Habitability Through Time: Photochemistry and Aerosols of Planetary Atmospheres

Citation

Adams, Danica Jeannine (2023) Habitability Through Time: Photochemistry and Aerosols of Planetary Atmospheres. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/za4k-hs94. https://resolver.caltech.edu/CaltechTHESIS:06032023-042339662

Abstract

The unique geologic preservation of much of Mars’ ancient surface provides a window into its earliest history, and hence the early history of the solar system. Extensive geological and mineralogical evidence suggest that ancient Mars once had large volumes of surface liquid water, which likely persisted over timescales of 10⁵ - 10⁷ years during the Noachian era (e.g., Carr et al., 2003; Clifford et al., 2001; Barnhart et al., 2009; Schon et al., 2012). Explaining this evidence for surface liquid water is challenging, however, because of Mars’s distant orbit and the lower luminosity of the young Sun 3-4 billion years ago. The faint young Sun paradox is an important problem in planetary science that challenges our ability to understand atmospheric evolution in general, including Earth, Mars, and rocky exoplanets (e.g., Sagan 1972).

The precise composition and climate of the early atmosphere overtime largely remains an open question. In 2014, it was first recognized that early Mars could have been episodically warmed by the greenhouse effects of H₂ in a CO₂ atmosphere (e.g., Wordsworth et al., 2017); however, no sustained source of H₂ was identified in the literature (noting that volcanism would have been short lived). Chapter 2 presents a solution: crustal hydration (the loss of surface water to reduced iron and hydrated minerals) likely supplied large fluxes of H₂. Over a timescale of 10⁷ years (the upper limit for the duration of large volumes of surface liquid water), crustal hydration provides a flux of H₂ into the atmosphere large enough to sustain a surface temperature >273 K in a ≥ 1 bar Noachian atmosphere. Importantly, Mars was likely warm over only a fraction of its early history, and cold early atmospheres likely also existed during early Mars’ history. In cool climates, I find that a loss of atmospheric oxidants to the ground (to oxidize surface reduced iron) caused CO₂ to convert to CO in agreement with the results of Zahnle et al. (2008). Furthermore, a warm and wet climate suggests early Mars may have been similar to early Earth; however, the climate alone is not enough to suggest that early Mars may have been habitable. In Chapters 3 and 4, I investigate whether Mars may have had a nitrogen cycle, which would be important for nitrogen fixation. I used KINETICS, the Caltech-JPL 1D photochemical model, to explain present day deposits in Mars’ soil samples. The Sample Analysis at Mars instrument onboard Mars Science Laboratory (MSL) has baked several volatile species out of the unique rock record at Mars, including nitrate (e.g., Sutter et al., 2017). The formation of these species would originate in the atmosphere as a result of photochemistry. In Chapter 3, I discover that nitrogen fixation in a warm and wet climate with lightning is able to explain the weight percent of nitrate measured by the MSL; lightning-induced NOx forms nitric acid in the atmosphere, and this nitric acid may dissolve in water, rain out to the surface, and undergo photoreduction in shallow surface waters. In Chapter 4, I discover a comparable amount of pernitric acid may be explained from formation in an icy climate; SEP-induced N(2D) attacks CO₂ to form NOx which reacts with HO₂ to form HO₂NO₂.

The relatively new subfield of comparative planetology between Mars’ evolution and exoplanet evolution (specifically, close-in super-Earths) will soon open, in the new era of the James Webb Space Telescope (JWST). I have prepared for this subfield by working with a suite of established numerical models to investigate the formation of prebiotic species in the reduced atmospheres of super-Earths (in Chapter 5) and the aerosols at warm gas Giants. In Chapter 6, I discover that aggregate hazes at warm sub-Neptunes, which result from methane photolysis, can explain the observed flat exoplanet spectra and muted spectral features, including the observations of GJ 1214b. In Chapter 7, I discover that the atmospheric dynamics of hot Jupiters cause patchy clouds of forsterite, iron, and titanium dioxide, and the 3D structure of the clouds helps explain the phase-integrated albedos of six worlds observed by HST.

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