atmosphere
- Two sources (probably some of each)
- outgassing from interior
- volcanoes, etc. give off gasses
- “heavy bombardment” -- comet nuclei
- comet = “dirty snowball”
- ices melted and vaporized
- H2O became oceans
- rest provided Earth’s “primitive atmosphere”
- Atmosphere modified by
- solar radiation (UV breaks up molecules)
- escape (e.g. H, He)
- life (photosynthesis started about 3 billion y. ago)
- On Earth, the 90 atmos. of CO2 on Venus stored as
- biomass (coal, oil, etc.)
- carbonate rocks (limestone, chalk, coral reefs)