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.
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.
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.
This is all very interesting and pertinent. I was wondering about the hadean period, and whether you could actually get to an ocean world without first having continents with a water cycle. I don’t know enough about planetary formation to conclude further. Thanks for pointing me to the hadean period, I will read more about that.
You might misunderstand my comment about the dead sea. The dead sea actually precipitates salt crystals onto the bottom of the sea. No land is required in this strange process. I don’t think it’s clear to say whether this happens because of the extreme salinity of the dead sea, or if the extreme salinity just makes it the only place we observe this rapid desalination on human time scales. I offered this as perhaps the most striking example that salts dissolved in water are not necessarily a stable state on a timeline of billions of years.
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.
Oh man! The fact that our current ocean isn’t getting more salty implies that the addition by rivers is very significant to the total saltiness of the ocean! Over billions of years with no rivers, the ocean must get significantly fresher! Wow! That is strong confirmation that an entirely ocean world would at least be significantly less salty!
Why wouldn’t it all best just considered salt water then?
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.
I think it would largely depend on whether or not there are any moons causing tidal forces.
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”.
