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I have a planet covered with a large shallow ocean, which extends over most of its surface. I want to turn this planet into a great desert, specifically a salty desert (the salts of this desert are a mixture of several chlorides, not just NaCl, there is also NH4Cl, MgCl2, CaCl2, etc.). On Earth there are small salty deserts (salt flats), which form when a lake evaporates and leaves behind all the salts that were dissolved in it. I wanted to do something similar to form the salty desert of my planet, but the question is, where will all the water go?

The surface, which is completely covered by large amounts of salts, must be depleted of water, since it would dissolve the salts in one way or another (with rain, for example), which I do not want. What mechanism could I use to remove all the water without affecting the salts?


Keep in mind that I don't want to eliminate the atmosphere (as happened with Mars) or add greenhouse gases to increase the average temperature of the planet and keep the water always evaporated in the atmosphere. Freezing the water and depositing it on the surface is not an option either. Sending the water underground is a good idea, but you should find a way not to send the salts too. It would also be possible to add some compound that reacts with water and depletes it. Things like that.

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    $\begingroup$ Turn it all into sugar through photosynthesis? $\endgroup$ – Muuski Oct 9 '19 at 21:52
  • $\begingroup$ @Muuski I forgot to mention that the planet is devoid of life. $\endgroup$ – URIZEN Oct 9 '19 at 21:59
  • $\begingroup$ So, was it a conscious entity that put the water into an escape trajectory, or a natural process? $\endgroup$ – Muuski Oct 9 '19 at 22:10
  • $\begingroup$ @Muuski A natural process (geological, chemical, etc.). $\endgroup$ – URIZEN Oct 9 '19 at 22:15
  • $\begingroup$ Are you sure you want to exclude freezing? An ice-age that leaves all the water in very thick ice sheets at the poles could give you a really big salty desert. $\endgroup$ – Patricia Shanahan Oct 10 '19 at 1:12

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Answer is very simple: Glacial Period. Most of the water would be captured in polar glacials, while at the equator, it would be a very dry, extremely hot at daytime, desert.

There was such a situation on Earth during the (last) glacial period (our oceans are too deep to dry out, but some water bodies did).

Edit: There would still be rivers flowing from glacials which will bring minerals with them. If there are vast shallow basins, where this river will flow, they will form vast salt deserts in mid and low-mid latitudes, exactly as you want it.

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  • $\begingroup$ Note if the planet is similar to earth you can't freeze most of the water without covering the entire planet in ice. you don't get a salty desert you get an iceball. $\endgroup$ – John Oct 10 '19 at 10:33
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    $\begingroup$ Note that freezing was specificaly excluded by the OP $\endgroup$ – Slarty Oct 10 '19 at 13:28
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    $\begingroup$ @Slarty, yes, I know. But this is the only way of mass desert creation that we realy have in our reality. Even Mars has polar caps (with water). Venus has no cups, but no deserts. Water is so common in our Universe, that having Earth-like planet without large amounts of free water in some form is completly unrealistic. And question is marked as science-based. $\endgroup$ – ksbes Oct 10 '19 at 13:48
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Earth has a lot of water because it is made from the remains of old stars that spit out tons of Oxygen and Hydrogen. If your planet is in a star cluster where none of the right stars have ever exploded for there to be much Oxygen, there would not be water in it's makeup.

So at first, your planet is just a boring, dry as a bone world. Then one day it collides with a massive comet from a distant star cluster where water is plentiful. It explodes leaving a shallow ocean over parts of the planet and introduces large amounts of CO2 into the atmosphere. The carbonic acid needed to dissolve the salts forms as the rain water falls through that CO2.

This brings up one minor issue with your planet which is that ocean salts come from acid rain hitting land. Your world can not be completely covered in salty deserts, but they could cover a large % of the world.

On Earth, the water that slowly sinks into the ground is being spit back out just as fast by volcanoes, but on this world volcanoes don't spit out water because there is none in the mantle. This means that after some time frame, the water left behind by the comet would mostly dissipate into the ground. There would still be water on the planet, but by the time it is done defusing evenly into the planet's interior, surface water will be just as scarce as any desert.

