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According to Google, the sea is salty because the rainfall dissolves land minerals and salt, which then washes away to the oceans, and over billions of years that process raised the salinity of the oceanic water to the current undrinkable condition.

So is my thinking correct that if this is the case, then the water of a fully oceanic planet (60-90% made out of water) should be almost pure distilled H2O? Since there would be too little other stuff to begin with, let alone any continents that the rainfall could erode (Sadly, this probably means that such a planet can only be lifeless and without breathable atmosphere, since there also would be nothing for the life to evolve from). To add to that, the little stuff there are would probably sink to the bottom and become encased in the ice seafloor, hundreds of miles below the surface.

But I'm not so sure in the case of an otherwise Earth-like planet that "only" has too much water, with no or barely any land or shallows during its entire geological history. The same point about lack of continents to erode stands, but in addition to that it also has a proper seafloor and probably volcanism, so there lies much bigger uncertainty - would just simple lack of continents made water fresh?

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    $\begingroup$ Minerals from the solids under the ocean would still get into the water. They talk about runoff , but any eroding or volcanic process would expose minerals to the water and result in increased salinity. A primitive planet would have less salt. 0.9% saline (the salt concentration of our bodies) is believed to have been the salinity of the oceans when life evolved, and is conserved in life today. But no rock equals no salt equals no salinity. There could still be life at trace concentrations, but it would be different. $\endgroup$ – DWKraus Dec 5 '20 at 2:52
  • $\begingroup$ if i remember correctly most of our oxygen come from phytoplankton so i think your planet can still have oxygen, though i dont know is that require salt or not if thats what you imply. $\endgroup$ – Li Jun Dec 5 '20 at 3:32
  • $\begingroup$ I sorta duplicated this question on Astronomy.SE; I figure they would have some thoughts too. $\endgroup$ – KeizerHarm Dec 5 '20 at 20:41
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It could be, depending on currents.

So fun fact salt distribution isnt uniform. In calm water, salt molecules tend to sink creating a salt gradient. If the ocean is deep enough, all the salt will be biased towards the bottom, leaving only trace amounts at the top. The top few km would indeed be drinkable.

What would ruin this would be currents circulating deep water up high, these could be caused by topography (a gentle sloping surface from deep the shallow) or a heat source on the bottom like a volcanic vent.

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It's your world, but...

The erosion of dry land is not the only way to dissolve minerals into water. Volcanism, geological shifts (e.g. earthquakes), saturation of the ocean bottom, and undersea erosion caused by currents can also dissolve minerals into the water.

Distilled Water

Would the water be distilled or nearly distilled? Absolutely not. That would require no volcanism or geological activity, no silt or saturatable sea bed, and bedrock made of something like diamond that couldn't be trivially (by comparison) eroded or dissolved by water. It's important to remember that water is known as the universal solvent. It can't dissolve everything. But it comes close.

It's not just erosion. It's that the water has nowhere else to go

It's also worth remembering that oceans are salty because there's nowhere else the water can go to distribute the mineral. So-called "freshwater" lakes and rivers aren't devoid of salt. But because they have outlets, there's somewhere for the salt to go. It therefore doesn't build up. Oceans, of course, have only two outlets: evaporation and the process of charging aquifers. The former doesn't take salt or other minerals with the water and the later can't move water away from oceans fast enough to avoid mineral build-up. A world that's mostly ocean is, from this perspective, no different from Earth or a world with only 10% of its surface given over to water.

So, what's your world made of?

If there's salt in the crust, geological events like earthquakes and volcanism will put it into the water. If there's salt in or near the sea bed silt, then it will be stirred up slowly through saturation and more quickly through sea current erosion. Some kind of saturation will occur eventually, it's really just a question of what minerals are provident on your planet.

One last thing...

I wonder how long the landmass on a 90% water world would actually last. Such a world in the Goldilocks zone would have a ton of storms. Storms that would work hard to erode what little land exists and reduce it to, eventually, sea bed. Of course, on our 71% world that doesn't seem to happen because tectonic and volcanic activity builds mountains.

Why doesn't that happen on your world? Low tectonic and volcanic activity would suggest either a whomping thick crust or a cold core. Now you have me wondering if a 90% water world can even exist.

OK, really, just one last thing...

One thing you could do. A world with that much water cover would have more evaporation and, therefore, more rainfall. I don't know if the science would allow it (my gut tells me it probably wouldn't), but suspension of disbelief could be that the rainfall is constant enough to create a freshwater layer on top of the saline layer. It shouldn't be very thick. Let's call it 3-6 meters. But such a world description wouldn't make me raise my eyebrows when I read about it.

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    $\begingroup$ Oh, no, nono, when I was talking about a 90% water planet, I meant 90% of water by mass. Think mini-Neptune. There would be absolutely no land on the planet with oceans hundreds or thousands of kilometers deep. $\endgroup$ – Darth Biomech Dec 5 '20 at 12:27
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    $\begingroup$ OKAY! I misunderstood that in a wonderful way. However, what I've said still applies. The difference is where layers on a mere 90% coverage planet would be suspension-of-disbelief now they become an absolute reality. You'd have layers of salinity at the bottom (this actually happens in our ocean) and layers of fresh at the top. However, you also might have ice at the bottom due to pressure. Ugh. The rules change a lot when you bring that much water into play. Water is only a liquid as specific temps and pressures. $\endgroup$ – JBH Dec 5 '20 at 14:49
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We really don't know enough about the ice-moons in our own system to definitively answer this. It is thought for example, that liquid water could exist beneath the frozen exterior on the Jovian moon Europa. and that the icy crust drifts, floating on top of this watery layer. If true this suggests there is a source of heat -- if only from the extreme tidal forces at work in the Jovian system, and that there is still a rocky interior possibly even with an iron core. But we won't know details about that water until we get the right kind of probes there - salinity, pH, etc..

It's not a huge stretch to imagine something could be alive there. Life here started in the oceans, afterall. It's just a question of metabolism, respiration, food source, how diverse and complex it would be or what kind of adaptations it might have evolved to survive in an environment beneath that ice sheet.

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  • $\begingroup$ Life on Earth started in warm shallow parts of the seas, deeply saturated with all kinds of compounds and minerals washed away from the land. There would be no such conditions on a planet where even the seabed is made out of water, I presume. $\endgroup$ – Darth Biomech Dec 5 '20 at 2:45
  • $\begingroup$ I hesitate to remind that the "primordial soup" theory is just that -- we do not know the origin of life on Earth. $\endgroup$ – Adam Coville Dec 5 '20 at 11:52
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Depends. What impurities are there? Because if it was distilled dihydrogen monoxide, people would have no problem drinking it. And if there were space bacteria living in the space water, you'd probably be okay, as the bacteria wouldn't have evolved for infecting humans. The exception to that rule is if the bacteria produce a hazardous by-product.

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