Let's create an alien biosphere!

[Schiaparelliorbust]

Some would react with dioxygen more readily than others, so they wouldn't all be depleted at the same rate.

Ethane and hydrogen sulfide are both highly flammable/explosive, so they would probably be the first to deplete to non-spontaneously combustible levels.

[MathAndCode]

Ethane and hydrogen sulfide are both highly flammable/explosive, so they would probably be the first to deplete to non-spontaneously combustible levels.

The same is true of methane and hydrogen cyanide. The latter may even be depleted more readily than ethane or hydrogen sulfide.

[Schiaparelliorbust]

The same is true of methane and hydrogen cyanide, so they will probably be depleted next.

Actually all of these gases are below their explosive limits, so they will probably not spontaneously combust, unless the presence other flammable compounds could lower them.

[MathAndCode]

unless the presence other flammable compounds could lower them.

I suspect that that would be the case.

[Schiaparelliorbust]

I suspect that that would be the case.

I bet that the simultaneous presence of both CO and flammable compounds will make their depletion slower. I also think that ethane and hydrogen sulfide would burn at lower concentrations of oxygen than the other flammable compounds, so think the percentage of them will decrease somewhat, while those of ethane and hydrogen sulfide will almost become zero. What are your thoughts on this?

[MathAndCode]

I bet that the simultaneous presence of both CO and flammable compounds will make their depletion slower. I also think that ethane and hydrogen sulfide would burn at lower concentrations of oxygen than the other flammable compounds, so think the percentage of them will decrease somewhat, while those of ethane and hydrogen sulfide will almost become zero. What are your thoughts on this?

Any component of the atmosphere that can react exothermally with oxygen would eventually be depleted. Earth's atmosphere used to have a larger amount of ammonia and methane, but today, they are only present in trace amounts.

[Schiaparelliorbust]

Any component of the atmosphere that can react exothermally with oxygen would eventually be depleted. Earth's atmosphere used to have a larger amount of ammonia and methane, but today, they are only present in trace amounts.

Trace but important amounts. There will also probably be a very strong greenhouse effect right now. Do you agree with me that ethane and hydrogen sulfide will deplete faster?

[MathAndCode]

Trace but important amounts. There will also probably be a very strong greenhouse effect right now. Do you agree with me that ethane and hydrogen sulfide will deplete faster?

I agree that they'll deplete faster than ammonia and methane, but I think that hydrogen cyanide may deplete even faster.

[Schiaparelliorbust]

I agree that they'll deplete faster than ammonia and methane, but I think that hydrogen cyanide may deplete even faster.

Yes, that will probably deplete faster. Should the order be hydrogen cyanide > ethane ≈ hydrogen sulfide > methane?

[MathAndCode]

Yes, that will probably deplete faster. Should the order be hydrogen cyanide > ethane ≈ hydrogen sulfide > methane?

Yes. How about this for the complete order?

1. Hydrogen cyanide
2. Isocyanic acid
3. Ethane
4. Hydrogen sulfide
5. Methane
6. Carbon monoxide
7. Sulfur dioxide/sulfurous acid/sulfite/bisulfite
8. Ammonia

Does that sound right to you?

[Schiaparelliorbust]

Yes. How about this for the complete order?

1. Hydrogen cyanide
2. Isocyanic acid
3. Ethane
4. Hydrogen sulfide
5. Methane
6. Carbon monoxide
7. Sulfur dioxide/sulfurous acid/sulfite/bisulfite
8. Ammonia

Does that sound right to you?

Is this the order of depletion speed? If so, I would consider moving carbon monoxide upwards because it is unstable. It barely needs any heat to react. Other than that it seems fine. (Sorry I didn't respond sooner. I thought you hadn't responded.)

[MathAndCode]

Is this the order of depletion speed? If so, I would consider moving carbon monoxide upwards because it is unstable. It barely needs any heat to react. Other than that it seems fine.

Yes, the substances at the top will be depleted sooner/more quickly. How about this for the new order?

1. Hydrogen cyanide
2. Isocyanic acid
3. Ethane
4. Carbon monoxide
5. Hydrogen sulfide
6. Methane
7. Sulfur dioxide/sulfurous acid/sulfite/bisulfite
8. Ammonia

[Schiaparelliorbust]

Yes, the substances at the top will be depleted sooner/more quickly. How about this for the new order?

