Fig. 7.26 The effect of orbital distance on the synthetic spectra of an exo-Earth. These synthetic spectra have been calculated for an Earth located at different points in the Sun's habitable zone. The atmospheric profiles (pressure, temperature, and composition) correspond to the conditions shown in Fig. 7.24. Beyond 1.3 AU, the CO2 condenses in the atmosphere, forming clouds whose radiative properties are little known. So the outermost region of the habitable zone has not been considered in the particular study. In configurations at less than 1 AU, atmospheric oxygen has not been included, because the surface conditions are very different from those on our planet. The spectral windows of the future space observatories TPF-C (visible region - visible) and Darwin/TFP-I (IR region -thermal emission) are shown (After Paillet, 2006)
the amount of nitrogen found on Earth, and the CO2:N2 ratios in the atmospheres of Venus and Mars are practically identical. If we assume that this ratio remains true for Earth, there would be a significant amount of terrestrial nitrogen in the mantle -which is a very controversial suggestion. We shall not discuss this question of N2 any further, but it is important to note that the partial pressure of N2 is likely to vary from one habitable planet to another. In the majority of simulations of exo-Earths that have been carried out, an atmosphere of 0.8 or 1 bar of N2 has been adopted, by analogy with Earth. But it should be borne in mind for future modelling that lesser or greater pressures are also realistic and that, in particular, higher pressures may modify the spectral properties of the atmosphere (by increasing the albedo, and also by collisional broadening of the absorption lines).
The ideal model of a 'habitable planet' certainly does not reflect the great diversity of atmospheres of terrestrial planets, nor even those of habitable planets. We have just seen this in the case of N2, which remains a free parameter. But we should also consider the more-or-less massive presence of volcanic gases (such as sulphur compounds, for example), the accumulation of atmospheric H2 in the case of a massive planet where escape is ineffective, or the production of abiotic methane by the reaction between CO2 and H2O in cases where hydrothermal activity is very significant. These remain paths that need to be explored in future models.
In the atmosphere of a habitable planet, not modified by life, and consisting of N2, CO2, and H2O, photochemistry does not significantly alter the atmosphere's structure and spectral properties, except in some extreme cases studied by Selsis et al. (2002), where the abundance of O2 or O3 (or both) reaches values that cannot be neglected. Two cases should be noted: the first is that of a planet in the inner region of the habitable zone, where the abundance of stratospheric water vapour becomes sufficient large to generate a gravitational loss of H as a result, leading to the accumulation of O2. In this situation, O3 does not become a significant component, because it is destroyed by the hydrogenated radicals produced by the photolysis of H2O. The second case is that of a planet in the outer region of the habitable zone, where the partial pressure of CO2 is around 1 bar. If the planet is no longer vol-canically active and is no longer releasing hydrogen or if H escapes effectively (or both) - these two conditions are found in the case of a planet that is significantly less massive than the Earth - then the CO2 photochemistry may generate an O2 partial pressure of about 10 millibars or more and an ozone layer that is sufficiently dense as to alter the structure of the upper atmosphere. However, the presence of a dense CO2 atmosphere and the absence of effective volcanism are two contradictory hypotheses. To conclude this discussion, let us simply say that the possibilities for the accumulation of abiotic O2 and O3 are restricted to fairly exotic cases.
Where the composition of the atmosphere is modified by the presence of a biosphere and deviates from our model of a 'habitable planet', which is dominated by abiotic geochemistry, photochemistry may then play a predominant role. In Earth's present-day atmosphere, it is the layer of ozone, produced from photolysis of O2, that creates stratospheric warming by absorbing UV and visible radiation. In the early atmosphere on Earth, at the time when oxygen was just a minor component, methane produced by the biosphere was probably the principal greenhouse gas and
H2O .HP H2O CS22O
CH4/H20 ch4 ch4 h O
2 CH4 CH4 H2O H2O
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