Can a perpetually darkening atmosphere exist and yet still allow life to evolve?

Question

I am creating a science fiction worldbuilding project, in which humanity has colonised distant exoplanets. One such planet is a Venus-like planet in the sense that it has a perpetual cloud cover, although on my world it is dense water-clouds instead of toxic sulphur. Although the surface has extreme pressure, temperatures are still Earth-like, and (maybe) suitable for sentient life to evolve (although no life features eyes as there is no light at sea-level.)

Is this situation realistic, and what would the makeup of the atmosphere have to be to avoid the extreme greenhouse effect and yet allow life to evolve?

Answer 1

Venus is the hellish planet it is because it combines a massive greenhouse effect with a closer proximity to the Sun than Earth (sulfuric acid apart).

The very opposite happens on Mars: the lack of a thicker atmosphere and a further distance from the Sun make it a chilly retreat.

However a greenhouse effect can help if the planet is at martian distance to make it warmer.

And life doesn't need sunlight to develop, just a source of energy: we have found life in the depth of the ocean, close to hydrothermal vent, where light is non existent but the vent gives plenty of chemical and thermal energy to the organisms which can use it.

Answer 2

Atmospheric pelagics.

https://environment.bm/sargassum-seaweed

It is dark on the ocean floor. You can live there like Dutch's worms if there is a chemoautotroph to be the basis of your food web.

Or you can live up where it is light, like these seaweeds do. They do not have roots, but float loose in the water column. So too in your world - up where there is still sunlight, there are floating plant equivalents (maybe they generate hydrogen for their floats?) and ecosystems of flying and floating herbivores and predators based around them.

Under this floating ecosystem there would also be life below - scavengers like hagfish and worms, ready to eat whatever fell from the heights into the dark. I could imagine some animals might reside like sea monsters in the lightless depths, ascending to the lighted regions to hunt.

If the premise grabs you and shakes you (in a good way!) there is more to read about it on the WB stack.

Which conditions would make aerial filter feeding successful?

Could life develop above a planet's atmosphere?

Answer 3

Light seems mostly irrelevant to life except as a form of energy. So mostly you just need to provide an alternative source of energy.

• Stuff (living or not) above the clouds soaks up energy and drops it down below.
• Thermal energy from the interior of the planet. As on Earth, it won't run out if there's enough radioactivity to keep heating it up.
• Directly use energy from radioactive decay. Well, probably not directly, but in the form of heat. Though more direct forms are theoretically possible (I think it can produce electricity in certain configurations.)
• The atmosphere itself (clouds etc.) can soak up the solar energy and convert it to heat and motion. That's a complicated way to say "wind energy" or "storm energy".
• Tidal energy. This requires a moon or three to drag liquid around the planet. This is pretty theoretical.
• Chemical energy. I'm not sure how, but the planet is made out of reactive substances that haven't fully reacted over time. The whole crust is a mixture of them. (You could even have that be where the clouds came from. Perhaps it is cyclic -- you start out with clear skies, which triggers one set of reactions that gather lots of energy but produce corresponding amounts of atmospheric moisture, then the light shuts off and those reactions shut down and things run off of the stored energy until the water precipitates out. Maybe water is required to release the stored energy. When you run out of water, the skies are clear again and the cycle repeats.)