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I have actually two different questions that may result in the same answer.

The first one : if the Earth would have less gravity, would it result in less air density?

The second one : On a different planet, let's say on Uranus which is bigger than Earth, the gravity is less than 1g but does that automatically mean that the density of the air is less? Or in some circumstances it can behave the opposite?

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  • $\begingroup$ 1st: yes; 2nd: it depends on the chemical composition of the planet's atmosphere. $\endgroup$ – stafusa Oct 3 '17 at 16:45
  • $\begingroup$ Actually, Uranus has a much larger mass than Earth. Almost 15x as large. $\endgroup$ – Samuel Weir Oct 3 '17 at 16:48
  • $\begingroup$ Samuel Weir you were right, I edited my second question. $\endgroup$ – Citizen602214085 Oct 3 '17 at 16:51
  • $\begingroup$ I think that the density of the atmosphere at a planet's surface depends on more than just the gravitational force at the planet's surface. As statusa pointed out, another factor is the chemical composition of the atmosphere. Another factor would seem to be the overall temperature profile of the atmosphere (which would depend on how close or far the planet is from the sun). May also depend on the composition of the planet itself and its history of outgassing. Someone who is knowledgable about planetary science would be better qualified than me to comment on all that. $\endgroup$ – Samuel Weir Oct 3 '17 at 17:24
  • $\begingroup$ It is better if you ask multiple questions as different questions, and refer eachother in links. Contrary the common forums, mailing lists, here it is not a problem if you ask multiple questions at once, but you have to formulate them to be comprehensible also alone. | Formulate both questions to be a round one. My experience is that the best question size is roughly 10-15 lines. The questions will be much better received if you show your own research (and maybe also refer them in links), and explain in them what is not clear. $\endgroup$ – peterh Oct 3 '17 at 17:39
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Yes. Less gravity implies less pressure because the gravity "pulls" the air to the surface of the Earth. And less pressure implies more volume. Ergo, the density of the gases on the atmosphere are less with the new atmosphere pressure.

And your second question, the answer is no. It depends of chemical composition (as someone commented above). What we call air, here in the Earth is not the same that the Uranus people call air.

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Uranus is bigger than Earth but its surface gravity is less. Uranus is made up of gases and is not solid like Earth. This makes Uranus very light for its size so if your talking about at the surface then the density would be less (apples for apples atmospheres). Yes to your first question.

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  • $\begingroup$ -"this makes uranus very light for its size". $\endgroup$ – user95137 Oct 3 '17 at 23:00
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Does less gravity means less air density?

Yes, but only if all other things are equal (same atmospheric mass, same atmospheric composition, and same atmospheric surface temperature). However, all other things are never equal.

Venus, for example, has slightly less mass and a slightly smaller surface gravity than does the Earth (81% Earth masses, 90% Earth surface gravity). Venus's atmosphere, on the other hand, has a surface pressure and density that are much greater than those of the Earth: The surface pressure and surface density of Venus's atmosphere are 90 and 55 times that of Earth's atmosphere, respectively.

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