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!
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
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.
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.
As far as I can tell, salt precipitation in the Dead Sea is a result of evaporation. As the concentration of various ions increases, you eventually reach a point where water can not hold any more salt in it. If there’s too much, the excess gets pushed out into the solid phase as salt crystals.
It’s all about the solubility of each compound, which depends on all sorts of things such as temperature, pressure, pH, other ions, etc. As the conditions change, solubility changes, excess salts get precipitated and the solution finds a new equilibrium.
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.
I think it would largely depend on whether or not there are any moons causing tidal forces.
Why wouldn’t it all best just considered salt water then?
