Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Geology

Collaborative Specialization

Planetary Science and Exploration

Supervisor

Neish, Catherine D.

Abstract

Saturn’s mid-sized icy moons display a range of surface processes. In this dissertation, we used spacecraft data to explore the surfaces of two moons, Tethys and Enceladus. Both moons are small, inner moons made largely of water ice. However, Tethys is inactive, with only a record of past tectonic activity, while Enceladus has a water-ice plume. First, we examined the small linear surface features on Tethys. Tethys is not expected to be geologically active, but its surface shows evidence of tectonic features. To better understand the moon’s history, we used images from the Cassini mission and mapped six types of features (including pit chains, tectonic fractures, impact-related crater chains, and ridges) and analyzed their locations, orientations, and origins. The maps from this work can be used for future hypothesis testing, to understand which processes and tectonic forces influenced Tethys’s surface evolution. We then used change detection to study Tethys and to compare it against Enceladus. For both moons, we examined pairs of optical images taken between 2004 and 2017 during the Cassini mission. We overlaid images from different years to look for surficial changes. On Enceladus, we found two changing albedo features and one image pair with unusual lighting that may have produced a candidate change. On Tethys, we observed no obvious changes. One of the candidate changes on Enceladus, the Black Spot, resembles a recent impact crater. This places limits on the impact cratering flux in the Saturnian system. We concluded that the Black Spot is a true albedo feature fading with time as it is covered up by plume deposits. Comparing to the literature, we suggest that deposition rates may be higher than the lower limits previously placed. Overall, we have expanded our understanding of the historic and present surface processes on Tethys and Enceladus, placing limits on tectonic processes, impact cratering, and surface deposition there. These results help us to better understand other icy moons and create a foundation that can be expanded on as we continue to explore the outer solar system.

Summary for Lay Audience

Saturn’s moons are very different from Earth. Made primarily of ice, these surfaces have unfamiliar features, most have no atmosphere, and they do not have plate-tectonics the way that we have on Earth. Because we cannot visit these moons in person, we use satellite data to understand their history. In this dissertation, we studied two of these moons, Tethys and Enceladus. On Tethys, we mapped out small lines observed on the surface thought to be formed by pits, cracks, and lines of craters from impacting comets. In the future, we can compare the locations of these features against various predictions to tell us how the pits and cracks formed, and the source of the craters. To learn more about these moons, we also looked at images of them from different times. By comparing satellites images from two time periods we determine if anything changed. On Tethys, we do not see any new features. But we did see changes on Enceladus. Unlike Tethys, which is comparatively inert, we know that there are active geologic processes on Enceladus. At the south pole, ice particles erupt like a volcano and some settle back down on the surface. Looking at Enceladus, we found three spots that may have changed over time. Two of the spots turned out to be darker features on the bright ice of Enceladus that might be disappearing over time. The third is the result of different lighting in the two images. Looking closely at one of the possible changes, which we dubbed the “Black Spot”, we concluded that it is disappearing over time and is likely an impact crater. We think that some of the ice erupting at the south pole is covering up the Black Spot. After studying Tethys’s linear features, and potential changes on both Tethys and Enceladus, we have a better idea of which geologic processes are affecting these two moons. As a result of our work, we will be able to better explore what formed Tethys’s cracks and pits, how often we see a new impact crater on either moon, and how much ice is covering up the surface of Enceladus over time.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

Download

Share

COinS