If water flowing over continents in rivers is what concentrates salt in our ocean, would a planet that has always been covered in water just be freshwater? The water is just sitting there, not eroding through salts.
If water flowing over continents in rivers is what concentrates salt in our ocean, would a planet that has always been covered in water just be freshwater? The water is just sitting there, not eroding through salts.
It depends on the composition of the planet. If it is just a massive ball of water floating in space then it will be whatever purity that is, plus whatever space dust and impactors bring in.
If it is basically a terrestrial planet with water on top, say earth plus a lot of water, then it would be salty. The thing with salt water is contact between the water and rock. If there is sufficient heat it will circulate, so salty water from the bottom of the ocean may be heated by magma or similar and then it will be less dense, floating upwards to the surface. Along the way it will mix and cool, leading to dispersal of the dissolved salts.
The only way I can imagine a planet with a solid subsurface completely coated in freshwater would be if the planet snowballed hard, no radioactive materials left in the core making heat, no significant tidal pull on the core, and then after reaching a very cold temperature having slow addition of clean water from comets. That said, comets are dirty, they have lots of stuff, so you would need somehow clean comets. Still, at that point once sufficient water has hit the surface it could form a thick enough layer over the salty ocean below and start to melt, maybe from greenhouse effects. As soon as it runs away and keeps heating enough it will start to melt the core ice though, so you could have a short lived window in that freak occurrence but it will be very temporary and not at all likely.
Thanks for all the detail! Your observation about comets is really pertinent. Saltwater is probably itself a purer form of water than comets. Maybe an ocean planet is actually more like a muddy swamp of nasty dirty water than a lake.
Well it depends too on how long things take to settle out. Salt is easily suspended in water, but silt is not, so the water would be salty but not muddy. The water would also probably have lots of photosynthetic bacteria/algae in it, so you would probably have blooms of green, blue, red, and brown all over. Those blooms would uptake light and carbon through that process then as they died drop the content down the long water column. All sorts of feeding below that would create a full eecological web. If there were deep sea vents, volcanic activity breaking through the sea floor, you would have a second source of energy and chemistry at the bottom. That said, the over level of life at the surface would be limited by things like iron, phosphorus, copper, and so on. Any heavier ions would be less available at the surface because there is no surface erosion bringing them in at the top so as they are bound up in dead algae they will drop to the floor.
The rate limiting at the sea floor will be based on energy but not too bad, you would likely see a lot of diverse life around vents and it would have a fairly large complexity over time. That said, the depth would make for less complex life due to the lack of light and associated vision. Some things would make light but it would be dangerous to make and would not be super common.
Another interesting consideration is the geography of the sea floor. Would there be fault lines? If there are continental plates but way under the ocean they would still have movement, so subduction and so on would play out, so you would probably have chains of vents along the diverging or merging plate boundaries. Life would spread along these lines, so life would be closely related at nearby vents but distant over the surface of the planet. I would anticipate a fairly heterogeneous population over the surface of the planet in the deep, but far less so at the surface.