I'm pretty sure it is literally true that the oxygen in the atmosphere is there only because living things keep putting it there. You're right that without life, it would be sequestered in oxides pretty quickly. That's the local minima for energy dissipation. Life is good at breaking those minimas for cyclic matter dissipation. 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.
It's why it makes a good biomarker when looking at exoplanets. If we find an exoplanet with high amounts of oxygen in the atmosphere, we can be fairly confident
Here is a quote from Nick Lane’s great text: Power, Sex, Suicide (p 153):
> The early Earth, as envisaged by [Michael J] Russell, is a giant electrochemical cell, which depends in the power of the sun to oxidize the oceans. UV rays split water and oxidize iron. Hydrogen, released from the water, is so light that it is not retained by gravity, and evaporates into space. The oceans become gradually oxidized relative to the more reduced conditions of the mantle.”
Lanes cites this paper “On the origins of cells: A hypothesis for the evolutionary transition from abiotic to nucleated cells”, 2003, by Martin and Russell
There's nothing in it about ultraviolet splitting of water or oceanic oxidation. If Nick Lane meant to paraphrase the paper in that cited passage, he did a poor job.
Direct UV homolysis of water to release hydrogen requires a photon with more than 6.5 electron volts of energy [1], corresponding to a wavelength of 190 nm or shorter. As you can see here, solar irradiance is extremely low at wavelengths shorter than 240 nm:
There isn't enough energetic UV radiation emitted from the sun to directly oxygenate the Earth via water homolysis. It might be possible for an exoplanet in orbit around a hotter star that emits more energetic UV.
EDIT: I forgot to account that the sun may have had a very different UV profile billions of years ago.
"UV radiation from the young Sun and oxygen and ozone levels in the prebiological palaeoatmosphere"
UV measurements of young T-Tauri stars, resembling the Sun at an age of a few million years, have recently been made with the International Ultraviolet Explorer. They indicate that young stars emit up to 10^4 times more UV than the present Sun.
I'm a huge fan of Nick Lane, and I'm not an expert, so I may have misunderstood. I have not read "Power, Sex, Suicide" but have read "The vital question", "transformer," and most importantly in this context, "Oxygen."
My understanding, which could absolutely be wrong, is that there are pathways to where Oxygen can be generated without life, but for it to be maintained at high levels of concentration over time, that takes life. I would definitely defer to whatever Nick Lane has to say about it.
To reduce hydrogen, something needs to be oxidized, but it doesn't need to be oxygen. E.g. you could get more metal oxides. Not my field exactly, so I don't know either relative abundances of different stuff in early oceans or the exact ranking of which would be most readily oxidized, but that could explain the discrepancy.
Edit: rereading the passage you quoted does make me think that metal oxides are the important factor here.
And not true at all that all organisms need O2 to pump protons. Microbes have quite a few alternative ways. Even we do when we run the Krebs cycle backwards.
Yeah, that's an exciting discovery. I was excited by this, because I believe it demonstrates a dissipative pathway that could have contributed to abiogenesis. Energy gradients are a driver of emergent complexity.
> 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.
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.
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.
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).
I've never understood applying this idea to exoplanets. What if the life there puts sulfur into the atmosphere instead of oxygen? Why would the life elsewhere look anything like the life here in terms of gas use, etc
The point is that if a planet has oxygen, it's a potential marker for life. No one is claiming that a planet that lacks oxygen in the atmosphere must necessarily be lifeless.
It's true that if life exists on other planets, it may not look exactly like life here. But it's also true that there are a small number of elements in the periodic table, and only so many of those are even relatively common in the universe, and only so many of those are useful for reactions, etc, etc. The things that life on our planet use seem to be some of the most obvious candidates to use, if not the most obvious, so if life exists on other planets, it would be surprising if the things that we use are unique or even uncommon across the universe.
It's why it makes a good biomarker when looking at exoplanets. If we find an exoplanet with high amounts of oxygen in the atmosphere, we can be fairly confident