Clouds and hazes vertical structure of a Saturn's giant vortex from Cassini/VIMS-V data analysis

Abstract

We studied the evolution of a giant tropospheric vortex formed in the wake of the storm that encircled Saturn from December 2010 to July 2011 (Fletcher et al. [2011a] Science, 332, 1413-1417; Fletcher et al. [2012] Icarus, 221, 560-586; Sánchez-Lavega et al. [2011] Nature, 475, 71-74; Sánchez-Lavega et al. [2012] Icarus, 220, 561-576; Sayanagi et al. [2013] Icarus, 223, 460-478; Fischer et al. [2011] Nature, 475, 75-77) taking advantage of the observations acquired by the instruments on board the Cassini spacecraft. In particular, the Visual and Infrared Mapping Spectrometer (VIMS) imaged the vortex several times. In this work we analyzed two observations registered by the visual channel of VIMS (VIMS-V) on 08/24/2011 and 01/04/2012, both after the active phase of the storm, and characterized quantitatively the vertical structure of the clouds and hazes above the vortex. Until now, VIMS-V dataset has been scarcely exploited to perform such an analysis. The IR channel of VIMS has always been preferred since it covers wavelengths containing spectral information on a wider range of altitudes in the atmosphere. Nevertheless, in our analysis we investigate the information content of VIMS-V observations and demonstrate that the covered spectral range contains valuable information that are helpful to improve our knowledge on the properties of Saturn's upper atmosphere. <P />We developed a forward radiative transfer model to describe Saturn's atmosphere and simulate VIMS-V spectra in the 0.35-1.05 μm wavelength range. The analysis has then been performed by means of an inverse model that we built on the basis of the Bayesian approach. Spatial distributions of effective radii, column number densities and top pressures of the cloud decks have been mapped and as a by-product of our analysis we also suggest a modified spectral shape for the imaginary part of the refractive index of the tropospheric haze, with respect to the shape described in the study of Karkoschka and Tomasko ([2005] Icarus, 179, 195-221). <P />The results suggest that the processes responsible for the formation and persistence of the vortex weakened between August 2011 and January 2012, even if the differences that we observe could be due to the fact that the vortex has moved in different positions between the two dates. We found that in August 2011 the upper haze was arranged in a dome like structure with the center at 8 mbar and the boundaries at 12 mbar; moreover we detected a zone in the lower haze at 135 mbar characterized by higher optical thickness with respect to the surrounding regions located at 85 mbar. In January 2012 the dome in the upper layer has diluted into a more homogeneous structure and the haze appears to be overall shifted to less than 6 mbar. Similarly, the 135 mbar high optical depth zone previously detected in the lower layer has disappeared.