The Chemistry of Europa and Venus, and Characterization of Earth-Like Exoplanets

Citation

Li, Jiazheng (2022) The Chemistry of Europa and Venus, and Characterization of Earth-Like Exoplanets. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1q9j-y297. https://resolver.caltech.edu/CaltechTHESIS:05252022-004039193

Abstract

This thesis contains three parts of work, including oxidant sources on Europa, sulfur chemistry on Venus, and the characterization of Earth-like exoplanets. In the first part, we build two chemical-transport models to study the various oxidant-generation processes that occur in both Europa’s atmosphere and surface ice. The atmospheric model focuses on the role that water plumes play in the formation of Europa’s ionosphere. The simulation results, which show that the ionization reactions are initiated by electron-impact ionization and photoionization of water and continued by charge transfer between water and oxygen molecules, have successfully reproduced the observations. This model has also been used to study the dissociation processes of water molecules from the plumes, which can be regarded as an alternative source for the oxygen in the atmosphere. The chemical-transport model on Europa’s surface ice is built to simulate chemical processes occurring in the ice during irradiation by electrons and to describe how the chemical species of interest (oxidants) are formed, transported, and distributed in the ice. This model also has implications for the chemical composition of Europa’s subsurface ocean. Since the availability of oxidants could be the limiting factor for biologically useful chemical energy on Europa, the proposed research may give us insight into Europa’s habitability.

The second part of this thesis focuses on the unknown ultraviolet (UV) absorber(s) in the atmosphere of Venus. Ever since the detection of the enigmatic ultraviolet absorption in the upper atmosphere of Venus, questions have been raised about the identity of the unknown UV-visible absorber(s) and how it is formed on Venus. Our recent photochemical modelling study suggests that SO dimers may not be the major UV absorber(s) in Venus’ upper atmosphere. However, SO dimers are important intermediaries in the formation of more complex S species (e.g., S_n (n = 1 to 7)). Polysulfur aerosol, which is formed from the nucleation process of S_n (n = 1 to 7), is a possible candidate for the unknown UV absorber(s). In this work, we compute that the mixing ratio of polysulfur aerosol is ~1.76×10^-14 in the upper atmosphere. By putting the polysulfur aerosol into the Spectral Mapping Atmospheric Radiative Transfer model (SMART), we find that the simulated spectrum of Venus agrees well with the observations. This result provides useful constraints for unraveling the identity(ies) of the unknown UV-visible absorber(s) on Venus.

The third part of this thesis is devoted to the characterization of Earth-like exoplanets. In this part, we study the glints, a possible phenomenon on Earth-like exoplanets and the rotation period detection for Earth-like exoplanets. Small flashes of reflected light—called glints—are found in images taken by spacecraft observing the Earth and occur due to specularly reflected solar radiation. These glints have been found over both ocean and land. Using Deep Space Climate Observatory observations, we show that glints over land are due to specular reflection off horizontally oriented ice platelets floating in the air, while glints over ocean have contributions from reflection off either platelets floating above the ocean or a relatively smooth ocean surface. We use a radiative transfer model to simulate different kinds of glints and to explore their properties. This technique of comparing observations of terrestrial glints with model simulations may provide new information relevant to atmospheric dynamics and the search for habitable exoplanets. A terrestrial planet’s rotation period is one of the key parameters that determines its climate and habitability. Here we demonstrate that, under certain conditions, the rotation period of an Earth-like exoplanet will be detectable using direct-imaging techniques. We use a global climate model that includes clouds to simulate reflected starlight from an Earth-like exoplanet and show that the rotation period of an Earth-like exoplanet is detectable using visible-wavelength channels with time-series monitoring at a signal-to-noise ratio (S/N) >20 with ∼5–15 rotation periods of data, while the rotation period of a planet with full ocean coverage is unlikely to be detectable. To better detect the rotation period, one needs to plan the observation so that each individual integration would yield a S/N >10, while keeping the integration time shorter than 1/6 to 1/4 of the rotation period of the planet. Our results provide important guidance for rotation period detection of Earth-like exoplanets in reflected light using future space telescopes.

Thesis Files