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 Post subject: Methane Rains on Titan
PostPosted: Tue Jan 15, 2013 1:56 am 
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Titan and Earth share a few similarities that make them unique among planets in the Solar System. Both planets are surrounded by dense atmospheres rich in nitrogen, which provides a medium to sustain circulating winds. Both planets also feature prominent cycles of evaporation of liquid, condensation into clouds, and rainfall to the surface that form major features of weather. Earth, however, resides at a cozy distance from the sun where liquid water from the oceans evaporates to form water clouds and rain water. Titan, on the other hand, sits much farther from the Sun so that methane, rather than water, forms lakes, clouds, and rain.

The primary source of energy for Titan, as well as Earth, is light from the sun. At it’s greater orbital distance, Titan receives about one percent as much sunlight as Earth, but this is still enough to heat the surface and begin a greenhouse effect. Methane plays the role of the primary greenhouse gas on Titan by absorbing and re-radiating the energy coming from the surface, which provides an additional source of warming. In short, the methane in the atmosphere of Titan keeps the planet at least a few degrees warmer.

At Titan’s cooler temperatures, methane ventures beyond its gaseous phase and condenses into rain, which falls and collects on the surface as rivers and lakes. The speed of this process depends on how quickly the liquid methane lakes evaporate, which in turn depends on the amount of energy reaching Titan’s surface. But if methane is an important greenhouse gas on Titan, and therefore provides some warming to the surface, then what happens when the methane rains out to the ground? The answer is that Titan gets even colder when it rains.

Because rainfall on Titan means that an important greenhouse gas is washed away, this creates the possibility for violent seasonal rainstorms. Once methane has rained out of the atmosphere, Titan’s surface will overall be lower, and so the liquid methane lakes will evaporate relatively slowly at first. As methane accumulates in the atmosphere, the rate of evaporation will increase--because the greenhouse effect of methane is now present again. This in turn will lead to an even more rapid accumulation of atmospheric methane, which provides further warming, until methane clouds develop into strong rainstorms. As the storms release energy, they will also return the greenhouse gas methane--now in liquid form as rain--back to the surface. Following the storms, a quiescent period will again follow, awaiting the slow buildup of methane in the atmosphere again.

The overall effect of this story is that Titan’s rainfall will be more like the monsoons of Asia than the drizzle of London.


Last edited by jacobHM on Mon Dec 16, 2013 1:36 pm, edited 1 time in total.

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PostPosted: Fri Jan 25, 2013 9:00 pm 
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Hi Jacob, I have a question, could it be possible in billions of years that Titan can get warmer or that it couldn't because it doesn't receive much light from the sun to cause a run away greenhouse effect. And with that, the speculation that maybe we can terraform a planet one day, could that be a possibility to create a warmer environment on the moon? And if it was possible, since Titan doesn't have a magnetic field of its own and is covered by Saturn magnetosphere most of the time will it always be protected from solar flares? So if we had machinery on Titan to terraform ( if that was possible) our equipment wouldn't be affected. Will the moon always be protected? This is probably more syfy-ish question I guess lol.


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PostPosted: Sun Feb 17, 2013 12:26 pm 
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Hi Devioussqurl,

Titan actually experiences a fairly strong "anti-greenhouse effect" that acts to cool the planet. This occurs because the thick hazy upper atmosphere of Titan contains big organic particles known as "tholins". These aerosol particles reflect incoming sunlight away from Titan's surface, so that Titan ends up being cooler than it might have otherwise been. This effect, and the regular greenhouse effect, contribute to the overall temperature on Titan's surface.

As the Sun evolves and expands, this anti-greenhouse effect will still be present, so the expansion of the Sun will be unlikely to terraform Titan on its own (although maybe it could help out a little...). Saturn's magnetic field certainly will provide some shielding and other atmospheric effects on Titan, but even so, some solar flares may not be sufficiently deflected. Nevertheless, the expansion of the Sun and presence of a magnetic field may help the sci-fi scenario a little bit.

Cheers,
Jacob


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