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The surface of this planet is about 3/4 the one of earth. It has no oceans or seas, its landscape defined by huge mountain ranges between 15 and 25km high. Most mountain ranges are latitudinal (N-S), so that traveling longitudinally (E-W) becomes very difficult. The main exception is the equator mountain range, which surrounds the planet. Mountains are made of materials with very high latent heat of fusion (e.g., Silicon Carbide, rhenium, zirconium, tungsten or graphite) to enable such size. The equator is a dry and hot natural barrier, which cannot be crossed with basic technology. Living conditions are impossible at the equator and poles, and possible elsewhere.

Liquid water flows towards the poles, creating lakes along the way. Mountains dig deep into the poles, before disappearing in them, together with what is left of frozen rivers. Humidity from the poles is picked by circulating winds, creating moisture clouds that travel towards the equator, precipitating upon reaching the gigantic mountain ranges, and becoming dry by the time they reach the equator (Orographic lift). This creates a desert belt, which in turn heats the air and makes it circulate dry towards the poles again at high altitude. I'm assuming earth standards for extreme heat (50C) and cold (-50C) are sufficient to enable this water cycle.

Orographic lift

The question is what could cause this water cycle and geography, while keeping the planet habitable.

My current approach is to have the planet spin rapidly, making it of oval shape, pushing water towards the poles. Mountain ranges caused by tectonics. If this is indeed feasible, can a planet spin fast enough to cause this water cycle while still host human-like life? Is there an alternative reason for water to flow to the poles, beyond faster spin?

Edit: The final solution is a merge of most proposed ones. Both a Mars-like structure, and a large lake of liquid water around the pole to load the winds with water

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    $\begingroup$ Water flows downhill. Spinning fast won’t help you here. Your watersheds would need to span entire hemispheres, with the highest elevations near the equator, and the lowest at the poles. $\endgroup$ Nov 30, 2022 at 14:27
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    $\begingroup$ One more thing worth considering: cold air doesn't carry moisture well. If you're relying on cold air picking up moisture from ice at the poles, you're going to end up with a very dry climate across the rest of the planet: if your poles are just freezing (0C) and 20C is a typical temperature in your habitable regions you're not often going to get days above 26% relative humidity, except right by rivers or other water sources (e.g. plants). (It's not quite so simple, altitude matters too, but it's an ok estimate.) For context, Phoenix AZ has a daily average RH of about 37%. $\endgroup$ Nov 30, 2022 at 15:58
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    $\begingroup$ The reason a fast-spinning planet becomes oval shaped is that an oval is the surface of constant gravitational potential in that case. So water will not flow towards or away from the poles - as far as anyone on the planet is concerned, the surface is level. (An object will still weigh more at the poles than the equator, though.) $\endgroup$
    – N. Virgo
    Dec 1, 2022 at 1:51
  • $\begingroup$ Have you considered using several small moons orbiting at the planet spin speed, making them stationary, would generate pull on the regions you want to have water concentrated, similar to tidal forces. $\endgroup$
    – Sxubach
    Dec 1, 2022 at 14:45

7 Answers 7

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Look at Mars: Map of Martian elevation

It's pretty obvious that the two hemispheres are of remarkably different altitudes. The difference between the median northern uplands elevation and the southern lowland elevation is almost 6 kms.

enter image description here

This is called the Martian Dichotomy. There are a number of competing hypotheses as to what caused it; the main debate is between mantle convection vs a massive impact. Interestingly, because of Mars rotation, even if the dichotomy was not originally centered on the poles, the planet's axis would have moved to what is now the poles.

Additionally, you can look at the graph and see that on the Martian equator, there are already a set of shield volcanoes that form the Tharsis Mons region. The tallest, Olympus Mons, has a height of 18 km from base to peak with respect to the Northern lowlands. Lined up parallel to the equator, they already would cover 1/10 of the planet.

If Mars had a bit more internal heat, than there might be a bunch more volcanism. Which if it lines up around the equator, boom you got your 15-25 km mountain range.

And if impactor that created Hellas crater, that low elevation blue circle in the south, was a bit bigger and centered on the southern pole, your planet could exist without having to spin faster than 24 hours per rotation.

