DDA 2015 – How massive is Saturn’s B ring – Clues from cryptic density waves

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Matthew M. Hedman (Cornell)


The B ring is the brightest and most opaque of Saturn’s rings, but it is also amongst the least well understood because basic parameters like its surface mass density are still poorly constrained. Elsewhere in the rings, spiral density waves driven by resonances with Saturn’s various moons provide precise and robust mass density estimates, but for most the B ring extremely high opacities and strong stochastic optical depth variations obscure the signal from these wave patterns. We have developed a new wavelet-based technique that combines data from multiple stellar occultations (observed by the Visual and Infrared Mapping Spectrometer (VIMS) instrument onboard the Cassini spacecraft) that has allowed us to identify signals that may be due to waves generated by three of the strongest resonances in the central and outer B ring. These wave signatures yield new estimates of the B-ring’s mass density and indicate that the B-ring’s total mass could be quite low, perhaps a fraction of the mass of Saturn’s moon Mimas.


  • B ring long assumedto be the most massive ring structure
    • essentially opaque
  • Density (and bending) waves
    • $k(r) = \dfrac{3(m-1)M(r-r_L)}{2 \pi \sigma_0 r^4_L}$
    • Wavenumbers can be quantified using wavelets
    • frequency chirping at moon (e.g., Prometheus, Pandora, Enceladus) MMRs
  • Few waveshave been identified in Saturn’s B ring(!)
    • $\rightarrow$ mass density poorly constrained
    • Expect to see density waves, but…
      • resonances in opaque region
      • a lot of the structure in the rings is of unknown origin
        • some are likely density waves, some not
    • Wave-like signatures not obvious in wavelet transforms
  • Solution? Include phase information inwavelet analysis
    • Different occultations cut through the spiral pattern at different places
      • Noise fluctuations confuse the signal
      • Normally ignored
    • Calculate what phase shifts ought to have been and remove them
      • can average components
      • Ideally, noise averages to zero
      • $\rightarrow$ suppresses background mess
    • Cannow measure wavenumber of resonances!
      • even in region where opacity is ~3
      • $\rightarrow$ mass density
    • Regions with same mass density can have very different optical depths
      • (from scatter in the data)
      • Don’t know why
    • Indications: B ring mass density lower than expected

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