Is there a non-biological way for an atmosphere to retain a high percentage of free oxygen?
It is thought unlikely, because oxygen is highly reactive with anything on the surface of a planet (i.e. rust) so it has to be constantly replenished in any planetary atmosphere. I am not sure we are aware of any non biological process that could do this.
It would be very strong to extremely strong evidence of life on that planet … but it wouldn’t be required for life to be on that planet … O[sub]2[/sub] is a product of respiration, and this didn’t start on Earth for a couple billion years after she formed … before then life used a fermentation process to extract energy from carbohydrates …
I’m not sure you’re right there. Stromatolites, for example, go back at least 3.7 billion years so are one of the earlier forms of life, and they emit oxygen.
Stromatolites are not lifeforms; they are geological formations produced by the layering of microbial mats and sedimentary grains. It is possible for stromatolite-like features to be produced abiotically, so their existence alone does not demonstrate oxygen production. However, we have geological evidence of free oxygen production in the geological record in the form of banded iron formations starting at about 3.7 Bya, and becoming more prominent around 2.45 Bya when the “Great Oxygen Catastrophe” occurred. Production of free (diatomic oxygen and ozone) does occur abiotically due to lighting discharge as well, but only at very small concentrations (~1%).
If we found a planet with a ~20% or higher level of diatomic oxygen based upon spectral absorption, it would not be a guarantee of extraterrestrial life, but we’d be at a loss for any other known natural phenomenon which could produce and sustain that much free oxygen on a planetary scale. The same would be true for any other highly oxidizing element (i.e. the halogen group) or compounds (hydrogen peroxide, nitrogen tetraoxide). However, the electronegativity of the halogens makes them somewhat unlikely candidates for a respiration medium (I, for one, do not want to meet aliens which breathe fluorine or chlorine, and I certainly wouldn’t share an elevator with them), and complex compounds seem less likely unless alien life evolved in a far more complex hydrocarbon-rich environment, which is entirely possible if they evolved in a Titan-like moon of a warm supergiant planet.
On the other hand, not finding a highly oxidizing chemical species in the atmospheric spectra does not establish that there is not life; just that anything that may exist does not rely upon oxidative phosphorylation to drive their metabolisms. That there is no current persuasive hypothesized alternative in xenobiology is largely a matter of not having anything other than life on Earth to study; there may be some chemical pathway to complex chemical mediation and storage of energy that we have yet to conceive of, particularly if it occurs in environments that would be lethal to terrestrial life.
It would be strong evidence for, but not proof. The first reason is simply just because we don’t have a good non-life explanation for it does not mean that one doesn’t exist. The second is that we’re aware of non-life processes that produce oxygen, the electrolysis of water being one example. I would say that the most likely explanation would be life, but the most likely reason and the true reason are not always the same.
Photodissociation of water vapor will also produce free oxygen along with hydrogen. If the gravitation of the planet is low enough (as it is on Earth), some of the hydrogen will escape, leaving a surplus of O[sub]2[/sub] in the atmosphere. That oxygen will combine with rocks on the surface. But if the surface is covered in H[sub]2[/sub]O, either liquid or solid, then it has nothing to react with and could build up. So the answer is, no, it’s possible for there to be lots of free oxygen with no life.
I agree with those above that it’s strongly indicative of life, but not quite proof.