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**BEyng6hj** · 08-15-2012, 06:10 AM

Bit of a science-fiction question: are there any circumstances in which a completely lifeless planet/moon could sustain enough atmospheric oxygen to support some humans for some time? (I can't think of any). Give it water or ice, or whatever elemental weirdness you can think of, as long as humans could conceivably walk around there without vac-suits. But no native life, not even bacteria. There are many circumstances that immediately occur to me:

There was life there but oxygen production killed it off. To all the original organisms on Earth, oxygen was a deadly poison, so it makes sense that when photosynthesis started oxygen production it killed off everything on the planet.
There was life there but the planet's crust completely melted either because of internal heat or because of a major impact. This melting killed off everything but left the chemical composition of the atmosphere largely intact.
The planet began cold, so had plenty of water ice and ammonia around. Ultraviolet radiation from the newly forming sun split the water into hydrogen and oxygen and the hydrogen being less dense escaped into space. The resulting atmosphere would be an oxygen-nitrogen mix. Oxygen is deadly to primitive life so no life would have formed.
The atmosphere came from de-gassing from the interior, allowing buried oxygen-containing compounds to release oxygen into the environment.

Perhaps the most telling argument is the following. The elements in the universe are, in order of abundance down from highest abundance: Hydrogen, Helium, Oxygen, Carbon, Nitrogen, Silicon, Magnesium, Neon, Sulphur, Argon, Aluminium, Iron. Through photo-dissociation and low density, hydrogen and helium are lost. The atmosphere would then be largely carbon dioxide and nitrogen. Then with heat the carbon dioxide dissociates and the carbon is deposited on the surface and buried leaving an atmosphere dominated by oxygen and nitrogen. No life required.

To me, that seems a far more likely scenario than that of a world with both life and an oxygen atmosphere. I expect life on planets with methane atmospheres to be far more likely than on planets with oxygen atmospheres.

08-15-2012, 06:10 AM	#22
BEyng6hj Join Date Oct 2005 Posts 495 Senior Member	Bit of a science-fiction question: are there any circumstances in which a completely lifeless planet/moon could sustain enough atmospheric oxygen to support some humans for some time? (I can't think of any). Give it water or ice, or whatever elemental weirdness you can think of, as long as humans could conceivably walk around there without vac-suits. But no native life, not even bacteria. There are many circumstances that immediately occur to me: There was life there but oxygen production killed it off. To all the original organisms on Earth, oxygen was a deadly poison, so it makes sense that when photosynthesis started oxygen production it killed off everything on the planet. There was life there but the planet's crust completely melted either because of internal heat or because of a major impact. This melting killed off everything but left the chemical composition of the atmosphere largely intact. The planet began cold, so had plenty of water ice and ammonia around. Ultraviolet radiation from the newly forming sun split the water into hydrogen and oxygen and the hydrogen being less dense escaped into space. The resulting atmosphere would be an oxygen-nitrogen mix. Oxygen is deadly to primitive life so no life would have formed. The atmosphere came from de-gassing from the interior, allowing buried oxygen-containing compounds to release oxygen into the environment. Perhaps the most telling argument is the following. The elements in the universe are, in order of abundance down from highest abundance: Hydrogen, Helium, Oxygen, Carbon, Nitrogen, Silicon, Magnesium, Neon, Sulphur, Argon, Aluminium, Iron. Through photo-dissociation and low density, hydrogen and helium are lost. The atmosphere would then be largely carbon dioxide and nitrogen. Then with heat the carbon dioxide dissociates and the carbon is deposited on the surface and buried leaving an atmosphere dominated by oxygen and nitrogen. No life required. To me, that seems a far more likely scenario than that of a world with both life and an oxygen atmosphere. I expect life on planets with methane atmospheres to be far more likely than on planets with oxygen atmospheres.
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