Doctor of Philosophy
Planetary Science and Exploration
Neish, Catherine D.
Icy worlds have attracted a lot of attention from planetary scientists over the last century because they exhibit an array geophysical and chemical processes that both contrast and mirror those on terrestrial worlds. Nitrogen and carbon molecules drive some of the most unique processes on these icy worlds. The central focus of this thesis is to study the role of carbon and nitrogen rich molecules in different icy body environments, both geologically and chemically. These molecules drive fluvial and aeolian processes on Titan in the form of organic dunes and methane rain, and on Pluto, these molecules are present as volatile ices that go through cycles of deposition and sublimation. On both Pluto and Titan, these molecules also contribute to the formation of prebiotic compounds on the planet surfaces. These prebiotic compounds are often associated with the origin of life on Earth and may have provided the building blocks of life on icy worlds. I investigate both the geologic impact of these molecules and the evolution of the prebiotic chemistry when mixed with liquid water in an impact crater melt lens. Organics that mix into impact crater melt lens are shown to freeze throughout the depth of the melt sheet. The highest concentration is near the center, but it is likely that there will be large enough concentrations for the Dragonfly mission to sample. The concentration will vary, based on the buoyancy of the organic impurity. Hydrolysis can also delay when more complex organic molecules appear, but in most cases, I predict Dragonfly should be able to detect more complex organics within the first meter of the ice. I find that geologic impact of nitrogen and carbon molecules on Pluto creates a range of crater morphologies from entirely degraded to anomalously deep, and there does not appear to be a correlation between the surface age and the level of degradation. There is also a longitudinal relationship with the level of crater degradation, which may hint that Pluto underwent a significant change in conditions in its past (e.g., polar wander).
Summary for Lay Audience
Icy worlds have beautiful water ice surfaces that often hide a liquid ocean underneath. Some of the most majestic surfaces are those covered with a lot of unique chemistry such as carbon and nitrogen rich molecules. My work studies how these molecules shape the surfaces of these planets, and I also study how these molecules might fuel the origin of life itself. Titan, Saturn’s largest moon, is covered in giant sand dunes made or organics, and its surface is covered with riverbeds and lakes made, not by water, but of liquid methane. On Pluto, the temperatures are so cold that molecules like methane freeze to form unstable ices that can slowly collect like snow or turn to gas. On both Pluto and Titan, the sun’s light can transform these molecules into more complex prebiotic chemistry. Prebiotic chemistry is the stuff that scientist suspect led to the origin of life on Earth, so some scientists think icy worlds might have had their own origin of life because of it. I study how the surface is changing, and I study how the origin of life may have happened and where we could find evidence of it. Pluto’s surface ranges from very old to very young. There are parts of Pluto that appear virtually new and other parts that are almost entirely erased. This shows that there is a wide range of things happening on Pluto that vary regionally; this contrasts with past work that showed Titan has few regions of ancient surface. For the origin of life, one of the best places to look is in impact craters where a large liquid water lake would have formed when the crater was new. Any evidence of the origin of life that forms in it is going to freeze into the frozen lake as it cools, with the highest concentration in the center. NASA’s upcoming Dragonfly mission is going to Titan to sample one of these frozen impact lakes, and my results suggest they should be able to find at least some early signs of life-like molecules (like the amino acids found in life).
Hedgepeth, Joshua E., "Using Craters to Study Carbon and Nitrogen Compounds on Icy Worlds" (2022). Electronic Thesis and Dissertation Repository. 8744.
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