# If there are water oceans, must there be oxygen in the atmosphere?

I am trying to create a planet that either has no oxygen in the air, or it has very little; i.e. not enough to sustain humans without the use of a breathing apparatus. Large portions of the planet (approximately 50-60% of the surface, compared to Earth's ~70%) are covered with oceans, seas, lakes, and other bodies of water, much like those that are found on Earth. The planet has no native life forms, but in all other respects, assume that the planet is like Earth.

Is this scenario scientifically feasible? Or will the presence of large volumes of water inevitably result in some of the oxygen making its way into the atmosphere?

• "If there is water, that means there is oxygen. If oxygen, that means we can breathe." Feb 27, 2018 at 13:43
• Data point: Europa has lots of water (ice), and a very tenuous Oxygen atmosphere. Feb 27, 2018 at 14:14
• If there is oxygen, you can breath isn't true. If there is too much, you'll die quickly ! Feb 28, 2018 at 11:40
• For breathing, the thing to consider is free oxygen, specifically O2. Water is actually a good example of why you wouldn't have free oxygen. Oxygen has a propensity to interact and bind to other elements; hydrogen, carbon and iron being perfect examples. Feb 28, 2018 at 16:57
• In Interstellar, they wore spacesuits on the water planet. And most of the science in it is pretty good. Feb 28, 2018 at 17:11

Possibly.

There are two mechanisms for producing oxygen on Earth (and other similar planets)

1. Photosynthesis. Water is split and combined with CO2 to make (roughly) CnH2nOn and O2n. Some reduced carbon ends up buried to give a net contribution of O2 to the atmosphere. Note that the combination of photosynthesis and metabolism - using carbohydrates for fuel - has no net contribution of oxygen.

2. Photodissociation. In the upper atmosphere, ultraviolet light splits water molecules into O2 and H2. The H2 is lost to space, being too light to be retained by gravity, and the O2 is retained. This is why, for example, both Venus and Mars have highly oxidising environments (Venus via SO4, Mars via perchlorates and Fe3+). Note that both Venus and Mars lose H2 more readily than Earth, but still, this process produces significant amounts of net oxygen on Earth.

So.. Any planet with water oceans will have some oxygen production via photo-dissociation, even with no life.

The question is, will this oxygen accumulate? Well, we can see from the Early Earth what happens, even with photosynthesis: Oxygen cannot accumulate until all the available Iron (II) has oxidised to Iron (III) in the oceans and near surface. On Earth, this process gave rise to the Banded Iron Formations. As long as this process is ongoing, there will be no oxygen in the atmosphere, even with photosynthetic life.

• I haven't studied chemistry since school, but I would expect there to always be some $O_2$ left in the atmosphere, if only at noble gas levels. Dynamic equilibrium means that the reaction goes both ways depending on the conditions, so a complete lack of molecular oxygen would force the reverse oxidation reaction. It's part of why we have molecular ozone present in the atmosphere, even though it's so reactive Feb 27, 2018 at 13:29
• What is the composition of the atmosphere likely to be before water vapor is photo-dissociated? Feb 27, 2018 at 13:55
• @Muuski - an Earth-like planet with no life would likely have a predominantly N2+CO2 mix, with much higher CO2 than Earth (because life has captured a large proportion of the available CO2 and converted it into biological molecules). Feb 27, 2018 at 14:40
• This is a better answer than mine. Thanks for teaching me something new.
– IEW
Feb 28, 2018 at 6:19
• To add to this, if the crust is tectonically active, wouldn't the surface constantly be recycling? Volcanoes and subduction will constantly refresh the surface and create new sinks for oxygen. So it seems like basically the oxygen would never get a chance to catch up. (Plus, don't other things besides iron oxidize?) Feb 28, 2018 at 21:01

will the presence of large volumes of water inevitably result in some of the oxygen making its way into the atmosphere?

No
The Earth has oxygen in its atmosphere because of cyanobacteria which produced most of earths oxygen through photosynthesis. However, your planet has "has no native life forms". Without any kind of life, it would be very unlikely for your planet to have oxygen.

