The aftermath of the Moon-forming impact left Earth with a hot, CO2-rich steam atmosphere. Water oceans condensed from the steam after 2 Myr, but for some 10-100 Myr the surface stayed warm (∼500K), the length of time depending on how quickly the CO2 was removed into the mantle. Thereafter a lifeless Earth, heated only by the dim light of the young Sun, would have evolved into a bitterly cold ice world. The cooling trend was frequently interrupted by volcanic- or impact-induced thaws.

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