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How can I arrange for a planet to have a much greater temperature range between the poles and the equator without it being tidally locked?

Edit: By “much greater” I mean the temperature at the poles in my fictional world should be 20 degree Celsius colder than the poles in our real world. And the temperature at the equator in my fictional world should be 20 degree Celsius warmer than the equator in our real world.

The planet should be very similar to earth, although variations can be made provided the planet is still capable of sustaining human life in the open on at least some parts of its surface. Some details of the oceans, land masses, atmosphere, gravity and orbital parameters can be adjusted within this broad requirement if that helps.

The situation needs to have arisen naturally and no magic is involved.

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    $\begingroup$ Could you give absolute values for your temperatures and not numbers relative to nothing? Mars is quite extreme btw, I'd start there $\endgroup$ – Raditz_35 Nov 2 '17 at 12:05
  • $\begingroup$ "20 degrees colder" than what, exactly? And is it important to keep ranges the same width they are on (presumably) Earth? $\endgroup$ – Mołot Nov 2 '17 at 12:07
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    $\begingroup$ Agree with @Raditz_35. By the way, when it's a hot summer somewhere in Africa, we're practically freezing here in Russia, let's say, in February. So it's 30C - (-25C) = 55C degrees of difference already on Earth. Or you want 20 degrees higher/lower than on Earth? $\endgroup$ – user2851843 Nov 2 '17 at 12:08
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    $\begingroup$ @Mołot I have clarified the second paragraph. Is that sufficient or do still think there are issues? $\endgroup$ – Slarty Nov 2 '17 at 17:45
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    $\begingroup$ @VilleNiemi thx - also kind of annoying that it got closed. Trouble is what is "obvious" to the person asking (me) is sometimes not so clear to those reading it. I have amended it, hopefully it will be reopened. Which points do you have issues with (other than Dannyboy's option 1!) $\endgroup$ – Slarty Nov 2 '17 at 17:51
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1. Smaller oceans.

Reverse the ratio of land to water. 75% land, 25% water will mean there is no continuous path for water to flow from equator to poles. This results in much lower heat transport. Consider the temperature difference between Hudson Bay and Scotland, both at the same latitude. Look at the climate of Rome compared to New York, aslo at the same latitude. This would create a climate with smaller climate zones. Should get faster evolution (more isolated populations) with overall greater diversity.

2. Less water in the air.

Water is a significant green house gas. And water has a huge latent heat between liquid and vapour phases.

3. Smaller green house effect.

#2 does some of this, but lower CO2 would do it too. During the Earth's warm periods, the equator was not much warmer than today, but palm trees grew up to the Arctic Ocean.

4. Faster rotation -- shorter day.

Stronger coriolis effect. This would make for a larger number of Hadley cells between equator and pole, requiring more heat exchange cycles for energy to move from equator to pole

5. Dispersed land -- more islands, fewer continents

Even if you don't go with more land relative to water, break it up more. With more obstacles and edges, there will be more friction on ocean currents.

6. Shallower seas.

More friction again. Less heat storage.

7. More mountains.

Or just run more ranges east-west. Mountains contain water movement by wringing out the moisture. At present the Americas span the equator. Turn the continents sideways, at latitude 30, and the mountains would block a lot of water vapour moving away from the equator. This would contain heat at the equator.

8. large continents at the poles.

More continental climate with greater temperature swings on an annual basis. There's a reason that Antarctica is colder than the Arctic.

It would be quite easy to create a world that had such extremes that anything that could live at mid latitudes would perish at the equator from heat.

If this isn't extreme enough, give the planet an eccentric orbit. This may mean that you can only cross the equator at aphelion, when the planet is farther from the sun. Give the planet some tilt too, then for one hemisphere, the orbit and axial tilt induced seasons partially cancel, and for the other hemisphere they reinforce.

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Specially configured continents

On Earth, Hadley cells and ocean currents are the primary means of moving warm water and air from the equator to the poles. Earth has several big opportunities for warm equatorial water to go pole-ward. They are: all of the Pacific, the Atlantic and the Indian oceans.

