Re: How would it be possiable to create the greenhouse effect on Titan?

Hello Judah,
The quick answer: It might be possible! But not easy.

Titan's weird in that it already has a thick atmosphere, consisting mostly of nitrogen but with a few percent methane and argon. The presence of all that methane suggests that Titan is already experiencing the greenhouse effect, since methane is a good absorber of heat.

A common physicist's calculation of the temperature of a planet is as follows: estimate the total sun-energy incident on the surface. Estimate, (using the Stephan-Boltzmann law) how much heat energy is reradiated into space by an object at that temperature. At equilibrium, since the planet is at a stable temperature, these two energies should be equal. You can find the (very simple!) math behind this in any intro astronomy text, like Zelik and Gregory (or if you're interested, ask MadSci again!) The gist is that any body with a certain temperature will radiate a certain amount of energy; the higher the temperature, the faster the energy loss. Over time the temperature will settle down somewhere such that the body radiates energy as fast as it absorbs it ... if, for any reason, you heat the planet up, it will be radiating faster than it's absorbing, and tend to cool down.

Doing a bit of math, you get that any planet's temperature should be around Earth's temperature (in Kelvin, about 290), times the square root of (distance to Earth/distance to planet). So when you get out as far as Saturn, this temperature (the "blackbody temperature") will not get above around 100 K. That's pretty chilly!

This calculation is for a bare black rock, though, and the greenhouse effect is sort of a correction to it. The radiation rate that you get from the Stephan-Boltzmann law depends on temperature---but what temperature? The ground? The outer atmosphere? In fact, the "apparent temperature" will be different at different wavelengths (moreover, the emission spectrum is more complicated than the blackbody spectrum). If the atmosphere is transparent to visible light, then the "blackbody temperature" at visible wavelengths will be the temperature of the ground. If the atmosphere is opaque to infared, then your blackbody measurement at infared wavelengths will see the temperature of the upper atmosphere. And this is the key to the greenhouse effect; you can get a higher temperature on the ground IF the atmosphere tends to absorb (and thus retain) the characteristic radiation at the ground temperature. However, you need the atmosphere to be transparent to some radiation, such that you can bring energy to the ground to begin with.

So yes, it may be feasible to warm up the surface of Titan via the greenhouse effect. Titan's atmosphere today is somewhat transparent in the near infared and very opaque in the visible; thus I might guess that a lot of the Sun's radiation (which peaks in the UV) is "wasted" heating up the upper atmosphere (which then radiates in the far infared and equilibrates to 100K). If you could get the atmosphere more transparent to UV and visible, while more opaque to infared, you'll get more solar energy delivered to the surface, while keeping the upper atmosphere cool. (Although I can't think of a gas that might pass UV and absorb infared; the usual greenhouse gases tend to absorb both.) This is all just physicist's speculation, though; the greenhouse effect is complicated, and this is all hugely oversimplified - indeed, even on Earth no one fully understands all of the mechanisms at work. But it's clear from the radiation arguments that Titan's greenhouse effect needs to be much better than Earth's, or even Venus', to bring the surface from 100K to 270K.

The Huygens experiment, due to arrive at Saturn in 2003, should teach us a lot about Titan's atmosphere.

Hope this is useful!

-Ben Monreal

(Also, many thanks to Joseph Lazio and Reyco Henning for their helpful comments on this answer.)