[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_*]

While there's still a knock-down fight over why we aren't seeing much liquid on Titan's surface, there's no chance that it's because the surface is cold enough to freeze methane or ethane -- it isn't. One parricularly convincing new model comes from Mitri and Lunine ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1962.pdf ). This is simply that Titan, in terms of surface liquid methane, is a desert world similar to Earth's deserts, with only small lakes of methane scattered across the surface -- despite the fact that its atmospheric humidity of methane is quite high.

The reason for the paradox is that the difficulty we have seeing Titan's surface from above through the haze is deceptive -- Titan's atmosphere is actually much cleaner particle-wise than Earth's is. Its haze is very rarified; the only reason it blocks our view of Titan's surface features is that Titan's atmosphere, and thus the haze in it, towers up to such a huge height above the surface. Earth's lower atmosphere, by contrast, has a far denser accumulation of solid aerosol particles -- not just windblown continental dust, but vast quantities of microscopic salt crystals whipped off the froth of its ocean waves.

And so, with far fewer solid particles in its air to serve as nuclei around which its atmospheric humidity (water for Earth, methane for Titan) can condense into liquid cloud droplets, Titan -- despite its high methane humidity -- has very few clouds. And on those infrequent occasions when cloud droplets do start to condense in its air, the initial tiny liquid droplets serve as condensation nuclei around which very large additional amounts of methane vapor suddenly condense, so that the cloud droplets very quickly grow in size to methane raindrops and thunder down copiously on the local surface, producing brief local flash floods and arroyos. However, there is never very much liquid methane on Titan's surface at any one time, any more than there is in Earth's deserts despite their own occasional local flash floods. "We have demonstrated that the high relative humidity of ~50% on the surface of Titan can result from lakes evaporation covering only 0.004 – 0.04 of the whole surface. Even if only a small value of the surface of Titan is covered by hydrocarbon lakes, the atmosphere is nearly saturated and, therefore, can generate thunderstorms." (Ralph Lorenz also saw this possibility coming in his book "Lifting Titan's Veil".)

As for the ethane: the theory has been that most of it sinks down through Titan's porous upper surface to form an underground "aquifer" -- and ethane virtually does not re-evaporate at all in Titan's air. Moreover, Sushil Atreya et al conclude in a new paper in "Planetary and Space Science" ("Titan's Methane Cycle": http://www-personal.umich.edu/~atreya/Arti...tan_Methane.pdf ) that the total amount of liquid ethane smog that has been slowly manufactured in Titan's atmosphere over the eons has been seriously overestimated, and we are probably talking about a total global layer only about 200 meters thick manufactured over Titan's lifetime, making it much easier for most of its ethane to permanently settle underground.

[Greg Hullender]

Great article! Thanks for the link.

I gather we still haven't actually seen anything that looked like a storm (producing rainfall) in progress. If I read it correctly, the article appeared to suggest that storms might be seasonal and that we might have a better chance of seeing one as Saturn approaches its equinox. I like that idea, since it's hard to believe that so much erosion can be accounted for by small local storms that are so rare there may not be one on the whole planet (er, moon) in a year. Do we at least seem to be seeing more clouds on Titan as time goes by?

The paper's model for replenishing the methane left me with one question: where does the hydrogen go? The photolysis from CH4 to C2H6 loses H2. It would seem that it would be lost to space, yet the article describes the system as closed, and it concerns itself only with how the Ethane gets turned back into CH4. Did I miss something?

Finally, since the surface of Titan is close to the triple-point of Methane, why does it seem so unlikely that a deep pool of it might freeze at the bottom? It should be both colder and under more pressure.

--Greg