Cassini ISS observation of Saturn's north polar vortex and comparison to the south polar vortex

We present analyses of Saturn's north pole using high-resolution images captured in late 2012 by the Cassini spacecraft's Imaging Science Subsystem (ISS) camera. The images reveal the presence of an intense cyclonic vortex centered at the north pole. In the red and green visible continuum wavelengths, the north polar region exhibits a cyclonically spiraling cloud morphology extending from the pole to 85 degrees N planetocentric latitude, with a 4700 km radius. Images captured in the methane bands, which sense upper tropospheric haze, show an approximately circular hole in the haze extending up to 1.5 degrees latitude away from the pole. The spiraling morphology and the eye-like hole at the center are reminiscent of a terrestrial tropical cyclone. In the System III reference frame (rotation period of 10h39m22.4s, Seidelmann et al. 2007; Archinal et al. 2011), the eastward wind speed increases to about 140 m s(-1) at 89 degrees N planetocentric latitude. The vorticity is (6.5 +/- 1.5)x10(-4) s(-1) at the pole, and decreases to (1.3 +/- 1.2)x10(-4) s(-1) at 89 degrees N. In addition, we present an analysis of Saturn's south polar vortex using images captured in January 2007 to compare its cloud morphology to the north pole. The set of images captured in 2007 includes filters that have not been analyzed before. Images captured in the violet filter (400 nm) also reveal a bright polar cloud. The south polar morphology in 2007 was more smooth and lacked the small clouds apparent around the north pole in 2012. Saturn underwent equinox in August 2009. The 2007 observation captured the pre-equinox south pole, and the 2012 observation captured the post-equinox north pole. Thus, the observed differences between the poles are likely due to seasonal effects. If these differences indeed are caused by seasonal effects, continuing observations of the summer north pole by the Cassini mission should show a formation of a polar cloud that appears bright in short-wavelength filters. (C) 2016 Elsevier Inc. All rights reserved.