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Take a planet like Gliese 1214 B, which has no land, an ocean 100s of kilometres deep and a seabed of Ice VII. For the purposes of the question, let's assume that the pressure and/or temperature near the bottom makes it simply impossible for any form of life to survive. Maybe it does.

Of all the plausible posited models for abiogenesis, excluding panspermia, could any occur in these conditions? If the deep truly is uninhabitable, then abiogenesis will have to happen without any solid seabed, and therefore without hydrothermal vents, clay etc.

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    $\begingroup$ Is black goo from a dissolving white alien a sufficient answer for you? $\endgroup$ – The Nate Feb 15 '19 at 1:06
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    $\begingroup$ @TheNate No, that would be by definition panspermia, which is excluded in the question. $\endgroup$ – worldbuilder Feb 15 '19 at 23:26
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Organic molecules delivered by comets.

https://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_s_comet_contains_ingredients_for_life

Ingredients regarded as crucial for the origin of life on Earth have been discovered at the comet that ESA’s Rosetta spacecraft has been probing for almost two years.

They include the amino acid glycine, which is commonly found in proteins, and phosphorus, a key component of DNA and cell membranes.

Scientists have long debated the important possibility that water and organic molecules were brought by asteroids and comets to the young Earth after it cooled following its formation, providing some of the key building blocks for the emergence of life.

Your water world suffers a constant and generous rain of greasy organic space shmutz. The long chain hydrocarbons all float, entrapping other hydrophobic molecules. Rafts of this waxy filth accumulate on the gentle oceans of your world. These serve as a substrate for tholins and other organics from space to combine and recombine under the gentle radiant proddings of your star. I envision certain molecule might avoid sinking into the deep by virtue of their ability to accumulate their own buoyant collection of hydrocarbons. Just as with earth, selective pressure will favor molecules of this sort which can replicate themselves and their ability to stay at the resource-rich surface. These go on to be your waterworld's life forms.

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    $\begingroup$ Your theory is called panspermia. This is excluded in the question. Definition: Panspermia is the hypothesis that life ... distributed by space dust, meteoroids, asteroids, comets ..." Your answer is the exact definition of panspermia. Question: "excluding panspermia". $\endgroup$ – worldbuilder Feb 15 '19 at 23:19
  • $\begingroup$ Dear question asker @SealBoi: Why did you accept an answer that does not answer your question? You asked excluding panspermia, and this answer specifically describes panspermia. $\endgroup$ – worldbuilder Feb 15 '19 at 23:20
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    $\begingroup$ @worldbuilder - panspermia implies distribution of actual life forms through space, not distribution of space shmutz. From Wikipedia. "Panspermia hypotheses propose (for example) that microscopic life-forms that can survive the effects of space (such as extremophiles) can become trapped in debris ejected into space after collisions between planets and small Solar System bodies that harbor life." $\endgroup$ – Willk Feb 16 '19 at 0:26
  • $\begingroup$ Incorrect. ORGANIC means LIFE You are talking about INORGANIC. Definition: Organic: of, relating to, or derived from living organisms. Carbon is called "organic" but it is NOT "organic" in the same manner, since carbon alone cannot become life. Ever seen a pencil stand up and start walking around? Neither have I. Graphite (pencil) is carbon, a lifeless mineral. $\endgroup$ – worldbuilder Feb 16 '19 at 14:46
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    $\begingroup$ @299NeandertalVariants - an interesting idea. For a terrestrial type planet you might be right. Lucky that this is a water world and water has a very high thermal mass / conductivity. I think the immense oceans will act as a mega heat sink. $\endgroup$ – Willk Feb 17 '19 at 3:11

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