Also the second sentence in the intro does not seem to be true (so the whole 1st 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.