Modeling radiation from the atmosphere of Io with Monte Carlo methods

Conflicting observations regarding the dominance of either sublimation or volcanism as the source of the atmosphere on Io and disparate reports on the extent of its spatial distribution and the absolute column abundance invite the development of detailed computational models capable of improving our understanding of Io's unique atmospheric structure and origin. To validate a global numerical model of Io's atmosphere against astronomical observations requires a 3-D spherical-shell radiative transfer (RT) code to simulate disk-resolved images and disk-integrated spectra from the ultraviolet to the infrared spectral region. In addition, comparison of simulated and astronomical observations provides important information to improve existing atmospheric models.;In order to achieve this goal, a new 3-D spherical-shell forward/backward photon Monte Carlo code capable of simulating radiation from absorbing/emitting and scattering atmospheres with an underlying emitting and reflecting surface was developed. A new implementation of calculating atmospheric brightness in scattered sunlight is presented utilizing the notion of an "effective emission source" function. This allows for the accumulation of the scattered contribution along the entire path of a ray and the calculation of the atmospheric radiation when both scattered sunlight and thermal emission contribute to the observed radiation---which was not possible in previous models. A "polychromatic" algorithm was developed for application with the backward Monte Carlo method and was implemented in the code. It allows one to calculate radiative intensity at several wavelengths simultaneously, even when the scattering properties of the atmosphere are a function of wavelength. The application of the "polychromatic" method improves the computational efficiency because it reduces the number of photon bundles traced during the simulation.;A 3-D gas dynamics model of Io's atmosphere, including both sublimation and volcanic sources of SO2 gas, is analyzed by simulating spectra and images from the model corresponding to three important observations: (1) simulations of the mid-IR disk-averaged observations of Io's sunlit hemisphere at 19 mum, obtained with TEXES during 2001-2004; (2) simulations of disk-resolved images at Lyman-a (1216 A or 0.1216 mum) obtained with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) during 1997-2001; and (3) disk-integrated simulations of emission line profiles in the millimeter wavelength range (1.2-1.4 mm) obtained with the IRAM-30 m telescope in Oct.-Nov. 1999.;We found that our atmospheric model generally reproduces the longitudinal variation in the strength of absorption band from the mid-IR data; however, the best match is obtained when the simulation results are shifted ∼ 30° toward lower longitudes. The simulations of Lyman-alpha images do not show the mid-to-high latitude bright patches seen in the observations, suggesting that the model atmosphere predicts column number densities that are too high at those latitudes. The simulations of emission line profiles in the millimeter spectral region support the hypothesis that the atmospheric dynamics favorably explain the observed line widths, which are too wide to be formed by Doppler broadening alone.;The computational modeling and simulation tools needed to study light scattering from volcanic plumes, which play a significant role in structuring the atmosphere on Io, is developed. The radiative transfer code is applied to the simulation of the brightness in scattered sunlight from a Prometheus-type plume on Io, as observed in limb-viewing geometry by the Galileo Solid State Imager (SSI). The computations are performed utilizing the "polychromatic" method, thus calculating the plume brightness for the entire filter bandpass in a single simulation. Such simulations account for multiple scattering and reflection of sunlight from the surface, not included in previous studies, and may serve as a powerful tool for simulating the plume observations necessary to understand the extent and composition of the plume atmospheres on Io.