carbon dioxide

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Evolution of Planetary Atmospheres and Climates François Forget, Université Paris 6 Are there other Earths? One of the major questions in planetary science is to know whether the conditions that have made possible the birth and the evolution of life on our planet are common in the universe. The discovery of hundreds of planets around other stars suggests that terrestrial Earth-class sized planets are common. However, life as we know it requires liquid water (in fact, we cannot imagine any kind of chemistry approaching life in complexity without liquid water). Geophysical evidence shows that for at least 3.8 billions of years, the Earth has enjoyed a mild climate and oceans of liquid water suitable for life on its surface. This is because our planet has always maintained an atmosphere with the right greenhouse effect to cope, for instance, with the variations in the solar flux. Such a stability seems to have resulted from geophysical processes such as the “carbonate-silicate” cycle involving plate tectonics and the chemical weathering of continents. A key issue is thus to understand if the long term presence of liquid water is a likely characteristic of a planet (assuming that it more or less enjoys Earth-like solar flux conditions) , or if it requires an exceptional combination of geophysical properties. In the Solar system, the present-day conditions on our two neighbors Venus and Mars are far from being compatible with liquid water. Venus is now an extremely dry and hot place, with a 90 bars CO2 atmosphere warming the surface above 700 K because of its strong greenhouse effect. Mars is a frozen planet with a thin CO2 atmosphere. However, Venus, Mars and the Earth are thought to have originated from the same “mixture” during the formation of the solar system, with in particular a similar amount of water and carbon dioxide. Moreover, it is likely that three planets may have been quite similar 3 or 4 billions years ago with liquid water flowing on their surface. What happened? For Venus, one possible scenario is that, as the Sun's luminosity slowly increased billions of years ago, its water was lost by photodissociation in the upper atmosphere and escape of hydrogen to space. From then on, any carbon dioxide exhaled by volcanoes could no longer be removed from the atmosphere by chemical weathering because of the lack of liquid water. As carbon dioxide accumulated in the atmosphere, the greenhouse effect grew ever more intense, leading to the planet that we know now. As for Mars, it seems that the planet may have been warmer billions of years ago thanks to a thicker atmosphere containing greenhouse gas and clouds Later, Mars lost most its atmosphere, probably because of a high rate of atmospheric escape to space, or the lack of tectonic activity able to recycle CO2 after incorporation in carbonate rocks (although carbonate rocks have not been detected in large quantities so far). In any case, in this evolution, the small size of Mars clearly played a more crucial role than its distance from the Sun. Our studies of the Earth, Mars and Venus thus show that in order to maintain liquid water on its surface, it may takes more than being at the right distance from a star. A planet must be able to avoid the loss of its water or even of its entire atmosphere to space. An active geology, possibly involving plate tectonic, may be necessary to recycle the atmosphere or stabilize the climate like on Earth. In fact, Earth is the only planet that we know with plate tectonic. Is it likely to find it on a planet around another star? After a general introduction of the subject, the session “Evolution of planetary atmospheres and climates” will include two lectures presenting our current understanding of the two key geophysical process that are though to control the long term “habitability” of a planet: atmospheric and water escape to space on the ...