Expert: Jayendra Upadhye - 1/21/2009

Question
All the buzz surrounding Titan a few years ago had me wondering about possible storms in the satelite's atmosphere.  As I understand it, pressure differentials combined with the spin of the Earth produce large rotational storms such as hurricanes and anticylones.  The Earth's rotational axis lies within the planet.  In contrast, Titan's rotational axis lies outside of the satelite due to the fact that it is tidally locked.  Rotating storms on Earth can not cross the equator due to the Coriolis Effect 'canceling out.'  It seems to me that if Titan had rotating storms, they would not be able to cross from the hemisphere that always faces Saturn into the hemisphere facing away from Saturn or vise versa.  Do you agree with this?

Answer
Hi,
All phase-locked satellites (including our moon), experience a gravitational gradient from the point closest to their parent planet to the point farthest from the parent planet.
If the moon had gaseous atmosphere, and the earth were sufficiently far away from the sun, you would find the atmosphere would assume a tear-drop shape. It would be thicker on the earthward side and thinnner on the other side.
Consequently an atmospheric pressure gradient would be generated from the point nearest to the earth to the point farthest from the earth.
Over and above this gradient line (actually a plane), there would be thermal pressure gradient, that would propel gas away from the sunward side to the farthest side from the sun (day side to the night side.
(these winds are observed on earth too).
As titan turns synchronously around saturn once every 15 days, it has to turn around itself too in that time. This has a very low but definite coriolis component like the one on the earth.
But it may be submerged by the violence of the sun's heat propelling gases from sunward to leeward side.

In the end what you would see is the leeward migration of hot hases in the upper atmosphere and the sunward migration of the lower gasees "high to low pressure".

In a circular hotspot spanning say 25% of the sunward area, huge updrafts would be continually rising up, and in a similar area on the night side, huge downdrafts would be continually falling surface wards!
While all along the daynight line, surface air would be gushing sunward, and higher atmospheric air would be rushing nighwards.

This would lead to a doughnut type of eddy all around the daynight line, with the inner gases rushing towards the sunlit side and outer gases rushing towards the night side.

A succession of such "cells" would be tiered towards the sunside hotspot and a likewise succession on the leeward side too.

All this would lift surface dust (or particulate condensates based on local triple point conditions for methane droplets to form), up into the sky and may be that is why we see the lower part of titan always so hazy.

Doppler studies should be easily able to establish this movement of gas (the doughnut planetwide eddy centered on titan itself).

In a cold dense atmosphere such as one on titan, "storms" would be pale ghosts of their earthly cousins. But due to its density, even slight wind would have a strong abrasive effect if it carried crystallized hard matter like methane and other ices.

regards
Jayen