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Planetary Atmospheres: Astrobiologically Relevant Icy Worlds and Earth as a Proxy Exoplanet


Fan, Siteng (2021) Planetary Atmospheres: Astrobiologically Relevant Icy Worlds and Earth as a Proxy Exoplanet. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/47nh-ze14.


"How did we get here?" is a long-standing question in planetary science. Characterizing the pre-biotic atmospheric environment in which life may emerge is critical and increasingly urgent. Given the fact that the Earth provides the only ground truth of habitable worlds, most of the characterizations are based on the current Earth. However, life did not emerge on the modern Earth. It instead took place in a prebiotic environment, which includes a nitrogen-dominated, methane-abundant and oxygen-negligible reducing atmosphere. Therefore, this type of planetary atmospheres has great significance in the context of astrobiology and the search for life. Despite that real time observations can not be obtained for Early-Earth, spacecraft observations of the atmospheres of two icy worlds in the solar system, Titan and Pluto, can provide such valuable constraints. The theme of Chapter 2 and 3 of this thesis focus on this topic of investigating the atmospheres of Titan and Pluto using spectroscopic analysis. Chapter 4 studies the search for life also across the spectrum from a prospective the other way. It characterizes the Earth, the only known inhabited planet, as an exoplanet proxy, to derive observational benchmarks for habitability assessment.

Chapter 2 studies Titan. It retrieves the hydrocarbon and nitrile species in Titan’s upper atmosphere using stellar occultation observations obtained by Cassini UltraViolet Imaging Spectrograph (UVIS) during its Titan flybys. An innovative method is introduced to consider the pointing issue of the instrument, which prevents most of the previous spectral analyses. Combing an instrument simulator for handling the pointing motion and the Markov-Chain Monte Carlo (MCMC) method for parameter searching, species abundances in Titan’s atmosphere are successfully retrieved during occultations with large pointing motions. The method also obtains the altitude range where the abundance of each species could be constrained.

Chapter 3 studies Pluto. It investigates the morphology and microphysical processes of Pluto’s haze particles in the lower 50km of its atmosphere using observations obtained by multiple instruments onboard the New Horizons spacecraft during its Pluto flyby in 2015. It suggests that Pluto’s haze particles have a bimodal distribution: a large-size population of ~1μm fractal aggregates, which consists of ~20nm monomers, and a small-size one of ~80nm. This result successfully addresses the disagreement among the instruments, and provides important constraints on transport and dimensional transition of haze particles in Pluto’s atmosphere.

Chapter 4 studies exoplanets. It evaluates the observational baseline for Earth-like exoplanets using the Earth as a proxy. Observations of the Earth’s images obtained by the Deep Space Climate ObserVatoRy (DSCOVR) are integrated to one single point to generate light curves of the "proxy" planet. Using the singular value decomposition (SVD) method, we found that the surface information of the "proxy" planet is in the second principal component (PC) of its light curves, while the first PC mainly consists of that of clouds. Using the strong linear correlation between the time series of the second PC and the corresponding land fraction, we constructed the first two-dimensional surface map of the Earth seen from a hypothetical distant observer, an observer who treats the Earth as an exoplanet.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Planetary atmosphere, Titan, Pluto, Exoplanet, Photochemistry, Haze
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Planetary Sciences
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Yung, Yuk L.
Thesis Committee:
  • Knutson, Heather A. (chair)
  • Yung, Yuk L.
  • Ingersoll, Andrew P.
  • Batygin, Konstantin
Defense Date:19 November 2020
Record Number:CaltechTHESIS:12142020-085746258
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Chapter 2. adapted for Chapter 4.
Fan, Siteng0000-0002-3041-4680
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14027
Deposited By: Siteng Fan
Deposited On:18 Dec 2020 17:31
Last Modified:08 Nov 2023 00:36

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