George D. McDonald
National Defense Science & Engineering Graduate Fellow
Georgia Tech

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Localized studies of water vapor in Mars's atmosphere
The water cycle on Mars has important effects on the atmosphere, in affecting the distribution of dust aerosols, and through radiative effects in the form of clouds. While the dominant behavior of the seasonal water vapor cycle is well studied in terms of its relation to summer sublimation of the northern polar cap, the roles of exchange with the surface and cloud condensation are less understood. I'm determining localized abundances of water vapor with data from the Thermal Emission Spectrometer on Mars Global Surveyor to investigate these small-scale phenomena. (Image Credit: Malin Space Science Systems, NASA)
Photoevaporation of sub-Neptune sized exoplanets
Sub-Neptunes with short orbital periods of less than 10 days are among the most common type of known exoplanets. However these planets typically receive high irradiation from their host stars (100s to 1000s of times that of Earth) where they are vulnerable to atmospheric photoevaporation. I am using data from the Kepler mission to constrain the conditions for a sub-Neptune sized exoplanet to lose its primordial hydrogen/helium atmosphere. (Image Credit: Rodrigo Luger, NASA)
Near-surface atmospheric transmission on Titan
Saturn's largest moon Titan has a nitrogen atmosphere four times denser than that of the Earth's. The atmosphere contains 1.5% methane, which plays a role analogous to water in the Earth's atmosphere, existing in both vapor and liquid form. Methane is a strong absorber at near and mid infrared wavelengths. In turn, infrared spectrometers studying the composition of Titan's surface, such as the Visual and Infrared Mapping Spectrometer (VIMS) aboard the Cassini orbiter, are hindered by methane absorption across much of the spectrum, and can only view the surface at a handful of 'methane transmission windows.' Future balloon and lander missions to Titan may attempt to remedy the situation by operating below orbit, thereby decreasing the methane column through which spectrometers must make observations. I am helping in the development of a new radiative transfer model (which calcualates the interactions of infrared light with gases and particles in the atmosphere) to determine the exact wavelengths in the infrared that would be available to future near-surface missions for the study of complex organics and other compounds on Titan's surface. (Image Credit: Tibor Balint)
Titan's aeolian processes
Dunes dominate Titan's equatorial region. Although dunes are found across the solar system, on no planetary body are they as prevalent as on Titan. These dunes are inferred to be composed of organics (carbon and hydrogen bearing molecules, think plastics) rather than silicate sands, and also serve as an important record of Titan's climate. I am investigating how these dunes may be influenced by changes in Titan's orbital configuration (changes that are analogous to those of the Earth's orbit that cause the ice ages) using general circulation models. I am also helping to design and run experiments that study the electrical charging behavior of these dune materials, and am involved in modelling efforts examining how these dunes are affected by topographic obstacles such as small hills. All of these studies will help us to maximize the information we can tease out about Titan's modern-day and historical climate. (Image Credit: George McDonald, NASA/JPL)