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This is why Sagan was a great scientist.

BTW, an oxygen atmosphere cannot be a requirement for life, as life on Earth created the oxygen atmosphere.

I think it’s precisely because life created the oxygen atmosphere that it’s interesting as a potential sign of life, no?

Oxygen is an indicator of life, but not a strong one, as there are many processes which could create an oxygen atmosphere. What is a strong indicator is the presence of both oxidizing (oxygen, ozone) and reducing (hydrogen, methane, dimethyl sulfide) molecules, in sufficient quantities. That suggests something driving the equilibrium away from all-oxidizing or all-reducing atmospheres.

I heard that he turned Galileo around for publicity: NASA got more funding because there was this picture that showed exactly how small we were compared to the universe, inspiring people for exploration.

That was Voyager 1 [0].

[0] https://en.wikipedia.org/wiki/Pale_Blue_Dot

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