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.
Fresh water is because of rain and snow. You get fresh lakes and rivers because rain and snow melt washes any salt and minerals out into the ocean. If you didn’t have land as a buffer, the rain would just fall into the salty ocean.
Very true, but I think the root of their question is: if there was no land above the surface, would the oceans be salty to begin with?
Exactly. If a planet ever had a salty ocean, adding more water probably wouldn’t dilute it in any meaningful way, so it would need to be a planet that never had continents.
Overall composition of a planet is what would matter, not whether there is land. If there is salt on the planet, it would almost assuredly have salty oceans. Salt diffuses in water. If you put salt into a glass of water and leave it sit, eventually the salt would dissolve and mix completely. Salt water has a different density than water. The act of dissolving involves energy changes. These create small eddies and currents that would mix the water until it was in equilibrium. If there is salt in any form on your waterworld, the only way it wouldn’t be salty is if the salt was permanently separated from the water physically.
Continents and the surface are just areas of the planet that don’t have water covering them up.
If Earth’s oceans rose only a few miles up, it would be a water planet, but these things would still exist. Including plate tectonics and the circulation of magma / molten core.
Water circulates due to pressure, temperature, and impurities, each having their own positive feedback loop into the system before it finds a balance.
