What concentration of oxygen in a planetary atmosphere would be indicative of life?

Question

Astronomical spectroscopy can be used to determine the chemical composition of distant bodies. In the related question Can we detect atmosphere on exoplanets? we learned that, even now, we are studying exoplanet atmospheres.

We know that currently the atmosphere of Earth is about 21% oxygen and about 0.035% carbon dioxide. As I understand it, Earth's early atmosphere had a much higher concentration of carbon dioxide, created by volcanic activity.

What atmospheric concentration of oxygen (O₂ or O₃) can be explained by tectonic or chemical actions, and what concentration could only be achieved by some kind of life?

Answer 1

I would argue that no specific level of molecular or atomic oxygen in atmosphere is indicative of carbon-based life (i.e. life as we know it on Earth). A planet could have oxygen rich atmosphere which could be due to naturally occurring thermo-chemical reactions (e.g. Sabatier/Bosch reaction), loss of hydrogen in water vapor through atmospheric escape and managing to hold onto the heavier oxygen, even by thermal / magmatic release of oxygen in oxide minerals.

On the other hand, we also know of anaerobic organisms, some of which (obligate anaerobes) don't tolerate oxygen rich environment and are essentially killed by its presence in notable concentrations. So we have both control ends with a non-indicative correlation; carbon-based life could exist even without molecular or atomic oxygen present in the environment and life forms develop that don't utilize it directly in their metabolism, and don't release it as a metabolic byproduct, and at the same time high concentrations of atmospheric oxygen could be due to other, inorganic causes.

More interesting than measuring atmospheric composition and amount of oxygen in it on average might however be measuring changes in atmospheric composition during planet's diurnal cycles, seasonal changes, and otherwise finding that it potentially has a role in metabolism of carbon-based life. Other metabolic byproducts might potentially be more indicative of activity of life, such as trace amounts of methane that could originate via methanogenesis that change in concentration as the environment changes daily or seasonally. Periodically changing ratio of other atmospheric gases, even in trace amounts and the changes can't be explained by inorganic processes, could also be more indicative of life than the presence of oxygen itself.

Answer 2

As @TidalWave said, no specific level of oxygen is by itself sufficient to potentially indicate life. However, based on this article, what astronomers are looking for is the combination of both oxygen and methane in a single atmosphere.

Both oxygen and methane can be created independently by nonliving processes, so their individual presence is of little interest. What scientists are looking for is both of them in the atmosphere of a single body. If these reactive gases are not constantly replenished by living things, they will react with each other, creating carbon dioxide and water. As a result, we should not observe them in the same atmosphere without a large, living source.

The article itself is about a study which suggests that we can be fooled by detecting both signatures from an exoplanet with an exomoon with opposite atmospheres (i.e. one is O2, the other is CH4), and is quite interesting.

Answer 3

The first third of Earth's own existence had neglible atmospheric oxygen. (Simple) life existed during a few hundred million years of this period. Thus, atmospheric oxygen does not correlate with life on a planet.

Answer 4

Oxygen is a highly reactive element. Gaseous oxygen rapidly combines with other elements. If there's any large amount of free oxygen in the atmosphere, after any significant amount of geological time, it's reasonable to assume that there is some process continuously releasing it to make up for what's being lost in oxidation.

On our planet, that process is photosynthesis done by living plants.

That doesn't mean no other mechanism is possible, but it does suggest that SOMETHING unusual and interesting is going on which is driving a reaction backward against its natural flow.

Answer 5

Oxygen is about 1% of the universe's mass. So that suggests that looking at any random sample of mass throughout the universe you would expect to see 1% oxygen. Now of course you wouldn't be surprised to see higher concentrations and lower concentrations in any given sample, but the larger your sample size the more likely it is that the value will approach 1%.

The atmosphere of a rocky planet like Earth is a tiny fraction of the planets mass (8.6134×10^-5% for Earth). However if we assume that oxygen content in the core and on the surface of a rocky planet without life is similar we can easily assume that any given lifeless planet should have around 1% oxygen in the atmosphere.

Now finding out the variance or standard deviation of oxygen in planetary atmosphere is a problem. I've found this paper that seems to be based on data for a large number of exoplanets, the problem is the data isn't shared, just the results. In Table 1 of that paper data is shown for three different types of exoplanet. We could calculate the variance based on this but variance from 3 data points isn't very accurate.

It comes down to two aspects in my eyes. First can you attribute the oxygen content to another source? Second is the oxygen content high enough to suggest life. It will never be a guarantee from this type of observation, so you really need to know how confident you want to be. Once you know you're confidence level you could calculate your confidence interval based on standard deviation of exoplanets oxygen content. However even this is dependent on how likely you think exoplanets are to support life, if it's very unlikely then you can be more confident if you have just a few similar oxygen content observations; if it's very likely then you would expect the majority of observations to have a high oxygen content.