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  • $\begingroup$ It is a good idea, but if there is not enough oxygen to form water in large quantities, there will be less to form silicate minerals, carbonates and oxides... and the salts of the planet come from the dissolution of these minerals. $\endgroup$ – URIZEN Oct 10 '19 at 1:44
  • $\begingroup$ @URIZEN That is a good point. However, a comet carrying large amounts of water should also have large amounts of CO2. Amended my answer accordingly $\endgroup$ – Nosajimiki Oct 10 '19 at 14:32
  • $\begingroup$ No evidence that this is the origin of Earth $\endgroup$ – Firestryke Dec 4 '20 at 0:23
  • $\begingroup$ @Firestryke The Nebular Hypothesis is the most widely accepted model in the world explaining the origin of Earth. While it may be hard to "prove" any model of creation, it is the most consistent with Geologic and Astronomic observations. Also, the question is tagged for Science Based which generally means that any widely accepted scientific explanation of a phenomenon applies, even if there are other credible theories that exist which might contradict it. en.wikipedia.org/wiki/Nebular_hypothesis $\endgroup$ – Nosajimiki Dec 4 '20 at 14:47
  • $\begingroup$ Most widely accepted doesn't mean correct. It just means a lot of scientists like it, but there are a lot who are against it as well $\endgroup$ – Firestryke Dec 4 '20 at 16:00
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One option would be to have an huge geological formation that formed a completely closed bowl with mountains on all sides a little like the ring of fire but on a smaller scale perhaps a few hundred - a thousand miles across. This could be in a remote but wet area.

Over time this area would fill up with rain water to form a very deep wide inland sea and the rest of the planets shallow oceans would be depleted of water.

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    $\begingroup$ You may want to check my math but...The volume of the Earth's ocean is approximately 1.35 billion cubic kilometers. A hole with a thousand mile radius covers about 8 million square kilometers (The size of Canada), which means it must be another 125 kilometers deep to hold the world's ocean. (a third of the thickness of the atmosphere and 40% of the mariana trench) $\endgroup$ – Muuski Oct 9 '19 at 22:33
  • $\begingroup$ OP said the starting ocean would be shallow. $\endgroup$ – Nosajimiki Oct 10 '19 at 0:31
  • $\begingroup$ I do not uderstand why rain would prefer this enclosed region and not all the other area of the planet. Described formation is most likely to form deserts in the middle - not a sea (see Sahara or Death Valley). "A completely closed bowl with mountains on all sides" will not allow any wet air inside at first place. $\endgroup$ – ksbes Oct 10 '19 at 9:59
  • $\begingroup$ It wouldn't prefer the enclosed region, but if clouds can get in water will build up in that area at the expense of the rest of the planet as it won't be able to get out again. Cloud access depends on mountain height and winds. $\endgroup$ – Slarty Oct 10 '19 at 22:04
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I don't think that you can get rid of the water without getting rid of the atmosphere. You just need a mechanism to replace the atmosphere.

Hit it with a big enough rock.

Theoretically Earth was an ocean world before it was hit by a rogue planet about the size of Mars. A lot of the lighter stuff was blown out at or above escape velocity. So that blew off most of our ocean (or all of it and it was then repopulated via comets). If the strike happens after most of the planetary afterbirth has been swept up or kicked out by other planets, you would end up with a dry world. Out gassing from volcanoes may be enough to restore an atmosphere (or just one or two comets after the big hit).

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  • $\begingroup$ Almost 80 % of the composition of volcanic gases is water vapor, therefore, if the atmosphere is replenished with volcanic gases after the impact, there would be a large amount of water vapor in it, which will condense on the surface forming an ocean again. $\endgroup$ – URIZEN Oct 10 '19 at 0:19
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Does the entire surface of the planet need to be desert, or just most of it?

If your world with a shallow ocean experienced some form of massive, sudden continental shift that then formed one very deep ocean, in theory all of the water could over time evaporate from the shallow ocean and rain over the deep one. Of course there'd be some redistribution, but if you enclose your deep ocean inside a rim large mountains, it could cause a sufficiently contained localised climate that would keep most of the water there. And if that ocean formed in one of the colder regions of your planet, there would be far less evaporation there than on the rest of the world.

Even with all of that, it would still rain from time to time on the rest of the planet. I think it would be impossible for this to keep rain away forever. But it could create massive expanses of salt deserts that experience rainfall less than once a year, and probably less than once a decade if you force the system enough.

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Electrolysis

The water gets broken down into hydrogen and oxygen by lots of lightning strikes. The hydrogen escapes the planet's gravity. Salt water actually aids in the process -- most hydrogen gas generated on earth by electrolysis is a byproduct of producing chlorine.

It requires A LOT of lightning strikes, of course.

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Turn your large shallow ocean into one small, very deep one. And cover it in ice.

To make a world mostly dry, and the surface into a lot of desert, you just need to reduce the surface area of exposed water. The less surface is exposed to air, the less evaporation, the less rainfall.

If you want to make the drought even worse, put this new, deep ocean on one of the poles. So what surface water is exposed, is not water but ice.

