[X] My Assistant

[Guest_AlexBlackwell_*]

Caitlin Griffith et al. have a real interesting abstract for the upcoming DPS meeting.

[remcook]

OK, this is how I understand it:

In general, condensation occurs when a certain gas has a higher concentration than its saturation level. This saturation level is dependent on temperature and pressure and the gas properties. Besides this, there should be consensation nuclei on which the gas can condense. In Titan's case these are mostly the aerosols.

The idea of methane supersaturation in the troposphere comes from the fact that large concentrations of methane (above its saturation level) are required to fit far-infrared Voyager data. If this is true, a lack of condensation sites or something more complicated is make condensation impossible there. Huygens also found methane values close to saturation.

There seem to be a range of different clouds on Titan. Firstly there are the mid-latitude clouds. Complex computer models of Titan's atmosphere show that there is rising 'air' in the troposphere at these latitudes, which brings enriched air higher up. There, it is colder and the methane will pass its condensation level and condense if there are condensation nuclei. Because the methane in the high troposphere are close to condensation anyway, once condensation starts droplets grow very quickly and fall quickly downwards, making the clouds changing rapidly. Whether this is the real process behind the mid-latitude clouds is not sure. These clouds tend to form preferably at specific longitudes, making some people believe cryovolcanism is bringing extra methane there, causing the clouds.

Then there is the big South Polar cloud. The same Titan model predicts these as caused by a circulation cell there.

Then a range of gases condense in the North Polar stratosphere. The mean circulation on Titan right now goes from pole to pole: air is brought up from the south and descends at the north. On Titan, most gases are produced high in the atmosphere, and so there is more of it the higher you go. Now, the downward motion brings air from high altitudes, which thus is enriched in gases, down to colder levels lower down. This causes condensation as well. It's absurdly ocld in the north anyway, which makes things even worse. The ethane cloud discussed in this topic is one of this category.

Hope this is clear. Sorry for the long post.