The atmosphere of a planet plays a major role for the development of life (Brack, 1993). A habitable planet (Kasting and Catling, 2003) needs moderate surface temperatures to maintain liquid water on its surface, an atmospheric composition suitable for the developing life forms, and stability against atmospheric loss processes for time periods sufficient to develop life. Early Venus, Earth and Mars had CO2-rich atmospheres, and possibly all of them once had liquid water on their surface. All these planets had organic molecules and an energy source. Therefore, the conditions for life were present, but these planets evolved very differently. The figure on the front cover summarizes the main phenomena controlling the Earth’s climate system. Similar figures may have applied to early Venus and Mars, but today their climate control system is certainly very different.
The main energy input into the atmosphere of the terrestrial planets is solar radiative energy. Important atmospheric constituents controlling the surface temperature, however, are the socalled “greenhouse gases”, which act as efficient absorbers of the re-radiated planetary IR radiation. Major greenhouse gases for terrestrial planets are CO2 and H2O, as well as O3 and CH4 (IPCC, 2007). A moderate greenhouse effect on Earth of 33 K leads to a mean global surface temperature, Tsurf, of 288 K, while in Venus’ dense atmosphere it is mostly efficient CO2 greenhouse absorption which results in Tsurf = 700 K. Mars’ thin CO2 dominated atmosphere is insufficient to warm its surface above 220 K, which does not allow water to be liquid. However, like on early Earth, the atmospheric structure and composition of early Venus and Mars may have been different. Detailed modelling of their present and past atmospheres, based on climate indicators from their geological record, is needed to investigate potential conditions for habitability during their evolution.
The atmospheric composition is largely controlled via catalytic photochemical cycles (WMO, 2006), which are fundamentally required to explain the ratios of CO2, CO and O2 on Marsand Venus. They also control the formation of O3, which is not only an important greenhouse gas, but also provides an important UV shield against harmful radiation for the biosphere on Earth. While such chemistry and dynamical transport in the atmosphere are well studied for Earth, substantial work still needs to be done for Venus and Mars.
One of the most important stabilizing long-term feedback loops on Earth is the carbonatesilicate cycle, linking the atmosphere to the tectonic and volcanic activity of the planet. Bycomparing the climate evolution of planets without active tectonics and different atmosphere structure with such processes active on Earth, we will gain insight into the importance of such a feedback loops and its role in defining habitability of terrestrial planets. Additional feedback loops include e.g. albedo variations of a planet by cloud and ice coverage. Life developed on Earth soon after its surface cooled. Today, life is part of the complex control mechanisms that determine the climate evolution of our planet. After the formation of life, biogenic chemistry can alter the composition of a planetary atmosphere substantially. While the early Earth probably had a CO2-rich atmosphere, as Venus and Mars today, its oxygen content rose after oxygen producing life forms developed about 2.5 Gyrs ago. Biomarker molecules in the atmosphere, e.g. O3, CH4 and N2O, may then be used to detect life, both on Earth and on other planets, including extra-solar terrestrial planets. Furthermore, recent results of terrestrial biology show that life can exist in a much wider range of environmental conditions (Cavicchioli, 2002) than previously expected. Expanding the study of atmosphere biosphereinteractions by including such extremophiles will further expand the range of possible habitable conditions.
Atmospheric loss processes (Shizgal and Arkos, 1996) over evolutionary timescales have had a substantial impact on the terrestrial planets of the solar system. If Mars had a dense atmosphere in its early history, it has subsequently lost most of it. If an ocean was present on early Venus, the water has been lost by evaporation, photochemical dissociation and subsequent loss of hydrogen to space. Loss processes in the planetary exospheres include thermal (Jeans, hydrodynamic) and non-thermal (ion pick-up, plasma instabilities) escape processes. The latter link the processes in the upper atmosphere to the interior of a planet via the planetary magnetic field. Furthermore, the solar radiation and particle environment plays a major role in all atmospheric processes. Thus, the long-time evolution of planetary atmospheres and their water inventories can only be understood within the context of the evolving Sun. In addition, the atmosphere of a planet can be removed by large meteorite impacts. On the other hand, smaller impacts deliver material to a planet and can play an important role in building up their atmosphere. The frequency and strength of impacts istherefore an additional important boundary condition (Morbidelli et al., 2000) for the atmospheric development of terrestrial planets.
The climate evolution of terrestrial planets is complex. However, we plan to significantly improve our understanding of the atmospheric conditions supporting habitability by taking advantage of what we have learned from Earth and our sister planets in the solar system. This knowledge will ultimately allow us to better understand also terrestrial planets outside our own planetary system, which are expected to be discovered in the very near future.