Figure 18. The habitable zone in the vicinity of Earth (adapted from Wikipedia). The zone is limited by only about 10–15 million km toward the Sun and extends outward to nearly the Mars orbit. The planet shifting closer to the Sun would meet runaway greenhouse similar to that on Venus (Ts = 735 K; Ps = 92 atm), whereas at Mars’s position, greenhouse gases would condense out and the atmosphere would collapse. Unlike Earth, the atmospheres of Venus and Mars consist mainly of carbon dioxide; their abundance is controlled by the carbonate-vollastonite equilibrium between atmosphere and crust, which is strongly dependent on surface temperature Ts . CO2 storage in the Venus atmosphere is the same as what is locked in the Earth’s crust, and it would release into the atmosphere amounting to Ps ~ 90 atm if the clement climate on the home planet irreversibly changed (temperature dramatically raised) and the CO2 balance maintenance ceased.