Investigating Europa’s Atmosphere: Hubble Space Telescope Analysis and Europa-UVS Stellar Occultation Preparations

Europa’s tenuous atmosphere remains poorly constrained to this day. It is primarily composed of O₂ with a concentration of H₂O near the subsolar point when the trailing hemisphere is illuminated and surrounded by an extended neutral cloud with an abundance of H₂. One of the main tools used to study this atmosphere has been the Hubble Space Telescope (HST), as it covers many important wavelength ranges that are essential for atmospheric research. NASA’s Europa Clipper mission is scheduled to arrive at the system in the upcoming decade, and will enhance our understanding of Europa. This dissertation focused on 3 main objectives working toward further understanding the atmosphere and preparing for the upcoming mission. The first objective was to establish a stellar occultation quality algorithm to ensure that Europa Clipper’s Ultraviolet Spectrograph (UVS) will achieve its optimal science goals. The second objective was to analyze Europa’s optical aurora using HST visible spectrograph images during times when Europa is in Jupiter’s shadow. The final objective was to characterize and constrain H₂ and other trace species at Europa by using a large number of HST UV spectrograph images to compile a spectrum with a high Signal-to-Noise Ratio (SNR).

Objective 1

Ultraviolet spectroscopy is a powerful method to study planetary surface composition through reflectance measurements, and atmospheric composition through stellar/solar occultations, transits of other planetary bodies, and direct imaging of airglow and auroral emissions. The present generation of UVS instruments on board ESA’s Jupiter Icy Moons Explorer (JUICE) and NASA’s Europa Clipper missions will perform such measurements of Jupiter and its moons in the early 2030s. This work presents a compilation of a detailed UV stellar catalog, named Catalog of Ultraviolet Bright Stars (CUBS). This catalog is composed of targets with high stellar flux (O,B,A types) in the 50–210 nm wavelength range with applications relevant to planetary spectroscopy. These applications include (1) planning and simulating occultations, including calibration measurements; (2) modeling starlight illumination of dark, nightside planetary surfaces primarily lit by the sky; and (3) studying the origin of diffuse Galactic UV light as mapped by existing data sets from Juno-UVS and others. CUBS includes observations from the International Ultraviolet Explorer (IUE) and additional information from the SIMBAD database. We have constructed model spectra at 0.1 nm resolution for almost 90,000 targets using interpolated Kurucz models (which have a resolution of 1 nm) and, when available, IUE spectra. CUBS also includes robust checks for agreement between the Kurucz models and the IUE data, with validation using Juno-UVS comparisons. Our catalog can also be used to identify the best candidates for stellar occultation observations with applications for any UV instrument. We report on our methods for producing CUBS and discuss plans for its implementation during ongoing and upcoming planetary missions.

Objective 2

We analyzed HST Space Telescope Imaging Spectrograph (STIS) observations of Europa’s optical aurora, yielding further insight into the composition of its tenuous atmosphere and its time-variable interaction with Jupiter’s magnetospheric plasma. We obtained these observations of auroral emissions while Europa was in solar eclipse behind Jupiter to avoid reflected sunlight as a background signal and source of noise. The focus of this study are the oxygen 630.0 nm and 636.4 nm emission line brightness profiles across the disk in order to constrain O₂ abundances. Analyses of time-varying brightness ratios across different regions of Europa were compared with previous auroral studies in the ultraviolet (e.g., Roth et al. 2016) and in the visible (de Kleer and Brown 2018; 2019). We find a loose correlation with auroral brightness and relative distance from Jupiter’s plasma sheet crossing. We find that the auroral brightness diminishes with longer eclipse durations, suggesting a possible partial collapse of Europa’s atmosphere when in eclipse is worthy of further investigation.

Objective 3

We used the UV auroral emissions of Europa to investigate the composition of the atmosphere including O and H, as well as searching for trace species such as S, C, and Cl. HST observations of Europa’s UV aurora, including the STIS data used in this study, have led to some important findings about the moon including evidence for plumes. These emission observations have also been essential in constraining the abundances of O₂, H₂O, and H at Europa. Since 1999, repeat observations of Europa in the far-UV using STIS have amounted to a large total integration time on the target. By compiling all the observed spectra, we can increase the SNR and place new constraints on species that are not yet detected, such as S, Cl, and H₂. To extract the spectrum, we start by subtracting the detector dark and sky background signals for the entire STIS image in each dataset. We then isolate the rows in the image that correspond to Europa’s disk and the immediate surrounding environment. Combining the spectra of each row, we then subtract the contribution from surface reflected sunlight in the non-eclipse observations (when Europa is not in Jupiter’s shadow) to arrive at a final extracted auroral spectrum. All spectra across all datasets are then combined to one total spectrum that we use for our constraints. We also divide the spectra into regions of different hemispheres and orbital geometries on Europa. We report on our findings of the derived species abundance limits in Europa’s atmosphere.