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The current mean temperature in Titan is −180 ºC (93 K), the same as in Saturn.

But I was wondering what would be the case if the atmosphere of Titan would be like the Venus atmosphere, whose composition is mainly CO2 and has a pressure of 90 bar, instead of approximately 1.5 bar.

What temperature could Titan reach in that case? Would the greenhouse effect allow the moon to be heated enough to maintain liquid water or is it too far from the Sun to be affected by any kind of greenhouse effect? Maybe the latest question allows to ask another one: How far from the Sun could Venus be so it can have temperatures above 0 ºC?

The question may be answered given two possible alternatives:

  • Same composition as Venus, but same pressure as in Titan.
  • Same composition and pressure as in Venus.
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  • $\begingroup$ I find this subject interesting, but you're asking more than one question and that's discouraged. You should probably separate your Titan question and your Venus question into two questions. $\endgroup$
    – userLTK
    Commented Jul 19, 2019 at 22:19
  • $\begingroup$ At minus 179 degrees Celsius, most of the components of Venus' atmosphere would fall out as snow. $\endgroup$
    – Wayfaring Stranger
    Commented Jul 21, 2019 at 0:46

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If the pressure were the same as the current pressure on Titan, the planet would be slightly warmer for a while, but not for long. The CO2 would soon be frozen out. It is probable that 4 billion years ago the atmospheres of Venus,Earth,Masrs and Titan were much more similar. A hotter Saturn may have warmed Titan in those days. It probably,like the others, had a lot of CO2 in its atmosphere,but that has long ago been frozen out. There is plenty of frozen water on Titan, and if gelogists ever dig down far enough,they are likely to find a layer of frozen CO2 there as well.

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One thing to keep in mind is that planet temperature doesn't follow a neat equation when an atmosphere is involved. Without an atmosphere, it's pretty straight forward and there's even an equation for it. See here, or here.

enter image description here

Note, the first link above has an error in it. It says Venus' clouds block 10% of the sun. I think it's closer to 80% of the sun, but apart from that error it's a nice article.

Titan's effective temperature, no atmosphere, albedo of 0.22 works out to about 84 kelvin, 11 degrees less than it's surface temperature. Earth's atmosphere, by comparison traps about 31 degrees and Venus' atmosphere, about 500 degrees.

Titan, being an icy-moon, likely has more CO2 than Venus. It certainly has much more water and much more methane than Venus, but only it's Methane is currently able to exist in the gaseous state. Even so, with about 5% methane in it's atmosphere, Titan has a strong greenhouse gas atmosphere already. Considerably stronger than Earths. The reason it traps less heat (about 11 degrees to 31 is because there's less heat to trap).

Atmospheric equilibrium temperatures get hugely complicated and way beyond my paygrade.

Take Earth, for example. If you move Earth 1/2% further from the Sun, so it gets about a 1% drop in solar energy due to the inerse square law, and using the effective temperature formula,the drop in temperature would be the 4th root of that, or about 0.25%. But we know studying Earth's history that ice ages can be triggered by roughly the equivalent of a 1% drop in energy, leading to as much as 4 degrees globally, about 1.25% of Earth's actual temperature and that's because of various feedback mechanisms like greater oceanic absorption of CO2 and greater ice cover, lowering Earth's albedo.

The transition of gases from ice in the upper atmosphere to liquid would affect the equilibrium temperature. It's no longer a neat equation, but a very complicated one.

Titan gets very little heat from the sun. At it's distance, it gets about 1/90th the heat that Earth receives per square meter, so if it did have a thick enough greenhouse gas atmosphere that trapped heat very effectively, it would still take a very long time for Titan to heat up.

Not doing any math, I'd say offhand that Titan would need a Venus pressure atmosphere to begin holding significantly more heat, because it has a pretty strong greenhouse gas atmosphere already, being 5.6% methane. You wouldn't need to bring CO2 to Titan though, you could simply heat up Titan's surface and initially some CH4 would boil off and if you got the atmosphere hot enough, some CO2 would begin to bubble up from it's frozen ground.

But I'm not sure how effectively, even a very dense greenhouse gas atmosphere would keep Titan warm, because it only receives 1/90th the sunlight that Earth receives per square meter, and that's a very small amount of heat going in. I'm not sure where the warm point would be, but it would take a considerable amount of greenhouse atmosphere.

Maybe there's someone here who can do a calculation about surface temperature equilibrium under a thick atmosphere. Mine is just a partial answer.

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