“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).