Pursuant to our discussion(s) of interstellar travel, what if that Terrestrial Planet
Finder (if it ever gets built) is able to image a planet and discovers an atmosphere
(via spectroscopic analysis) very similar to ours? Does that mean it must have life?
What other markers should we look for to confirm its presence? Are there ways for
nonliving processes to generate (and maintain, natch) such an atmosphere?
I’ll note Europa naturally secretes oxygen, so at least we might conclude that any
such planet is an ice world if nothing else…
Maybe. But only because life exists in Earth’s oxygen/nitrogen atmosphere.
On the other hand, life can exist without oxygen. Even here. Therefore, no matter what kind of atmosphere might be indicated by spectral analysis, nothing can be concluded about the presence or absence of life.
I though the presence of free oxygen in an atmosphere would be s strong indicator of carbon-based life. if there was no life, wouldn’t the free oxygen get bound up in compounds? like the iron oxide dust on Mars-it seems to indicate no plants are photsynthesizing (O2 would be a product of that reaction). of course, if the life is silicon-based, maybe there is no free oxygen).
If it wasn’t for all the plants dumping oxygen into the atmosphere it would disappear very quickly. Now, there could conceivably be some exotic chemical process dumping 02 into some planet’s atmposphere, but the most likely explanation is that the “exotic chemical process” could be termed “life”.
But other thermodynamically improbable atmospheric compositions could indicate other types of life too, just not standard terran photosynthesis. Life existed on earth long before photosynthesis evolved, so it wouldn’t be surprising to find planets that have life but no photosynthesis.
Roog, you’re quite correct that life can exist without free oxygen. But free oxygen cannot, so far as we know, exist in any significant quantity without life. It’s one of the most reactive elements known, and unless there’s something continually replenishing it, it’ll all be bound up quickly.
Where did all the iron oxide (on Mars0 come from? Is it possible the marian civilzation had steel buildings…and all of them crumbled into rust? Seriously, the planet is covered with the stuff-is Fe2O3 all that common on earth?
It is wherever unalloyed iron is found. Oxygen also forms other oxides with virtually every element save for the noble gases, and is found in many complex organic compounds. Next to fluorine, it is the most electronegative oxidizer and its universal availability (it’s formed as part of the CNO fusion cycle in high mass stars) makes it both useful for the highly active chemical processes of life and unlikely to be found uncombined in nature.
As Chronos says, life can exist without oxygen (and it’s actually toxic in quantity to many primitive forms of life), but free oxygen is unlikely to exist without life processes producing it. If we find a planet that shows an O[sub]2[/sub] band in spetroscopy, it would virtually be a given that life, of some kind, must exist there.
If all life suddenly stopped, would the accumulated oxygen react and go away? I was under the impression that pretty much anything that can oxidize on Earth already has.
Eventually. Continuing volcanism will constantly spew gasses and other materials which are capable of being oxidized, slowly taking the oxygen out of the atmosphere and replacing it with other stuff.
As plant matter decayed and/or burned it would be converted into mostly CO[sub]2[/sub] and a smattering of organic compounds. Existing compounds (including those in organic matter) are in a constant state of oxidation and complementary reduction, the latter mostly powered by photosynthesis.
According to most of the world’s biogeochemists oxygen can and did exist in significant quantities without life. Photosynthesis didn’t create an oxygen atmosphere, an oxygen atmosphere permitted photosynthesis. Without a significant amount of atmospheric oxygen their can be no ozone layer, and without an ozone layer photosynthesis is extremely difficult to evolve.
The reactivity of diatomic oxygen is constant regardless of whether it is created through organic or inorganic means. The molecule obviously can’t know how it was created and somehow alter its reactivity. As such free oxygen will get bound up in compounds regardless of how it originated.
The only point of interest is whether oxygen is being released faster than it is being bound up. If so the atmosphere will become increasingly oxygenated. If not it will not. It doesn’t make a scrap of difference how the oxygen is being released.
That’s going to come as news to all those biogeochemists who know that oxygen was (and indeed still is) produced in appreciable quantities by high energy radiation shattering water and other compounds in the atmosphere.
An oxygen rich atmosphere only tells us that the rate of consumption of oxygen is lower than the rate of production. It tells us zip about the actual means of production and certainly isn’t a sure sign that life exists. The only way we could even make oxygen into a vague guide to the presence of life is if we knew that the levels were far higher than could possibly be sustained given the atmospheric composition, temperature, geological composition of the crust, degree of geological activity, radiation shielding from ozone, van Allen belts etc. and numerous other factors. IOW we’d basically have to do a close flyby if not land on the planet to be able to use the presence of oxygen as a pointer to the presence of life.
Not to hijack the thread, but I have a question that I think fits better here than in its own thread.
I’ve heard that one reason Earth’s atmosphere is so thick is because of our strong magnetic field. I’ve heard that Mars’s magnetic field is weak, and that the solar winds gradually scoured the atmosphere away because the magnetic field was not strong enough to deflect it. (So what’s Venus’s magnetic field like, and why is its atmosphere so thick?)
If we found planets that from a distance resemble Earth (e.g., they’re about the right size and position from a suitable star), and could somehow detect a strong magnetic field on one of them, would that planet be more likely to have a thick atmosphere and life?
Cite? Every explanation I’ve seen credits early photosynthetic bacteria (as well as other archaic bacteria with more exotic reducing metabolisms) as the source of oxygen in our atmosphere.