On a planet without those currents, it would be much more difficult for warm water to make it to the poles. Conveniently, those currents can be blocked by continents. Design your planet in such a way that the major ocean currents either can get to or away from the equator.

Slowing down the atmosphere

According to this paper on Hadley cell width on exoplanets, if a planet rotates more slowly than earth, the Hadley cells will grow latitudinally. I don't have a good intuitive grasp for whether a longer day on this planet would make it cooler or warmer. But, this is certainly a tool that could be used. Maybe moving the continents would be enough and the Hadley cells are required to keep the planet from turning into a snowball.

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You did not mention seasons.

Earth has four seasons because of its tilt.

A planet with no tilt would have the temperature differential between poles and equator more stable. Thus, the equator would not go through cooling and heating phases during the planetary year. At the equator, it would always be high noon summer, and at the poles it would always be full dusk winter.

However, a greater tilt would place one or the other pole in shadow from the sun for a greater period of the year. In fact, a complete 90 degree tilt would mean one pole would be in continuous daylight and the other pole would be in continuous darkness for one quarter of the year, and for two quarters, the poles would both be equal in daylight with the equator.

So if you are prepared to accept seasonal variation, you can achieve your goal in the extreme by adjusting the tilt. If you don't want seasonal variation, eliminate the tilt and you would get an extremely hot equator all the time and an extremely cold poles all the time.

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Oceans absorb a lot of heat from sunlight, and oceanic currents, aided by the distribution of the landmasses and the rotation of Earth, transport this heat between the warmer equatorial waters and the colder arctic waters.

If you arrange your continents in a way that these currents cannot form because the lands are in the way, then it will make your equatorial area a lot hotter and your poles a lot colder. (I can't give you numbers, sorry.)

It might also widen the temperature difference if your poles have a lot of high mountains and your equatorial area is rather flat.

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Option one

Give the planet greater volume and decrease the density to keep the same gravitational pull (1G for example). This way, the equator is closer to the sun than before. You could also move the planet accordingly to move the poles "away" from the sun and the equator will be "closer".

Option two

Change atmosphere composition. An atmosphere with lower greenhouse effect (gas composition) will be more exposed to the sun at the equator, increasing the temperature while the sun shines on it and the poles will be colder because the heat will more quickly radiate into space.

Option three

Remove a lot of the oceans or move them. Oceans store vast amounts of thermal energy that are moved with the currents across the planet. This means that the poles will be warmed and the equator will be cooled. (to varying a varying degree) For instance, having no oceans and just thousands or millions of small lakes would keep the equator warmer since the energy will not be moved about like with oceans that are all interconnected like here on earth.

Option four

Eliminating the planetary tilt would also remove the seasonal shifts in temperature furthering the temperature "locking". Seasons seasonally alter the temperature of the poles, having no seasons would result in the poles being constantly colder and the equator warmer.

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    $\begingroup$ Option one is unlikely to work. Distance between Earth and Sun is so big that difference in this distance is insignificant. Earth radius / mean Earth-Sun distance is like 4.26 × 10^-5 (or 0.0000426%), and your increase would be only a small fraction of that. $\endgroup$ – Mołot Nov 2 '17 at 13:00
  • $\begingroup$ It would all depend on the size of the planet. If you were to double the radius of the planet and lower the density accordingly to maintain 1G, we would be 6371km closer to the sun. (earth radius) This is the difference in distance to the sun during mid-day and sunset. That difference is substantial. However, a few meters or even a few kilometers are negligible. $\endgroup$ – DannyBoy Nov 2 '17 at 13:31
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    $\begingroup$ That difference is not substantial. It's still 0.0000852% of Earth radius / mean Earth-Sun distance. Comparing it to the difference during mid-day and sunset is a fallacy : the difference in temperature during these two times of the day is due to the increased portion of atmosphere through wich the sun rays have to pass. People need to understand that the Sun is really, really far away, and that a 6371km distance will not make any difference. If it would well... keep in mind that the difference between aphelion and perihelion is of 5 millions kilometers ! $\endgroup$ – Keelhaul Nov 2 '17 at 13:48

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