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A [microns]

Fig. 7.19 Comparative synthetic spectra for different Pegasids, using a cloud-free model and with complete redistribution of the incident stellar flux (f = 0.25) (After Burrows et al., 2006)

they condense at lower levels, but the diagram illustrates the great theoretical (and perhaps even real) diversity in possible spectra.

7.3.2 Terrestrial Planets and Habitable Planets

The atmospheres of the terrestrial planets primarily arise from the degassing of volatile components that were trapped in the planetesimals that were involved in the accretion of the planet (Fig. 7.22). This atmosphere, degassed either by impacts during the accretion phase, or by volcanism, was subsequently partially lost and fractionated by gravitational escape (Selsis, 2006) under the influence of stellar radiation. The importance of the escape varies from one planet to another as a function of its mass, its magnetic field (itself a function of the mass), and its orbital distance. The D:H ratio measured in the atmospheres of Venus and Mars shows that they have undergone significant escape (see Sect. 4.4.3.3). As for the role of escape in the evolution of the Earth's atmosphere, it is poorly constrained. The depletion of the Earth's atmosphere in certain volatile elements, as well as the fractionation of rare gases certainly bears witness to escape phenomena (Zahnle et al., 2007) but these probably took place before the formation of the Earth proper, in the fragile atmospheres of the planetary embryos, which were eroded by X-ray and EUV radiation from the young Sun, and by the bombardment by planetesimals. The atmospheres

Fig. 7.20 The effect of atmospheric circulation on the horizontal distribution of temperatures and on the CO:CH4 ratio. This simulation, carried out by Cooper and Showman (2006) shows how the atmospheric circulation evens out temperature (top) and the abundance of CO (bottom). The atmospheric level represented is at a pressure of 1 bar and the planet's parameters are those of HD 209458b. The initial state in this simulation has a day-night contrast of 1000 K at the top of the atmosphere, and an equilibrium chemical composition (enhanced CO:CH4 ratio on the day side and negligible on the night side). The planet is assumed to be in synchronous rotation. After simulating a period of 1000 days, the atmosphere is in the state shown here, where the violent winds (of several kilometres per second) have evened out the temperature and the CO abundance

Fig. 7.20 The effect of atmospheric circulation on the horizontal distribution of temperatures and on the CO:CH4 ratio. This simulation, carried out by Cooper and Showman (2006) shows how the atmospheric circulation evens out temperature (top) and the abundance of CO (bottom). The atmospheric level represented is at a pressure of 1 bar and the planet's parameters are those of HD 209458b. The initial state in this simulation has a day-night contrast of 1000 K at the top of the atmosphere, and an equilibrium chemical composition (enhanced CO:CH4 ratio on the day side and negligible on the night side). The planet is assumed to be in synchronous rotation. After simulating a period of 1000 days, the atmosphere is in the state shown here, where the violent winds (of several kilometres per second) have evened out the temperature and the CO abundance

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