# Is the air mass density in the International Space Station (ISS) higher than that on the earth at sea level?

I have read that the atmosphere in the ISS is pressurized to a standard sea-level atmosphere and also of the same composition (ratio of nitrogen and oxygen etc.). What I am struggling to find online is whether the air mass density in the ISS is higher than that on the ground. I think the air density up there is higher because here on the surface we have a large altitude of air in the atmosphere sitting and pressuring on us, where up there in the ISS it virtually is zero gravity, let alone having an atmosphere as thick as that on the surface. So my logic is that the pressure in the ISS has to fully rely on the momentum of free-moving air particles without gravity i.e. temperature and density of air in order to achieve an atmospheric pressure comparable to that on the surface, is that true?

• If held at the same temperature, pressure, and composition, the number density of a gas will be the same, regardless of where you are in the gravity well. Jun 13, 2019 at 17:23
• @JonCuster Does it mean the air can be treated as an ideal gas?
– Y.JQ
Jun 13, 2019 at 17:35
• No. But, the equation of state of a gas will depend only on temperature, pressure, and composition. An ideal gas just has a nice simple description. A van der Waals gas not so much... Jun 13, 2019 at 17:36
• @JonCuster Actually I had this original question in the beginning: If a rigid tank is sealed in the sea just below sea level, and another sealed in the Challenger Deep, is the water pressure inside the two tanks equal? Provided temperature is equal and water is virtually incompressible.
– Y.JQ
Jun 13, 2019 at 17:44
• Yet, water is compressible. But, even beyond that, the pressure (and therefore number density) at the bottom of the Challenger Deep is higher than at the surface. If you brought the tank up from the bottom of the sea, it had better be designed to handle the pressure difference. Jun 13, 2019 at 17:47