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Venus (0.815 Earth mass) supports a CO2 atmosphere of 93 bars at its surface. Earth's N2/O2 atmosphere has a surface pressure of one bar. What are the factors that limit the mass of atmospheric gases on a planet?

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One would start by the molecular composition of what constituted finally a planet. 1 bar is approximately 1 kilogram per square centimeter. The heavier the gases found on a planet the larger the pressure on the surface given the same size.

Now the reason why Venus has heavier and/or more gases and earth less so, is another story. Pick in the planetary lottery :).Planets do not have the same molecular composition.

Extra heat from the sun with respect to earth and closer approach should play a role in light gases escaping from the atmosphere. In the wikipedia article on Venus there are more explanations offered.

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Large planets tend to be gas giants, where Jupiter is gas all the way to a core made of metallic hydrogen and some rock. The pressures inside there are very extreme. A gas giant up to some 50 times the mass of Jupiter is a brown dwarf, and more massive than that and you start getting fusion.

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@Lawrence-- Are you saying fusion limits the mass of atmospheric gases since then the object would not be a planet, but a star? This answer compels me to extend your concept of metallic hydrogen to include degenerate gases in white dwarfs and neutron stars. –  Gordon Feb 15 '11 at 7:44
    
What limits the density of these electron and neutron gases is the Pauli Exclusion Principle since they are spin one half fermions. More pressure simply fills up all available quantum states (to the Fermi level). For electrons, this can continue to the Chandrasekhar limit before the repulsive pressure is overwhelmed by gravity and the white dwarf becomes a neutron star. –  Gordon Feb 15 '11 at 7:45
    
In the collapse process, the electrons interact with protons to produce neutrons . For a neutron star the Tolman-Oppenheimer-Volkoff limit (vague) leads to black hole production, so, I guess, if one stretches or drops the words atmosphere and planet from the question, the density of a degenerate gas is limited by the Pauli Exclusion Principle, and ultimately, by a singularity :) BTW a neutron star has been found with a thin carbon atmosphere (accreted)--+1 –  Gordon Feb 15 '11 at 7:46
    
""where Jupiter is gas all the way to a core "" depending on temperature profile one cannot exclude some region where supercritical gases are compressed to densities makeing them to practical liquids. –  Georg Feb 15 '11 at 12:13
    
The definition of a planet is somewhat imprecise as it is. Remember the whole kafuffle over Pluto. I suppose if we were to be precise a gas giant is really a very different object than a rocky terrestrial planet. Maybe the point where one gets fusion is the ultimate limit. Of course we are talking about planets, which pretty much excludes neutron stars and white dwarfs. –  Lawrence B. Crowell Feb 15 '11 at 19:41

It mainly has to do the the planets distribution of volatile (gaseous at ambient temperature) substances, and the distribution of them. In the earth most of the CO2 is bound up in carbonate minerals. High temperatures favor silicates over carbonates, and drive off the CO2 as gas. The reverse reaction, CO2 plus silicates is said require liquid water (I don't know the reason), and hence isn't available on dry Venus (I think the hydrogen was lost because Venus lacks a magnetic field to shield it from the solar wind), and the CO2 ends up in the atmosphere. On mars, CO2 is a major component of the atmosphere, but the low temperatures mean it condenses out as frost during the polar winter, so the atmosphere can't contain very much. Also the history of the planet can also have an effect on the amounts of volatiles. If the planet was very hot during its some phase ot its history the volatiles may have been lost at that time. The earth probably lost most of its volatiles during the massive moon forming collision, and the current inventory of volatiles probably came from asteroid and comet impacts since that time.

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