UC Santa Cruz

This thesis is composed of two distinct themes. The first concerns the chemical structure of Venus's atmosphere. Venus's atmosphere can be roughly separated into lower and middle regions separated by a thick cloud deck. When modeling the chemistry of Venus' atmosphere past researchers have focused on these regions separately. In doing so they have made conflicting assumptions about the cloud region that connects them. In Chapter 2 I present the first detailed chemical model of Venus' atmosphere that includes both the lower and middle atmosphere. This model is used to characterize the chemical recycling pathways of observed trace species. In this study I find that there also exists a yet unidentified sink of sulfur-dioxide in the Venusian clouds.

The second theme of this thesis concerns understanding the interior structure and history of Kuiper Belt objects. In July 2015, NASA's New Horizons spacecraft made its close flyby of Pluto. This opened a new era for understanding not only of Pluto, but also the nature of Kuiper Belt objects generally. To this end I developed a model of how the bulk density of Kuiper Belt objects would evolve through time due to changes in porosity and the melting and refreezing of a subsurface ocean. In Chapter 3 I apply this model to Pluto and its largest moon Charon. I found that the density contrast between Pluto and Charon is large enough that it can only be reasonably explained by a difference in bulk composition (eg. rock to ice ratio).

In Chapter 4 I apply this model to bulk density measurements of Kuiper Belt objects (KBOs) generally. It has previously been observed that small KBOs have a much lower bulk density than their larger counterparts. I have found that this can naturally be explained by smaller KBOs being more porous. This difference in porosity is due to the longer cooling timescale of large KBOs causing them to warm more from the heat of radioactive decay. This in turn causes the ice to viscously relax away porosity. Small KBOs in contrast can efficiently conduct out this heat while staying cold and rigid. Because this depends on the abundance and heat production of radioactive elements, I have used this density information to place a constraint on when these objects formed.

In Chapter 5 I revisit Pluto examining what the observed tectonics imply about the history of Pluto's subsurface ocean. Namely I consider whether these observations are more consistent with Pluto having that ocean shortly after formation or developing it over a longer timescale through radioactive decay of long lived radioisotopes. If Pluto did form its ocean slowly, this would have caused global compression for which we find no tectonic evidence. If, however, Pluto started with an ocean we predict two stages of extension which is far more consistent with the observed geology.