how do you freeze an ocean to get a desert planet?

Question

So I'm in the middle of writing my desert planet, let's call it Thermos for now, and I'm having a bit of trouble balancing some aspects which I'll get to later. Thermos is an earth like planet with an axis tilted in such a way that most of the planet is desert-like at its equator but the poles are frozen. This could be achieved by having the planet tidally locked or without rotation but that would make it uninhabitable. Instead the tilt of the planet is lower than that of earths under 23.5 degrees. Possibly this would allow icecaps to form on both poles and reduce the oceans making a mostly dry planet.

Now onto the later part: Thermos has to be within habitable temperature ranges. The concept was to have life adapted to the hot and cold regions, and since this is for a story there are humans living on this planet. Story wise Thermos was once covered by oceans but has devolved into its current state, with most water turned into ice which lowered the sea levels considerably. This question is a continuation of [what would cause oceans to disappear?]

Long story short how many degrees should the axis tilt be to get the desired result? Would it work?

Answer 1

How to build a habitable desert planet, maybe. Mostly speculation, since I'm far from being an expert*.

The fundamental problem here is that in order to have a breathable-by-humans atmosphere, you need photosynthetic life to generate atmospheric oxygen. (Since oxygen is highly reactive, it needs to be continually regenerated, too. See "carbon cycle".) Life as we know it needs water to exist. Even the most highly desert-adapted plants need water, and indeed, many of their adaptations are for obtaining and conserving the limited water in their environment.

Furthermore, in order for life to originate in the first place, you probably need an ocean environment. It's also arguable that you need some oceans to act as a CO2 sink over geologic time (limestone & chalk), and as a regulator in the shorter term. So your planet must have some ocean, or at least have had them in the past.

Another reason you need water is for plate tectonics to happen, which might well be necessary for habitability - think of volcanos adding all sorts of stuff to the atmosphere, and the ocean crust being continually recycled by subduction.

The bottom line here is that you're just not going to get a habitable planet that's entirely desert. You'll have to settle for one that's mostly desert.

So you have a planet that's midway between Mars & Earth, say 70-80% of Earth's mass, and similar composition, with a molten iron core to give you a protective magnetosphere. It either started out with less water than Earth, and/or lost much of it over the eons. It has active plate tectonics, and so mostly granitic continents and basaltic ocean beds. Oceans exist, but they occupy only trenches and the lower parts of what would be oceans** on Earth.

You still have evaporation from the oceans, and so rainfall and all the other weather that Earth has. However, orography ensures that almost all of the precipitation falls on the continental slopes (the area between the continental shelf and the deep ocean). Very little reaches the continents.

So you have Earthlike continents that are desert, rimmed by wetter areas. Your humans naturally inhabit the wetter areas (where they can grow food), but venture into the deserts for trade, mining, &c. A Columbus on your world would use camels, not ships.

Now about those ice caps. On present-day Earth, they're limited by the fact that ice flows, and when it reaches the ocean, chunks break off and float equator-wards, where they melt. On your desert planet, the oceans are much lower, so this won't happen. Instead, the ice will be more like that of Ice Age North America, flowing towards the equator until it reaches a balance with melting. Thus your continents might have a number of meltwater rivers flowing through the desert. (Like the Nile.) There might even be occasional lakes that would be the equivalent of islands in the ocean.

*Here's a book by someone who is: https://www.cambridge.org/core/books/principles-of-planetary-climate/5B5EEF0534CB6F69FB2E395DD21D3476

**Isn't it really an amazing coincidence that Earth's water fills the ocean beds so neatly, just barely lapping over the edges of the continental shelves?

Answer 2

Simply have a large landmass, ideally even with mountains shading it from rain, to create deserts around the equator. If you want that area to be hot, you can also play with the planet's distance to its star or the star's heat output to increase average temperature slightly without getting rid of the polar caps. Axial tilt then only comes into play when thinking about your seasonal migration.

Answer 3

Opening: "Story wise Thermos was once covered by oceans but has devolved into its current state, with most water turned into ice"

There's a lot of water

Available land mass will depend on sea level. Most water is frozen you say. Your planet has large, ice age like polar regions. In that case, the equatorial regions would become moderate rather than dry, because liquid oceans and clouds exist there. If you would introduce more summer and winter differences, by increasing the planet's tilt, more water would melt and freeze periodically, you'd get more water circulation.. but you'd get no deserts completely devoid of water, or freezing oceans.

Your ocean did not freeze, sea level went down

One desert forming scenario I could think of: suppose as a result of some geological change involving shifting continental plates, erosion would cause large holes in the planet's crust, that can gather liquid water. With each earthquake, oceans get deeper in these locations and sea level lowers.. sea will retract from the coast. In land hot zones will be the result. Circulation broken, land mass dries out. Eventually, deserts will form.