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While thinking about Starfish Prime's answer to the question Algae using UV light from auroras for photosynthesis, I considered the possibility of an alternate Earth which has a normal, Earth-like atmosphere throughout most of the globe, but a thin atmosphere at the poles. This would allow for higher amounts of ultraviolet light to penetrate at high latitudes (say, > 60$^{\circ}$ N and 60$^{\circ}$ S). The problem is, I have absolutely no idea how this could happen.

Here are my specifications and requirements:

  • It's an Earth-like planet - that means the same mass and mean radius.
  • The atmosphere is like Earth's: roughly 78% nitrogen and 21% oxygen.
  • At all latitudes of less than 60$^{\circ}$, the atmospheric structure should be identical to Earth's.
  • At all latitudes greater than 60$^{\circ}$, the atmosphere should become less and less dense until, at the geographic poles, the column density becomes approximately 10% of the normal value.
  • The terrain at the poles should be approximately the same as it is on Earth right now. No gigantic mountain ranges, for instance.
  • The atmospheric structure should be stable on timescales of a few billion years.
  • Whatever mechanism causes the underdensity should be natural (and, ideally, abiotic).
  • No magic, please. Let's stay within the confines of the laws of physics as they exist in our universe, too. Please try to minimize or avoid speculation.

Is this weird polar atmospheric structure possible, within the requirements set forth? I'm not requiring hard science answers, but answers should be hard science-y enough that they can justify that they meet the requirements - especially the column density.

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    $\begingroup$ Penetration of UV light is mostly blocked by the ozone layer, which is practically destroyed over the south pole - even though atmospheric pressure there is not much affected by this. $\endgroup$ – Renan Aug 22 at 15:02
  • $\begingroup$ "Earth-like atmosphere throughout most of the globe, but a thin atmosphere at the poles." So you mean Earth? $\endgroup$ – David Robie Aug 23 at 19:29
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If you want to have a substantial pressure gradient, and you want it stable over geological eras, you need something to oppose the gradient.

What can you use? But of course the planet rotation, which when fast enough will tend to spread liquids, gases and even solids along the equator.

Since you want it to be stable over billion of years, you cannot afford a Moon, since it's tides inducing influence would slow down the planet rotation like it did with our planet.

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  • $\begingroup$ The speed of rotation needed to lower the pressure at the poles as much as OP has specified is not achievable before friction between the air and the planet's surface is overcome. You'll have a planet spinning under its atmosphere, without providing the effect you want. Uranium enrichment centrifuges must spin at super sonic speeds, and they're working with extremely heavy nuclei. $\endgroup$ – stix Aug 22 at 19:51
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If you wish for more UV light to reach the poles, you can deplete the ozone layer over them. This is exactly what happened on our world - ozone depleting chemicals reached the upper layer of the atmosphere and due to wind currents they concentrated over pole over the decades, specially the south pole.

The hole over the south pole is shown as the blue-ish hues below:

Ozone hole over south pole

Source: https://earthobservatory.nasa.gov/images/38835/antarctic-ozone-hole-1979-to-2008

The ozone layer does not block all UV, and it is not the sole blocker of UV radiation. It does screen out most UV-C and UV-B radiation, which are the most harmful forms for life.

If you wish for a mechanism that does strip the poles of all gases, though, for a lower atmospheric pressure at the poles... I do not think such a thing would be feasible. The difference in pressure would mean wrath-of-god force winds at the polar boundaries. The pressures would either be equaled over relatively short geological time, or the permanent mega-hurricanes would make for a planet possibly devoid of life, with much different geological features due to erosion, most possibly a mix of both.

If you reduce the constraints to allow for planets similar to Earth during planet formation but different from current Earth: maybe a tectonic event that causes the plates on the poles to collapse, for a hadean climate over them. This might be stable for hundreds of millions of years. The exposed mantle would heat up the air above it, causing the air to expand. It will be forced out of the polar regions with a lot of force, which may create a pressure boundary. Where it meets colder air it will go over and above, reducing its pressure over the pole while also maintaining a vortex over the region. Such a planet would be hellish at the poles, so the whole point of the other question about having UV-feeding algae would be lost.

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Short answer, no, this is not possible. The column depth of an atmosphere is based on the gravity of the planet and the total mass of the atmosphere. A gas will expand to fill its container (that's the prime definition of a gas), and in this case the container is the extent that gravity can hold it to the planet.

However, all is not lost if you're willing to play with your planet's geology a little.

If you assume that above 60 degrees latitude there is a very large mountain range that increases in altitude until it gets to the poles, it becomes possible to thin your atmosphere out.

This somewhat stretches credibility from a plate tectonics standpoint, but it shouldn't be impossible. What you need to do is have two large plates at either end of the planet, then along the equator have rifts that spread outward. This is somewhat opposite from Earth's plate tectonics, where the rifts and subduction zones generally travel along north-south longitudes, but there's no real reason that you couldn't turn everything 90 degrees and have spreading along latitudes.

On Earth rifts and subduction zones are generally in the oceans, but not always. Iceland is a prime example of where rifts come up onto land, but subduction zones are almost invariably under the ocean (they push down the crust, and eventually it falls below sea level). However, the Himalayan mountains are very similar to a subduction zone, and it's no coincidence that the Earth's tallest mountains are in the Himalayas.

So your planet would probably have the following characteristics: A large equatorial sea containing a rift zone around the equator of the planet. This pushes new plate material north and south, where it collides with the polar plates at around 60 degrees latitude. Because the collisions happen along the entire circumference of your planet, they push inward on the plate, forcing massive mountains up at the poles. These mountains are extremely tall, perhaps 1.5 times as tall as Everest, but much wider, covering nearly the entire polar continents, so, like Olympus Mons on Mars, you can't really tell you're on top of a mountain at the poles. Because they rise so much in altitude, the pressure drops dramatically. In fact, at 10% pressure it is nearing the Armstrong limit, where water boils at the temperature of the human body (in reality on Earth this limit is around 60,000 feet, so a mountain twice as tall as Everest would be well above it).

Because of the massive subduction zones surrounding your polar continents, the coast lines have a reputation for producing massive tsunamis every few hundred years. Most people avoid living by the seas because of this. Those that do build on high ground and have learned to construct dwellings that can either withstand large earthquakes or be easily rebuilt after them.

The equatorial seas have no real land masses to obstruct winds, so extremely high winds are able to form. Maybe the areas below 30 degrees latitude are known as "The Banshee's Tempest" due to the strong storms that are almost constantly forming. Below 10 degrees latitude however, is the "Central Oasis Belt" where strong storms are unable to form due to Hadley cells and the intertropical convergence zone suppressing them. Since this area is also near the spreading rifts, small island chains would likely form when the rift mountains get large enough to peak above sea level, perhaps resulting in many small island nations around the belt of your planet. Because the islands are bordered to the north and south by the Banshee's Tempests, the island nations are mostly isolated from the rest of the world.

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