Exploring Habitability Markers, Biosignatures, and Their False Positives Using Spectral Models of Terrestrial Exoplanets

Thesis (Ph.D.)--University of Washington, 2016-08

In the coming years and decades, we will obtain our first opportunity to spectrally characterize potentially habitable worlds outside our solar system. These planets will be at the right distance from their host star and possess the correct range of atmospheric compositions to have surfaces conducive to maintaining liquid water, the key requirement for habitability and life. For example, the James Webb Space Telescope (JWST), set for launch in 2018, could observe the transmission spectra of a handful of terrestrial planets orbiting late type stars. Direct-imaging telescopes currently in the design phases, including the Large UltraViolet Optical InfraRed (LUVOIR) Surveyor, will possess the ability to spectrally characterize planets in the habitable zones of up to hundreds of nearby planetary systems. The goal of this work is to advance our ability to recognize whether an exoplanet can or does support life by exploring a range of spectral habitability markers and astronomical biosignatures. As our best, and currently only, example of a habitable planet, Earth provides a fiducial point for studying the possible spectral manifestation of habitability markers and biosignatures for exoplanets. This thesis includes studies in four areas related to this theme. First, I build a high fidelity, high cadence spectral Earth database from the far UV (0.1 μm) to the far Infrared (200 μm) using the VPL 3D, line-by-line, multiple scattering spectral Earth model. This database furthers our understanding of Earth as an exoplanet, illustrating spectral changes as a function of phase and rotation. Second, I demonstrate the detectability of N2 using the (N2)2 collisional pair, which has implications for characterizing the bulk atmospheres of terrestrial exoplanets and may be a strong biosignature in combination with detection of O2 or O3. I use data model comparisons to show that (N2)2 produces a ~35% reduction in Earth’s spectral flux at 4.1 μm. I quantify the strength of the (N2)2 feature in a variety of simulated atmospheres with different N2 abundances with both synthetic direct-imaging and transit transmission spectra. Third, I investigate observational indicators of planetary mechanisms that may generate abiotic oxygen (O2 or O3) on exoplanets, leading to potential “false positives” for life. Abiotic production of O2 from CO2 photolysis potentially leads to detectable amounts of CO as a byproduct. Oxygen build up from massive H-loss during a runaway greenhouse could leave behind more O2 than could be plausibly produced by biology. In this case density dependent O4 features would be strong and potentially indicative of this process. I investigate the strength and detectability of CO (at 2.35 and 4.6 μm) and O4 (at 0.345, 0.36, 0.38, 0.445, 0.475, 0.53, 0.57, 0.63, 1.06, and 1.27 μm) absorption for these abiotic oxygen scenarios in both transmission and direct-imaging spectroscopy. Finally, I present an interdisciplinary study of nonphotosynthetic pigments as alternative surface reflectance biosignatures, in contrast to direct photosynthetic signatures like the vegetation red edge. This study includes reflectance measurements of a variety of pigmented organisms in the laboratory, illustrating a wide range of spectral diversity. I also model the spectra of hypothetical planets containing nonphotosynthetic pigment biosignatures including the confounding spectral effects of the atmosphere. I find that these signatures could potentially be observable in disk-averaged spectra depending on the fraction of the planet containing the signature. Organisms with nonphotosynthetic pigments will produce reflectance signatures different than that of the commonly referenced vegetation red edge, and push us to broaden our understanding of what surface biosignatures might look like on Earth-like exoplanets once remote characterization of these worlds becomes possible.