Dust and Devils, Microbes and Mass Loss Radiative Transfer in the Atmospheres of Mars and Rocky Exoplanets.

Abstract

Planetary atmospheres are fundamentally shaped by their interaction with stellar radiation, governing climate, atmospheric dynamics, and planetary habitability. This dissertation examines the role of radiative transfer in shaping the atmospheres of Mars and rocky exoplanets through four complementary projects: dynamics in the dust loading and lifting in Gale Crater, the survivability of Earth-based microorganisms on the Martian surface, and the escape of hydrogen from rocky exoplanets observable in the Lyman-alpha line. Airborne dust strongly modulates the Martian energy balance by altering the radiation budget at the surface. To reduce temporal constraints on previous observations of dust dynamics, a correction to the radiative transfer equation used in line-of-sight extinction measurements was applied. This modification revealed a diurnal cycle in dust opacity, with a peak in dust extinction near solar noon that is larger in amplitude during southern summer. Linking these results with an analysis of dust devil optical depth in Gale Crater suggests that dust devils produced under strong solar forcing contributes to the midday peak observed in dust loading, providing new insights into the dust cycle in Gale Crater. To estimate the potential forward contamination of Mars, the survivability of Earth-based microorganisms on the Martian surface was investigated. By combining laboratory results of Bacillus subtilis with forward models, it was shown that UVC radiation sterilizes exposed spacecraft surfaces within 24 hours on Mars, reducing bioburden by up to 15 orders of magnitude. However, regolith and dust strongly dampen UVC radiation, meaning buried microbes or those embedded within a spacecraft may persist for up to 25 Mars Years before succumbing to low-pressure effects. Finally, hydrogen escape from rocky exoplanets was modelled under Jeans and hydrodynamic regimes to quantify Lyman-alpha transit depths as a function of key planetary and atmospheric parameters. Results demonstrate that the hydrogen mixing ratio at the homopause is the main contributor to large transit depths, with temperature, planet mass, and photoionization introducing secondary variations. These results establish detectability thresholds for current and future UV telescopes. Ultimately, we find that Lyman-alpha transits can meaningfully constrain the atmospheric composition of rocky exoplanets, particularly when combined with multi-wavelength observations.