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  • $\begingroup$ mars is the right shape, but no matter how warm you make it it will have basically no rainfall unless it has an ocean in the northern hemisphere. moisture does not travel from colder to warmer areas well. $\endgroup$
    – John
    Dec 1, 2022 at 22:31
  • $\begingroup$ I accepted this answer because the Martian Dichotomy solves my problem. It is possible. Combined with a little trick to overcome the issue pointed by @John , which is to have a circle of water around the south pole loading the cold dry winds that blow from the high pressure system at the pole. Together with the latitudinal mountain ranges, clouds get directed and rivers happen along the way. $\endgroup$
    – Dario
    Dec 3, 2022 at 23:04
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    $\begingroup$ @Dario except th moisture will never make it to the equator , you wont have circulation cells running from pole to equator. you just get a small one around the pole. think about the earth today the equator is far hotter than the pole but the polar Hadley cells don't make it past the artic. $\endgroup$
    – John
    Dec 4, 2022 at 0:54
  • $\begingroup$ Thanks a lot for this comment @John. Got me reading on Hadley cells, until I found a 1990 paper which hypothesizes single pole-to-equator cells (Equable Climate) happened, and how was this possible. journals.ametsoc.org/view/journals/atsc/47/24/… $\endgroup$
    – Dario
    Dec 20, 2022 at 13:59
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I doubt the spin of the planet would have this effect on its own. When a planet "bulges" from its spin, it's really just evening out the combined effects of gravity and angular momentum. Any net force that moves water will also, over geological time scales, move rock. Earth bulges, for instance, but there's no net preference for water to accumulate at the poles vs at the equator.

If the planet had recently and dramatically decreased its spin (leaving its internal structure alone), then you might expect water to accumulate at the poles. Under this scenario, the planet will "want" to become more spherical, and water is faster to adapt to this than rock. (I can't offer much to explain how a planet's spin might be dramatically altered though.)

If it helps, planets that are tide-locked to their stars may (as I understand it) develop a convection cycle that transports hot air from the sunlit side to the night side, and cool air from the night side to the sunlit side. (This is basically explained by the same diagram you included!)

Another possibility to explore -- just at a guess -- might be a subterranean ocean (underneath the solid crust) that transports cold water from the poles to the equator, within the planet. En route, the water heats up from geothermal energy and erupts in huge geysers at the equator.

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    $\begingroup$ + for subsurface ocean moving water back to equator. $\endgroup$
    – Willk
    Nov 30, 2022 at 15:54
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    $\begingroup$ Good answer. It clarifies that planet spin wont have a distinct effect on water and land, and the subterranean currents is a cool idea. However I think these currents and geysers wouldn't be enough to channel the majority of the water cycle. $\endgroup$
    – Dario
    Nov 30, 2022 at 18:25
  • $\begingroup$ "Any net force that moves water will also, over geological time scales, move rock." In fact, this is the definition of a planet. Rocks which do not have sufficient gravity to move the rock into an oblate spheroid do not get to be called a planet (sorry Pluto). $\endgroup$
    – Aron
    Dec 1, 2022 at 8:01
  • $\begingroup$ I assume that the N-S mountains were carved by the rivers, so I think the rivers need to be there for the formation of the mountains. $\endgroup$ Dec 1, 2022 at 16:16
  • $\begingroup$ @LittleEndian in the millions of years scale, I'd say the chicken or egg paradox works, literally. $\endgroup$
    – Dario
    Dec 3, 2022 at 23:18
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have the planet spin rapidly, making it of oval shape, pushing water towards the poles.

If the planet is spinning so fast that it is oblate, water would accumulate at the equator, not at the poles, for the very same reason why the rocks are bulging at the equator and not accumulating at the poles.

If you want water to accumulate at the poles, use only orography.

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  • $\begingroup$ It's hard to imagine a planet-wide orography where lower land is at the poles, where planetary features like spin are not a key factor. Would a very slow spin help the water move towards the poles then? $\endgroup$
    – Dario
    Nov 30, 2022 at 12:59
  • $\begingroup$ @Dario no the rock is heavier than the water it will display the water in whatever direction the gravity gradient is stronger, that is why the planet is round. the only way aorund it is to have diffrent kinds of rock of with a small enough difference in density gravity is not a major factor at that level. just like continents vs oceanic crust on earth. $\endgroup$
    – John
    Dec 1, 2022 at 22:38
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It is fine just as you lay it out.

  1. Planet spins fast: groovy.

2 Equatorial mountain belt as a consequence of spin: extra groovy. Related idea: What Could Create the Remaining Two Types of Single-Biome Planets?

  1. If equator is high altitude it is downhill to the poles. That works fine. You don't need anything else to have water flow downhill.

