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I want my hypothetical planet to have a sort of ocean to filter out radiation, but density is problematic. There would have to be an outer atmosphere, and then some form of ocean-like liquid that is more dense than that outer atmosphere, but less dense than the inner atmosphere, so that the ocean stays within the ‘walls’ of the two atmospheres. So, is it plausible that a liquid could be less dense than a gas atmosphere?

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    $\begingroup$ There are no liquids that are less dense than any gas; that follows from the very definition of gas. You might be able to use fuzzy physics and get two liquid "atmospheres", one slightly denser but not at all viscous that one can breathe and move through with only a little resistance; and a slightly less dense but more viscous one on top that has more resistance and feels more like a liquid - but I don't have the physics knowledge to tell if that would be in any way more feasible. $\endgroup$
    – KeizerHarm
    Apr 5 at 22:44
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    $\begingroup$ You accepted an answer 15 minutes after posting your question. Are you sure about that? $\endgroup$
    – rek
    Apr 5 at 23:24
  • $\begingroup$ In theory, a substance with a low density and high melting point could be denser than a low-melting, high-density gas. In practice, this seems impractical. Radon gas under kerosene comes closest from this page, but not within the right order of magnitude: examples.yourdictionary.com/examples-of-density.html See also "what's the densest fairly-stable gas" here: chemistry.stackexchange.com/questions/16439/… $\endgroup$
    – Anon
    Apr 7 at 3:06
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Really technically speaking, there can be liquids that are less dense than gasses. We know this because we (by we, I mean scientists - I'm shamelessly taking credit as a human being for other people's achievements) have created such liquids.

With careful study of their drop images, Tarruell and colleagues could estimate the density of their liquids. Their result, about 10^14 atoms per cubic centimeter, is five orders of magnitude below the density of an ideal gas at room temperature and pressure.

But obviously this is under really strict laboratory conditions, and at quantum level.

So unless your planet is an artificially created one by a super advanced alien race (and one that is being maintained, too, I guess), under natural circumstances, I would have to say this would be a no.

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