Views of an Ocean World: The Signatures of Internal and External Processes on the Surface of Europa

Citation

Trumbo, Samantha Kathleen (2021) Views of an Ocean World: The Signatures of Internal and External Processes on the Surface of Europa. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/d63b-y030. https://resolver.caltech.edu/CaltechTHESIS:01082021-174930437

Abstract

Beneath a comparatively thin ice shell, Europa harbors a global, salty, liquid-water ocean in contact with a rocky seafloor, making it an exciting target for exploring habitability in the Solar System. The potential habitability of Europa's ocean depends on its composition, which may be reflected in that of Europa's geologically young, fractured surface. However, two intertwined uncertainties are the degree to which the ocean and the surface are in contact, and the degree to which surface materials truly represent oceanic signatures. The latter is complicated by the fact that Europa’s surface is continuously altered by sulfur plasma and particle irradiation due to its location within Jupiter’s magnetosphere. In this thesis, I utilize a variety of multi-spectral, Earth-based observations of Europa to explore the balance and interplay of internal and external processes in shaping its surface.

Chapters II and III focus on using visible-wavelength spectroscopy from the Hubble Space Telescope (HST) to understand the chemistry of Europa's surface salts. In Chapter II, I present the detection of irradiated sodium chloride (NaCl) and show that its distribution correlates with geologically disrupted chaos terrain, suggesting an ocean source. In Chapter III, I investigate multiple spectral features across Europa's sulfur-bombarded trailing hemisphere. In comparing their geographies with the distributions of large-scale geology, magnetospheric particle bombardment, and surface color, I identify some features as reflective of purely exogenous sulfur radiolysis products and others as indicative of radiolysis products formed from a mixture of endogenous material and magnetospheric sulfur.

Chapters IV and V further consider the effects of radiolytic processing through the analysis of infrared spectra obtained with Keck NIRSPEC. In Chapter IV, I report a previously unseen spectral feature at 3.78 µm in disk-integrated spectra of the trailing hemisphere. Using Hapke spectral modeling, I demonstrate that it represents an unidentified radiolytic product of potential relevance to understanding the alteration of endogenic material. Chapter V considers a radiolytic species thought to be independent of endogenic material -- hydrogen peroxide (H₂O₂), a species relevant to the oxidation state and habitability of the ocean in the case of mutual exchange through the ice shell. Contrary to laboratory expectations, I observe the largest H₂O₂ absorptions within salty, low-latitude chaos terrain. I hypothesize that this distribution may reflect decreased hydrogen peroxide destruction due to electron scavenging by CO₂ within these same regions, which would suggest an internal carbon source.

Finally, Chapters VI and VII present preliminary studies of Europa's thermal emission using four images obtained with the Atacama Large Millimeter Array (ALMA) and a global thermophysical model developed to simulate Europa's expected thermal emission. In Chapter VI, I combine a single ALMA image with an observation from the Galileo Photopolarimeter Radiometer (PPR) to show that a thermal anomaly seen by the PPR and associated with two potential plume detections is better explained by a locally high thermal inertia than by geologic heating. Chapter VII considers all four ALMA images. While much of the large-scale thermal structure can be readily attributed to albedo variation, modeling of the images reveals a number of localized anomalies, which may indicate variations in geothermal heat flow, thermal inertia, or millimeter emissivity. In the absence of the additional observations needed to distinguish between such possibilities, I construct hypothetical maps presenting the ranges of possible thermal inertia and emissivity values.

Thesis Files