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Very early it would have been mainly hydrogen & helium,

then the sun lit up (fusion started) and they would have been lost leaving mostly Argon & nitrogen with traces of stuff from volcanic out-gassing: H2O, CO2, SO2, H2S, & HCl.

After life formed there would have been methane,

but not until after chlorophyll was invented was there any oxygen.

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17y ago

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