Morphology and Landscape Evolution of Icy Worlds
Birch, Samuel Patrick Dennis
The outer solar system contains dynamic worlds with active sedimentary cycles that govern their overall appearances. Two of these in particular, Titan and Comet 67P/Churyumov-Gerasimenko (67P), though vastly different in terms of size, thermal history, and composition, offer a diverse array of landscapes for study, and are among the most active bodies in our solar system. Titan, the largest, icy moon of Saturn, has a surface that is covered with organic sediments. These sediments, of unknown origin, are transported by both the wind, and through an Earth-like hydrological cycle across the entire surface. As revealed by Cassini's RADAR instrument, Titan's polar regions maintain a stable inventory of volatile liquids (methane and ethane) that collect into a complex network of lakes and seas. With the completion of the Cassini mission, we show in Chapter 2 the overall geomorphology of these terrains to elucidate the origin and evolution of not only the lakes and seas, but the surrounding landscapes as well. What became immediately clear from this study, was that the majority of Titan's liquids (97%) reside in the north polar region, even though the overall landscapes appear quite similar. We show in Chapter 3 that the presence of large, now-drained sedimentary basins provide additional evidence for a long-term climate cycling of liquids between the poles, akin to the Earth's Croll-Milankovitch cycles. Currently, Titan's orbital configuration favors north polar liquids, where ~10^14 kg^3 of liquid methane is transported northward each Titan year. Evidence of this surface-atmosphere interaction are also recorded in Titan's alluvial fans, sedimentary structures that are predominantly found in Titan's mid-latitudes. This latitude range correlates with the regions of Titan where precipitation discharges are predicted to be highest, the implications of which we discuss in Chapter 4. 67P, though <4-km in size, is another geologically-active world where sublimation erosion drives a complex sedimentary cycle. After entering the inner solar system, where sublimation of water ice is possible, large gradients in thermal insolation promote mass-wasting of the previously consolidated portions of the primitive nucleus. Owing to its low gravity (~0.16 mm/s^2), the products of this failure are ballistically transported to colder, gravitational lows. However, perhaps surprisingly, this is not a uniform process. Instead, the obliquity and eccentricity of 67P's orbit result in a seasonal dependence; northern latitudes, in polar winter at perihelion, are blanketed by sedimentary materials liberated from the warmer southern latitudes. In Chapter 5, we study 67P's global landscapes to understand how this process proceeds, and what it implies for cometary surface geology more generally.
Comet 67P; Geomorphology; Hydrology; Landscape Evolution; Titan; Planetology; Astronomy
Hayes, Alexander G.
Pritchard, Matthew; Jordan, Teresa Eileen; Squyres, Steven Weldon
Ph. D., Geological Sciences
Doctor of Philosophy
dissertation or thesis