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Titan (moon of Saturn) is unique in that it possesses a very thick atmosphere. However, Titan is certainly is not the largest of the moons - Ganymede being larger.

What is the current accepted theory as to why Titan has retained its thick atmosphere?

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It's not really known why only Titan has a thick atmosphere while moons like Ganymede don't.

One thought is that temperatures may have been too high (well above ~40K) in the Jovian subnebula of our solar system due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the $NH_{3}$-hydrate inventory accreted by both Callisto and Ganymede. The resulting $N_{2}$ atmospheres might have been too thin to survive the atmospheric erosion effects that Titan has withstood.

EDIT: some links: (though it's from 2002). (this paper critiques the previous idea of a $CH_{4}$ supported atmosphere)

I would also think the NASA/JPL Cassini pages would have any up-to-date info in regards to Titan.

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Very nice answer - do you have any links to add to your answer? – user8 Dec 9 '13 at 22:59
Thanks! I've went ahead and edited in some links. – Lame-Ov2.0 Dec 9 '13 at 23:37

It has retained it's atmosphere because there is enough water (or ice) to keep it still. That's also how Earth can retain it's atmosphere. If it is a planet or moon like Venus, then the clouds and the greenhouse effect would block the atmosphere, and therefore retain the atmosphere.

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I fail to see how that makes sense. I don't believe water is relevant. Venus doesn't have much water, but it retains a very think atmosphere. Titan has water, but it's cold enough that it's essentially a mineral. And the greenhouse effect doesn't "block the atmosphere"; it blocks outward infrared radiation, thus making the atmosphere hotter. I'd think that Venus loses more of its atmosphere to space than it would if it were colder. – Keith Thompson Oct 27 '14 at 18:47

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