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How would living organisms develop/evolve in a very-low g ocean environment?

The environment would be a small radioactive core surrounded by a small mass of water, less than 100 km in diameter, in the center of the L4 or L5 Lagrangian point between a very large planet and a large moon.

The body of water would be held together by surface tension, the forces of the Lagrangian point, and a thin outer layer of ice.

The water is kept liquid from the heat of the radioactive core.

The moon provides sufficient amounts of radioactive material, water, nitrogen and carbon in a similar fashion to how Enceladus provides the material of Saturn's E Ring, such that the body of water keeps a stable amount of water, the core receives new radioactive material as the old one decays, and life has the materials to develop.

The large planet has a strong magnetic field protecting the body of water.

How could/would solid or liquid multicellular life forms develop in a body of water without substantial gravity?

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    $\begingroup$ I'm not sure, but I suspect the water would ablate off the surface until it had dissipated into space if it was warm enough to stay liquid. It would be in constant flux at best, which isn't great for evolution. You'd at least need a containment for the surface. Anyone know this better? How does new material (especially heavy radioactive material) add here? Enceladus is cold and slow, as is Saturn's rings. Also, that's a lot of radiation - how big is this thing? I have a feeling exchance w the moon is critical here, but can't quite see the pieces. Is there life on the moon? $\endgroup$ – DWKraus Jul 24 '20 at 11:57
  • $\begingroup$ Have you ever read The Integral Trees by Larry Niven? $\endgroup$ – DWKraus Jul 24 '20 at 11:58
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    $\begingroup$ Where is your intended energy input? chemicals from the moon, or light from a star? $\endgroup$ – DWKraus Jul 24 '20 at 12:08
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    $\begingroup$ So what's preventing water from boiling? $\endgroup$ – user6760 Jul 24 '20 at 12:36
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    $\begingroup$ Okay, had a chance to look for a reference. I think the whole thing is doomed without a different heating mechanism. Hot core surrounded by hot air pocket surrounded by ice shell is what we'd get. aps.org/publications/apsnews/200703/…. $\endgroup$ – DWKraus Jul 24 '20 at 14:46
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On one side for a micro organism floating in water, gravity is almost non existent. Being made mostly out water, they simply float in the liquid and are carried around by currents.

On the other hands the lack of gravity will greatly influence the distribution of nutrient: while on Earth any dropping falls to the bottom sooner or later, in microgravity everything, from a rotting corpse to the feces of any other organism would stay in place.

This would mean that transport of nutrients would be greatly limited by only intermolecular diffusion.

As a reference, stagnating waters on Earth are often abiotic or very poorly livable, due to this very issue.

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  • $\begingroup$ I was trying to understand the heat/radiation cycle, so bear with me. I don't know how new radioactives get added to a core, but wouldn't the hot core lead to currents circulating all materials in & out by pressure? I think everything would have hot/radioactive and cold/(sunny?) cycles. $\endgroup$ – DWKraus Jul 24 '20 at 12:06
  • $\begingroup$ @DWKraus with no gravity there will be no buoyancy induced by density difference $\endgroup$ – L.Dutch - Reinstate Monica Jul 24 '20 at 12:21
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    $\begingroup$ But we're not looking at buoyancy, but instead he specified a hot core. Water heats and expands, displacing volume and generating turbulence. cooler material moves in, is heated, and expansion continues. I suspect the outer surface would freeze, slowing material loss,and the ice might be kept on the surface not due to gravity but lower density and pressure. Pressure doesn't need gravity to work. Something is sufficient to keep the matter in place. $\endgroup$ – DWKraus Jul 24 '20 at 12:27
  • $\begingroup$ @DWKraus, hot water moves only because it is less dense than cold water and has a different weight. With no gravity there is no weight difference, ergo the hot water stays there. For the same reason astronauts in the ISS need to sleep in a room with forced ventilation, else the hot air rich in CO2 that they exhale would suffocate them. $\endgroup$ – L.Dutch - Reinstate Monica Jul 24 '20 at 12:41
  • $\begingroup$ Okay, finally had a chance to dig for references. Expansion from the heat likely doesn't go quick enough to give good circulation. (see reference above). Thx. You get a pocket of hot steamy air around your heat source. $\endgroup$ – DWKraus Jul 24 '20 at 14:50
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Life growing from the cracks.

The stuff to build meat will be scarce in these waters. As it is in open ocean, but maybe scarcer than that - at least in open ocean there is the possibility of blown dust, or elements from the sea floor welling up. On this lonely droplet there is none of that.

Perhaps you could have this water in a cloud of space carbon and nitrogen; tholins raining down onto the planet surface. I could imagine they might accumulate on the ice, as might be the case for Saturn's icy moon Enceladus. https://astronomy.com/news/2017/01/gunk-enceladus-surface

enceladuse https://www.nasa.gov/mission_pages/cassini/media/enceladus_cracks.html

When a crack forms in the ice, that material becomes available to the life beneath. It is an upside-down deep smoker. Instead of rich materials ejected from the deep earth and made available to the sea dwellers, rich materials will fall thru the ice cracks and become available.

I could imagine a tangle of life hanging down from these cracks as they open or close. Primary producers work chemistry on the tholins as they thaw. Heterotrophs feed on the primary producers. Spores or planktonic forms are released into the bottomless ocean, to drift until they encounter another lifegiving rift in the surface.

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