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Cassini UVIS Observations of Saturn’s Rings Josh Colwell, UVIS Co-Investigator Larry Esposito (UVIS P.I.), the UVIS Team, and Glen Stewart, Heather Tollerud, Jeff Cuzzi (Rings and Dust IDS) Saturn’s Rings: Age and origin unknown Cassini ISS image: SSI (Boulder), NASA/JPL. Cassini Division Approach picture from Cassini: F May 10, 2004 Dist: 27 million km. Pixel: 161 km. C B A Moon: Prometheus Cassini ISS image: Space Science Institute (Boulder), NASA/JPL. Encke Gap W~350 km 3 Types of Ring Observation with UVIS •Spectra and images from 550 -1900A. •Stellar occultations with spatial resolution of 10 m. •Meteoroid impact detection. “Spokes” Observed by Voyager QuickTime™ and a YUV420 codec decompressor are needed to see this picture. But not by Cassini. SSI, NASA, JPL. It may not be spoke season yet due to the photoelectron layer over the rings. QuickTime™ and a DV/DVCPRO - NTSC decompressor are needed to see this picture. Detecting (spoke-forming) meteoroid impacts on the rings: 28 readings of 6 2 readings of 7 3.18 million samples Mean = 0.454 SQRT(mean) = 0.674 Std. Dev. = 0.676 Mean = .526 Std. Dev. = .721 Root Mean = .725 2 readings of 8 10 readings of 7 4.98 million readings Poisson Statistics e P(x) x! UVIS_00CRI_IMPACT001: Mean = 0.454 SQRT(mean) = 0.674 Std. Dev. = 0.676 UVIS_00CRI_IMPACT002: Mean = 0.526 SQRT(mean) = 0.725 Std. Dev. = 0.721 x Expectation values: m(6) = P(6)xN = 24.6 m(7) = P(7)xN = 1.6 m(8) = 0.09 Observed values: 28 readings of 6 2 readings of 7 0 readings of 8 Expectation values: m(7) = P(7)xN = 6.5 m(8) = 0.43 Observed values: 10 readings of 7 2 readings of 8 So far, no smoking gun (or ring particle) •Distance to Rings: 6.3 million km. •Radial Coverage: –Inner C ring –Outer B ring (mostly opaque to this star) –Cassini Division –A ring (to 135,500 km). •Sampling Interval: –0.88 to 1.0 km/s. –8 msec sampling. –7-8 m samples. –~30 m Fresnel zone radius ( d ). Stellar Occultation Raw Data Density Waves in Saturn’s Rings – Separation of azimuthal (Ω), radial (), and vertical () orbital frequencies around an oblate planet. (RL ) m(RL ) mM nM pM m, n, p are integers, and M refers to the moon, and RL is the resonance location. Strongest horizontal forcing when n=p=0: (RL ) m M m 1 - Packing density of ring particles varies with the gravity of the ring propagating the wave. - Propagation of wave gives ring surface mass density. m = 2 streamlines m = 2 streamlines affected by perturber and self-gravity More on resonances (RL ) m(RL ) mM nM pM Suppose m=1, n=p=0. Then: M (RL ) (RL ) In other words, the azimuthal motion of the moon is equal to the apsidal precession rate of ring particles at RL. Suppose m=2, n=p=0. Then: 2M 2(RL ) (RL ) (RL ) Which is therefore called a 2:1 resonance. Now suppose m=3, n=0, p=1, then: 3M M 3(RL ) (RL ) 2(RL ) Which is therefore called a 4:2 resonance. 4:2 resonance ≠ 2:1 resonance. Cassini ISS image: SSI (Boulder), NASA/JPL. Ring Plane Radius (km) Ring Plane Radius (km) Ring Plane Radius (km) Bending Waves in Saturn’s Rings (RV ) m(RV ) mM nM pM Strongest vertical forcing when n=1, p=0: m(RV ) (RV ) mM M - Vertical corrugation or warping of the ring. 5:3 BW 5:3 DW Cassini ISS image: SSI (Boulder), NASA/JPL. Density Waves Wave dispersion relation: ( m)2 2 2G k 0 Dispersion of wave spreads power over many frequencies Period (km) Wavelet Power Spectrum Estimation for Wave Dispersion Provides local surface mass density. Ring Plane Radius (km) New Density Waves Atlas 5:4 in Cassini Division shows << (A Ring) Atlas 5:4 Measuring Particle Size or Clumps from Occultation Statistics Particles << sample size. Particles ~ sample size. Observed = Poisson Observed > Poisson Region of ring observed in one sample. Titan 1:01:0 Ringlet Titan Inner Inner Edge Edge 7.2 m res. 29 m resolution Same Edge at Higher Resolution Sharp Edges in the Rings What Does It All Mean? • No clear signal of meteoroid impacts yet: – Detection may not work as expected; – May be fewer meteoroids than expected. • Ring edges unresolved at 30m resolution: – Particle traffic jams; – Direct observations of large clumps or particles? • Composition, particle size, surface mass density recent resurfacing or creation of some ring regions. • And certainly much more… Future • 100’s of hours of impact observations ahead. • 100’s of hours of ring UV images ahead. • 6 more stellar occs between May and September. • >60 stellar occs by the end of the nominal mission on June 30, 2008. • Two-year extended mission anticipated. • Best views of rings still to come. • Stay tuned… Rings Summary • A Ring has cleanest water ice signature: less contaminants than other rings, particularly C Ring and Cassini Division. • Density waves galore: – Dispersion of waves gives ring surface mass density: (Cassini Division) << (A Ring). – New waves seen in Cassini Division and new second order waves observed. • Large particle or clump size distributions from occultation statistics: – Largest particles or clumps (~ 10 m) in A ring, increasing outward to Encke Gap, then decreasing. – No significant number of large particles or clumps in C ring, Cassini Division. • Correlation between surface mass density and largest particle sizes and (to a lesser extent) ring ice purity. • Abrupt density transitions observed (r<50 m): particle “traffic jam” at perturbed ring edges. • Unexplained features observed at high resolution.