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I'd like to explore some details on a so-called water-ball planet (i.e. a planet in the habitable zone of a sun, covered by a 100 km deep ocean). I am particularly interested where life could develop on such a planet. I am afraid that it will be a planet full of de-mineralised water where no life form can survive.

  • Will there be enough salts and other minerals in the ocean for it to be able to host life forms?

  • What kind of atmosphere do we expect? It could be oxidizing like Earth's atmosphere containing oxygen and nitrogen gas, reducing containing methane, or neutral with almost only water vapor ?

  • What is on the ground of the ocean? I can think of rock, some kind of ice, or methane hydrate.

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    $\begingroup$ I'd strongly suggest asking the question you want answered. If you're wanting to prevent people from answering it in certain ways explicitly rule out those possibilities in the question text. In your case you seem to be asking "Where would life exist on a water ball planet? Would there be dead zones on such a planet?" $\endgroup$
    – sphennings
    Dec 18, 2017 at 14:50
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    $\begingroup$ Hi, I threw some heavy editing to narrow the question. In my opinion, the three bullet points are so intertwined that they have to be answered together given the hard-science scope. I removed questions about life developing. That is too broad and should be asked in a follow up question. Good question, I hope we get some good answers! $\endgroup$
    – kingledion
    Dec 18, 2017 at 15:20
  • $\begingroup$ @kingledion Your edits have my explicit approval $\endgroup$
    – Sir Cornflakes
    Dec 18, 2017 at 15:21

2 Answers 2

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As you can read in this paper, a planet only water is not the best place for life.

It turns out that water worlds may be some of the worst places to look for living things. One study presented at the meeting shows how a planet covered in oceans could be starved of phosphorus, a nutrient without which earthly life cannot thrive. Other work concludes that a planet swamped in even deeper water would be geologically dead, lacking any of the planetary processes that nurture life on Earth.

On Earth, rainwater hitting rocks washes phosphorus and other nutrients into the oceans. But without any exposed land, there is no way for phosphorus to enrich water on an aqua planet over time [...] There would be no ocean organisms, such as plankton, to build up oxygen in the planet’s atmosphere, she says — making this type of world a terrible place to find life.

So your water would likely host no life. You may have dissolved salts in the water, sure, but no life for the lack of phosphorus. Lack of life means also no oxidizing atmosphere, as you will have no mechanism to replenish the oxygen involved in oxidation. With no life will come also no methane (not in big amounts, at least) for the bottom of your ocean.

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    $\begingroup$ Isn't Europa a big ball of liquid that is thought to have geologic events and could potentially harbor life? If there were underwater volcanoes, could these supply phosphorus? $\endgroup$
    – Muuski
    Dec 18, 2017 at 19:14
  • $\begingroup$ With some imagination, I can envision life forms without phosphorus (using self-replicating proteins, for instance) though terrestrial life depends heavily on that element (ATP, RNA, DNA). Or we have to look for hydrothermal vents and the bottom of the ocean for life forms. $\endgroup$
    – Sir Cornflakes
    Dec 19, 2017 at 9:15
  • $\begingroup$ Can you elaborate on why a planet covered in water would be geologically dead? It it wasn't and had volcanism, that might be able to solve the phosphorus problem, i guess? $\endgroup$
    – Burki
    Dec 20, 2017 at 14:21
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You are thinking about nutrients and salts being washed off dry land and into the sea. But you could have those nutrients and salts expelled from the crust up into the water. You can have life arise at hydrothermal vents on the sea floor. It turns out there are a lot of those on our own planet.

https://www.sciencenewsforstudents.org/article/seafloor-hosts-surprising-number-deep-sea-vents lots of deep sea vents

People think that something like this could give rise to life on Jupiter's ocean moon Europa. http://www.divediscover.whoi.edu/ventcd/vent_discovery/index_sr_future_et.html

It could work on your watery world too!

Other idea: Pumice raft! https://en.wikipedia.org/wiki/Pumice_raft

Pumice raft

These are floating mineral islands spewed up from undersea volcanoes. They can be big. This Home Reef is a related floating island phenomenon and it looks plenty big. If your world has undersea volcanoes, in addition to powering hydrothermal vents as above they could create floating pumice islands to serve as a substrate for life in the sun.

Would the islands last long enough? If you had them continually replenished they might. Would there be enough of them? You can make up as much of them as you want!

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    $\begingroup$ I'm not sure that would work here - OP stated that the oceans are 100km deep. Also, something's weird about that source - 3 to 20 km is 1.8 to 12.4 miles. $\endgroup$
    – Rob Watts
    Dec 20, 2017 at 18:19
  • $\begingroup$ @Rob Watts - I agree math is wonky. But why would 100km preclude vent life? Too far down for vents? $\endgroup$
    – Willk
    Dec 20, 2017 at 20:47
  • $\begingroup$ There is still oxygen around the thermal vents, an ecosystem with tube worms and shrimps yet depends on photosynthesis. $\endgroup$
    – Sir Cornflakes
    Dec 21, 2017 at 17:31
  • $\begingroup$ @jknappen: it does not depend on photosynthesis! It depends on chemosynthesis. Read up. It is super cool. $\endgroup$
    – Willk
    Dec 22, 2017 at 13:01
  • $\begingroup$ @Will: Well, it does chemosynthesis (gaining energy to fixate CO2 by oxidating sulfide), but the chemosynthesis depends on oxygen produced by photosynthesis in a remote place. $\endgroup$
    – Sir Cornflakes
    Dec 28, 2017 at 17:25

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