If all life on Earth was exterminated, how long would oxygen remain in the atmosphere?

Question

There are numerous other question on this site about "If oxygen production stopped...?" but all of them appear to still have humans around.

This question is different. In my scenario, we may assume that every single living thing on earth has been exterminated instantly. There is absolutely nothing left alive that either produces or consumes Oxygen using biological processes. We may presume that somehow the processes of life simply stop.

I would presume that there will be all sorts of objects remaining on earth that are not alive that will consume oxygen, such as rusting iron from the multitude of abandoned cars, dried-out deceased organisms and left-over refined fuels combusting and other natural non-biological processes.

While the world in which I am interested is not Earth, it is functionally identical to Earth, so Earth may be assumed to be the planet in question.

The questions:

1. How long will Earth retain molecular Oxygen in its atmosphere? I am looking for answers that can give a ballpark figure... for approximately how many days/weeks/months/years/centuries/millennia/aeons etcetera will there be detectable levels of oxygen in the atmosphere, i.e. >= 0.5% Oxygen? Answers with an error of up to ±10% will be considered acceptable.

2. How will the levels of oxygen decline? Will loss of oxygen occur at a constant rate, begin slowly and accelerate, or begin quickly and occur more slowly as time passes?

The purpose of this is to be able to calculate the approximate percentage of oxygen remaining in the atmosphere at any time between the extinction of all life and the point at which the amount of atmospheric oxygen is negligible (i.e. < 0.5%).

Answer 1

So assuming we're starting with an atmosphere weighing (5E18 kg) of 21% oxygen (1E18 kg), and ending about 0.5% atmosphere (about 2E16) kg), and the oxygen cycle continues functioning minus anything living:

At current air pressure, the lithosphere will absorb 6E11 kg of oxygen a year. Lightning also removes about 1E11 kg of oxygen a year by combining it with nitrogen. Source

It gets a bit more complicated - as the removed oxygen lowered the air pressure, which slows down the removal of oxygen.

• After 104,000 years, earth is at 20% oxygen.
• After 609,000 years, earth is at 15%.
• After 1,087,000 years, earth is at 10%.
• After 1,537,000 years, earth is at 5% oxygen.

Spreadsheet with the maths

It will hit 0.5% oxygen after 1,923,000 years.

Answer 2

How long would humans or aliens with human environmental requirements be able to live on the dead planet and breathe the air?

The total atmosphereic pressure at sea level is 760 mmhg, and oxygen at 21 % is 160 mmhg.

According to Habitable Planets For Man Stephen H. Dole, 1964, pages 13-18, humans nead a partial pressure of 60 mmhg to 400 mmgh of oxygen to survive.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf[1]

According to ASH's answer after 104,000 years Earth would be down to 20 % oxygen or 152 mmhg, after 609,000 years down to 15 % oxygen or 114 mmhg, and after 1,087,000 years down to 10 % oyxgen or 76 mmhg, close to the lower limit.

So it should take approximately a million years for the concentration of oxygen to become so low that humans acclimitized to high altitude low air pressure environoments would not be able to function well at sea level.

Thus one can write about human space travellers returning to Earth, or time travelers from before all life was exterminated, or alien visitors with similar atmospheric requirements, and they should be able to breathe the air fairly well for about a million years.

This question and the answers are mentioned in my post number 578 at:

https://www.trekbbs.com/threads/name-that-star-trek-object.302370/page-29[2]

This link seems to be broken. But you can go to Trek BBS and select Star Trek - the Original & Animated Series and select Name That Star Trek Object and go to page 29 of that thread and post number 578.