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So, in my work, there is this terraformed Venus in the future.

Venus is a sunny, warm tropical world with oceans, islands, and small continents, and is basically the Caribbean/Hawaii/Polynesia of the future.

The thing is, while Venus is very similar in size to Earth (similar gravity, after terraforming, similar day-night cycle and atmosphere), it is a lot closer to the sun and it receives a lot more sunlight and, well, radiation.

One of my solutions was having the ozone layer being a lot denser or thicker than Earths, so as to absorb much of the UV radiation before it comes down to the humans living below.

However, one thing I like to do is to give many planets unique traits. (Terraformed) Mars has an orbital ring similar to Star War's Kuat Drive Yards, for example.

For Venus, I envisioned something I called the "Aeroalligant Layer." Basically, below the ozone layer, Venus has a large ecosystem of photosynthetic, unicellular algae. These algae, called aeroalligants, are largely transparent, with internal, prismatic-shaped crystalline structures designed to break dow the light spectrum from the sun. The Aeroalligant layer feeds off the UV frequencies of broken down light, CO2, and atmospheric water vapor, releasing oxygen into the atmosphere.

After the planet's thick ozone layer and the aeroalligant layer bellow absorbs much of the intense solar radiation directed to the planet, Venus' surface is normally bathed with a similar amount of UV radiation and overall light as Earth.

Those two layers would also be responsive to Venus' characteristic sky. Because of the denser ozone layer, it is of a more intense blue than Earth's, and the prismatic action of the trillions of aeroalligants would give it a crystalline sparkle, as if the sky was full of countless, shining diamonds.

Well, the thing is: I like to imagine and then try to explain it afterwards. And I truly don't know how to explain these crystalline airborne algae. Since UV light is invisible to the naked eye, can transparent (to the naked eye) structures capture UV light? How would that work? Could a layer of those organisms exist as a natural UV blocking mechanism for a planet? It doesn't need to be completely hard science, but I would like it to at least seem plausible.

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    $\begingroup$ Minor point, but if the "aeroalligant" layer and ozone layer absorbed the excess solar radiation, Venus would still heat up. If anything on a planet absorbs solar radiation, it is eventually released as heat. A thicker ozone layer might absorb dangerous radiation, as it does for us on Earth, but it would probably work against reducing the temperature of the planet, since it would both convert that radiation to heat and insulate the atmosphere against the heat radiating back into space. You'd need to increase Venus' albedo for this to work. $\endgroup$
    – jdunlop
    Dec 6, 2023 at 2:22
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    $\begingroup$ Also a minor point that just now jumped out at me: "similar day night cycle"? Speeding Venus' rotation up to have a normal day would take a lot more energy than merely terraforming it (and would take hundreds or thousands of years to do non-destructively). $\endgroup$
    – jdunlop
    Dec 6, 2023 at 2:23

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The whole reason an ozone layer is important is that UV light would otherwise shred biological structures. If what gets through the ozone layer doesn't sterilize those "aeroalligants", they probably aren't needed to block it, if they could even do an effective job of doing so.

Ozone itself is also extremely reactive and disruptive to biological systems. Enough ozone to actually color the atmosphere visibly, in a layer adjacent to those living organisms, would be counterproductive to them remaining alive.

If you want exotic aeroplankton, fine, but it doesn't make sense to link it to ozone and UV in that way. You shouldn't need to do anything so exotic to deal with the UV anyway, there's only about twice as much of it as what Earth receives. It's way, way down the list of problems with terraforming Venus.

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No, because...

Before getting into the main question, it's necessary to look at a couple of the assumptions listed in it.

The first of these assumptions is the statement that terraformed Venus is "a sunny, warm tropical world". Due to varying levels of incident radiation at the equator relative to the poles, it is not possible for a science-based inhabitable world in orbit around a star to have a single biome. Even if you adjusted Venus' axial tilt from its current 2.64 degrees to 90 degrees (making everything "tropical" by definition) there would not be a single biome. There simply are no "jungle planets", "desert planets", "forest moons" etc.

The second problematic assumption is that having prismatic algae will "break down" UV light. A prism will split light into a spectrum of its component frequencies, but it does not eliminate it. It is obviously possible to absorb UV light with substances that are transparent to the visual spectrum, otherwise we could not buy UV-protective sunglasses/windows or apply non-opaque sunscreen, but that is not the same thing.

With that out of the way, can you have a floating layer of algae suspended in the atmosphere? It may be possible, but like all questions about flying lifeforms it is hard to keep them floating at the right level indefinitely. It becomes even harder if you want there to be so many of them that they change the very appearance of the sky. No organism can live on nothing but a human-breathable atmosphere, as has been tragically proved by some breatharians. The algae would need other nutrients, yet suspending enough nutrients in the sky to nourish the algae would require the atmosphere to be a dense soup that would eliminate the blue sky you are trying to create. (It would also require massive winds scouring the surface to throw the nutrients into the sky, which is a twofold problem in that it both makes the surface less inhabitable and the winds would make it impossible for the algae to maintain a coherent layer in the atmosphere.)

If you really want optically peculiar organics in the upper atmosphere, the best I can suggest is a weird plant that originates on the ground (or surface of the ocean), then releases some kind of lighter-than-air spore(?) with the required optical properties that floats up into the air and stays there for a long time absorbing UV before releasing its seeds to be scattered across the surface. My gut feel without running the numbers is that the quantity required would require that the majority of the surface of Venus would need to be covered with the spore-creating plants in order for there to be enough spores in the atmosphere. I also cannot come up with a good rationale for why they need UV to reproduce. However, it is more plausible than a self-sustaining layer of algae that solely exists in the upper atmosphere.

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  • $\begingroup$ By "sunny and tropical" I mean more than Earth. There is still variation of biomes, deserts and all, but Venus' poles would be warmer than Earth's (if still frozen), and the temperate zone would still be mediterranean-like to equatorial-like. By "breaking down" the light, I mean that the structures that absorb UV light would also break the visible spectrum in a prismatic way. On your other points. I understand it. It seems I didn't think all that too well. Thank you for the comment. It was really helpful. $\endgroup$
    – JJ2161
    Dec 6, 2023 at 3:47
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This won't work for one very simple reason (and a bunch of others). The deal breaker is that UV is inimical to life in your scenario. So you cannot use a lifeform to block it.

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