How do you explain that the atmosphere composition is so uniform?
convection, not diffusion, is dominant in the atmosphere, It is a well mixed system. Also, chemical energy considerations will predominate over gravitational energy difference effects in the absence of convection. Otherwise the entire world, every planetary body, would be expected to be segregated according to atomic density, and it isn’t even close to that. There is definitely a molecular density segregation (gases tend to be at the surface and solids down below) but it is secondary to chemical segregation and only manifests when unit density differences are significant and mixing (convection) is slow (core gets denser over time and crust less dense would be an example of that sort of long term molecular segregation). Actually not a bad question and leads to some interesting thoughts.
