its atmospheric abundance profile was determined by photochemistry. The CO2 was not regulated by the carbonate-silicate cycle, and Earth must then have been hotter. Estimating the structure and the detailed composition of the atmosphere on a primitive Earth at different stages in its evolution is an extremely difficult task in the absence of any firm constraints. We can, however, model the Earth's spectral evolution based on a standard atmospheric evolution, although this is likely to be modified by future work on the primitive terrestrial atmosphere. It is this spectral evolution that is shown in Fig. 7.27. The spectra there are shown at a moderate resolution (high resolution spectra are shown in a grey tint) and in the spectroscopic windows that will be available to the future space observatories TPF-C (visible and near infrared: from 0.6 to 1.2 |m) and Darwin/TPF-I (thermal infrared: from 0.5 to 20 |m).

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