[X] My Assistant

[Webscientist]

The color images taken by the Huygens probe during its descent reveal a major contrast between a dark area apparently made up of wet sand and white and bright elevated terrain composed of multiple channels. I have the feeling that those white and bright hills consist of ice.What kind of ice? perhaps water ice, or a mixture of hydrocarbon molecules, methane or ethane for instance.
And if it's really composed of ice, it's not forbidden to think that the channels on its edges are fractures rather than rivers, especially if those ice blocks or "icebergs" are moving on the wet sand.
Regarding Titan meteorology, if the hypothesis is correct, it's likely that this supposed ice can evaporate and form clouds.And then, it can rain or snow depending on the environmental temperature.

http://www.titanexploration.com

[Guest_BruceMoomaw_*]

The clouds are not the smog of other organic compounds (including ethane) which is made (extremely slowly) out of Titan's methane by solar and Saturnian radiation -- they are separate, infrequent, and small clouds of genuine condensed liquid methane droplets, with those droplets being vastly bigger than the smog particles. Never confuse Titan's omnipresent but rarified non-methane smog haze with its small but concentrated liquid methane clouds.

And it is now considered a safe conclusion that those clouds are indeed methane, as was originally thought. Remember that -- because of the ironically rarified nature of Titan's smog as compared to the number of solid particles in Earth's lower atmosphere -- Titan's high atmospheric concentration of methane vapor has difficulty condensing into liquid droplets at all because of the shortage of smog particles to act as condensation nuclei. And so -- on those rare occasions when methane droplets do start to condense in some local area -- they themselves serve as nuclei for more methane to quickly condense around them; and so the cloud droplets very rapidly grow to large size and thus tend to rain quickly and forcefully out of the atmosphere, accounting for Titan's rare but violent rainstorms. So, whenever you do see a methane cloud on Titan, its droplets are likely to be unusually large in size -- and they are also not likely to remain suspended in the air very long.

Finally, to answer "Messenger's" question: ethane is extremely reluctant to vaporize at all at Titan's surface temperatures -- its vapor pressure there is (I believe) about 1/1000 that of methane, even in Titan's warmest equatorial region. So it is simply impossible that the clouds should contain any significant trace of liquid ethane mixed in with their liquid methane.

[Guest_Richard Trigaux_*]

Bruce, with your theory of rare condensation nuclei we could imagine the following scenario:
-lower Titan atmosphere is relatively free of haze. So methane "humidity" could accumulate there.
-this lower "damp" layer is unstable, as methane vapour is lighter than nitrogen (16 to 28 molecular mass). So upwelling and convexion can occur (On Earth too damp air rises like hot air, and sometimes more strongly than hot air).
-When convexion towers up to the higher altitude haze layer, it finds condensation nuclei and rain falls.
This mechanism could govern the formation of small but violent rains. Of course solar warming could act too, favouring the summer hemisphere and day side. But basically the rythm of the formation of rain would be governed by the rythm metane vapours raises into the hazy upper atmosphere.

This process would clean the lower atmosphere of haze. But this haze could continue falling from the upper atmosphere, or mix with the lower atmosphere, driven by convexion (most of the solar energy being absorbed into the upper haze layer, we can expect that this layer has a convexion pattern, explaining the winds at high altitude).