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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?

Mathew Mahindaratne
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jvriesem
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  • I wouldn't expect much of a difference. – Ivan Neretin Jan 31 '19 at 19:33
  • As long as somehow all the hydrogen on the world is deuterium, no problem. Deuterium is stable. But if there is protium in the rocks, weathering will mix the isotopes over time. – Jon Custer Jan 31 '19 at 19:38
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    The Oklo phenomenon would have looked different. –  Jan 31 '19 at 20:06
  • @Loong - true, and a number of biological reaction pathways would be a bit different to account for the isotope effects... – Jon Custer Jan 31 '19 at 20:59
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    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. – Nicolau Saker Neto Jan 31 '19 at 21:36
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    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. – Jakub Muda Jan 31 '19 at 22:30
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    $\ce{D2O}$ 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. – Buttonwood Jan 31 '19 at 22:39
  • @JakubMuda There’d be little reason for sea ice and sea water to have different kinds of water though (right?). I’d expect sea ice to come from the ocean, so they’d have similar compositions. – jvriesem Jan 31 '19 at 22:41
  • It would be easier to answer general question like how deuterium would change chemistry if all hydrogen-1 would "magically" transform into deuterium :) – Jakub Muda Jan 31 '19 at 22:49
  • While for example https://chemistry.stackexchange.com/questions/71894/is-it-true-that-heavy-water-is-not-blue looks OK for the site, I think this question is far beyond the scope of the site. – Mithoron Feb 01 '19 at 00:02
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    I think the OP's multiple questions are not actual questions, but examples to help a community member answer the question. – A.K. Feb 01 '19 at 03:25
  • Life would be much more difficult because biochemical synthesis of almost anything involves many sequential steps many of which would be just a little slower due to isotope effects. Although each step would be slower buy only a small amount when multiplied over a sequence of steps the effect will be be significant. – porphyrin Feb 01 '19 at 09:35
  • I doubt that - most biochemical reactions are tuned to specific rates and subject to various regulatory mechanism beyond the catalytic mechanism, therefore while things might slow down due to the higher mass of deuterium, it is likely organisms could find ways to compensate as necessary. – Buck Thorn Feb 01 '19 at 15:29
  • I don't agree, things would be more difficult just because each reaction is that bit slower, natural selection is very nearly optimal, on average, and with D instead of H, synthesis would be slower, not impossible. Side reactions would play a greater part lowering yields. Why would organisms find a better way when they could do so at present? – porphyrin Feb 01 '19 at 16:49
  • @porphyrin nature is what it is because that's what it had to fool around with, not because it had a choice.... if it had to work with deuterium, it might find different solutions to the same problems, you would not regard them as better or worse. You might as well ask what would happen if the mass of the proton were lower, what would organisms look like? (such a world might be impossible due to physics, but not due to chemistry or biology) – Buck Thorn Feb 01 '19 at 17:54
  • yes it 'might find different solutions' but it might not, that is my point, you cannot simply assume that it will work because it is natural selection; this cannot be divorced from chemistry. – porphyrin Feb 02 '19 at 10:15

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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.

Oscar Lanzi
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  • 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. – jvriesem Feb 01 '19 at 18:26
  • Some studies (e.g. https://www.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. – jvriesem Feb 01 '19 at 18:30