On the idea of heavy atmospheric Xenon

I am wondering about the plausibility of an idea for a Si-fi planet, mostly seeing if it would be like I imagine, and what else would be needed for what I want.


I am thinking of a planet with large amounts of atmospheric Xenon. For an explanation I have the idea of uranium under the surface being put under intense pressure, forming into iodine-129, which flows out of geothermal vents, volcanos, and hot springs as stabilised Xenon-129. Is my chemistry there correct? It seems correct that Iodine-129 can form from spontaneous fission of Uranium, and that Xenon can form from Iodine-129, and is often found flowing out of springs.


I want this to be a case where it flows into the atmosphere from this, in very large amounts, and makes it highly toxic. But reading into Xenon more it seems like it might be too dense to actually be able to spread through the atmosphere and become a part of it in high numbers, instead of sinking to the ground. Is this true, or is my idea plausible?




The degree to which gasses mix and stratify in an atmosphere is dependent on a number of parameters for the planet (e.g. surface gravity, average temperature, length of day, tidal forces...). But all of these complications aside, there is a notion of [scale height](https://en.wikipedia.org/wiki/Scale_height), which indicates roughly how high you might expect a gas to reach in an atmosphere. Importantly, this height is inversely proportional to the molecular weight of the gas. The molar weight of xenon is about four times heavier than a typical terrestrial planet atmospheric gas (e.g. nitrogen), so it would concentrate near the ground much more. However, for Earth-like conditions, the scale height of xenon is still measured in kilometers.


The only real "problem" I see is that [xenon is a fairly rare element in the Universe](https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Solar_system), and uranium much more so still, *especially* considering fissile U-235 (the proposed source of your xenon) has a half-life of 700 million years, which is reasonably long but not outrageously so. It seems fairly unlikely there is any natural mechanism by which a planet could have a substantial atmosphere composed of even 1% xenon. But then, these details can be just brushed away.


You might be interested to know that under the right conditions on Earth, even CO2 (less than half the weight of xenon) [can concentrate enough near the ground to be deadly](https://en.wikipedia.org/wiki/Mazuku), and for xenon this would be much easier. This means that even a relatively small abundance of xenon could still have deadly effects.


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