Response of upper atmospheres of terrestrial planets to extreme solar conditions: Implications for atmospheric evolution

The aim of the proposed project is a coordinated study of the behaviour of the thermosphere, ionosphere, and exosphere and of thermal and non-thermal atmospheric loss processes from Venus, Earth and Mars during "quiet" and "extreme solar events". Such events can serve as proxies for the influence of the active young Sun with important implications for the evolution of planetary atmospheres, water inventories, and habitability. Extreme events involve enhanced solar EUV and X-ray radiation, coronal mass ejections (CMEs) and related intense solar proton/electron fluxes, auroral phenomena, etc. Responses of planetary atmospheres to them include: thermospheric and ionospheric density variations, enhanced photochemistry and changes in atmospheric composition leading to changes in heating and cooling due to IR-, optical and UV-emissions, and related temperature perturbations, as well as the formation of "hot" neutral atoms and planetary coronae. In the project we shall develop and apply up- to-date diffusive-photochemical models of the neutral and ion composition and self-consistent hydrodynamic models of the photochemical and thermal balance in the thermospheres of terrestrial planets. Our studies will include calculations of the gas heating rates caused by the EUV-radiation at varying solar activity levels, as well as cooling processes. Photochemically produced "hot" atoms, their transport, including collisions and energy transfer, as well as the formation of planetary coronae, will be studied by means of coupled Monte Carlo - exosphere test particle models developed for this aim. Our models will be validated by using thermospheric density and temperature data inferred from the CHAMP satellite orbiting around Earth and plasma data from the VEX and MEX instruments at Venus and Mars. The final aim of the project is to derive accurate estimations of the escape rates of the atmospheres and hydrospheres of Earth, Venus, and Mars (hydrodynamic outflow and/or Jeans evaporation, nonthermal loss) under the impact of the solar EUV-radiation during 4.6 Gyr since the arrival of the Sun at the Zero-Age-Main-Sequence till present time.

The project focused on the response of the upper atmosphere of Earth-like planets against solar activity or higher EUV fluxes as observed at young Sun-type stars. The obtained results based on satellite observations, empirical and thermospheric, Monte Carlo and test-particle models indicate that the evolution of the atmosphere of early Earth and of terrestrial planets which may be capable of sustaining liquid water and continents where life may originate depend on: the formation age, its mass and size, and the lifetime in the EUV-saturated early phase of its star. To validate theoretical thermosphere models under extreme solar conditions we studied the response of the EUV radiation during an extreme X17.2 flare event, which could be separated from CME-related particle events on the Earths upper atmosphere. By analysing GRACE satellite drag data and comparing them with empirical thermosphere models MSIS00 and JB08, our study indicates a significant change in the total mass density during the event. By analysing the observed EUV flux during the flare peak we found that the peak EUV flux during the event agrees with the EUV flux of a Sun-like star or of the Sun at the age of about 2.3 Gyr and the upper atmosphere temperature raised from about 900 K to about 2000 K and the exobase expanded from 500 km to about 1000 km. Our results agree with theoretical models and indicate that for higher EUV fluxes the exospheric temperature can reach several thousand Kelvin so that the exobase, cools adiabatically but expands dynamically above the magnetopause and the magnetosphere had not been able to protect the upper atmosphere against strong non-thermal erosion by the solar wind. Our results indicate that early Earth had a different atmosphere composition or was protected by a hydrogen envelope which remained from its protoatmosphere during the first hundred million years (Myr) after the planets origin. We further developed a Monte Carlo hot atom model and applied it to Venus, Earth, and Mars to study the influence of hot atom coronae and hot atom escape against solar activity. We found that hot atom escape from Mars may be the main atmospheric escape process and is about an order of magnitude larger compared to ion loss. Furthermore, low gravity planets such as Mars most likely never build up a dense atmosphere during the first few 100 Myr after their origin. During our studies we began to develop an innovative new idea on how EUV heated and expanded upper atmospheres can be characterized by hydrogen cloud and Energetic Neutral Atom observations and advanced numerical modelling around transiting Earth-like exoplanets by future space observatories such as the WSO-UV, can be used for validating our findings related to atmospheric evolution hypotheses. We showed that such observations would enhance our understanding on the impact on the activity of the young Sun/stars on the early atmospheres of Venus, Earth, Mars and other Solar System bodies as well as exoplanets.