hydrocarbon surface unroughened by waves) but also requires that the liquid be deep and/or lossy enough to suppress a bottom reflection.


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