4 ms·

> If there was a natural source for oxygen, it would have to come from some sort of cycle in order to be maintained, and there's not a energetically favorable c

> If there was a natural source for oxygen, it would have to come from some sort of cycle in order to be maintained, and there's not a energetically favorable cycle for that. Life is the energetically favorable natural source of oxygen. Or more accurately, it's thermodynamically favorable. Photosynthesis uses light to create intermediate products (eg carbohydrates) which are later metabolized in a way that releases chemical energy and heat. If you consider the incoming light as part of a system including Earth, and not as something acting on a system, you can see that it ultimately increases entropy despite being chemically endothermic. It converts fewer, higher energy photons (in the visible light range) into a higher number of lower energy photons (most of the energy being infrared as a result of the incremental increase in blackbody radiation from the heat generated from metabolism of the intermediate food products) and drives the conversion of simple chemical compounds like urea into highly complex ones like proteins. In other words, the oxygen in the atmosphere is an energetic byproduct of all the light colliding with the surface of earth. There are processes which do essentially the same thing without life. The atmosphere of the moon includes trace amounts of elemental sodium gas from very high energy photons colliding with sodium rocks in a way that cleaves away sodium ions. And the atmosphere of earth contains the even-more-reactive form of oxygen Ozone because of ultraviolet light doing the same thing to molecular oxygen.

andrewla 2y ago
I don't think this interpretation is correct -- at least on Earth, there are no fundamental geophysical processes which can sustain oxygen in the atmosphere at anything but trace levels. Producing oxygen is not energetically favorable under basically any circumstances. Free O2 production was "invented" (as it were) as a way of murdering almost all other life on earth at the time. It's a mistake to look at life in a thermodynamic equilibrium sense unless your time scales are ridiculously long (i.e. burn-out-of-the-sun long).
jmcqk6 2y ago
Yes, you are exactly right, that is my awkward wording for exactly the point you're trying to make. I should have added, "besides life" to the end of my sentence. The fact that there is not a more thermodynamically favorable pathway besides life is probably what allowed life to emerge in the first place. If there was a more efficient way to dissipate that energy, earth would probably be dead. Or put another way: life emerged because it was the most thermodynamically favorable way to dissipate the available free energy in our system.
adrian_b 2y ago
You are right that life does not provide any source of energy, so any results of life activity could also appear when there is no life, including free elemental oxygen. What life does, is changing by many orders of magnitude the speed of certain chemical reactions, including the reaction by which water is decomposed, releasing free dioxygen. The concentration of any substance in our environment is normally an equilibrium concentration, which is determined by the speeds of the chemical reactions that produce and that consume that substance. On a planet without life, the speed by which the stellar light produces free dioxygen is very small in comparison with the speed at which the dioxygen is consumed by oxidizing various substances available in the environment. Therefore the concentration of dioxygen stays extremely small. On a planet with life forms that have developed catalysts (enzymes) for oxygenic phototrophy, like the blue-green algae (cyanobacteria) of Earth, the speed of producing dioxygen increases by many orders of magnitude. The consequence is that the ambient concentration of dioxygen increases until the speed by which it is consumed balances the production speed. The existing dioxygen is consumed by the oxidation of the magmatic rocks that are brought from higher depth to the Earth surface, by the respiration of a number of living beings that increases with the available amount of dioxygen, by fires and nowadays by the oxidation of various reduced substances, such as metals, which are produced by human industry. In conclusion, life alters the speeds by which various chemical substances are produced or consumed, and this greatly alters the equilibrium concentrations of those substances in any environment where life exists.

andrewla 2y ago

I don't think this interpretation is correct -- at least on Earth, there are no fundamental geophysical processes which can sustain oxygen in the atmosphere at anything but trace levels. Producing oxygen is not energetically favorable under basically any circumstances. Free O2 production was "invented" (as it were) as a way of murdering almost all other life on earth at the time. It's a mistake to look at life in a thermodynamic equilibrium sense unless your time scales are ridiculously long (i.e. burn-out-of-the-sun long).

jmcqk6 2y ago

Yes, you are exactly right, that is my awkward wording for exactly the point you're trying to make. I should have added, "besides life" to the end of my sentence. The fact that there is not a more thermodynamically favorable pathway besides life is probably what allowed life to emerge in the first place. If there was a more efficient way to dissipate that energy, earth would probably be dead. Or put another way: life emerged because it was the most thermodynamically favorable way to dissipate the available free energy in our system.

adrian_b 2y ago

You are right that life does not provide any source of energy, so any results of life activity could also appear when there is no life, including free elemental oxygen. What life does, is changing by many orders of magnitude the speed of certain chemical reactions, including the reaction by which water is decomposed, releasing free dioxygen. The concentration of any substance in our environment is normally an equilibrium concentration, which is determined by the speeds of the chemical reactions that produce and that consume that substance. On a planet without life, the speed by which the stellar light produces free dioxygen is very small in comparison with the speed at which the dioxygen is consumed by oxidizing various substances available in the environment. Therefore the concentration of dioxygen stays extremely small. On a planet with life forms that have developed catalysts (enzymes) for oxygenic phototrophy, like the blue-green algae (cyanobacteria) of Earth, the speed of producing dioxygen increases by many orders of magnitude. The consequence is that the ambient concentration of dioxygen increases until the speed by which it is consumed balances the production speed. The existing dioxygen is consumed by the oxidation of the magmatic rocks that are brought from higher depth to the Earth surface, by the respiration of a number of living beings that increases with the available amount of dioxygen, by fires and nowadays by the oxidation of various reduced substances, such as metals, which are produced by human industry. In conclusion, life alters the speeds by which various chemical substances are produced or consumed, and this greatly alters the equilibrium concentrations of those substances in any environment where life exists.