Mesoscale structures in Saturn’s rings from UVIS autocorrelations

The Cassini Ultraviolet Imaging Spectrograph (UVIS) highspeed photometer (HSP) measured photon count rates in the wavelength range of 110–190 nm with sampling rates of 1–8 ms during its lifetime. Over the course of the Cassini mission to Saturn, the UVIS HSP recorded photon count rates for 276 stellar occultations of Saturn’s main rings. These stellar occultations trace a 1 dimensional path through the rings, and due to the high sampling rate of the UVIS HSP, the azimuthal and radial resolution are on the order of the Fresnel scale (√λD), about 10 m [1, 2] in the frame of the ring particles. Stellar occultations are offset widely in time and azimuth, but by combining autocorrelations of the occultation time series, we develop a 2-D autocorrelation of the temporally and azimuthally averaged structure at a given ring radius. 2-D autocorrelations allow us to study the rings on the scale of the largest individual particles in the rings. Ring particles aggregate and disperse creating self-gravity wakes [3, 4] as well as what has been referred to as “streaky texture”, “feathery texture”, and “straw” [5] in Cassini images which have pixel scales as low as 300 m in the frame of the ring particles. By comparing autocorrelations from the UVIS HSP stellar occultations with images from Cassini Imaging Subsystem (ISS), and n-body simulations we determine the properties of the mesoscale structures at a given ring radius. Mesoscale structure refers to agglomerations of particles which are bigger than the largest individual ring particles but smaller than a few km. We directly measure the length, width, and orientation of self-gravity wakes as well as the wavelength of viscous over-stable waves both of which are significantly smaller than the pixel scale in Cassini ISS images.

1. Jerousek et al. 2020, Icarus, 344, 113565.

2. Colwell et al. 2018, Icarus, 300, 150-166.

3. Colwell et al. 2006, Icarus, 190, 127.

4. Hedman et al. 2007. Astron. J., 133(6), 2624-2629.

5. Tiscareno et al. 2019, Science, 364, 6445.