UC Santa Cruz

The principal theme of this thesis is to see the planetary processes underlying observable variations. Various planetary processes in atmosphere, surface and interior exert long-term or short-timescale influence on the superficial properties that can be easily observed. This thesis combines observations with theoretical modelling to mine out the essential information of the Venus atmosphere and Enceladus’s ice shell and promote the understanding of variations and evolution of our Solar System.

The Venus atmosphere is essential for understanding why Earth and Venus have evolved so differently even though they are similar in mass and radius. However, the complicated coupling among atmospheric dynamics, chemistry and clouds on Venus is still not well investigated. Using chemical-transport models (CTMs), I aimed to disentangle the effects from various atmospheric processes and guide observations of future Venus missions (DAVINCI+, VERITAS and EnVision).

Recent ground observations from TEXES/IRTF have for the first time revealed the co-evolution of SO2 and H2O at the cloud top of Venus. The two species exhibit a temporal anti-correlation. I used a one-dimensional CTM to investigate the mechanism of this anti-correlation. I found that the anti-correlation can originate from the sulfur photochemistry in the middle atmosphere, while the variations can be caused by the lower-atmosphere perturbations. Eddy diffusion alone cannot explain the observations. This study emphasizes the urgent need of detecting the cloud layer and the deep atmosphere of Venus.

The instrumentation TEXES/IRTF also found a two-peak feature in the local-time distribution of SO2 at the cloud top, consistent with SPICAV/VEx observations. I developed a two-dimensional CTM and connected it to a Venus GCM to investigate this feature. My work revealed that the two peaks can be explained by the combination of the semi-diurnal tides and the retrograde superrotating zonal (RSZ) flow. SOIR/VEx also observed a statistical difference between terminators for CO in the upper atmosphere. From my simulations, this difference can be explained by the transition from the RSZ flow to the sub-solar to anti-solar (SS-AS) circulation. My work also discussed mechanisms underlying the local-time distributions of other species and implied a complex coupling of photochemistry and dynamics in the Venus mesosphere.

The Cassini flyby observed that Enceladus currently experiences a high surface heat flow. This leads to the question whether its ice shell is in steady state or its sub-surface ocean is freezing with time. To support the steady state of the ice shell, amounts of endogenic heat are required, which are currently thought coming from tidal dissipation. However, the exact process that produces sufficient tidal dissipation to match the observations remains elusive. I used a libration model to investigate the heating effect of the diurnal forced libration. I found that although the forced libration enhances the tidal dissipation in the ice shell, the total heating in the shell is still insufficient to match the observed surface heat loss. If Enceladus is in steady state, there should exist a large heat source beneath the shell, either in the ocean or in the core. If in steady state, Enceladus is likely to be in a stable thermal equilibrium, which resists small perturbations on the ice shell. This implies that thermal runaway or episodic heating is unlikely to originate from the librations of the ice shell.