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Forget space whales, forget star-trek-style creatures like the crystal entity, forget space creatures so massive that they have their gravity to hold on to an atmosphere around them. The issue with all of those living things is how they started to evolve in the first place. (My question is in the last paragraph)

Instead, go to Europa. Europa, a moon of Jupiter, is a moon with an ocean under a thick ice crust. Tidal forces between it and the other moons often break the ice, spewing out water with its organic content. Evaporating water deposits organic matter, be it of geochemical or biological origin, giving it the tell-tale orange stripes along the ice cracks. Now, what if some of the life carried to the surface manages to survive in the vacuum of space, evolving to live its entire life scouring the organic matter left by the evaporating water instead of floating aimlessly in space?

Where that life may come from

Life under the ice crust, in the liquid ocean underneath, is relatively in a safe place. However, the temperature and pressure right at the ice crust's bottom edge correspond to the water phase diagram's solid/liquid boundary. So organisms holding on to the ice "ceiling" must evolve to use anti-freeze compounds to prevent freezing over. Some of those organisms may be unfortunate to be brought up to the surface when a crack forms in the ice crust above. The imminent danger is the water in their cells boiling before cooling down, the second danger is lack of oxygen, and the third is radiation. No water, no oxygen, and no protection from radiation are why we don't see those coveted "space whales" that would turn any zoo into a multi-million dollar business.

How this life may evolve

First, I would say that this life would stay on the outer surface of the ice crust. Any organism ejected at escape velocity and flung out into space is likely to die.

FOOD - The surface would provide food in the form of minerals and organic matter deposited by the water evaporating through the cracks.

OXYGEN - To have sufficient oxygen, it is possible to assume that the first organisms surviving on the surface are photosynthetic and are capable of scrubbing water molecules from the ice surface, splitting the water molecules, and extracting oxygen. Europa orbits beyond the frost line, so the organisms do not need to worry about their water evaporating (unlike the hotter water that spawns out through the Geysers), so they must use an antifreeze that keeps the water in a liquid or gelatinous phase which still allows chemical processes to take place.

RADIATION - Surface radiation on Europa is too high for humans, but it is possible to assume that elsewhere a Europa-like moon or its parent planet has a stronger magnetic field offering some protection. Tardigrades, for instance, are unique in having the Dsup protein which offers substantial protection from radiation, a solution that may work on the hostile surface of Europa.

The question: KEEPING WATER LIQUID IN A VACUUM - Here comes the hard part. At first, I thought about Glycerol with a vapor pressure of just 0.4 Pa, but its freezing point is too high, being 17.8 °C (64.0 °F). I did not find if it forms a eutectic solution with water (the mix having a freezing point lower than that of either water or Glycerol) at any given rate. So I must find an anti-freeze that keeps liquid water stable in a vacuum and at the cold temperature on the surface of Europa. Is there such antifreeze? Since life begins as single-celled and multicellular organisms come later, pressure containment via strong membranes is not the solution here. You may suggest an anti-freeze that works at warmer temperatures closer to the frost line without crossing it.

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    $\begingroup$ No liquid can exist in vacuum: liquids and vacuum are incompatible. (Because any liquid would boil, as its vapor pressure is above the zero pressure of the vacuum. What happens in reality is that as the outer layers boil off, the rest of the liquid cools down and freezes.) If you want to keep any liquid liquid, you must keep it enclosed in a container. $\endgroup$
    – AlexP
    Jan 14 at 20:32
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    $\begingroup$ Side note: Although, at first glance, surface tension might be an answer to this issue, it isn't. Surface tension is an interaction between the liquid and the atmosphere, and doesn't happen in a vacuum. $\endgroup$ Jan 14 at 21:29
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    $\begingroup$ Surface tension is a product of the geometry of the surface and the attraction of the molecules of liquid for each other, and exists as long as there's a surface, vacuum or not. Also, numerous liquids have low enough vapor pressures to exist in vacuum without boiling. They evaporate, but slowly from their surfaces. $\endgroup$ Jan 14 at 21:57
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    $\begingroup$ This is a very messy question, and I am reading this over and over, but finding there is so much information that does not help me think about and answer the question. I mean... take the "Radiation" paragraph for instance... what does that have to do with the actual query, how does it help me answer the query? Nothing, and not at all, as far as I can tell. Here is my advice to you, on how to formulate a questions... (continued) $\endgroup$
    – MichaelK
    Jan 14 at 22:32
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    $\begingroup$ (continuted) A post should consist of three parts: 1) Background 2) Problem 3) Query. Background is the minimum information needed to understand the problem, and why the problem is a hiderance. Problem is the thing that is stopping you, and makes you unable to proceed in your world-building. And Query is the actual question that, if it is answered, will clear away the problem. Try shaping your question as such, and you are more likely to get a valuable answer. Good luck! :) $\endgroup$
    – MichaelK
    Jan 14 at 22:34

2 Answers 2

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KEEPING WATER LIQUID IN A VACUUM

It has an impermeable, insulated outer layer and generates its own heat. Pretty much what a spaceship would do except its energy source is renewable from the environment not a finite resource stored.

Both of these could have reasonably evolved in its original environment.

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I am going to answer the question in the title...

I wonder whether it would be possible to make life in very low G made of very open molecules, and no water.

Life on Earth is probably descended from black smokers on the sea-bed. The equivalent in space might be the outgassing jets from comets as they warm up. Or something similar from planets too small to retain an atmosphere. The green tail of some comets is the C2 radical. This would readily attach itself to other organic molecules.

Water-based electrostrictive gels contract at a certain pH or with an electrical stimulus. They do this by expelling water when the structure becomes neutral. I see no reason why we should not make the equivalent with an open molecule without water than ca pack tightly when neutral, or open out when it has charges that repel each other.

Aromatic compounds can conduct in the electron pi-systems. Graphite conducts. It would be possible to make conductors entirely out of carbon. We could have molecular-level nerve signals.

The life form might look like a piece of aerogel or a dust-bunny. It might be less vulnerable to radiation damage as it has much less matter to it. It could be quite tough: carbon aerogel has been proposed for laser-driven solar sails.

This is all very speculative, and very unlike any life we know about. I wonder whether any complicated life-form could evolve amid the constant risk of getting blown away by the smoker it was feeding off, or just floating off into space at random. If it lived on a comet, there would be long periods when it was in deep space, and the smokers were dormant. These dust bunnies would have to hibernate.

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