Cloud-tops and vertical structure of Saturn's 2011-2012 giant vortex by means of Cassini/VIMS-V data analysis

In this work, selected regions of the giant vortex observed at Saturn's North hemisphere since January 2011 [7] have been investigated using observations from VIMS-V [4], the visual channel of the Visual and Infrared Mapping Spectrometer on board the Cassini spacecraft. Its wavelength range is 0.4-1.0 µm. By studying the 2D images acquired by VIMS-V, especially comparing those relative to the transmission windows wavelengths to those relative to the strongest CH4 absorption bands in this range, we can postulate how the vortex can be vertically structured. A forward radiative transfer model has been developed based on the LibRadtran code [11], and adapted to Saturn's atmosphere. Then an inverse code, that takes advantage of the optimal estimation technique [12], has been implemented to retrieve the microphysical and geometrical properties of the clouds. Best fits of the radiance spectra relative to the different selected regions are produced by means of a least square analysis and then the cloud top pressures are estimated along with other microphysical properties. Different vertical structures have been tested to define the most probable one. This work is the first step in the direction of monitoring the characteristics of the vortex's clouds versus time.