The atmospheric and surface conditions of Earth, Venus, and Mars formed as a result of a chain of astrophysical and geophysical/chemical processes. The end results were the formation of a habitable environment on the Earth and highly inhospitable environments on Venus and Mars. Understanding these processes will improve our understanding of the formation of life on Earth and help us achieve a general understanding of how habitable planetary environments form. In our ISSI team, we study the early evolution of the atmospheres of Earth, Venus, and Mars as test cases for the formation of habitability on terrestrial planets. Our team contains a unique combination of expertise in the evolution of planetary interiors and surfaces, atmospheric formation processes, the Sun’s activity evolution, planetary atmospheric loss mechanisms, and atmospheric chemistry.
Many of the most critical processes took place within the first 100-200 Myr of solar-system formation and were highly intertwined. Initial H/He envelopes (protoatmospheres) may have remained on planets from their formation during the protoplanetary disk phase, preventing rapid cooling and the formation of a secondary atmosphere. However, solar radiation and the solar wind should have eroded such envelopes, and the cooling magma ocean surfaces eventually led to crust formation, outgassing of secondary atmospheres including heavier species such as H2O, CO2, and N2, and the collection of liquid water oceans, thus setting the stage for the operation of plate tectonics on Earth; the latter in turn helped remove potentially massive amounts of greenhouse-active CO2. A delicate timing of events and well adjusted amplitudes of solar and geophysical influences were thus critical for the Earth – and may have failed on Venus due to its closer proximity to the Sun, preventing rapid cooling, the formation of oceans and therefore plate tectonics. The atmospheres of Earth and Venus thus evolved very differently, yielding a CO2 dominated atmosphere on Venus, and an N2 dominated habitable atmosphere on Earth. How these mechanisms interact to form habitable planetary environments, or why they may fail to do so, is the major focus of this ISSI team.