Yes, but what it explicitly does NOT say is that all atmospheric oxygen derived from organic processes. Life did initiate a rapid transformation to an oxidising atmosphere, or more correctly it infinitely accelerated a process that had been ongoing for billions of years. That’s very different form saying that before photosynthesis there was no free oxygen and no way of producing free oxygen. We know that simply isn’t true, and even Mars has around 5% atmospheric oxygen IIRC. AFAIK nobody seriously suggests that Mars’ significant oxygen concentration is due to the presence of life. It’s due to the breakdown of water and carbon dioxide in the upper atmosphere, just as was the case on Earth pre-photosynthesis.
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Also, there are other even more ancient photoautotroph bacterial lineages like the green non-sulfur bacteria.
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Not sure what the relevance of this is. The sulfur bacteria use sulfur as the electron donor, not oxygen.
No, it is saying that initially any free oxygen would have been rapidly bound up with sulfur and carbon as well as other elements. Only after all those elements had been fully oxidised could oxygen start accumulating in the atmosphere.
Of course that is the case no matter where we postulate the free oxygen came from. The more easily oxidised elements had firs dibs on the gas, and only after they had been taken care of could the atmosphere start accumulation oxygen.
Broeker’s point being that photosynthetic life only developed after both the atmosphere and the crust ceased to be strongly reducing. That being the case we would then have expected to see accelerating levels of atmospheric oxygen accumulation even on a sterile planet. However as Stranger points out the predicted rate would only have been a tiny fraction of the observed rate. We assume the difference was made up by photosynthesis.
What about the abundance and proportions of oxygen, methane and nitrous oxide all at the same time. Can you have an atmosphere with the same proportions of those gases as we have on earth due to abiotic processes?
I can’t see why not. Methane and nitrous oxide are produce through volcanic activity and presumably with equivalent oxygen and equivalent production of volcanic methane it will reach the same equilibrium seen on Earth.
I guess it all depends on knowing the exact composition and activity of the planet crust beforehand.
Yeah, I failed to preview a last time after editing, and didn’t catch it.
This contradicts what I’ve previously read on the topic, but as you note, our understanding of planetary conditions and atmospheric composition 2Byr+ ago are limited at best, and obviously subject to revision as we gain more knowledge and better fitting hypotheses are proposed. Can you point me toward some more technical references on the topic?
I hate to hijack this thread back to the question, but…
A highly oxidizing environment (whereever the O2 came from), adjacent to environments that are strongly reducing, means that you have a zone between that is in great chemical disequilibrium.
In the absence of an absolute definition of what “life” is (since we only have one data point from which to extrapolate), I propose that the best place to look for life would be a place that displays this type of disequilibrium. Lots of chemical reactions, transferring lots of electrons, making lots of energy available for exploitation.
This doesn’t restrict us to looking for O2-N2 atmospheres (some levels of Venus’ atmosphere are highly active, with great disequilibrium), but if I found a planet that had high concentrations of gases (> than one or a few percent) in that great a disequilibrium that’s the first place I would look.
Perhaps we’re using different standards of a “significant amount”, Blake? It looks like you’re talking about abiotic processes leading to an equilibrium level of elemental oxygen around 1% of the present levels. I wouldn’t call that “significant”, but perhaps you are?
Johnny L.A., there’s no puzzle about why Earth’s atmosphere is so thick; if anything, the puzzle is why it’s so thin. Earth is both more massive and cooler than, say, Venus, both of which factors should mean that we could support a thicker atmosphere than Venus’. But in fact, Venus has an atmosphere nearly a hundred times thicker than Earth’s. I don’t know how current this is, but the best explanation I’ve heard is that we lost most of our atmosphere in the same cataclysmic event which produced the Moon. Mars also has a relatively thin atmosphere, given its temperature and mass, and I think that one is explained by Mars’ weak magnetic field, which is not enough to shield it from the solar wind, which blows away the atmosphere.
It looks like Blake and I are the only ones who are of the opinion that the presence of oxygen is not a sign that life must exist. The chances are very high, but not 100%. But the OP also had a question about what other indicators to look for. I would think that water is also important, maybe even more important than oxygen.
I disagree here. Water is a good medium for life because it is a good solute, but there are a number of other good solutes out there, including ammonia and liquid methane.
Life (at least as we know it) is a process that exploits the energy difference between points of chemical disequilibrium. O2, when present, always represents disequilibrium. No matter where the O2 comes from it is always anxious to react with something with electrons to donate. Practically the entire universe is a reducing environment therefore whereever the oxygen is, it is never far from an element or compound with which it can react.
