Re: Gases in primordial atmospheres

Hi Tomas,

Thanks for your very interesting question. It's certainly true that if you mixed the gases that you list in a sealed container, you would have quite a reactive mixture. Add the effect of sunlight to this (UV radiation destroys molecules such as methane over time periods that are short geologically) and we have a problem: if these gases did make up the ancient atmosphere, how did they persist?

I think there are 3 parts to your question:

• How do we determine the composition of the early atmosphere?
• How did the early composition affect development of the earth?
• How can we explain apparent anomalies such as those that you pointed out?

1. How do we determine the composition of the early atmosphere?
It's very difficult to know for sure what the early earth was like, but there are two things of which we are fairly sure. First, there was far less oxygen than there is today. When we look at rocks of different ages, we see a sudden increase in oxidised compounds beginning around 2.5 billion years ago and so far as we know, the only significant oxygen source on earth is produced by photosynthesis. Second, we believe the concentration of greenhouse gases was higher. According to current theories of stellar evolution, the sun would have been around 30% less luminous when the earth formed and so the average surface temperature of the earth should have been below freezing. Yet there is geological evidence of liquid water far back in the history of the earth - so the atmosphere must have been able to trap heat.
In addition, volcanism is thought to have been active since the earth formed - probably more so on the early earth - so we assume the same sorts of gases emerged from volcanoes as do today - listed on this page.

2. How did the early composition affect development of the earth?
So the big question - why are we interested in this question at all and how have we addressed the issue? In a word, life. The chemistry of the early atmosphere may have had a profound influence on the origin of life on earth.
I'm sure you are familiar with the famous Miller-Urey experiment. This work has been a major influence on how people have thought about the early atmosphere and also a source of intense controversy. In the original experiments, Miller and Urey assumed a reducing atmosphere - rich in ammonia and methane. There has since been a lot of debate about the extent to which the early atmosphere of earth was reducing and the relevance of these experiments. However, my feeling is that many of these criticisms miss the main point of the work, which was to demonstrate that organic molecules could be synthesised from simple molecules under physical conditions that may have existed on the early earth.

3. How can we explain anomalies and controversies?
In science, nothing is ever simple and there's always debate. Here's a brief summary of ways in which our thinking has changed since the Miller-Urey days.

• The early atmosphere is now thought to have been less reducing than that used in the original Miller-Urey experiments. Ammonia in particular was probably not a major component. Recent simulations suggest that hydrogen could have been a major reducing component (you may need a subscription to access the article).
• Volcanoes are normally considered to be the only significant source of outgassing to the atmosphere and they generate predominantly neutral gases - carbon dioxide, nitrogen, water vapour. However, ocean floor vents can be a significant source of reducing gases. This is discussed in depth in this article at Talk.Origins, which I recommend as a great source of information on all these topics.
• Recent theories suggest that life may have originated not in Darwin's "warm pond" at the surface, but in subsurface or subocean environments, perhaps similar to deep ocean ridges or vents. This could mean early atmospheric chemistry was much less relevant to the origin of life question.
• Going back to the start of my reply - the earth is a dynamic place, not a sealed chemistry experiment. It would be wrong to think that the atmosphere was uniformly mixed or that materials could not be added continuously to it, from inside or outside of the earth. There may be localised chemical environments, near to volcanoes for example that would not reflect the atmosphere as a whole. We also think that the early earth was bombarded by comets and meteorites and we know that these objects can contain water and organic molecules in abundance. This not only demonstrates that prebiotic chemistry is common to other parts of the solar system but suggests that this material could be the source of molecules precluded by the chemistry of the early earth, or could replenish those molecules if they were destroyed or consumed.

I hope this helps answer your question. It's a fascinating and important topic with lots of active research, so a bit of Googling with some of the terms in this reply should bring up lots of information,