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I have a planet about the size of earth. It is a desert planet with an atmosphere at 0.1 atm, made of 90% co2 and 10% n2. Water at this pressure should freeze at 35℉ and boil at 125℉. The planet has a magnetic field, a day of 28 hours, an axial tilt of 28°, a year of 3267.5 hours and an average temperature of 130℉.

I would like water at the poles to be liquid during the winter and evaporate during the summer. What changes should I make to my planet to allow this to happen?

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    $\begingroup$ The title question is quite different from what's in the body; It would be better to edit and ask the same question ^^. Correct me if I'm wrong, but I picture your goal is more to change water state at the poles than to just know the temperature ranges, so your title would be in the like of "How can I have water evaporating and liquify at a planet's pole periodically?" $\endgroup$ Nov 16, 2022 at 15:24
  • $\begingroup$ Water can evaporate at temperatures lower than its boiling point, you know. Even ice can sublime into vapor without melting. Presumably you are specifically asking for a maximum temperature >boiling point of water at some point in the planet's orbit? $\endgroup$ Nov 16, 2022 at 16:03
  • $\begingroup$ Not enough for an answer: but determining what your planet needs to ensure the water conditions you're asking about requires some additional detail. We need to know how the water is getting there in the first place. Since you don't have freezing temperatures at the poles, it won't be condensation. That means rainfall, most likely as a pole transitions into it's autumnal period. Do you have an idea of how much rainfall? How deep/large are the water storage (e.g., lakes) areas? Perhaps more simplistically, how much water and how are you getting it? $\endgroup$
    – JBH
    Nov 16, 2022 at 16:41
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    $\begingroup$ @Trish On a whim I asked my wife if she knew -273.15 was the offset to Kelvin for Celsius. No surprise, she didn't know - and didn't care. If you want to argue with the OP about using Farenheit, please do. But try to keep your focus on the ball. It's a ridiculously small thing. $\endgroup$
    – JBH
    Nov 17, 2022 at 3:27
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    $\begingroup$ It's often helpful, when addressing an international audience to use international units (the metric system, i.e. degrees Celsius or Kelvin) as well as your local imperial system. $\endgroup$ Nov 17, 2022 at 7:03

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(..) an average temperature of 130℉ (...) I would like water at the poles to be liquid during the winter and evaporate during the summer. What changes should I make to my planet to allow this to happen?

NOTHING

Your planet is already too hot.

The average temperature of the Earth right now is 57F. This is what global temperatures look like throughout the year:

A map of the world showing global temperatures through the year

Source: https://sci-web46-v01.ocio.monash.edu/mscm/greb/cgi-bin/scny_i18n.py?scenario=37&variable=01 - this is a tool that simulates Earth temperatures should we increase or decrease its distance to the Sun. The image above is with its current distance averaging 1 AU through the year.

The white color in the map is a temperature range going from -5K to 5K above the current global average. 130F is almost exactly 40K above 57F, so it would be the darker reddish hue in the map's scale. To turn the whites of the above map into reds, we would make everywhere else much hotter too. Like this, is the Earth were to orbit at 0.91 AU (the lower limit on the simulator):

A temperature map of a much hotter Earth, to match the OP's description

The water will simply never freeze anywhere. You may have it evaporate in the summer if it is accumulated in shallow pools in the poles, so it is more about geography than temperature now.

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  • $\begingroup$ Melt? Isn't it about evaporating back and forth ^^'? $\endgroup$ Nov 16, 2022 at 18:18
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    $\begingroup$ @Tortliena sorry, I was thinking in reverse. I meant freeze. Fixed that :D $\endgroup$ Nov 16, 2022 at 18:40
  • $\begingroup$ The OP never asked about freezing. He/she merely pointed out the changes to freezing/boiling temperatures due to the change in atmospheric pressure. The OP only asked about what could be done to the planet to get liquid/vapor water at the poles. Did you answer that question? $\endgroup$
    – JBH
    Nov 16, 2022 at 21:16
  • $\begingroup$ Another question: I looked at that simulator, which I thought would be a cool addition to the worldbuilding resources page. What do you think they mean by "K?" It isn't Kelvin. 50K is -370F. When they say "K" do they actually mean "C?" 50C is 122F and makes a lot more sense. In which case the OP is looking for 54C as an average and your image for 0.81AU showing polar temperatures with liquid water would make a lot of sense and the planet wouldn't be too hot. Add Will's axial tilt and we're done. $\endgroup$
    – JBH
    Nov 16, 2022 at 21:25
  • $\begingroup$ @jbh I did answer, from the OP's data water will be liquid on the poles all year round. And yes K is Kelvin. The numbers in the images are offset from current Earth's global average temperature. $\endgroup$ Nov 16, 2022 at 23:09
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Increase axial tilt to make polar winters colder.

