Modeling 3D radiative-transfer on earth-like (exo)planets with Phoenix 3DRT and PUMA

Abstract

The present work arose from the intention to apply the possibilities of the radiative transfer framework PHOENIX 3DRT to the spectrum synthesis of extrasolar planets. As almost always in the context of astronomical investigations, light is the dominant source of information and in this respect it makes sense to examine the fingerprints of the atmospheres of planets in the spectrum of their host stars. During the development of the work, however, it has also become apparent that the pure radiative transfer theory, which has long been used in the study of stellar atmospheres, reaches its limits when studying exoplanets. Hydrodynamic processes in the atmosphere, which are often ignored or limited as a downstream influencing factor in the spectrum synthesis of stars, cannot be excluded in connection with the modeling of planetary spectra. This is not surprising since the spectra primarily reflect thermodynamic properties that are largely determined by hydrodynamic processes in the planetary atmospheres. This fact has long been known and well understood: they are the basis of all meteorological models and no weather report would be able to make meaningful predictions without taking them into account in detail. In this sense, the author sees this work primarily as a bridge between the two disciplines of astrophysics and meteorology, and accordingly the main focus is on harmonizing the underlying theoretical principles and making them compatible with each other, so that a combined numerical model is created that connects both worlds. As a result of this work, a combined model of 3D radiative transport and hydrodynamic atmospheric calculations is presented. Both models are examined for their theoretical foundations in order to determine which implicit assumptions are contained in the respective models and which limitations arise in relation to their general validity. Since the atmosphere model already has a very simple radiative transport mechanism, this part is replaced by the much more powerful radiative transport from the PHOENIX 3DRT-model. In order to limit the scope of the work and to focus on the fundamental question of whether both models can be combined, the calculations are carried out iteratively and the respective results are exchanged manually between the two models. Using the example of Mars, it can be shown that the combination of both models can reproduce the observable properties such as atmospheric temperature distributions with astonishing accuracy in view of the described limitations of the atmospheric model.