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A trimodal optical depth model for mesoscale structure in Saturn’s rings: wakes and ghosts fit data better than wakes alone

Presentation #110.08 in the session Many Planets, More Rings Posters (Poster)

Published onOct 23, 2023
A trimodal optical depth model for mesoscale structure in Saturn’s rings: wakes and ghosts fit data better than wakes alone

Mesoscale structures are larger than the largest common ring particles but smaller than ~1 km and include self-gravity wakes (ephemeral aggregates elongated by Keplerian shear) and ghosts (azimuthally limited transparent regions) [1,2,3,4,5]. In the A ring where, the structure is dominated by self-gravity wakes, [6] finds that a third, transparent mode is necessary to characterize the measured optical depth distribution while [2,3,4,5] used bimodal models. We present the best-fitting parameters of a trimodal rectangular box optical depth model [7]. We fit the suite of viewing-geometry dependent optical depths measured by the Cassini UVIS, VIMS, and RSS at a variety of ring radii. The model fits 7 free parameters which constrain the 3 optical depth modes, the fraction of the average wavelength associated with each mode, the vertical extent of the most opaque mode, and the angle of the long axis of the structure from the outward radial direction, φwake (i.e. For self-gravity wakes, φwake ~ 110º). We constrain the sum of the widths associated with each optical depth mode to equal the average wavelength, which is determined from high-resolution UVIS occultations independently of surface densities derived from the dispersion of nearby density waves. We compare two versions of the model: one where the elongated structures are regularly spaced, such as in [2,3,4,5,6], and another where the structure is irregularly spaced so that the widths are Poisson distributed within each observed area. In the middle A ring, the best-fitting model finds highly flattened opaque wakes where transparent ghosts occupy about half of the space between them. In the C ring plateaus where λToomre is smaller than the average ring particles, we find that only two modes are necessary to fit the measured optical depths: a transparent mode corresponding to ghosts and a mean plateau optical depth mode which is ~100 times wider. This limit on the number density of ghosts in the plateaus suggests that if the ghosts in the plateaus are caused by propeller lobes around 5 – 10 m moonlets [1], the presence of these moonlets reduces rather than enhances the optical depths.

  1. K. Baillié et al. Astronomical Journal, vol. 145, pp. 2624-2629, 2013.

  2. J. E. Colwell et al. Geophysical Research Letters, vol. 33, 2006.

  3. M.M. Hedman et al. Astron. J., 133(6), 2624-2629.

  4. P. D. Nicholson and M.M. Hedman, Astronomical Journal, vol. 133, pp. 2624-2629, 2009

  5. R. G. Jerousek et al. Icarus, vol. 279, pp. 36-50, 2016.

  6. M. Tiscareno et al. The Astronomical Journal, vol. 139, p. 492, 2010.

  7. L.W. Esposito et al. Abstract submitted to DPS-EPSC 55, 2023.

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