Earth's first atmosphere was made of hydrogen and helium accumulated before fusion started in the Sun. As soon as this happened it was stripped away by the Sun. How many atm of hydrogen and helium had Earth accumulated at the peak pressure of the first atmosphere?

  • $\begingroup$ You think that the Earth formed before our sun? That's a new one to me. $\endgroup$ – Phil N DeBlanc Aug 8 '19 at 8:04

We have no way of knowing how much hydrogen and helium was in the Earth's primordial atmosphere. What is more important is what the atmosphere consisted of when the primeval soup from which life arose was formed, just over 4 billion years ago. We have a fairly good idea what this atmosphere consisted of: mainly carbon dioxide and nitrogen, rather like Mars at the present time, but with substantial amounts of ammonia and methane. Also very small amounts of hydrogen and helium, most of which had by then been blown away by solar radiation. There was hardly any oxygen. As for pressure, all we can say for sure is that on Earth it was a good deal higher than it is today. Conditions at that time were similar, though not identical, on Venus, Earth and Mars, perhaps on Titan as well, so it is possible that all three terrestrial planets developed a version of the Earth's primeval soup, which was essential for the creation of life. Whether life emerged on any planet other than Earth, we don't yet know. Because all three planets had oceans, water vapour would also have been an important atmospheric constituent.

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    $\begingroup$ [citation needed] $\endgroup$ – Mike G Aug 7 '19 at 2:19
  • $\begingroup$ Whether Titan was ever warm enough to have oceans of H2O depends on how much heat it received from the a much hotter Saturn and how much CO2 and methane was in its atmosphere. These are things we don't know for certain. 4 billion years ago,Titan also had some primordial heat left over from its recent formation. $\endgroup$ – Michael Walsby Aug 7 '19 at 7:04
  • $\begingroup$ I think the answer would benefit greatly by having less speculation. "No way of knowing" is probably not true. $\endgroup$ – BMF Aug 7 '19 at 17:13
  • $\begingroup$ @-BMF OK, explain how we could find out what the pressure of hydrogen and helium was 4.5 billion years ago shortly after the formation of the Earth, and how would we separate pressure due to H2 and He4 from that due to other gases. I'm all ears! $\endgroup$ – Michael Walsby Aug 8 '19 at 19:13

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