Saturn’s largest moon, Titan, provides a new perspective on planetary climate. It is larger than Mercury, has a 16-day rotation period, 29.5-year annual cycle, and a ~1.5-bar nitrogen atmosphere. Titan has a fully developed atmosphere, analogous to that of Earth, and methane plays a similar role to water in the hydrological cycle on our planet, generating clouds, storms and precipitation. Titan’s clouds have been under investigation since their detection in 1995 with ground-based telescopes and were observed in detail during the Cassini-Huygens mission to the Saturn system. “Cassini” orbited Saturn and its moons from 2004 to 2017, giving unparalleled views that have led to countless discoveries and clouds are one of many fascinating Titan phenomena revealed by it. To this day, cloud formation mechanisms, dynamics and duration of the associated storms are still not fully understood and are the subject of ongoing study. The central goal of this work is to provide a general physical interpretation of observed storms and their relation to atmospheric dynamics of the moon. Two previous studies are the foundation for this research: Mitchell et al. (2011), who developed a process for interpreting Titan’s cloud morphologies and precipitation through a combined analysis of observations and general circulation model (GCM) simulations; and Turtle et al. (2011), who reported the first evidence of seasonal changes on the moon obtained from Cassini cloud data.
This dissertation presents a survey of Titan’s mid-latitude clouds, as seen from space by the Cassini Imaging Science Subsystem (ISS) instrument, compares a subset of the observed clouds to the methane storms produced in a climate model of Titan, and infers the underlying storm dynamics by connecting the two. ISS is a multi-wavelength – ultraviolet to near-infrared – camera specifically designed to take high resolution images from the top of the atmosphere to the surface of Titan piercing through the thick haze located in the stratospheric layer of the moon, which typically blocks the view of tropospheric clouds underneath it.
This study starts with an analysis of the physics of clouds applied to Titan’s conditions and a microphysical cloud scheme to show how the abundant haze particles in the atmosphere are likely the seeds for methane droplets that catalyze cloud formation. Next, the ISS image archive is searched for cloud phenomena and various types of storms are surveyed. This is followed by Image processing, that require the conversion of raw images into maps with global locations of the clouds and the production of enhanced views of cloud features against the surface background to reveal their morphology. We then search for storms with temporally resolved observations and use their spatio-temporal distributions to identify the atmospheric dynamics behind them, including Rossby and gravity waves. Although many clouds/storms are identified, only two of them provide clear spatial and temporal information that allow this type of analysis.
The manuscript then pivots to analysis of methane storms in model simulations of Titan’s climate using the Titan Atmosphere Model (TAM; Lora et al. 2015) with full surface hydrology (Faulk et al., 2019). The spatio-temporal features of observed clouds combined with the simulated storms at the same season as the observations suggest that just as waves organize storms on Earth, they do so on Titan as well. The results of the study strongly indicate that Titan’s cloud formation and propagation are associated with Rossby and equatorial Kelvin waves, and perhaps combinations thereof, and that the clouds/storms can persist for weeks and perhaps much longer as they propagate around the moon’s globe – a phenomenon referred to in this work as “persistence”.
These findings offer a glance into the complex phenomenology, dynamics and persistence of Titan’s clouds. The methodology developed in the course of this work for comparing the spatio-temporal distribution of observed clouds to analog storms in TAM is novel, while also being consistent with previous studies focused either on the spatial distribution or seasonal evolution of observed clouds. Future missions to Titan, including the funded Dragonfly mission, will facilitate further model-data comparisons, for instance the long-term persistence of the Kelvin wave in TAM. This methodology, documented in detail in a “cookbook”, provides a set of useful tools and guidance for future explorations of the clouds and storms of Titan.