Here is one of the better ones for a layman. I’ve never actually seen a reputable reference suggest that life is solely responisble for the oxygen in our atmosphere. That particular belief seems to stem from junior high science books, usually the same type that teach that forests produce oxygen, that Columbus discovered America and Henry Ford invented the motor car.
At peril of crossing swords with the learned Blake, I am, like lazybratsche, going to have to ask for a a cite here. It is my understanding that it is firmly established that early life–that which proceded cyanobacteria–were anaerobic in their respiration mechanics and could not tolerate more that an small fraction of oxygen in the ambient environment. It is true that diatomic oxygen (as well as a small but important amount of ozone) is produced by bombardment in the upper atmosphere, but only at a minor rate (1-2%) compared to oxygen separated by noncyclic photophosphorylation, insufficient to explain the amount of diatomic oxygen in the atmosphere, particularly coming from a reducing atmosphere where oxidative recombination must have been a major hurdle.
It’s certainly true that a screening ozone layer permitted photosynthesis by filtering out the most harmful radiations and leaving a useable amount of energy, but there’s no contention that ozone is produced anywhere in significant quantities other than the upper atmosphere. I don’t know of any natural nonorganic processes that produce free oxygen in the quantities required to explain a high (>20%) oxygen content, and in a reducing atmosphere such processes would have to be accompanied by others which bound up the carbon and nitrogen which would otherwise recombine with oxygen.
If my understanding of the basic biochemistry and composition of the primitive atmosphere is out of date, I’d welcome sources and information that are more current.
That seems in line with what they’re saying over at this U of Michigan site: Evolution of the Atmosphere; photolysis of CO[sub]2[/sub] and H[sub]2[/sub]O in the early atmosphere leading to ozone formation, and maybe 0.1% atmospheric O[sub]2[/sub].
Stranger I posted a reference to the world leader in this field 5 minutes before your last post. I’m assuming you missed it.
Note that nobody disputes that life is responsible for a sizable portion, probably most, of the oxygen in our atmosphere. The important point in addressing the OP is that the Earth’s atmosphere contained a significant amount of oxygen well before life started producing any. IOW we know that significant amounts of oxygen are produced through normal inorganic processes.
What that means is that, even if the Earth had never developed life, any spectral analysis would still have detected large amounts of diatomic oxygen. If we accept that other planets need not have anything like the same atmospheric composition, the same degree of radiation shielding, the same amount of geological activity and so forth then we are forced to conclude that an abundance of free oxygen in the atmosphere tells us approximately nothing about the presence of absence of life.
A geologically inactive planet with massive amounts of water and large amounts of ionising radiation could presumably have a higher oxygen atmosphere than the Earth itself. Indeed it has often been suggested that the only reason the Earth’s oxygen content remains so low is because once it increases much it promotes massive conflagrations which suck the oxygen out of the atmosphere. A sterile yet geologically stable world OTOH can continue to add oxygen to the atmosphere almost indefinitely since there is simply nothing available to sequester the free oxygen.
As for oxidative recombination being a major hurdle, that requires numerous untestable assumptions about the geological activity of the primitive Earth. While the rate of oxygen production through inorganic means is low compared to organic the time span was also much longer. 2% of the annual production for 2 billion years is exactly the same as 100% for 20 million years. The only proviso is that production outstrips the amount being tied up by geological activity. Aerobic respiration can be ignored because it evolved from photosynthetic pathways after atmospheric oxygen concentrations were already elevated. Of course we have no real handle on the geological activity of the early Earth, but given 2 billion years there is no reason why oxygen levels of >10% couldn’t be achieved purely by inorganic processes.
I think the point to stress here is that rate of production is rather irrelevant so long as production outstrips destruction. Inorganic oxygen production had 2 billion years to oxidise all the crust and atmospheric material. It may have been a slow process but it was also very steady.
Well, I have an undergrad level textbook on evolution right here that explicitly states: “After photosynthesis evolved, life began a major transformation of the atmosphere, removing carbon dioxide and adding oxygen.” It places the evolution of photosynthesis at around 3 billion years ago (when cyanobacteria diverged from the rest of eubacteria), which is also when oxygen concentrations started rising. Also, there are other even more ancient photoautotroph bacterial lineages like the green non-sulfur bacteria.
While the link you provided seems to be an excellent article about why we don’t have to worry about running out of oxygen, it only talks about the origin of atmospheric oxygen briefly in the last paragraph:
[QUOTE=WALLACE S. BROECKER]
Perhaps the most amazing thing about our planet is that we have any O2 at all . . . Just how O2 came into being remains a mystery. The most likely explanation is that water molecules that wandered to the outer edges of the atmosphere were knocked apart by ultraviolet rays from the Sun. The light hydrogen atoms were able to evaporate to space, while the much heavier oxygen atoms were bound to Earth by gravity and mated with the reduced sulfur and carbon exposed at the Earth’s surface. Only when this conversion had been completed could O2 begin to accumulate in our atmosphere. Records kept in sediments tell us this task took at least 2.5 billion years (more than half of geologic time). The evolution of multicellular organisms, and hence of our ancestors, awaited this transition from an O2-free to an O2-bearing atmosphere.
[QUOTE]
Even then, if I’m reading that right, it seems to be saying that oxygen liberated from water by UV light was then bound up by sulfur and carbon on the earth’s surface. Wouldn’t that imply that diatomic oxygen was then created by life from carbon/sulfur oxides?
Ah, fair enough. It seems that we are mostly all vigorously agreeing with each other. Admittedly, I did not know about the role of photolysis for the initial production of oxygen.