Prof. Patrick Irwin

Abstract:

In this paper we present a study of the vertical haze and cloud structure over a triple vortex in Saturn's atmosphere in the planetographic latitude range 55°N-69°N (del Río-Gaztelurrutia et al., 2018) using HST/WFC3 multispectral imaging. The observations were taken during 29–30 June and 1 July 2015 at ten different filters covering spectral range from the 225 nm to 937 nm, including the deep methane band at 889 nm. Absolute reflectivity measurements of this region at all wavelengths and under a number of illumination and observation geometries are fitted with the values produced by a radiative transfer model. Most of the reflectivity variations in this wavelength range can be attributed to changes in the tropospheric haze. The anticyclones are optically thicker (τ ~25 vs ~10), more vertically extended (~3 gas scale heights vs ~2) and their bases are located deeper in the atmosphere (550 mbar vs 500 mbar) than the cyclone.

Abstract:

Current observational data of exoplanets are providing increasing detail of their 3D atmospheric structures. As characterisation efforts expand in scope, the need to develop consistent 3D radiative-transfer methods becomes more pertinent as the complex atmospheric properties of exoplanets are required to be modelled together consistently. We aim to compare the transmission and emission spectra results of a 3D Monte Carlo Radiative Transfer (MCRT) model to contemporary radiative-transfer suites. We perform several benchmarking tests of a MCRT code, Cloudy Monte Carlo Radiative Transfer (CMCRT), to transmission and emission spectra model output. We add flexibility to the model through the use of k-distribution tables as input opacities. We present a hybrid MCRT and ray tracing methodology for the calculation of transmission spectra with a multiple scattering component. CMCRT compares well to the transmission spectra benchmarks at the 10s of ppm level. Emission spectra benchmarks are consistent to within 10% of the 1D models. We suggest that differences in the benchmark results are likely caused by geometric effects between plane-parallel and spherical models. In a practical application, we post-process a cloudy 3DHD 189733b GCM model and compare to available observational data. Our results suggest the core methodology and algorithms of CMCRT produce consistent results to contemporary radiative transfer suites. 3D MCRT methods are highly suitable for detailed post-processing of cloudy and non-cloudy 1D and 3D exoplanet atmosphere simulations in instances where atmospheric inhomogeneities, significant limb effects/geometry or multiple scattering components are important considerations.