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?