Welcome! Below are select samples of published work (papers, conference abstracts, conference slides). For a complete publication list please see my page “CV”. Thanks!
Crustal Hydration Primed Early Mars with Warm and Habitable Conditions
Draft Manuscript (in review, Nature Geoscience)
Geological evidence suggests that ancient Mars held substantial volumes of surface liquid water, lost over time due to crustal hydration and atmospheric escape. Resolving the Mars Faint Young Sun Paradox, or explaining how the atmosphere was kept warmer early on despite a less luminous sun, has remained a challenge. Reduced greenhouse gases like H2 in a CO2 atmosphere are known to induce warming through collision induced absorption, but H2 is short-lived and no long-lasting source at early Mars has been identified yet. Here we show that crustal hydration would have supplied significant H2 fluxes, which, built up in the atmosphere over 1e5-1e7 years, could have facilitated a warm, wet climate through collision-induced absorption with CO2 and N2. This would only occur during a few major events lasting ~ million years, while shorter events could not have warmed the climate. Our results are the first to explain how long-lasting warm climates could have persisted at early Mars. As CO2 was lost and the atmosphere cooled, it transitioned to a CO-dominated state in the Hesperian era, leading to a colder, drier climate. We anticipate that sharp climate and redox state changes may have been triggered by geological or obliquity changes.
PhD Thesis
Nitrogen Fixation at Early Mars
Astrobiology Paper PDF (Astrobiology)
The Mars Science Laboratory (MSL) recently discovered nitrates near Gale Crater (eg., Stern et al., 2015; Sutter et al., 2017). One possible mechanism for ancient nitrate deposition at Mars is through HNOx formation and rain-out in the atmosphere, for which lightning-induced NO is the fundamental source. This study investigates nitrogen fixation at early Mars in order to probe early Mars habitability. We consider a 1-bar atmosphere of background composition CO2 with abundances of N2, H2, and CH4 varied from 1-10% to explore a swath of potential early Mars climates. We derive lightning-induced thermochemical equilibrium fluxes of NO and HCN by coupling the lightning-rate parametrization of Romps et al. (2014) with Chemical Equilibrium with Applications, and we use a Geant4 simulation platform to estimate the effect of solar energetic particle (SEP) events. These fluxes are used as input into KINETICS, the Caltech/JPL coupled photochemistry and transport code, and we consider the chemistry of 50 species linked by 495 reactions to derive rain-out fluxes of HNOx and HCN. We compute equilibrium concentrations of cyanide and nitrate in a putative northern ocean at early Mars, assuming hydrothermal vent circulation and photoreduction act as the dominant loss mechanisms. We find oceanic concentrations of 1-100 nM cyanide and ~0.1 nM nitrate. HCN is critical for protein synthesis at concentrations > 0.01 M (eg., Holm and Neubeck, 2009), and our result is astrobiologically relevant if secondary local concentration mechanisms occurred. Nitrates may act as high potential electron acceptors for early metabolisms, though the minimum concentration required is unknown. Our work derives concentrations that will be useful for future laboratory studies to investigate the habitability at early Mars.
Spatially Resolved Modeling of Optical Albedos for a Sample of Six Hot Jupiters
Optical secondary eclipse measurements made by Kepler reveal a diverse set of geometric albedos for hot Jupiters with equilibrium temperatures between 1550-1700 K. The presence or absence of high altitude condensates, such as Mg2SiO4, Fe, Al2O3, and TiO2, can significantly alter optical albedos, but these clouds are expected to be confined to localized regions in the atmospheres of these tidally locked planets. Here, we present 3D general circulation models and corresponding cloud and albedo maps for six hot Jupiters with measured optical albedos in this temperature range. We find that the observed optical albedos of K2-31b and K2-107b are best matched by either cloud free models or models with relatively compact cloud layers, while Kepler-8b and Kepler-17b's optical albedos can be matched by moderately extended (fsed=0.1) parametric cloud models. HATS-11b has a high optical albedo, corresponding to models with bright Mg2SiO4 clouds extending to very low pressures (fsed = 0.03). We are unable to reproduce Kepler-7b's high albedo, as our models predict that the dayside will be dominated by dark Al2O3 clouds at most longitudes. We compare our parametric cloud model with a two-zone microphysical cloud model (CARMA). We find that even after accounting for the 3D thermal structure, no single cloud model can explain the full range of observed albedos within the sample. We conclude that a better knowledge of the vertical mixing profiles, cloud radiative feedback, cloud condensate properties, and atmospheric metallicities is needed in order to explain the unexpected diversity of albedos in this temperature range.
Aggregate Hazes in Exoplanet Atmospheres
Photochemical hazes have been frequently used to interpret exoplanet transmission spectra that show an upward slope towards shorter wavelengths and weak molecular features. While previous studies have only considered spherical haze particles, photochemical hazes composed of hydrocarbon aggregate particles are common throughout the solar system. We use an aerosol microphysics model to investigate the effect of aggregate photochemical haze particles on transmission spectra of warm exoplanets. The wavelength dependence of unit nadir optical depth is steeper for spherical hazes than for aggregates since aggregates grow to larger radii. As a result, while spherical haze opacity displays a scattering slope towards shorter wavelengths, aggregate haze opacity is gray in the optical and NIR, similar to those assumed for condensate cloud decks. We further find that haze opacity increases with increasing production rate, decreasing eddy diffusivity, and increasing monomer size, though the magnitude of the latter effect is dependent on production rate and the atmospheric pressure levels probed. We generate synthetic exoplanet transmission spectra to investigate the effect of these hazes on spectral features. For high haze opacity cases, aggregate hazes lead to flat, nearly featureless spectra, while spherical hazes produce sloped spectra with clear spectral features at long wavelengths. Finally, we generate synthetic transmission spectra of GJ 1214b for aggregate and spherical hazes and compare them to space-based observations. We find that aggregate hazes can reproduce the data significantly better than spherical hazes, assuming a production rate limited by delivery of methane to the upper atmosphere.
Using Magnetic Topology to Probe the Sources of Mars’ Nightside Ionosphere
We combine thermal electron densities in Mars' ionosphere with magnetic topology information to investigate the sources of the nightside ionosphere. Thermal electron density is measured in situ by the Langmuir Probe and Waves (LPW) experiment onboard MAVEN (Mars Atmospheric and Volatile EvolutioN), while magnetic topology is simultaneously inferred from suprathermal electron energy-pitch angle distributions measured by the Solar Wind Electron Analyzer (SWEA) and the Magnetometer (MAG). Topologically closed regions inhibit electron impact ionization (EII), allowing us to isolate the effects of plasma transport from the dayside, which exhibits a dawn-dusk asymmetry. Pressure gradient forces on open magnetic field lines connected to the dayside ionosphere source the high-altitude nightside ionosphere, resulting in higher densities. Regions that are topologically open to the nightside ionosphere allow us to assess in situproduction by EII, which is responsible for ~50% of the nightside ionosphere below ~160 km and ~25% above ~220 km (on average).