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Question is rather self-explanatory. An answer touching upon how/why atmospheres are formed in the first place would be ideal as well. An example of such astral bodies would be The Earth and it's moon. Earth has an atmosphere, while the moon does not. Why is that so?

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2 Answers 2

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The amount and kind of gases a body can trap depends on the object's surface temperature, and its density & radius (which refers to it's gravity).

An object with high gravity and low surface temperature will be able to hold more gases in it's atmosphere. In the case of the Moon, due to its low gravity it could barely trap an atmosphere of Xenon.

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You can check this thanks to this amazing plot: http://astro.unl.edu/naap/atmosphere/animations/gasRetentionPlot.html

But this is not all you need to hold an stable atmosphere like the Earth does. The object will also need some protection against the solar wind. In the Earth we have a magnetic field that prevents the solar wind to reach our atmosphere, but in the case of Mars it has a weak and poor magnetic field to defend itself against the solar wind that rips it's atmosphere.

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Other factors might be outgassing from the interior, heating from collisions, volatiles from comets which form temporary atmospheres. My impression is that it is poorly understood why Titan has an atmosphere. And many aspects of a celestial body seem to interact such as its interior composition, magnetic field, tidal forces, distance from the star and more which causes great variation and makes it complex to explain. –  LocalFluff Aug 27 at 7:21
    
You are right @LocalFluff , that plot gives some important clues but it seems no to be 100% reliable. As you say in the plot Titan is supposed to only be able to hold Xenon (Xe), Carbon dioxide (CO2) and Oxygen (O2) but it's actually full of Nitrogen (N2) and Methane (CH4) which seems impossible in that plot. –  Joan.bdm Aug 27 at 7:45

There is a certain sense in which the moon does have an atmosphere.

Even Mercury has an atmosphere in this sense.

Though the sense is mostly in a "technically there are particles floating around the solid surface", but at densities which would make for very good vacuums on earth.

The more practical/intuitive definition of atmosphere excludes such meager densities. What it takes to acquire one of those is addressed in the other answer.

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