You want to make sure it is cold at the pole in winter. You can tilt your planet on its rotational axis to make sure this happens. The more axial tilt, the less sun a given pole gets in winter and the colder winter is. Axial tilt is why the sun does not even rise in the polar midwinter - and also why it does not set in polar midsummer.

https://blogs.nasa.gov/pluto/2015/10/23/a-planet-for-all-seasons/ axial tilt

Changes in axial tilt of earth over long periods account for changes in seasonal temperature extremes.

https://earthobservatory.nasa.gov/features/Milankovitch/milankovitch_2.php

Obliquity (change in axial tilt) As the axial tilt increases, the seasonal contrast increases so that winters are colder and summers are warmer in both hemispheres. Today, the Earth's axis is tilted 23.5 degrees from the plane of its orbit around the sun. But this tilt changes. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degrees. Because this tilt changes, the seasons as we know them can become exaggerated. More tilt means more severe seasons—warmer summers and colder winters; less tilt means less severe seasons—cooler summers and milder winters. It's the cool summers that are thought to allow snow and ice to last from year-to-year in high latitudes,

You might not need to tilt it that much if your planet has a long year = big orbit. A big orbit means longer seasons means more time in winter means more time for the water to condense and rain down on your poles.

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    $\begingroup$ OP isn't asking how to change the temperature variation. They're asking how to determine the temperature variation. As written this does not answer their question. $\endgroup$
    – sphennings
    Nov 16, 2022 at 15:46
  • $\begingroup$ @sphennings - /OP isn't asking how to change / OP is asking how to change. The clue is in the text. /I want to find out what (if anything) I need to change to make liquid water exist at a pole during winter, and evaporate in the summer./ Emphasis mine. Now maybe you will see fit to change that pesky downvote? $\endgroup$
    – Willk
    Nov 16, 2022 at 16:10
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    $\begingroup$ @sphennings - JBH fixed it! Now we can all agree on what the question is asking. About that pesky downvote... $\endgroup$
    – Willk
    Nov 16, 2022 at 16:24
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    $\begingroup$ I think the question's better objectively, but I'm not sure JBH's change is what the querent want. Still, I'm waiting to vote (up or down) here because we just can't really know if the answer's answering or not the querent's question ^^. And I'm expecting your answer to change if @Sphenning was right :p. $\endgroup$ Nov 16, 2022 at 17:19
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    $\begingroup$ @Tortliena The OP went to great length to explain what he/she wanted in the body of the text. The assumption that the title question better reflected the intent is not based on evidence. Nevertheless... If the OP wanted to ask the question the other way, there's nothing stopping him/her from asking another question. $\endgroup$
    – JBH
    Nov 16, 2022 at 18:56
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So, first of all, earth has three types of poles: True Grid North, Astrological True North and Magnetic North and corresponding Southern equivalents. True Grid North is the point where a cartograph grid of a planet would place the prime meridian's intersection with the 90th degree of latitude. Astrological True North is the point around which the planet's rotation occurs, and Magnetic North is the location where a planet's magnetic field is north.

It's important to note that while the first two are fixed locations, the later one is constantly shifting over time and will one day be located close to either True South.

Now, as I said in comments, on earth the poles aren't ice and snow because of atmospheric water, but rather are snowy and ice because it's cold. Both polar regions get very very little rainfall (well, snow, sleet, hail) annually, despite being absolutely covered in the stuff. This is because they are not in positions that would favor perciptation, because, being so cold, the atmosphere cannot hold as much water, so very little water vapor will reach these locations before precipitating.

This is the problem with your entire model. Water vapor is atmosphere is measured by humidity and how much water can be held in atmosphere in a given location is dictated by a number called the "Dew Point" which is a 100% saturation. When this occurs, water vapor turns to liquid and falls from the sky... or forms droplets on items close to the ground. The dew point is not a fixed point, as it changes not only based on the temperature of an area, but also the air pressure. The colder and denser the air.

The other thing about deserts is that they are very hot... but only during the day. Remember deserts lack the amount of atmospheric water to precipitate. Well in Earth's Atmosphere, atmospheric water works to keep heat trapped... but since deserts lack that, they get cold at night.

YOu also have a problem that your atmosphere looks remarkably like Venus, which, thanks to its ~96% CO2 and ~4% N2 atmosphere, it has an impressive heat retention that makes the surface of Venus is a balmy 867 °F. We don't know if Venus had a sufficient amount of water comparable to Earth, but if it did, it's long stopped being able to rain it out.

All that said, if you want polar liquid water, it's not that hard. All you have to do is make the lowest point on the planet the polar regions. Water flows down hill, so as long as your poles are the lowest point, it will go to those spots.

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    $\begingroup$ Astronomical True North? $\endgroup$
    – Alexander
    Nov 16, 2022 at 18:41
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    $\begingroup$ The atmosphere doesn't look very much like Venus, on account of having ~0.1% of the surface pressure. $\endgroup$ Nov 16, 2022 at 21:04

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