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Discussion lead to citing Why does Earth's atmospheric density have a big "knee" around 100 km? Is there a good analytical approximation? who's answer is "monatomic oxygen". For some reason above 100 km O2 tends to split into two single atoms of oxygen and because they are lighter the exponential drop-off of density with altitude slows down.

Apparently this is not happening so much for N2 compared to O2 which suggests that that 1:4:9 monolith that the aliens put in LEO that was supposed to stay there ended up reentering the atmosphere a few billion years ago when the Great Oxidation Event happened, so they had to come back and put a new one on the Moon.

If we had a nitrogen atmosphere without such a pronounced knee the ISS wouldn't have to boost itself nearly so often.

Question: How many solar system bodies have pronounced "knees" in their atmospheres? These could be due to monatomic oxygen or any other atmospheric constituent. I'm just curious if this anatomical-like feature is unique to Earth or if it's been seen elsewhere in the solar system.


From Why does Earth's atmospheric density have a big "knee" around 100 km? Is there a good analytical approximation?:

Earth's atmosphere's "knee" at around 100 km due to monatomic oxygen

Earth's atmosphere's "knee" at around 100 km due to monatomic oxygen Earth's atmosphere's "knee" at around 100 km due to monatomic oxygen

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    $\begingroup$ Thinking out aloud. I'm wondering if part of the reason for monatomic oxygen at height is due to the lower bond strength of O2 compared to the bond strength of N2. UV energy from the Sun splits O2 to 2O. The monatomic oxygen atoms then either stay monatomic or they combine with O2 to for ozone. $\endgroup$ – Fred Jan 19 at 8:57
  • $\begingroup$ @Fred I don't know my entropy from my enthalpy from my chemical potential from my half-reactions so I haven't a clue how to look those up. What are the bond strengths for O2 and N2? $\endgroup$ – uhoh Jan 19 at 10:19
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    $\begingroup$ Bond strength & energy, in kJ/mol, O2: single bond 140, double bond 498; N2: single 160, double 418, triple 946. Also this $\endgroup$ – Fred Jan 19 at 10:25
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    $\begingroup$ I provided an alternative answer to the linked question. $\endgroup$ – David Hammen Jan 19 at 12:53
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Any planet or moon that has an atmosphere that is sufficiently dense at some point to support turbulent mixing will have a density knee above which the atmosphere fails to be dense to support turbulent mixing. This density knee is the object's turbopause. The Earth's atmosphere has a turbopause, as do the atmospheres of Venus, Mars, the giant planets, and Titan. The Earth's Moon does not have a turbopause. The extremely rarified atmosphere of the Earth's Moon does not support turbulent mixing, even at the Moon's surface.

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