Modeling of non-thermal processes in upper atmospheres exposed to the young Sun

Multi-wavelength observations of solar proxies of different ages have clearly shown that the emissions in X-ray to UV of young Sun-like stars might have been considerably stronger than that of the present Sun. This extreme radiation in short wavelengths together with the enhanced plasma environment of the young Sun or star has important implications for the evolution of planetary atmospheres both in our solar and in extraterrestrial planetary systems. In particular the energy budget, thermal structure, and photochemistry of the upper atmosphere will be affected by these extreme conditions and can result in significant thermospheric expansion. These extended atmospheres can well exceed the average magnetopause distance of a possible intrinsic magnetic field and are thus subject to consider-able solar/stellar wind erosion. In addition, the hot particles originating from various exothermic photochemical processes constitute an extended exosphere and are in turn exposed to non-thermal loss processes. The production of these suprathermal particles significantly depends on the solar/stellar EUV radiation and influences the thermospheric energy balance and hence the exobase temperature and density. The effect of the hot particles on the thermospheric structure is supposed to be particularly important at the very early ages when the solar/stellar EUV fluxes were considerably stronger than today. Thus the investigation of the impact of hot atoms on the thermosphere may prove to be essential in view of the long-term stability of a planet`s atmosphere and is therefore certainly of fundamental significance for the question of habitability of planetary surfaces. The project is aimed to address these questions by developing a self-consistent coupled ionosphere-thermosphere-exosphere model which includes the effects of suprathermal particles and the proper treatment of the transition region between the thermosphere and the exosphere. Numerous photo-chemical processes will be investigated in order to determine their significance for the energy balance of the early thermosphere and to figure out their impact on hydrodynamic outflow and adiabatic cooling. The model will allow us to study in detail the interaction between the upper atmosphere and the extreme solar/stellar particle and radiation environment that might have prevailed during the early stages of planetary evolution. We should be able to identify the main physical processes that govern the long-term evolution of terrestrial atmospheres and to find out the major constraints that lead to the formation of a possible habitable planet.

Certain chemical reactions in an atmosphere can supply a relatively large amount of energy to atmospheric species. When these species arrive at altitudes, where collisions among particles become rare, they may have sufficient energy to escape from the planet. The most important reaction producing such "hot" particles in present terrestrial atmospheres is believed to be the dissociation of O2+ ions into two O atoms. By investigating various possible chemical reactions in the martian atmosphere, we showed that dissociation of CO2+ ions into CO and O is almost equally important, a fact not known so far. Moreover, we found that dissociation of CO into C and O is the main source for C escape on Mars. To study the long term stability of the atmosphere of Mars, both the early atmosphere as well as the evolution of the solar radiation with time must be known. For this purpose we used simulation results representing the martian atmosphere up to 4 billion years ago. Observations of solar like stars of different ages indicate that the solar radiation was significantly stronger in the past than it is today, these observations were taken to estimate the time evolution of the solar radiation. Our studies suggest that the importance of different production mechanisms for hot O and C change with increasing solar flux. In particular, due to the complex interplay between varying solar radiation, atmospheric response, chemical reactions, and production of hot particles, a simple interpolation from present atmospheric escape rates to those in the past are misleading. By summing the loss rates over time, we estimated the change in the martian atmospheric CO2 pressure due to loss hot atoms. We found that an amount of CO2 corresponding to an atmospheric pressure of about 200--400 mbar could have escaped via hot particles during the last 4 billion years. Besides internal chemical reactions, the atmosphere will also be influenced by the steady energetic flux of electrically charged particles streaming away from the sun (the solar wind). When these particles arrive at a planet, they may not directly hit the atmosphere but may be deviated around the planet by local magnetic fields. However, part of the solar wind can be converted by charge exchange into neutral energetic atoms and can then directly hit the upper atmosphere, since neutral particles are not influenced by magnetic fields. By knocking off atmospheric species, these particles will contribute to atmospheric erosion. By simulating these processes for early Venus we found that this kind of loss is of much less important than thermal escape processes -- in contrast to what was expected. However, these escape processes are probably relevant for exoplanets exposed to strong stellar winds.