Pulling Back the Veil: The Characterization and Habitability of Enshrouded Worlds

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

This dissertation explores global atmospheric haze and cloud layers and shows that they are not impenetrable barriers to information about the lower atmosphere and surface environment of planets. Uncovering techniques to probe beneath these aerosol decks can reveal a wealth of knowledge about planetary environments that would otherwise be inaccessible. Hazes themselves may reveal much more than they conceal because photochemical haze layers can constrain important planetary processes and characteristics such as atmospheric redox state, surface gas source fluxes, planetary habitability, and possibly even signs of life. The spectral consequences of global aerosol decks can be significant. Unfortunately, based on historical efforts to understand cloud- and haze-covered solar system worlds, we know there is a high potential for spectral misinterpretation of globally-enshrouded worlds (e.g. Kuiper 1944). Therefore, understanding how to properly interpret the remote observables of enshrouded exoplanets will be important to future exoplanet characterization efforts that will lack the advantage of ground-truthing within our lifetimes. In the first section of this dissertation, I discuss sub-cloud observations of the closest globally-enshrouded planet: Venus. Venus has near-infrared spectral windows observable on the planet's nightside that allow remote sensing of thermal radiation emanating from below the cloud and haze deck. We observed Venus with the Apache Point Observatory 3.5m telescope TripleSpec spectrograph (R = 3500, λ=0.96-2.47 μm) on 1-3 March 2009 and on 25, 27, 30 November and 2-4 December 2010. With these observations and synthetic spectra generated with the Spectral Mapping and Atmospheric Radiative Transfer (SMART) model, I produced the first simultaneous maps of cloud opacity, acid concentration, water vapor (H2O), hydrogen chloride (HCl), carbon dioxide (CO), carbonyl sulfide (OCS), and sulfur dioxide (SO2) abundances in the Venusian sub-cloud atmosphere. Water measured at wavelengths near 1.18 μm (near-surface) averages 29±2 ppm (2009) and 27±2 ppm (2010), and measured near 1.74 μm (15-30 km) averages 33±2 ppm (2009) and 32±2 ppm (2010). Water in both of these altitude ranges is spatially homogeneous. Water measured near 2.4 μm (30-45 km) averages 34±2 ppm (2009) and 33±3 ppm (2010) and is spatially inhomogeneous and variable. Estimates retrieved from measurements of HCl near 1.74 μm indicate mixing ratios near 0.41±0.04 ppm (2009) and 0.42±0.05 ppm (2010). CO and OCS, (2.3-2.5 μm; 30-45 km in altitude), are spatially inhomogeneous and appear to be anticorrelated. CO (35 km) averages 25±3 ppm (2009) and 22±2 ppm (2010). OCS (36 km) averages 0.44±0.10 ppm (2009) and 0.57±0.12 ppm (2010). SO2 measurements indicate average mixing ratios near 140±37 ppm (2009) and 126±32 ppm (2010). Many species display a hemispherical dichotomy in their distributions, and there is considerable spatial variability suggesting active processes with conservation between species. The most variable regions are just below the Venus cloud deck, and these may be related to changes in atmospheric circulation or virga events. While Venus may be the closest current example of a world covered by haze and cloud, Earth itself may have hosted a global organic haze early in its history. This is significant because recognizing whether a planet can support life is a primary goal of future exoplanet spectral characterization missions. However, past research on habitability assessment has largely ignored the vastly different conditions that have existed in our planet's long habitable history. My study of hazy early Earth presents simulations of a habitable, yet dramatically different phase of Earth's history, when the atmosphere contained a Titan-like, organic-rich haze. Prior work has claimed that a haze-rich Archean Earth (3.8-2.5 billion years ago) would be frozen due to the haze's cooling effects (e.g. Haqq-Misra et al. 2008). However, no previous studies have self-consistently taken into account climate, photochemistry, and fractal hazes. Using coupled climate-photochemical-microphysical simulations, I demonstrate that hazes can cool the planet's surface by about 20 K, but habitable conditions with liquid surface water could be maintained with a relatively thick haze layer (τ~5 at 200 nm) even with the fainter young sun. I find that optically thicker hazes are self-limiting due to their self-shielding properties, preventing catastrophic cooling of the planet. Hazes may even enhance planetary habitability through UV shielding via their broad UV absorption signature, which can reduce surface UV flux by about 97% compared to a haze-free planet, and potentially allow for survival of land-based organisms at 2.6-2.7 billion years ago. Hazy Archean Earth is the most alien world for which we have geochemical constraints on environmental conditions, providing a useful analog for similar habitable, anoxic exoplanets. To examine how organic haze may impact exoplanet habitability, I compared the production of fractal organic haze on Archean Earth-analog planets around several spectral types of stars: the sun at 2.7 billion years ago and at present day; the highly flaring M3.5V dwarf AD Leo; the M4V dwarf GJ 876; a modeled quiescent M dwarf; the K2V star ε Eridani; and the F2V star σ Boötis. In my simulations, planets orbiting stars with the highest or lowest UV fluxes did not form haze. Low UV-stars are unable to drive the photochemistry needed for haze formation. High UV stars generate photochemical oxygen radicals that halt the buildup of this haze. Hazes can impact planetary habitability via UV shielding and surface cooling, but this cooling seems unimportant for hazy M dwarf planets because the bulk of the M dwarf spectral energy arrives at longer infrared wavelengths where organic hazes are relatively transparent. I simulated hazy planet spectra for these exoplanet-analogs in reflected light, thermal emission, and transit transmission and found that the spectral features of organic hazes should be detectable with future telescopes. For 10 transits of a hypothetical Archean-analog planet orbiting GJ 876 observed by the James Webb Space Telescope (JWST) over 0.8 - 14 μm, haze, methane and carbon dioxide are detectable assuming photon-limited noise levels. For direct imaging of a planet at 10 pc using a coronagraphic 10-meter class ultraviolet-visible-near infrared telescope, a shortwave haze absorption feature would be strongly detectable at >12 σ in 200 hours. The apparent prevalence of hazy worlds in the known exoplanet population and in our solar system suggests that the impact of haze on planetary habitability and spectra are crucial to consider for future characterization of terrestrial exoplanets. The impact of haze on habitability may have been far-reaching, and haze in the Archean could even have impacted the evolution of photosynthetic pigments because the spectrum of light reaching the planet's surface would have been reddened. I explore the consequences of this and show the spectrum of photons at the Earth's surface beneath a haze. In addition to haze, other types of UV shields would have been present in the Archean. I present spectra at several depths under water with and without dissolved Fe(II), a UV shielding compound that may have been in the Archean oceans. UV-tolerant phototrophs like Chloroflexus aurantiacus could have received a survivable level of UV irradiance under a haze and 10 cm of water containing 5 ppm dissolved Fe(II). With haze and other types of biochemical, chemical and physical UV shields, such organisms may have been protected even directly at the planet's surface. Besides UV shielding and possible impacts on photosynthesis, there are other ways that an Archean haze the evolving biosphere were connected. Any haze in Archean Earth's atmosphere would have been strongly dependent on biologically-produced methane, and hydrocarbon haze may be a novel type of spectral biosignature on planets with substantial levels of CO2. On planets with high levels of biogenic organic sulfur gases, photochemistry involving these gases can drive haze formation at lower CH4/CO2 ratios than methane photochemistry alone, providing another means to argue for biological activity on a haze-rich planet.