Convection in Planetary Atmospheres: Titan's Haze, Saturn's Storm and Jupiter's Water

Abstract

Atmospheric convection is a profound topic. Numerous books have been written on the consequences of convection. Yet, the picture of convection is still far from complete because of its high nonlinearity, multi-scale coupling and complex interactions with other systems. The theme of my dissertation is to investigate three aspects of atmospheric convection on three different planets. This dissertation is multi-disciplinary and includes scientific topics like photochemistry, dynamics and radiation, and methodologies like information retrieval, theoretical calculation and dynamic modeling. Chapter 1 and 2 study Titan. It focuses on how to infer the strength of convection from the vertical distribution of chemical species. In a photochemical model, convection is parameterized as eddy diffusion and the strength of convection is proportional to eddy diffusivity. We developed an inversion method to retrieve the vertical profile of eddy diffusivity directly from the Cassini observations and found out a stable layer in the atmosphere which may give rise to the detached haze layer on Titan. In addition, new observation from Cassini/CIRS limb sounding came a few month later. C 3 H 6 was detected for the first time in the stratosphere. Our new photochemical model with the updated eddy diffusion profile successfully explained the observed vertical distribution of C 3 H 6 . Chapter 2 explains the modeling result and does a systematic study on all C 3 -hydrocarbons. Chapter 3 studies Saturn. It investigates the role of convection on regulating Saturn’s giant storms. Six giant storms, called Great White Spots, have erupted on Saturn since 1876 at intervals of about 30 years. The most recent one occurred on Dec. 5th, 2010 at planetographic latitude 37.7°N. It produced intense lightning, created enormous cloud disturbances and wrapped around the planet in 6 months. We proposed the water-loading mechanism to explain the periodicity. Moist convection is suppressed for decades due to the larger molecular weight of water in a hydrogen-helium atmosphere. We show that this mechanism requires the deep water vapor mixing ratio to be greater than 1.0%, which implies Saturn’s O/H to be at least 10 times the solar value. Chapter 4 studies Jupiter. It proposes an inversion strategy for the upcoming Juno microwave observation based on the modeling results and the theoretical arguments developed in Chapter 3. We extend the Juno/MWR’s functionality by retrieving both the deep water mixing ratio and a few dynamic parameters representing subcloud meteorology. This proposition will contribute substantially to achieving the Juno/MWR objectives and shed light on the functioning of convection on planets with deep atmospheres.