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.
The reason water is concentrated in oceans isn’t specifically due to continents existing. Salt doesn’t evaporate so all rain is fresh water. That fresh water falls. When it falls over land it flows to the lowest point it can go. This leads to all flowing water flowing towards oceans and seas. Salt won’t travel upstream. Ergo salt simply stays in oceans and seas.
Now consider a world with no land. This wouldn’t really differ from a single ocean on earth. Currents and waves will move in all directions at some point which should mix the salt all around. You could get some differences if there were ice caps or icebergs. Those could behave similarly to continents depending on size.
Sure, I get that, but without land for rivers to essentially mine salt from, the equation changes a lot. Underwater erosion is dramatically less destructive than above water erosion.
Earth’s oceans are in a steady state, where all the addition of salt by rivers is balanced by loss of salt in the ocean. If you removed all the rivers from the equation, Earth’s oceans would find a new balance at a point significantly less salty than they are currently. Though I have little idea if that would be something we consider freshwater, or just “less salty” saltwater.
But that’s not what’s happening. There’s no “mining” of salt. There’s no significant addition or loss of salt to the ocean. Salt just stays in the oceans here. Freshwater will evaporate and return through rivers and rain. On a planet without land, the salt would still remain in the ocean.
Salt gets into rivers when material that can’t be dissolved is stripped away by erosion. This exposes new water soluble compounds to the water, where they dissolve into the water and are taken to the ocean.
Over millions of years erosion removes innumerable tons of material, essentially mining the subsurface soluble compounds and delivering them to the ocean. Once there, as you mention ,those salts remain in the ocean. On Earth, this process began billions of years ago and has been adding salt to the oceans ever since.
You can observe this happening in many rivers today. The Colorado River is a great one. If you measure is salinity at the headwaters (or heck, probably even the inlet of Lake Powell), and where it enters the Gulf of Mexico, you will observe an incredible increase in salt. There was an international treaty formed around the US delivering river water that is not too salty to grow crops in to Mexico. The US solved that problem by installing a desalination plant on the river!
However without that land based salt mining process, how salty would the oceans be? Lots of good clues in this thread, but I don’t think anyone has offered a definitive answer.
Ah. I see the angle you’re coming from. I had mentioned in another comment somewhere that essentially all salt without an impermeable barrier between it and the water on this planet would be dissolved (provided it doesn’t saturate the water which would be a horrifically enormous amount of salt). Salt is highly soluble in water and on any timescale that could be relevant would fully dissolve and achieve a general equilibrium. If the planet has water, then it has a star able to warm the planet. There’s no realistic scenario that wouldn’t result in the ocean fully mixing.
Disclaimer: Not an expert.
Thoughts: I think this would largely depend on multiple factors, such as the overall composition of the planet, a hypothetically almost perfectly spherical core underneath the water, and not having a moon to shift the water tides around.
And even then, solar gravitational tides are a thing, so the water would most likely still move. Also, I’m pretty sure there’s no perfectly spherical planet, so I assume there would still be some sort of underwater erosion going on.
All speculation though.
I was trying to figure out how much underwater erosion there is but if you compare the sandy and silty bottom of the ocean to like, Utah, it seems like continental erosion is orders of magnitude more significant.
Conversely, we know oceans deposit all sorts of stuff at their bottoms, which makes me think there is a small amount of salt being deposited. Would that cancel out significant underwater erosion?
Similarly, if underwater erosion was a big deal, wouldn’t old lakes (in geological time) be notably saltier than young lakes? But the only salty lakes we have primarily lose all their water through evaporation, basically ultra concentrated river water.
There’s also heat exchange so you’ll have deep sea vents where there could be all kinds of caustic stuff and/or minerals.
So it wouldn’t necessarily be fresh even if that stuff wasn’t saline
For those following along: SEAWATER IS NOT SALINE EITHER. Just making sure we’re clear on everything
I was thinking of the term salinity.
Hahaha I figured something like but couldn’t resist the opportunity :D
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.
probably not.
unless the planet is water all the way down, I don’t think it’s possible to have life or even submerged landmasses that don’t have the chemical elements that can create salts. dead things would dissolve in the water and chemicals in rocks will leach into water over time.
now if this water planet is far enough away from the sun to freeze, sure. the frozen ice should be all fresh. I’m not aware of any salts that stay in frozen water ice. the stuff underneath the frozen stuff most definitely will be salty.
not a chemist or chemistry major but I’m using the word “salts” deliberately. there’s more types of salt than NaCl.
If salts were present when the water froze, the salts would still be there. If the ice is pure water but you can’t microscopically brush away all the salts during thawing, can fresh water be extracted?
It’s all fresh if you adapt.
Why wouldn’t it all best just considered salt water then?
Enceladus, a moon of Saturn is actually mostly water, but salt has been found in volcanic emissions ejected into space.
That said it’s not impossible that conditions exist somewhere in the universe where you have H2O and no NaCl since that is the salt we usually mean when we talk about salt water. Unfortunately it is not the only thing found to be mixing with water as on Jupiter, liquid water does exist but it mixes with amonia.
I feel like I’d rather drink saltwater before I drink water with ammonia dissolved in it.
The water is just sitting there, not eroding through salts.
Is the ocean still or famously active?
But all jokes aside freshwater is salt free because it has been distilled by the evaporation/cloud/rainfall part of the water cycle. When rain falls in the ocean it mixes pretty quickly.
Running water causes a lot more erosion than stationary bodies of water. Consider lakes, which are still cycling water much like a river, but over thousands of years they deposit so much silt that they cease to exist. That’s the opposite of erosion.
Underwater erosion is certainly a thing, but in comparison to downhill water erosion on land, it’s pretty insignificant. It does not seem a given that it could significantly offset the processes that remove salt from salt water.
So let’s consider the premise that our oceans got their salt from water washing over the land in rivers after it rains.
On a world completely covered with water, are we presuming there is a solid ocean bottom? Because if so, that water is “washing over the land” 24/7, isn’t it?
Running water causes a lot more erosion than stationary bodies of water. Consider lakes, which are still cycling water much like a river, but over thousands of years they deposit so much silt that they cease to exist.
Underwater erosion is certainly a thing, but in comparison to downhill water erosion on land, it’s pretty insignificant. It does not seem a given that it could significantly offset the processes that remove salt from salt water.
A river will erode more than a static lake, true, but what about an ocean? They’re far from calm. And an ocean’s amount of water to rock contact is a couple of orders of magnitude greater than a rocky landscape with rivers in specific places, so more sites where salination can occur.
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.
Sure but once a continental plate is flooded, isn’t it by definition an oceanic plate at that point? A continent only exists if it isn’t flooded.
I mean, it’s basically arguing semantics, which was my point. Temperature, sediment, etc. transfer will still occur, and erosion will happen. It would just happen at different time scales.
