Aerosols and Methane in the Ice Giant Atmospheres Inferred from Spatially Resolved, Near-Infrared Spectra: I. Uranus, 2001-2007

Abstract: We present a radiative transfer analysis of latitudinally resolved H (1.487-1.783 micron) and K (2.028-2.364 micron) band spectra of Uranus, from which we infer the distributions of aerosols and methane in the planet's atmosphere. Data were acquired in 2001, 2002, 2004, 2005, and 2007 using the 200-inch (5.1 m) Hale Telescope and the Palomar High Angular Resolution Observer (PHARO) near-infrared adaptive optics (AO) camera system (Hayward, 2001). Observations sample a range of latitudes between 80-deg S and 60-deg N on the Uranian disk. At each latitude, a vertical distributions of aerosols was retrieved using a custom non-linear constrained retrieval algorithm. Two layers of aerosols are needed to match the observations: a thin upper layer peaking just below the 100-mb tropopause and a lower clouds at ~1.9 bars. Latitudinal variations in aerosols are interpreted in context of notional circulation models, while temporal changes suggest potential seasonal effects. We infer significant reduction in aerosol scattering optical thickness in southern latitudes between 2001 and 2007, in agreement with trends reported in studies covering part of the same period using different data and retrieval algorithms (e.g., Irwin et al., 2009, 2010, 2012; Sromovsky et al., 2009). Best fits to the data are consistent with proposed models of polar depletion of methane (e.g., Karkoschka and Tomasko, 2011). Finally, a discrete cloud from 2007 is analyzed in context of simple parcel theory, with the goal of identifying likely formation mechanisms. The low scattering optical thicknesses of the discrete high cloud are consistent with formation associated with vortices and shallow lift rather than deep convection.