All that is required for this sequence is some nicely accurate Geological shuffling, possibly a new tectonic plate movement away from the polar region?

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If for some reason you have a lot of alkali metals or alkaline earth metals (maybe underground deposits that at some moment get in contact with the water), water will react with them to form a hydroxide and H2, which in turn can escape from the planet. You will need a lot of them, though.

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Simple: You fire solar flares at it and weaken its magnetic field. That works. Just ask Mars.

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    $\begingroup$ Didn't you read the bit about Mars losing its atmosphere too? Something the OP doesn't want to happen. Full marks for creative thinking, but needs more focus on the actual question. $\endgroup$ – a4android Dec 4 '20 at 1:20
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The Dune Way:

Import Shai Hulud. Wait 200 years. Your planet turned into Dune II. Wait, that's already a book: Heretic of Dune.

The Space-travel way:

Your ships use water in their drives. They fill up on water, then blast that out of the engines as "reaction mass" when they fly. A lot will be used in interplanetary travel. Slowly, the planet will end up dry. There was a story about that, which I fail to remember, where Mars, to try to evade Earth's raising water tax, sent a ship to haul an ice asteroid in. Your planet? Was sucked dry instead.

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The water permanently drains into the mantle

Earth's mantle contains at least as much water as all of Earth's oceans, dissolved in various minerals. On your planet, make the crust very porous, and the ocean water will sink into the mantle at a high rate. If the mantle is able to retain all of it, then most of the water will be lost off of the surface and be trapped deep underground. In other words, the water table will start tens of thousands of meters under the surface. This will result in a planet-wide desert. Make sure your planet is not too geologically active, because vapor water might re-enter the surface water-cycle from volcanoes.

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Initially Earth was very close to a water world.

From about 4 billion to 2.5 billion years ago The early Earth was a lot more tectonically active than today, due to the combination of the energies of accretion, radioactive decay, and the formation of the metallic core. It was during this time that the Earth's crust cooled enough to be called solid, although again, there was quite a bit of tectonic activity.

This activity generated structures called cratons, which are the oldest rocks on earth, about 5-7% of the total crust. Cratons are like daggers of rock that reach deep into the mantle. The Canadian Shield is one example. These structures are also the place where diamonds can form, deep enough for the pressure to create them, and then they work their way up to us in a few places, such as the Kaapvaal Craton in South Africa.

Even at this stage there was evidence for deep oceans on Earth. Volcanic eruptions under water generate distinctive shapes known as pillow lavas. More to the point, these oceans had sky high iron content, not surprisingly. What they also had was some kind of photosynthesizing cyanobacteria, which is how the distinctive banded iron formations are created.

Now, there is a trick one can do, given tectonic activity, and moving plates. The plates will move around the planet, dragging the cratons with them. Cratons, for all their durability, are a fair bit lighter than the basalts that are erupted under the sea to create the sea floor. These eruptions happen along the fault lines that make up the oceanic ridges, pushing basalts outward.

As these basalts cool, they become denser and eventually slide under the cratonic plates, and sink towards the mantle, along with seawater, which makes the rock thinner.

These are the subduction zones. Alaska, California, Japan, Indonesia, the Andes, the Rockies, the Ring of Fire is where this process happens.

This thinner rock is heated and forced up to the surface. Some of it is molten, and gets erupted into igneous crust, like we see in Scotland, and the Black Mountains in England, when it passes over a hot spot, or mantle plume. Compared to the plates, this rock and water mix is frothy, and light.

This process is how continental crust is created, more or less, over geologic time. Fresh rock from the oceanic ridges, pushed outwards, cooling as it goes, until it is heavy enough to sink back to the mantle, except that the water makes the rock thin enough to be erupted via surface vulcanism, the lava flows become new land. This new land gets smashed into other new land at the subduction zones, or pulled apart which allows for more vulcanism, and more new land.

Now, Earth possibly went through something called the Late Heavy Bombardment which would have brought a great deal of water bound in the rocks themselves. However, your planet may not have gone through this. YMMV.

Over time the continents will become bigger and bigger, and drift will eventually create a super Pangea type continent.

This structure will last for millions of years. There will be heavy rainfall near the shores, and a vast, world sized desert in the rain shadow created by the uplifting of the crust to create the continent. The salts you require will be all over the interior of the continent, which will be vast, but most of it would have been sea floor a few million years ago. There'll be your salt, as well as limestone, everything you might want, but almost no water.

The oceans and coasts will generate enough oxygen to create an ozone layer, so your desert can be utterly dry, like the Atacama, or the Gobi, and yet not sleeted with killing UV so life can exist.

I think that covers most of the requirements?

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