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Millimeter and Near-Infrared Observations of Neptune's Atmospheric Dynamics

  • Author(s): Cook, Statia Honora Luszcz
  • Advisor(s): de Pater, Imke
  • et al.
Abstract

The atmospheric chemistry and dynamics of the ice giants are critical to comprehending planetary diversity within our Solar System and beyond. The bulk composition of these planets, which can be investigated through the composition of their atmospheres, is intimately tied to their formation in the circumstellar disk. In this dissertation, I take a multi-wavelength approach to studying chemistry and large-scale circulation in the atmosphere of Neptune, one of the two local examples of ice giant planets.

In equilibrium, the molecule carbon monoxide (CO) should be confined to the warm interiors of the giant planets. Its presence in the upper atmosphere, therefore, indicates disequilibrium processes at work: either rapid transport from deeper levels where the molecules are thermodynamically stable, or production high in the atmosphere as a result of infall of material from the planet's environment. Using millimeter-wave interferometry with the Combined Array for Research in Millimeter-wave Astronomy (CARMA), I observe Neptune in two rotational transitions of CO. Radiative transfer modeling yields a CO profile with 0.1+0.2-0.1 parts per million (ppm) of CO in the troposphere, and 1.1+0.2-0.3 ppm in the stratosphere. The stratospheric abundance implies an infall rate of oxygen-bearing material of 0.5–20×108 cm-2 s-1 to Neptune's upper atmosphere, which is consistent with supply by (sub)kilometer-sized comets. I also revisit the calculation of Neptune's internal oxygen abundance using revised calculations for the CO→CH4 conversion timescale in the deep atmosphere (Vissher & Moses 2011), in the context of my derived CO profile. The best-fit solution of 0.1 ppm of CO in the troposphere implies a global O/H enrichment of at least 400, and likely more than 650 times the protosolar value. This is one order of magnitude greater than Neptune's observed carbon enrichment relative to solar. However, the CO profile is also consistent with 0.0 ppm of CO in the troposphere, in which case no over-enrichment of oxygen in the interior is required.

Maps of Neptune in and near the CO (2–1) line from CARMA show spatial variations in the intensity at the 2–3% level. Variations at frequencies in the CO line are consistent with variations in zonal-mean temperature near the tropopause. At continuum wavelengths, I observe a gradient in the brightness temperature, increasing by 2–3 K from 40°N to the south pole. This corresponds to an opacity decrease of about 0.3 (30%) near the south pole at altitudes below 1 bar, or a factor of 100 decrease in the opacity at altitudes below 4 bar. A global circulation pattern in which moist air rises at mid- southern and northern latitudes and dry air subsides near the equator and south pole is consistent with the south polar brightening observed in the millimeter.

At near-infrared wavelengths, observations are sensitive to sunlight reflected from clouds and hazes in the upper atmosphere. These clouds act as tracers of atmospheric dynamics, which aid in the determination of Neptune's large-scale circulation. Near-infrared images of Neptune from the W.M. Keck II telescope from July 2007 show that the unresolved cloud feature typically observed within a few degrees of Neptune's south pole had split into a pair of bright spots. A careful determination of disk center places the cloud centers within 2 degrees of, but not directly at, Neptune's south pole. When modeled as optically thick, perfectly reflecting layers, the two features are found to be in the troposphere, at pressures greater than 0.4 bar. Images with comparable resolution taken two days later reveal only a single feature near the south pole. The changing morphology of these circumpolar clouds suggests they may form in a region of strong convection surrounding a Neptunian south polar vortex.

Additional near-infrared observations were performed with the OSIRIS integral-field spectrograph on the Keck telescope. I present three-dimensional data cubes covering more than 90% of the visible hemisphere of Neptune, with a spatial resolution of 0.035'' per pixel and spectral resolution of R∼3800, in the H (1.47–1.80 μm) and K (1.97–2.38 μm) broad bands. In my preliminary radiative transfer analysis of these data, I find that the observed spectra are generally well fit by models with three cloud layers: a stratospheric haze layer, a tropospheric haze layer and a low albedo, optically thick bottom cloud at 2 bar. Models are consistent with a north-south hemispheric asymmetry in the properties of the clouds, with both bright and dark features in the north at higher altitudes than equivalent regions in the south. The range of derived altitudes between the highest and deepest features is nearly 5 atmospheric scale heights. The highest concentration of features can be found within a bright band extending from 30 to 45°S. I find that these features can all be fit by hazes at the same altitude, by varying only the haze particle densities. In contrast, I find that two features near 60°S appear to be at very different altitudes, even though they are at the same latitude.

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