The Evolution of Earth’s Atmosphere and Surface
James F. Kasting (Penn State University)
Earth is the only planet known to harbor life and thus serves as our model both for a place where life may have originated and as a planet on which life could be remotely detected. Conditions on the early Earth, however, are not well understood, partly because of the lack of a rock record before about 4.0 Ga (billions of years ago). We deduce that the early atmosphere was a weakly reduced mixture of N2 and CO2, with smaller amounts of H2, CO, and some CH4. This composition could conceivably have been altered by large impact events, and so the question of whether the heavy bombardment occurred over an extended period, or as a pulse near 3.9 Ga, is critical.
Climate on the early Earth remains an enigma, as well. Despite the faintness of the young Sun, the early Earth appears to have been warm, or perhaps even hot. Taken at face value, oxygen and silicon isotopes in ancient cherts imply a mean surface temperature of 70(+/-15)oC at 3.3 Ga. A recently published analysis of the thermal stability of ancient proteins supports this conclusion. This evidence for hot early surface temperatures must be weighed against theoretical considerations, as well as geomorphic evidence for glaciation at 2.9 Ga, 2.4 Ga, and 0.6-0.7 Ga. Such models must also account for the well documented correlation between the rise of O2 at 2.4 Ga and the Paleoproterozoic glaciations which occurred at that same time.