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.
Even fresh water has stuff dissolved in it, just in lesser amounts. Pure water isn't a naturally occurring thing that lasts long. There are two components, water's polarity which grabs things that are available, and how water in a large system that is getting energy isn't going to stay still and "sit there".
Something interesting I learned the other day in following the recent launch of the Europa Clipper. One of the things they want to explore is how as Europa moves through the huge magnetic field of Jupiter it induces a magnetic field of its own. Why is this relevant? It's one bit of evidence that the waters under the ice have salts dissolved in them, giving them conductivity to produce this field. So even there water is not "fresh".
That is super interesting! I hope the Clipper gives us a definitive answer!
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.
There is salt in the earth.
Salt dissolves in water.
If earth was covered completely in water, there would be even more salt in it.
Yeah, but not all that salt is in contact with water. There are huge deposits of salt under the Mediterranean, yet they have not dissolved into the ocean. You have to actually strip away the non-salt components to get enough water to salt contact to actually dissolve enough salt to make that much water salty.
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.
Water and salts are a package deal. If you have a planet with one, you’re going to have all the others as well, because they all come from an exploding star.
When a star goes supernova, it creates oxygen, which can later combine with hydrogen to make water. That very same supernova also makes sodium, potassium, magnesium, chlorine, sulfur etc. so you end up with all the elements for making a bunch of different salts. Ask physicists why supernova does this sort of packaging.
The presence of sodium and chlorine on the planet makes sense to me, but that doesn't necessarily mean it's dissolved in the water. I think the key understanding is if the water cycle is the key component of dissolving salt in water, or if the much less dramatic erosion on the bottom of the ocean is sufficient to make the water notably salty.
So far the best answer I've got is that water in comets and otherwise outside the planet might actually be something like salty, so maybe freshwater is just a temporary aberration of the water cycle.
At the same time, we know there are some processes that remove salt from oceans (e.g. the salt formations at the bottom of the Dead Sea), so in the end I think it would come down to where that balance of salt in vs salt out. It's not totally clear to me that without the continental influx of salt from rivers, that that balance would result in something like freshwater or saltwater. This thread has highlighted several factors that come in on both sides, so it may be something we won't know until we've explored more planets.
According to NOAA, the ocean was originally not very salty but became saltier over time as rivers eroded the land and delivered the dissolved minerals to the ocean. At the same time, salts crystallize out of the water and are deposited on the ocean floor. This input and output are now more or less balanced so the ocean is not getting saltier. Apparently, this salt cycle involves about 4 billion tons of new dissolved salts being added to the ocean each year and about the same amount being deposited from the water to the ocean bottom.
So, why aren't rivers salty? Apparently, it is because rivers carry only a small amount of salt and are kept fresh by constant rainfall, whereas the ocean has been accumulating salt for the last 4 billion years.
Lakes that don't drain to the ocean, like the Dead Sea, can get salty over time, just like the ocean.
To some extent, these compounds will inevitably mix together. During the early stages of earth (hadean period), there was a time when it was raining all the time, which meant that all of the minerals on the surface were exposed to water. Naturally, some of those were water soluble, which changed the composition of the growing oceans at the time. Some minerals also underwent various other reactions, which caused them to crumble (weathering) which exposed even more reactive surface. In some cases, you ended up with cracks that allowed the rain water to penetrate deeper into to the crust and find its way to larger deposits of water soluble minerals, such as NaCl. The initial exposure to water only kickstarted the process, but later rain and rivers continued to deliver even more salt to the oceans, resulting in the current salinity over the course of billions of years.
In order to prevent the initial dissolution of salts, you would need to have a planet without oxygen in any form, so that there would not be any water. If your planet has oxygen and water, but no chlorine, you would still get various other salts such as sulfates, which would make the oceans salty. Either way, it would be a very exotic combination of elements, and might never actually happen.
If you’re ok with the initial dissolution of salts during the hadean era, but wish to prevent any later dissolution of salts, you could do that by evaporating all the water, just like Venus and Mars did. However, then you won’t have any oceans either, so that’s not ideal.
Another way would be to make the planet as cool as the moons of Jupiter and Saturn, so that there would be hardly any liquid weather. This way, the midly salty oceans produced in the hadean period would be covered with a sheet of ice, preventing any further weathering and dissolution. Also, a Water World (remember that movie) should produce a similar result, since rain and rivers aren’t in contact the rock surface. However, the salt from the hadean period would still be there, so this isn’t ideal either.
The dead sea mechanism is also an interesting alternative. Just replicate that mechanism at a massive scale, and you have relatively fresh water oceans and massive dead seas that just accumulate all of the salt from other bodies of water. Those surface salt deposits would need to be close to the equator so that the sun can evaporate all of the water that flows into them. Those deposits would also need to be lower than the rest of the terrain, and they would need to be connected to the surrounding oceans via rivers, which is a tall order IMO.
Over the course of billions of years, some of those salt deposits might get pushed into the fresh water oceans, which would mess up the whole thing. I think this setup is not stable for billions of years, but it could be possible for a certain period anyway. Maybe this could be a good place for a scifi story. Imagine a planet with massive fresh water oceans and several saturated salt pools near the equator.
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.
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?
In freeze desalination, the initial ice crystals before it freezes solid are pure water; you mostly freeze a volume of saltwater into slush, strain out the ice, and discard the liquid (which will be brine; higher in salt than your initial water).
Probably not super efficient, and probably needs multiple steps, but I dunno. Somewhere where the ambient temperature is below freezing, but geothermal is available, it could work at scale, but if you have to refrigerate, you're probably better off with regular distillation
It's all fresh if you adapt.
I think it would largely depend on whether or not there are any moons causing tidal forces.
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.
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?
The rivers bring salt to the lakes and ocean that they erode when passing. Those get saltier over time. Evaporation filters that out and feeds the fresh water to everything again, but the salt gets deposited in the more permanent structures.
Why wouldn’t it all best just considered salt water then?
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.
