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If a planet was completely covered in water, wouldn't it all be freshwater?

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.

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  • 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.

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