Loss of Atmosphere
Most of the creatures on this planet need a certain amount of oxygen to survive and develop, but this was not always the case. About 3.7 billion years ago this was not the case, earth having a mainly volcanic methane, nitrogen and carbon dioxide atmosphere, living on iron, sulphur and oxygen energised matter. Most of the oxygen was absent due to the high levels of iron that captured it. About 2.5 billion years ago cyanobacteria evolved big time and converted much of this carbon dioxide to oxygen using it as a part of their living process, possibly taking up to 100 million years. In both cases life was very simple and multi-celled organisms didn’t start to appear until about 1.2 billion years ago.
So, up until about 2.5 billion years ago most of the gasses present were what we currently get out of volcanoes. This means that probably the atmosphere consisted of something like 60% water vapour, 20% carbon dioxide and about 5% sulphur dioxide and hydrogen sulphide, the rest probably being hydrogen, carbon monoxide, methane, nitrogen, ammonia and oxygen.
The nitrogen probably built up over time as it’s harder to utilise and gets released back into the system quickly when it is, the other gasses getting attached to something, such as in the case of rocks. So, I wouldn’t be surprised if the early atmosphere was about 60% water vapour, 20% carbon dioxide, 5% hydrogen sulphide, 5% nitrogen, 5% oxygen and 5% methane.
We now have an atmosphere that is roughly 76.5% nitrogen, 20.5% oxygen, 2% water, 0.9% argon, 0.04% carbon dioxide and 0.03 trace gasses making up the difference. To not include water as a major part of the atmosphere is similar to not including the seas and oceans as part of the earth.
But over the last 500 million years oxygen as a component may have been as high as 35%, a value that allowed really massive insects to evolve and survive.
One interesting thing is that oxygen levels are constantly decreasing at the moment, not in a big way, just a trend, in the past 30 years being a constant 0.002% per year. This doesn’t seem to be an immediate problem, but over 500 years amounts to a 1% reduction. We are currently at about 20.5%, causing serious problems around about 14%, a figure that levels may reach after 19,000 years of this rate of decrease.
At the moment there is no indication that this rate will continue, except there is a problem with volcanic activity that seems to be on the increase that may accelerate this problem.
In an event such as the Deccan Traps were to happen today it could mean getting close to an extinction level event for the human race. If something like the Siberian Traps event were to happen most human society would likely disappear and it would be a global reset, few surviving the changes in atmosphere.
There is a large magnetic anomaly on Mars that suggests that at some time it suffered a major hit from a large body, but it could be that prior to that hit it had a thicker atmosphere and after that hit it didn’t, most of it being blown away from the surface with the impact. We don’t know what the conditions and atmosphere was like before and after a small event on the earth such as the Chicxulub impact that wiped out many of the dinosaurs, so there is no real way of knowing what a larger one would do, but I would expect that the levels of gasses in the atmosphere changed at least a small amount, the blast throwing into the atmosphere a large amount of dust, but also changing it and reducing the amount of oxygen by direct fire and indirect plant death.