FYI -- I'm still curious about the atmosphere in the 0.065 - 1 bya time period. This paper indicates that nitrogen was actively being removed from the early (2.7 bya) Earth atmosphere by *organisms* -- something that apparently never happened on Venus. https://www.washingtonpost.com/news/speaking-of-science/wp/2016/05/10/secret... Secrets of life on early Earth  and other planets  may be trapped in ancient gas bubbles The culprit was probably tiny organisms that sucked nitrogen  the element that makes up about 80 percent of the atmosphere  from the air. That process, called nitrogen fixing, still happens today, but the gas that is removed is replaced by other biological processes. The organisms that release nitrogen back into the air rely on oxygen, a gas that wouldn't become available in the atmosphere until the Great Oxidation Event a few hundred million years later. http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2713.html Earth's air pressure 2.7 billion years ago constrained to less than half of modern levels How the Earth stayed warm several billion years ago when the Sun was considerably fainter is the long-standing problem of the 'faint young Sun paradox'. Because of negligible1 O2 and only moderate CO2 levels in the Archaean atmosphere, methane has been invoked as an auxiliary greenhouse gas. Alternatively, pressure broadening in a thicker atmosphere with a N2 partial pressure around 1.6Â2.4 bar could have enhanced the greenhouse effect. But fossilized raindrop imprints indicate that air pressure 2.7 billion years ago (Gyr) was below twice modern levels and probably below 1.1 bar, precluding such pressure enhancement. This result is supported by nitrogen and argon isotope studies of fluid inclusions in 3.0Â3.5 Gyr rocks. Here, we calculate absolute Archaean barometric pressure using the size distribution of gas bubbles in basaltic lava flows that solidified at sea level ~2.7 Gyr in the Pilbara Craton, Australia. Our data indicate a surprisingly low surface atmospheric pressure of Patm = 0.23 ± 0.23 (2 ) bar, and combined with previous studies suggests ~0.5 bar as an upper limit to late Archaean Patm. The result implies that the thin atmosphere was rich in auxiliary greenhouse gases and that Patm fluctuated over geologic time to a previously unrecognized extent.