1. Hydrogen cyanide
2. Isocyanic acid
3. Ethane
4. Carbon monoxide
5. Hydrogen sulfide
6. Methane
7. Sulfur dioxide/sulfurous acid/sulfite/bisulfite
8. Ammonia

Yes, that seems good. How fast will these deplete?

[MathAndCode]

How fast will these deplete?

I'm not sure. It will depend on a variety of factors, with the most important being how quickly the photosynthesizers spread. We can probably use Earth as a guide.

[Schiaparelliorbust]

I'm not sure. It will depend on a variety of factors, with the most important being how quickly the photosynthesizers spread. We can probably use Earth as a guide.

I don't think the speed matters too much, but it would be good to at least get a rough estimate for it. On Earth, even without (AFAIK) any impeding reducing chemicals in the atmosphere, the oxygen still came relatively slowly. It did go up a lot, but over the span of 200-2300 million years (stage 4).

[MathAndCode]

I don't think the speed matters too much, but it would be good to at least get a rough estimate for it. On Earth, even without (AFAIK) any impeding reducing chemicals in the atmosphere, the oxygen still came relatively slowly. It did go up a lot, but over the span of 200-2300 million years (stage 4).

Yes, but oxygen was accumulating before then; it was just absorbed by various reservoirs first. Something similar would likely happen on this planet.

[Schiaparelliorbust]

Yes, but oxygen was accumulating before then; it was just absorbed by various reservoirs first. Something similar would likely happen on this planet.

Those were non-atmospheric reservoirs though. Once they were filled, oxygen stated accumulating in the atmosphere. The chart also says that that rocks and the seabed absorbed oxygen. How does that work?

[MathAndCode]

Those were non-atmospheric reservoirs though. Once they were filled, oxygen stated accumulating in the atmosphere. The chart also says that that rocks and the seabed absorbed oxygen. How does that work?

Dioxygen can dissolve in water in small concentrations. Because the oceans are large, they can dissolve a lot of it. Early Earth's seabed had large quantities of metals, such as manganese and iron, in non-oxidized forms; some of the dissolved dioxygen in the oceans formed stable oxides with them. Because our planet is more geologically active, and hydrothermal vents can extrude transition metals, the same thing would likely happen on the alien planet to a greater extent, and the graphite fields would also contribute to that. Certain substances on land could likely absorb oxygen in the same way.

[Schiaparelliorbust]

Dioxygen can dissolve in water in small concentrations. Because the oceans are large, they can dissolve a lot of it. Early Earth's seabed had large quantities of metals, such as manganese and iron, in non-oxidized forms; some of the dissolved dioxygen in the oceans formed stable oxides with them. Because our planet is more geologically active, and hydrothermal vents can extrude transition metals, the same thing would likely happen on the alien planet to a greater extent, and the graphite fields would also contribute to that. Certain substances on land could likely absorb oxygen in the same way.

We could have reddish oceans due to the iron oxide, which happened on Earth. Also, would the oxygen react with the graphite to form carbon dioxide?

[MathAndCode]

We could have reddish oceans due to the iron oxide, which happened on Earth. Also, would the oxygen react with the graphite to form carbon dioxide?

Yes, they would form carbon dioxide.

[Schiaparelliorbust]

Yes, they would form carbon dioxide.

Since one of the reactions used to make graphite is CO₂+2H₂→2H₂O+C, which uses CO₂, the microorganisms using it could multiply.

[MathAndCode]

Since one of the reactions used to make graphite is CO₂+2H₂→2H₂O+C, which uses CO₂, the microorganisms using it could multiply.

Yes, although only photosynthesizers and organisms near hydrothermal vents would likely be able to access dihydrogen.

[Schiaparelliorbust]

Yes, although only photosynthesizers and organisms near hydrothermal vents would likely be able to access dihydrogen.

Sorry, I forgot about that. What else would happen with oxygen?

[MathAndCode]

Sorry, I forgot about that. What else would happen with oxygen?

Creatures would probably learn to use it to generate energy eventually.

[Schiaparelliorbust]

Creatures would probably learn to use it to generate energy eventually.

Then we should figure out how long it takes for the substances in the atmosphere to deplete. We don't have to be too exact. We could maybe make it 100-300 million years? I don't know.