also the second sentence in the intro does not seem to be true so the whole st paragraph is misleading as far as i can remember i have seen experiments with a lighter gas on top of a heavier one with the interface behaving as a free surface a free surface is just an interface between two fluids which may be a gas gas liquid liquid or liquid gas interface or in mathematics and probably astrophysics an interface between a fluid and a vacuum one of the core elements on wikipedia is verifiability by reliable sources the nice thing of the article as it is now is that there are quite some footnotes to sources used although unfortunately either unreliable or misinterpreted by the authors of the opening paragraph so accompanying your edits with sufficient inline citations to journals and pages in books will be of great help