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Unraveling Neptune's Atmospheric Structure from Multi-Wavelength Observations

  • Author(s): Tollefson, Joshua Wayne
  • Advisor(s): de Pater, Imke
  • et al.
Abstract

Uranus and Neptune are representatives of the `ice giants', one of the most common classes of exoplanets \citep{Fressin2013}. Thousands of exoplanets have been discovered thanks to the \textit{Kepler} mission, and soon the James Webb Space Telescope will characterize their atmospheres in unprecedented detail. Such work will rely on the observations, techniques, and analysis used to study the Solar System's gas giants. However, in many ways our own ice giants remain poorly understood. In this dissertation, I use multi-wavelength observations of Neptune to better constrain the bulk properties and dynamic patterns within the planet's upper atmosphere.

At visible and near-infrared wavelengths, sunlight is reflected off the cloud tops and hazes populating the upper atmospheres of the giant planets. Bright cloud features can be tracked to extract velocities. By doing this over many latitudes, a global velocity field called the `zonal wind profile' can be made. Here, I present zonal wind profiles for Jupiter and Neptune. These are constructed from \textit{Hubble Space Telescope} WFC3 global maps of Jupiter taken between $2009-2016$ and Keck NIRC2 images of Neptune taken in the H-band ($1.4-1.8 \mu$m) and Kp-band ($2.0-2.4 \mu$m) in 2013 and 2014.

I show that Jupiter's zonal wind profile is stable throughout the observed period, apart from variations on the order of 10 m/s at the $24^{\circ}$N Northern Temperature Belt (NTB). These variations arise during periodic plume outbreaks at the NTB and are coupled to a decrease in the albedo. These findings suggest that material, normally unseen, is dredged upward due to these plumes. If plumes are a signature of deeper activity, the decrease in velocity we see at the NTB during outbreaks may be evidence of vertical wind shear.

I also find evidence of vertical wind shear at Neptune's equator, with the H-band zonal wind profile offset eastward by 100 m/s at the equator relative to the Kp-band profile. I apply a new thermal wind equation applicable at the equator to reconcile this observed vertical wind shear with Neptune's horizontal thermal and composition profiles. In order to match \textit{Voyager}/IRIS derived temperatures \citep{Fletcher2014}, the equator must be enriched in methane compared to the mid-latitudes at pressures greater than 1 bar. I discuss the implications of this finding with regards to global dynamics and compare and contrast to the other giant planets.

Radio wavelengths probe below the visible cloud deck. I analyze maps of Neptune taken with the Atacama Large Millimeter/Submillimeter Array (ALMA) and extended Very Large Array (VLA) to constrain Neptune's deep opacity sources. The opacity source at radio wavelengths is dominated by H$_2$S and NH$_3$ as well as the the collision-induced absorption of H$_2$ with H$_2$, He, and CH$_4$. Clear brightness temperature variations are present across Neptune's disk caused by variations in these trace gases. These observations are the first to achieve the sensitivity, resolution, and wavelength coverage required to simultaneously extract the abundance profiles of H$_2$S, CH$_4$, and NH$_3$. I retrieve disk-average properties assuming both wet and dry adiabats. The disk-averaged data are consistent with profiles where trace gases are enriched by $30\times$ their protosolar value, apart from NH$_3$ which is $1\times$ its protosolar value.

In both the ALMA and VLA maps, I identify seven distinct latitudinal bands with discrete transitions in the brightness temperature. I use the radiative transfer code Radio-BEAR to generate model spectra of Neptune's brightness temperature as a function of temperature and composition. I find best-fitting parameters to the H$_2$S, NH$_3$, and CH$_4$ abundance profiles in each of the seven identified latitude bands using $\chi^2$-statistics and MCMC retrievals. Of note, the equator is more complicated than expected. Trace gases are enriched in the $2-12^{\circ}$N band compared to neighboring latitudes. Here, the best-fit deep CH$_4$ abundance is $45\times$ the protosolar value (or 2.2$\%$ mixing ratio). H$_2$S is $30\times$ solar (or $7\times10^{-4}$ mixing ratio) and supersaturated at the H$_2$S-ice cloud formation. I relate these findings to my near-infrared work and present a new schematic of Neptune's global circulation structure.

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