Uncovering Peculiar Rainbows

Abstract

Saturn's E ring harbours faint, quasi-periodic, inclined brightness structures called luminous bands, first characterised from Cassini VIMS and ISS observations during Enceladus flybys. We present the first automated, catalogue-scale survey of luminous bands, drawing on 23 Cassini flybys. Detection exploits the chromatic character of the bands, whose apparent position shifts with wavelength. Because these bands present a quasi-periodic inclined brightness structure, a frequency-domain search using a zero-padded two-dimensional Fourier angular spectrum is applied, with candidates assessed against a phase-randomised null ensemble. Of 573 processed images, 62 yield positive detections across eight flybys. Six of the eight carry no prior published detection, including the first ISS detection for E13, and five are non-targeted flybys, demonstrating bands are present in more ISS images than initially found. The observed band directions are compared against a diffraction model: 39 of the 62 detections agree to within 2 degrees, confirming the grating interpretation; the remaining 23 show a systematic offset whose origin is not yet explained. Band contrasts decrease with Cassini–Enceladus distance, suggesting the strongest structure near the plume source. Notably, no detections occur in the morning ansa despite comparable coverage. A complementary VIMS spectral analysis reveals selective suppression of sub-micron grains on the morning-side ring, consistent with size-selective electromagnetic clearing by Lorentz forces, explaining the asymmetric detection rate.