Can my planet have a very thin atmosphere only at the poles?
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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