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While on earth it usually rains water, it usually isn't a sight to behold. To spice things up, I would like the rain to truly remind its colonists that they're on an alien world. However, this exoplanet has an oxygen and nitrogen based atmosphere. Which brings me to the question: What liquid might rain instead/alongside regular water? Note: solids such as snow or hail are valid aswell!

The requirements

  • It should not be (completely) acidic

  • It the gas which condenses should be able to be replenished in the atmosphere by natural processes

  • The gas should be able to condense in realistic, -75° to 50° or anything in between (for example -37° to 18°)

  • Preferably although unnecessary, the liquid or solid should not soluble in water

  • Preferably not rain naturally on earth

If you were able to give the chemical reactions to create the gas, liquid or solid it would be much appreciated as well!

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    $\begingroup$ Offering this as a comment because I don't know enough science to provide it as an answer. Perhaps the reminder comes from characteristics of the sunlight hitting the rain rather than the chemical makeup of the rain itself. What does a rainbow cast by a red-dwarf look like? $\endgroup$
    – Henry Taylor
    Aug 22, 2021 at 18:46
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    $\begingroup$ Do you have to be able to walk outside in the rain? $\endgroup$
    – Cadence
    Aug 22, 2021 at 19:10
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    $\begingroup$ Yes, to some extent. Be it a gas mask or an entire space suit. $\endgroup$
    – Explunky
    Aug 22, 2021 at 19:13

4 Answers 4

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Petrochemical Rain

Suppose some part of the planet in question (possibly an inaccessible region like a high plateau or the poles; whatever your plot dictates) is covered by what amounts to an enormous oil-field. Perhaps millions of years ago, a large and fertile algal sea dried up and was buried by some geological event.

Now, in the (geologically) recent past, something has suddenly uncovered a significant fraction of the oilfields. Could be tectonic activity, an asteroid impact, or anything else that suits your plot. After the really volatile stuff in the crude oil (methane, ethane, propane, butane, hydrogen sulfide, etc.) has evaporated (and perhaps been consumed by microbes or flora), there'd be a period when the slightly-less volatile hydrocarbons would evaporate. Hexane, for instance, fits your temperature requirements pretty well, it doesn't mix with water to any significant degree, and its solvent properties would make it very easy to distinguish from water rain. It's slightly toxic, and it's a nervous-system depressant, which would have to be worked around.

Another possibility, springing from the same root idea of suddenly-exposed petroleum reserves, is a massive oilfield fire in some inaccessible part of the planet in question. This could give you a couple of different types of rain/precipitation. The volatile stuff (anything natural-gas-, gasoline-, or diesel-fuel-equivalent) would boil off and burn pretty early on, but if the fire decreased to a lower intensity after that, it could evaporate analogues to heavy fuel oil, which could be carried by a jet stream or similar until the droplets coalesce sufficiently to rain out. Or, the precipitation in question could simply be "snowflakes" of fine soot which aggregated in the upper atmosphere. In both cases, it'd be very distinctive black precipitation. Tarry oil in the fuel-oil case, and black graphite snow in the soot case.

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Rain of animals

https://en.wikipedia.org/wiki/Rain_of_animals

rain of fish

Fortean rain is sweet. And you do not risk someone scoffing at the fact that you have bromine or siloxanes somehow be compatible with your otherwise earthlike atmosphere. Rains of animals really happen!

Such events are freaky enough on earth. On your alien planet it is extra freaky because it is a rain of alien animals; perhaps from a far ocean or high in the atmosphere. And also it is more fun to write about. After everyone collects siloxane from puddles to comb into their their hair you are kind of done. Alien animals falling everywhere provides much grist for your prose mill.

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There are some trees similar to poplars

Poplar trees have seeds that are covered by a fine hair, that allows them to fly and disperse. In many places, the poplar trees release in the spring a lot of fluff, which really seems like a (slightly ennoying) snowfall (I can confirm)!

In your planet, almost all plants need wind to disperse their seeds (maybe birds never evolved), so these fluff snowfalls are very common.

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A classic writing prompt used in elementary school is "The Day it Rained Lemonade". When I drew that, I set it on an space station like an O'Neal Cylinder where rain was artificial. Any issues with the pipes, or even something like a leak in the infrastructure in the "hub" can rain down to the living deck.

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