  2. I suspect not much water will make it to the poles. Cold dry air coming back from poles between longitudinal mountains will eat up water on the way as it warms. When air finally makes it to equator it will have warmed up and be carrying a boatload of water. That humid air will drop all of it off as rain on the equatorial bulge mountain as it rises and cools off.

  3. Longitudinal mountain ranges will be very dry on upper parts.

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  • $\begingroup$ the warming air can only hold a lot of water if there is water to pick up, AKA oceans, warming air does not magically create more water, it will actually make it drier without oceans. rain is unlikely to occur outside the artic air cell on such a world. this planet will be mostly desert $\endgroup$
    – John
    Dec 1, 2022 at 22:34
  • $\begingroup$ Water is not created, nor destroyed. The question is how much water there is, where is it, in which form and following which cycle. $\endgroup$
    – Dario
    Dec 3, 2022 at 23:27
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Why don't you flow the rivers towards the hot equator desert instead, where the water would evaporate and head towards the poles as clouds.

You could also just say that the planets density for whatever reason varies and along with the topography causes the equatorial area to be 'higher' in regards of water flow but you're still kinda screwed trying to explain where the water comes for the sources of the rivers, as the air near the equator can hold more moisture being hotter and it would rain down in the cooler zone, which is also where the rivers are taking it, so all the water would just end up on the polar ice caps locked as ice forever instead of being part of a cycle.

you could also have 2 moons and the planet has it's own spin that makes the poles be a thing, but that the moons are in stationary on top of the north and south pole in regards to the planet and they spin that way as a system to keep the moons from crashing into the planet, not sure what kind of day/ night cycle that would imply and lightfall on the poles and equator possibly ruining the equatorial desert from being a desert, but it would have essentially a stationary tidal effect pulling the water towards the poles - but for it to be feasible the day/night cycles wouldn't be as regular as they are on earth, at times either pole would be pointing directly at the sun, but also either pole could be on the dark side of the planet for large parts of the year if it's in a single star system.

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  • $\begingroup$ Geostationary orbits have to be equatorial, not polar... Unless something is holding up your moons to prevent them from crashing into the planet. $\endgroup$ Dec 1, 2022 at 14:09
  • $\begingroup$ Ah yeah you're right. It couldn't keep rotating on the axis between the moons while rotating with the moons as a system without the rotation having to be irregular and making no sense? like you put a spin on the sphere, then spin it while it's spinning along with the orbiting objects so they're geostationary. The daily spin would be on x axis and the monthly spin around perpendicular y axis. This would really need an animation to explain to myself. $\endgroup$ Dec 2, 2022 at 13:55
  • $\begingroup$ Equator is the high altitude in this planet. For the massive wall of rock effect. Thus, rivers must go to the poles. $\endgroup$
    – Dario
    Dec 3, 2022 at 23:29
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You may have created the planet with the largest percentage desert possible.

If it doesn't have oceans the air is too dry to support rain at anything but the most local distances from water sources. it doesn't matter what the temperature is. even large continents on earth, which is mostly oceans end up with deserts near the center, your planet is essentially nothing but continent with no way to supply moisture to warming air.

Worse Hadley cells means your moisture can't really escape the polar air cell. your rivers are short and and close to the melt line. There is no circulation taking moisture far from the poles. Big rivers really do require big oceans in the same latitude.

The difference in elevation has to be tectonic. Gravity can't be the dominate factor unless the planet has recently changed spin speed, and if you do it with spin change it will only be temporary and catastrophic, complex life will be unlikely.

tidal locking will not work, you get an eyeball world then, if it is cold enough for ice the cold side is too cold for rivers and were all the water is and you will not have significant amounts of water on the hot side.

If you want to maximize rivers, you want oceans and north south aligned continents with mountains on the downwind side, (opposite the direction of rotation) with gaps in the dry cells. then your planet will be basically nothing but ocean, rivers, and mountains. If you want rivers flowing to the poles you need oceans, some at the poles and some in each of the other Hadley cells. Long rivers require oceans to supply enough water to the air.

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Frameshift:

As many have said, you can't have a lot of water pickup from a cold environment. (In fact, this is essential to our survival--it's the very low water content of the stratosphere that's the only reason Earth can hold onto water in the first place. Water vapor that gets too high up disassociates due to UV radiation and Earth is too small to hold onto hydrogen very well.)

Thus let's not have a cold environment. The poles are massive geothermal zones where the lava is kept in check by the polar oceans but the oceans evaporate rapidly in doing so. The poles themselves are unapproachable without modern technology.

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