The Temperature Structure of Titan's Atmosphere
The surface of Titan is quite cold: 94 K (-290 F). Traveling
up in the atmosphere it continues to get colder, reaching a
minimum temperature of about 70 K (-330 F). After the
temperature goes through this minimum, at about 40 km on Titan,
it begins to rise again, at higher altitudes. This temperature
profile, first decreasing then increasing, is just like what
is seen on Earth (the Earth's minimum occurring at the base of
the ozone layer, in the stratosphere).
Although the atmosphere is thought to contain mostly N2, methane (CH4), ethane (C2H6), and C2H2 are also seen (or are expected) to be present.
Clouds on Titan
The haze seen on Titan is due to a couple of well predicted cloud decks,
based upon the temperature at which vapor will condense into droplets.
Both a cloud-deck of methane (30 to 50 km up) and a much lower cloud-deck
of ethane, just above the surface, are expected. Moreover, the ethane
clouds are predicted to actually rain on the surface of Titan.
Methane in Titan's Atmosphere
While there is clear evidence Methane is seen in the atmosphere of
Titan, scientists are puzzled by this. Ultraviolet radiation from the
sun easily dissociates methane, and would destroy all of it in the
atmosphere within just 10 million years. There must be a source
of methane at the surface to keep it replenished in the atmosphere.
This requires significant liquid methane on the surface.
Oceans of Methane?? Probably not..
Measurements of the temperature change in Titan's
atmosphere with altitude indicate methane is probably not at a
saturation level. And there is little to support a large liquid
methane ocean on Titan's surface (though maybe sizable seas might
exist). How do we explain the atmospheric methane abundance?
1) Titan's current state is transient. A recent event
forced a large amount of methane into the atmosphere.
2) A significant ocean of liquid volatiles is present in sub-surface 'aquifers'.
3) Titan's methane is supplied to the atmosphere only episodically via volcanic activity.
4) Methane might be delivered by the impact of comets throughout
Titan's history.