As Blake said, even if the O2 is the result of dissociation of, say, water molecules in the upper atmosphere, individual O2 molecules don’t hang around long before they are gobbled up by other reactions. If the O2 concentration is at some equilibrium it is because the reactions releasing it (whether organic or inorganic) are at equilibrium with the reactions using it. That means a source of energy to be exploited by some life form.
That depends on whether you’re looking for necessary or sufficient conditions. Water is probably by far the most common medium for life in the Universe, even if something can be contrived with ammonia or the like. So if you see a world without liquid water, it’s probably a safe bet that there’s no life there. But water is also one of the most common compounds in the Universe, so you’re going to see plenty of worlds which have water but no life (including probably some in our own Solar System).
So a world which has neither water nor oxygen (such as Mercury) would probably be dead, a world with both (such as Earth) would probably be alive, a world with water but not oxygen (such as Europa) would be of unknown status, and a world with oxygen but no water (if we were ever to discover one) would be an enigma.
I’ve seen this impact hypothesis proposed from time to time as well, and I don’t see why it’s necessary. On the surface it seems necessary to explain Venus’ thicker atmosphere, relative to Earth, since it’s much hotter. But the average molecular mass of Venus’ atmosphere is almost half again heavier than Earth’s. If you look at a heavy, non-reactive gas like Argon, you see that its total mass is about the same (again, within about 50%) on both planets. This suggests that, assuming similar initial chemistries, neither planet has lost a significant fraction of its atmosphere.
I think you meant to say solvent instead of solute. Water is special because of its hydrogen bonding ability. Ammonia can do this as well, but boils at -33° C. It also freezes at -77° C, so there is only a 44° range in which it is liquid. Methane has an even lower boiling point, is non-polar and not really a good solvent for anything but other non-polar stuff. This does not mean that there cannot be any life where the main solvent is ammonia or methane, but it would be very slow.
I think we all agree that neither water nor oxygen are sufficient for life. So the answer to the question is “no”. Furthermore, oxygen is not necessary and water seems to be necessary for the type of life that exists on this planet.
That depends on whether you’re looking at it from a causative or evidentiary point of view. If I fill up a tank (let’s say it’s lined with gold, so it won’t corrode) with pure oxygen, and let it sit for a very long period of time, it’s not inevitable that life will arise in that tank (in fact, I’d be pretty confident in saying that it’s impossible). So in a causative sense, oxygen isn’t sufficient for life. But if I discover another planet, and the only thing I know about it is that it has a lot (say, a fifth of the total) of oxygen in its atmosphere, I will conclude, based on that single piece of evidence, that that planet has life. So in an evidentiary sense, oxygen (in large enough amounts) on a planet is a sufficient condition for life.
All I am trying to say is that the planet with an atmosphere containing 20% oxygen could be like your fish tank. We all agree that if it’s just oxygen (and water), nothing will happen. Therefore oxygen is not sufficient. As far as being evidence for life, it is still not sufficient because something is required for oxidation along with the infrastructure to regenerate that something plus oxygen.
A planet with a significant amount of oxygen in its atmosphere would almost certainly have life and we would be disappointed and surprised if it did not. But to say that a planet with such an atmosphere **must **have life isn’t correct.
Hoo boy we sure have a bunch of nitpicky people here. All I wanted was to have a discussion
of what would happen if we discovered a planet (via the proposed Terrestrial Planet Finder
or the like) which had a number of spectroscopic signatures of key elements and compounds
(including but not limited to oxygen, nitrogen, and water vapor)-what would be our confidence
interval basically that it would have life? What else would we need as further evidence
that the planet in question does have life? Semantic nitpicking (“might” vs. “must”) doesn’t
serve the discussion well at all, even if I agree with Roog’s last line. Maybe you can fault
me for not spending 3 hours going over my OP with a fine tooth comb to eliminate such logical
and semantic errors and make it a bit more to the point. In other words focus on
the spirit of my question and not the letter.
Sooo…back to the topic. What else do you need? Can we build the right
instruments reasonably soon, or will we need something beyond near-future
tech to take it step further?
The sediments referred to are largely the remains of pre-aerobic unicellular such as cyanobacteria – unicellular photosynthetic bacteria which formed giant sedimentary structures <crudely akin to modern coral> called stromatilites, dating back to at least 3.5 billion years, a billion years before the transition to a “free oxygen” atmosphere.
For comparison, the oldest verified Earth rocks, the last I checked, were just 3.8 billion years old.