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I am trying to create a planet that rains acid, and that has acidic oceans of water, but a breathable nitrogen-oxygen atmosphere. This planet has native lifeforms that have evolved to withstand these acid rains and that have perfectly adapted to the acidic oceans.

Maybe the acid rain could be produced by volcanism, or the life on this planet somehow creates the acids in the form of waste or by expelling them. What type of acid could we be seeing here?

The air doesn't have to be fully breathable, maybe we could use filter masks to protect from harmful acid gas.

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  • $\begingroup$ How acidic do you intend to have your oceans? Because that alone is going to cause some major issues for how much oxygen there is cycling throughout your planet. $\endgroup$
    – FIRES_ICE
    Dec 22, 2022 at 0:58
  • $\begingroup$ Huh, this might be a long-shot question. What we know about rain formation means it can't be systemic to the rain cycle or the atmosphere wouldn't be breathable. What could exist is an atmospheric layer that doesn't mix well with the breathable atmosphere below, but for some reason the rain condenses above that layer, thus the rain becomes acidic as it passes through the layer. I don't know if it's possible (given wind, etc) to have layers like that on a planet as small as (e.g.) Earth. Jupiter, yes, but probably not earth. Good question! Is a believable answer adequate? $\endgroup$
    – JBH
    Dec 22, 2022 at 2:40
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    $\begingroup$ I'm tempted to answer "Earth." Our rain has a PH of around 5.6, and the increase in CO2 in our atmosphere is turning our oceans into carbonic acid. How acidic does it need to be for the story to work? noaa.gov/education/resource-collections/ocean-coasts/… $\endgroup$
    – Robert Rapplean
    Dec 22, 2022 at 3:58
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    $\begingroup$ Welcome to Worldbuilding! As it stands, your question is generating Votes to Close as it asks multiple questions and lacks some focus. I'd suggest editing it to focus on the question 'what kind of acid could this be?', and leave things like 'how could it be produced?' and 'how could we protect humans?' to further discussions. $\endgroup$
    – K. Morgan
    Dec 22, 2022 at 12:08
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    $\begingroup$ Realistic options for acidic rain is a legitimate worldbuilding question. So legit as to be almost banal. It is focused too: acid rain, breathable atmosphere. Vote to reopen. $\endgroup$
    – Willk
    Dec 23, 2022 at 20:27

2 Answers 2

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Short answer: Volcanic eruptions or nitric acid breathing by animals/funghi might be the best way to go.

I'm assuming you mean a pH of 0-2, this is would be really acidic. For this, a strong acid gas is needed. It must be a gas to cause acid rain and it must be created in the atmosphere/on land, not in the oceans because there it would react immediately with the water, leading to acidic oceans but not to acid rain. In the atmosphere the acidic gas would react with water and cause acidic rain. There are 3 common (and a lot less common) acid gasses.

Sulphuric acid (H2SO4) is quite common in volcanic eruptions leading to acid rain here on earth from time to time. Sulfur doesn't really play a major role in land biology (quantitatively), so there is no good reason why it would be mass-produced by land organisms.

Nitric acid (HNO3) contains nitrogen. Nitrogen is way too valuable for any primary producer (plants, some bacteria) to be emitted into the atmosphere, but heterotrophic organisms (animals,funghi) need to get rid of nitrogen. Animals usually do this by excreting it in a solution (e.g. urine), but I guess it would not be completely implausible to get rid of it by breathing out HNO3 just like they get rid of carbon with breathing out CO2.

Hydrochloric acid (HCl) contains chlorine. This is generally abundant in the ocean, but it requires a lot of enery to create HCl from it. I could imagine some plants at the coast (some sort of mangroves) that use some sort of biogenic electrolysis to get minerals from the seawater and create HCl in the process as waste-product. Although this would hardly be enough to create enough of it to alter the atmosphere significantly.

All of this gases are high-energy compounds, so organisms need a good reason to use energy on it, e.g. because they need to get rid of a waste-product, just like oxygen. The most plausible is probably the sulfuric acid by volcanic eruptions. The least implausible biogenic explanation is probably the breathing of nitric acids by heterotrophs. While the volcanic eruptions are a relatively solid explanation, the nitric acid breathing is speculative.

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Sulfuric acid in rain has been severe enough to damage marble building facades and artworks exposed to the weather, when coal burning was widespread and sulfur reduction wasn't yet available in the 19th and 20th centuries.

All that would be needed for this to be a "natural" problem is for lightning to ignite exposed coal seams (as is believed to be the case with the burning coal seam in North Dakota, which was already alight when the first settlers reached the region and was still burning as recently as a few years ago) over a significant area; soft coal, especially, tends to be high in sulfur and the combustion products will react with moisture in the air to produce sulfuric acid.

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