Hydrogen Loss Retrievals from MAVEN/IUVS via Temperature-Dependent Radiative Transfer Modeling
Résumé
Hydrogen loss from Mars has played a major role in the desiccation and oxidation of the planet over geologic time. Martian H is most easily detectable in Lyman alpha sunlight scattered by H in the upper atmosphere, but because this emission is optically thick prior models have had some difficulty in reaching definitive conclusions regarding the density, temperature, and escape rate of this important minor species. We present new retrievals of hydrogen in the Martian upper atmosphere using the Mars Atmosphere and Volatile EvolutioN Imaging UltraViolet Spectrograph (MAVEN/IUVS) data and a temperature-dependent radiative transfer model that more accurately incorporates the line shape of the emission. Improvements in the algorithm and a GPU implementation allow us to retrieve temperature and density of hydrogen using asymmetric models that more accurately reflect the physics of the upper thermosphere. We apply a Hamiltonian Monte Carlo retrieval technique to account for parameter degeneracy and systematic uncertainty in the brightness. These retrievals enable us to interpret more of the IUVS dataset, including data from the deep nightside, and more comprehensively address the question of how much photochemically produced hot hydrogen is affecting present day loss rates. In addition to retrievals from IUVS Lyman alpha measurements, we also present predictions for Lyman beta brightness that will be observed by future missions. We will discuss these results in the context of contemporary understanding of the Martian climate and its evolution.