The Eye of Saturn's North Polar Vortex: Unexpected Cloud Structures Observed at High Spatial Resolution by Cassini/VIMS

Abstract

Near-infrared spectral maps of Saturn's north polar vortex obtained at high spatial resolution provided by Cassini/Visual Infrared Mapping Spectrometer (VIMS) reveal localized ammonia clouds composed of unusually large particles exceeding 13 μm in radius, the largest cloud particles documented during the Cassini mission. Taken under near-optimum direct polar lighting and viewing conditions within a month of the summer solstice, these small ( 200 km in breadth) discrete clouds are located within the eye of the polar vortex, which otherwise is unusually clear of observable aerosols in reflected sunlight, with total 2-μm opacity <0.04 versus >1.0 elsewhere on Saturn. The dichotomy of large-particle condensate cloud features—indicative of convective upwelling—within a large ( 2000-km diameter) nearly aerosol-free region of downwelling characteristic of the core of a polar vortex reveals surprising polar dynamics on Saturn.