Cassini ISS observation of Saturn’s String of Pearls

Abstract

We present the dynamics of the String of Pearls (SoPs) feature observed by the Cassini spacecraft’s Imaging Science Subsystem (ISS) camera between 2007 and 2010. The SoPs was originally discovered in the 5 μm images captured by Cassini VIMS instrument, where it appeared as a chain of infrared-bright spots (Momary, T.W., et al. [2006]. The Zoology of Saturn: The Bizarre Features Unveiled by the 5 Micron Eyes of Cassini/VIMS. AAS/Division for Planetary Sciences Meeting Abstracts 38, 499). Using ISS images of Saturn, we found a chain of 23–26 dark spots at 33.2°N planetocentric latitude with characteristics that are consistent with those of SoPs. Our measurements imply that the feature propagated at −2.26 ± 0.02° day^−1 in longitude (−22.27 ± 0.2 m s^−1, negative values denote westward) during the observed period that spans three Earth years. Our measurements imply that the SoPs is a chain of cyclones, which we infer from the motion of clouds on the periphery of the individual pearls. We tracked the motion of 26 pearls for 6 months in 2008 and noted a few pearls appearing and disappearing, all near the east–west termini of the SoPs feature. During this period, a few of the pearls, varying between 6 and 10, harbored a small circular cloud at the center, which we call the central peaks. In general, a group of vortices with the same sign of vorticity tend to merge; however, our measurements did not detect merger of pearls. The interest in the feature was heightened when the latest planet-encircling storm erupted from the SoPs on December 5, 2010 (Sayanagi, K.M., Dyudina, U.A., Ewald, S.P., Fischer, G., Ingersoll, A.P., Kurth, W.S., Muro, G.D., Porco, C.C., West, R.A. [2013]. Icarus 223, 460–478). The storm severely disrupted the region; the SoPs was last seen on December 24, 2010 in the turbulent wake of the storm, and has not reappeared as of August 2013.