[X] My Assistant

[dvandorn]

I've been pondering the fact that Titan's north pole is currently pointed away from the sun. It's northern winter on Titan, and southern summer.

At its distance, does the Sun actually provide significant heating on Titan? If so, what are the phase change temperatures of liquids (mostly liquid ethane, I imagine) on Titan? And do the temperature differences between the shadowed northern pole and the sunlit southern hemisphere range across those phase change temperatures?

What I'm wondering, here (and I freely admit that my thinking is mostly influenced by how water works on Earth and how CO2 works on Mars, each of which may be entirely misleading when it comes to Titan), is whether the liquids on Titan might not migrate to the winter pole and freeze out on the surface, leaving relatively little liquid on the rest of the globe. As Titan passes through its equinoxes, the ices melt and either evaporate or are entrained in the atmosphere as "humidity" and begin to migrate to the pole moving into darkness. During this period, the liquids are more generally distributed through the atmosphere and rain out in a "hydrologic" cycle that waxes through and past the equinoxes, and then wanes as we get closer to winter at one pole or the other. Whenever it's mid-winter at either pole, you get what we're seeing now -- a relatively "dry" environment, due to the freezing-out of the liquids at the winter pole.

If much of the liquid on Titan is currently bound up in a northern polar cap of some sort, we may see indications of a much "wetter" Titan as time goes on and we approach the next equinox.

-the other Doug

[JRehling]

At its distance, does the Sun actually provide significant heating on Titan?  If so, what are the phase change temperatures of liquids (mostly liquid ethane, I imagine) on Titan?

If much of the liquid on Titan is currently bound up in a northern polar cap of some sort, we may see indications of a much "wetter" Titan as time goes on and we approach the next equinox.
-the other Doug

In fact, both methane and ethane freeze out at temperatures just slightly cooler than what Huygens and Voyager both measured as Titan's surface temperature (94K). Ethane freezes at 90K; methane at 91K. Cassini has measured the stratospheric temperatures over the winter cap at 2 degrees colder than the daylight stratosphere. This is close enough to make for a lot of suspense, and close enough to make you wonder if there is a regulatory mechanism making this closeness more than a coincidence.

The idea, BTW, has been posted on before... by me!

Incidentally, seasonal changes are more than just a matter of more or less heat. Different circulation patterns can introduce nonlinear differences. If you note seasonal weather averages in the US, for example, a given point may experience a different mean wind direction in one season vs. another. For a dramatic example, Florida usually experiences west-to-east winds, but in summer can experience east-to-west as the tropical Hadley cell "rides up" into North America.

There should be no question that 15 *years* of darkness would drop Titan's temperature by the 4K needed to freeze its liquids... if convection weren't a factor. But it surely is. If we see great floods in the coming years, or if July's upcoming RADAR track over the near-north-pole shows a cap... we'll have the answer.