Sources:

• I was in the middle of typing up an answer that used the word endothermic perhaps a bit too much. +1 for simplicity alone. :) Feb 27, 2018 at 4:42
• Thanks! That's what I was thinking but I'm not very well-versed in this area of science.
– ECP
Feb 27, 2018 at 4:44
• There is more to it than that, see below. Feb 27, 2018 at 13:25
• Also worth mentioning: the Earth itself had oceans but no oxygen for about 1.4 billion years before the great oxygenation event. Feb 28, 2018 at 14:33
• A source of free oxygen (cyanobacteria, photosynthesis, geochemical processes, etc.) is not enough. First all of the exposed iron on your planet's surface must be oxidized. This may not sound like much, but there is a huge amount of exposed iron in the earths exposed rocks, mountains and soil. Feb 28, 2018 at 17:45

You don't have a problem.

Not only is free oxygen only produced in large quantities (so far as we know) by life, but also, once produced, oxygen is extremely reactive.

To explain why that's important - on earth, even after it began being produced by life in world-changing quantitites, it still took something like a billion years for oxygen to start gathering freely as O2 in the atmosphere. Before then, (almost?) all the oxygen produced, simply reacted with iron in the earth's crust producing banded iron ore (or other reactions and combinations, or dissolved in the seas), instead of gathering in the atmosphere.

It takes quite a lot to get oxygen in the air :)

A lot of great info and input here but, to answer the question yes, you can have a planet with no oxygen or very little and still have water. Just because there is O^2 in H20 does not in any way mean there needs to be oxygen around it to thrive. There are planets in existence that have water or ice but no sustainable oxygen or plants to produce or sustain life. For example: https://gizmodo.com/5887003/hubble-discovers-a-new-type-of-world-made-of-water and there is not oxygen to sustain life on this planet as of yet that we know.

To add to other answers, consider comets. Whether you call them a "dirty snowball" or a "snowy dirtball", the common feature is frozen water. As a comet approaches the sun, heating will somewhat melt the ice and cause gas jets to be emitted, but this does not result in any permanent atmosphere, never mind breathable O2.

What's the air pressure?

Humans don't only need oxygen in the atmosphere to breathe, we also need just the right amount of air pressure. If your world has Earth like 20% oxygen atmosphere and a sea level air pressure of 30 kPa (0.3 atm, lower than air pressure at Mt. Everest summit), the air temperature could still be a balmy 80°F (26.6°C) but no one would be able to breathe. On the plus side (for steam powered devices), water would boil at around 122°F (50°C).

Conversely, you could have too much pressure. Again with 20% oxygen and 340 kPa (3.4 atm), humans would suffer from oxygen toxicity. This would push the boiling point of water to around 280°F (140°C).

In case it was unknown, humans could easily survive at these different pressures indefinitely if we're introduced to them slowly (and obviously with the correct oxygen content in the air). Humans only really get in trouble from quick changes in pressure and at the extremes of pressure.

The high pressure case makes the breathing apparatus a bit easier. You just need to reduce the oxygen percentage, which can be done passively.

See Hal Clement's novel "The Nitrogen Fix" where a ecological catastrophy removes all O2 from the atmosphere.

I think that you received great answers to your question but there is something that you should take into consideration.

It is in my opinion very unlikely that a planet with oceans, similar to Earth's ocean, would not have any type of life. Water is important to create life and therefore, there will probably be a type of life present if the temperature isn't freezing like on certain moons. There is life pretty much everywhere on our planet and it doesn't always depend on oxygen. As pointed out before, oxygen is a byproduct of living organisms and the concentration that we have today is linked to biochemical cycles. So, you may have a planet without any dissolved oxygen in the ocean that has life based on an energy source such as sulfur.

Also, you can have oceans that are not made out of water. Matter can be found in different states and pressure as well as temperature may change a gas to a liquid or solid to a gas. It may be a point to consider...