Lots of biochemical processes are sensitive to pH, and most (all?) organisms maintain close-to-neutral internal pH, except for specific localized excursions for specific purposes (like producing acids for digestion).

If a world's oceans were consistently acidic, though, presumably primitive life would develop to prefer a different pH set point.

So: what sort of world could have consistently acidic oceans over a billion years or more, and still be compatible with organic biochemistry? (So, no anhydrous sulfuric acid oceans, for example.) It's fine if the equilibrium eventually changes, as long as it's stable long enough for life to become fixed at low pH. Ideal pH would be between 4 and 5, since that's where RNA is most stable.

  • $\begingroup$ By oceans you mean water correct? Because an ocean of an acidic liquid would be a very different dynamic than water that’s 6.9 on the pH scale due to dissolved sulphates. $\endgroup$
    – user71781
    Aug 14, 2022 at 21:13
  • $\begingroup$ @NixonCranium I expect the answer to involve water with stuff dissolved in it, but oceans don't have to be water, if you've got something else that nucleic acids and proteins will work in. $\endgroup$ Aug 14, 2022 at 21:31

1 Answer 1


Add CO2.

Atmospheres with lots of CO2 are not very weird. Venus has one. Mars has one. Earth had one until plants gobbled it all up. When CO2 dissolves in water it makes water acidic.

The pH of CO2-saturated water at temperatures between 308 K and 423 K at pressures up to 15 MPa

ph of co2 saturated water

In these experiments CO2 saturated water had a pH between 3 and 4 except for the hottest temperatures (423K) at lower pressures. You can have the pH be less acidic by not having the water be completely saturated with CO2. Some cooler temperature, less pressure, less CO2 in the atmosphere.

The early earth probably had a more acidic ocean than it does now but around 6. You want it slightly more acidic - so a little more pressure than early earth or a little cooler. Nothing crazy.

  • $\begingroup$ Oh, lovely! I suspected that would work, but I couldn't find the data--which you have! $\endgroup$ Aug 14, 2022 at 22:16

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