The water on Earth is primarily made of $\ce{H2O}$. I'm wondering about the conditions in a hypothetical $\ce{D2O}$ ocean world.

Would it be stable over geologic timescales (~$\pu{0.1–2 Ga}$)? What would be expected to be different, if anything? Would it affect the chemistry of its atmosphere, or appreciably change the incident radiation from its host star?

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    $\begingroup$ The Oklo phenomenon would have looked different. $\endgroup$ – user7951 Jan 31 '19 at 20:06
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    $\begingroup$ A deuterium-rich world would make some important gasses substantially heavier (e.g. $\ce{D2}$, $\ce{D2O}$, $\ce{CD4}$) which would affect their concentration profile with altitude in the atmosphere ("scale heights") and could have some significant effect over geological timescales, such as significantly less $\ce{D2}$ escaping Earth compared to $\ce{H2}$ and making the atmosphere more reducing. $\endgroup$ – Nicolau Saker Neto Jan 31 '19 at 21:36
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    $\begingroup$ The entire chemistry where hydrogen is involved could be different. Deuterium bond energy and length are different than those in hydrogen-1. It could affect the entire biochemistry. Consider also physical properties like viscosity and... ice made from heavy water SINKS in normal water. $\endgroup$ – Jakub Muda Jan 31 '19 at 22:30
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    $\begingroup$ $\ce{D2O}$ has a higher melting point (3.8 C) than the of $\ce{H2O}$ of 0 C. On chemical side, expect reactions with hydrogen to proceed slowlier (heavy isotope effect) than normal. $\endgroup$ – Buttonwood Jan 31 '19 at 22:39
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    $\begingroup$ I think the OP's multiple questions are not actual questions, but examples to help a community member answer the question. $\endgroup$ – A.K. Feb 1 '19 at 3:25

For one thing, it would be hugely more unlikely to exist. We are more likely to have worlds where there is a little more deuterium in the hydrogen, not 100%.

Most of the hydrogen in our Universe is protium (hydrogen-1) because the vast majority of unfused nuclear particles in stars are protons. Neutrons must either fuse with something or they decay to make protons with a half-life of ten minutes. Deuterium and tritium are formed initially when the protons are fused, but they are just intermediate towards the formation of more stable helium-4 nuclei (alpha particles) and so do not accumulate to large concentrations.

If there were a heavy water world, life would have to adapt to it. Prolonged consumption of pure heavy water is deadly to life on Earth.

  • $\begingroup$ As a planetary scientist, I know well that a D2O water world would be extremely unlikely for the reason you give. What I'm looking for is less how likely it would be and more what would be different. $\endgroup$ – jvriesem Feb 1 '19 at 18:26
  • $\begingroup$ Some studies (e.g. ncbi.nlm.nih.gov/pubmed/10535697) have shown that some life on Earth can adapt to D2O consumption and be fine! It is toxic to other forms of life in varying degrees, however. I've also heard that if humans were to very slowly replace their H2O intake with D2O, the mild toxicity effect would be reduced or perhaps eliminated — but I haven't seen a study that supports this. $\endgroup$ – jvriesem Feb 1 '19 at 18:30

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