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1. Evolution of the Earth’s atmosphere
- Venus & Mars: $\require{mhchem}\ce{CO2}$ (oxidized)
Jupiter, Saturn, Uranus, Neptune: $\ce{CH4}$ (reduced)
Earth’s atmosphere is not in chemical equilibrium. $\ce{N2}, \ce{O2}, \ce{CH4}, \ce{N2O}, \ce{NH3}$ are much higher. why?
biological process play dominantly
1.1 The primitive atmosphere
compare to Sun/cosmos, Earth atmosphere lack of $\ce{H}, \ce{He}, \ce{Ne}, \ce{Ar}, \dots$
Earth atmosphere was generated by degassing(脱气) of volatile(挥发性) compounds of the original solid materials (secondary atmosphere)
before the Earth’s core formed: gas highly reducing ($\ce{H2}, \ce{CH4}, \ce{NH3}$)
after core formed: similar to effluents from current volcanic activity ($\ce{H2O}, \ce{CO2}, \ce{N2}$, small quantities of $\ce{H2}, \ce{CO}$ and sulfur compounds)
more recent model: Earth’s interior was initially hot (tremendous bombardment 大撞击)
- due to this, Earth’s core formed earlier, volcanic gas at 4.5Ga similar to present emissions (more oxidized)
an atmosphere of steam during the period that Earth was accreting material
accretionary phase ended -> Earth cooled -> steam produce the oceans
- atmosphere $\ce{CO2},\ce{CO},\ce{N2}$ (~10bar, ~10bar, ~1bar)
Earth continue to be bombarded（撞击） until 3.8Ga. provide $\ce{CO},\ce{NO}$
1.2 Prebiotic atmosphere and the origins of life
- living cells present before 3.5Ga
- lab work: biologically important organic compounds (include amino acids) can form when $\ce{CH4},\ce{NH3},\ce{H2},\ce{H2O}$ irrated with UV and sparked
- $\ce{CH4},\ce{NH3}$ may not present at 3.5Ga ago
- even if released from volcanoes, $\ce{CH4},\ce{NH3}$ minor because of photolyzed
- early atmosphere dominated by $\ce{N2},\ce{CO2}$
- $\ce{CO2 +h\nu->CO + O},\ce{O + O + M -> O2 + M}$
- $\ce{O}$ increased sharply at altitude above ~20km because of solar radiation
- surface $\ce{O2}$ very low ($<10^{-12}$ present atmospheric levels, PAL) due to reactions with $\ce{H2}$
- two key compounds for formation of life: $\ce{HCHO},\ce{HCN}$
- $\ce{HCHO}$ could be formed by photochemical rxns
- formation of $\ce{HCN}$ difficult -> theories: comets bring biological precursor molecules / origins of life in oceanic hydrothermal vents
