It was an artificial atmosphere. The human races had bombarded the parched and dead surface of the planet with comet cores for hundreds of years just to bring the sea level up to where they wanted it. Then they had infected that water with organisms genetically engineered to produce the balance of gases needed to support life—and, having done that, to commit suicide, so that their biomass could be used as nutrients for the next wave of atmosphere-building creatures.
Do these physics work?
