Structure of self-gravity wakes in Saturnʹs A ring as measured by Cassini CIRS

The CIRS infrared spectrometer onboard the Cassini spacecraft has scanned Saturnʹs A ring azimuthally from several viewing angles since its orbit insertion in 2004. A quadrupolar asymmetry has been detected in this ring at spacecraft elevations ranging between 16° to 37°. Its fractional amplitude decreases from 22% to 8% from 20° to 37° elevations. The patterns observed in two almost complete azimuthal scans at elevations 20° and 36° strongly favor the self-gravity wakes as the origin of the asymmetry. The elliptical, infinite cylinder model of Hedman et al. [Hedman, M.M., Nicholson, P.D., Salo, H., Wallis, B.D., Buratti, B.J., Baines, K.H., Brown, R.H., Clark, R.N., 2007. Astron. J. 133, 2624–2629] can reproduce the CIRS observations well. Such wakes are found to have an average height-to-spacing ratio H / λ = 0.1607 ± 0.0002 , a width-over-spacing W / λ = 0.3833 ± 0.0008 . Gaps between wakes, which are filled with particles, have an optical depth τ G = 0.1231 ± 0.0005 . The wakes mean pitch angle Φ W is 70.70 ° ± 0.07 ° , relative to the radial direction. The comparison of ground-based visible data with CIRS observations constrains the A ring to be a monolayer. For a surface mass density of 40 g cm−2 [Tiscarino, M.S., Burns, J.A., Nicholson, P.D., Hedman, M.M., Porco, C.C., 2007. Icarus 189, 14–34], the expected spacing of wakes is λ ≈ 60 m . Their height and width would then be H ≈ 10 m and W ≈ 24 m , values that match the maximum size of particles in this ring as determined from ground-based stellar occultations [French, R.G., Nicholson, P.D., 2000. Icarus 145, 502–523].