Temporally and azimuthally averaged mesoscale structures in Saturn's rings from Cassini UVIS occultations

Abstract:

Over the 13 years Cassini spent in orbit around Saturn, the Ultraviolet Imaging Spectrograph (UVIS) highspeed photometer (HSP) measured 276 stellar occultations of the rings at 1 - 8 ms intervals corresponding to both radial and azimuthal sampling as high as one to several meters. The UVIS HSP measured starlight at an effective wavelength of λ ~150 nm giving an average Fresnel scale (√λD) of about 10 m in the local Keplerian frame. An autocorrelation of the time series of an individual stellar occultation provides information on the presence of periodic structure in the rings and the scale of that structure down to the Fresnel scale which for UVIS is on the order of the largest individual particles commonly found in the rings [1, 2]. But ring particles often aggregate to form ephemeral density enhancements such as self-gravity wakes [3, 4] as well as what has been referred as “straw”, “streaky texture”, and “feathery texture” in Cassini images [5]. We refer to these structures which are between the size of the largest common ring particles and a few km “mesoscale” structure. Autocorrelations of individual UVIS occultations are ideal for characterizing the scale of these structures and individual stellar occultations can also be summed to produce a 2-D autocorrelation of the temporally and azimuthally averaged ring at a particular ring radius. We characterize these small but ubiquitous features in the rings using these 2-D autocorrelation functions combined with Cassini ISS images and N-body simulations. We directly measure the characteristic length, width, and cant angle of self-gravity wakes in the A and B ring. We identify and measure the wavelength of viscous over-stable waves, and other features which are inaccessible to occultations by instruments or cameras with lower resolution.

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