Cassini UVIS results from Saturn, Titan, icy satellites and rings

1. Cassini UVIS results from Saturn, Titan, icy satellites and rings LW Esposito and the UVIS team 23 May 2005 American Geophysical Union

2. Cassini UV Imaging Spectrograph • Spectra and images from 550 -1900A • Hydrogen-Deuterium cell measures D/H • High speed photometer has 20m resolution • Chemistry of Saturn, Titan clouds • Exospheres of moons • Saturn’s magnetosphere neutrals; thermosphere airglow and aurora • Ring origin and evolution

9. 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.

10. Implications of UVIS observations • The rise and fall in abundance of OI between Dec 25, 2003 and May 13, 2004 amounts to ~500 Mkg of mass apparently lost from the system in this period. Mean inferred loss rate of bulge in mass during 2 months is ~4 X1027 atoms s-1. • Total mass of OI + OH in system is ~2200. Mkg. • The estimated micron sized particles in the E-ring involved in the Mimas – Tethys region is 600 Mkg.

14. Species found in Titan Tb occ • CH4 methane • C2H2 acetylene • C2H4 ethylene • C2H6 ethane • C4H2 diacetylene • HCN hydrogen cyanide

17. T-3 Titan FUV Observation February 15, 2005 Start Time: 10:26:52 End Time: 11:34:52 Lyman Alpha Image: 1146 –1271 Å LBH Image: 1286 – 1427 Å Solar Image: 1661 – 1913 Å

18. Titan Results • Differences with INMS show horizontal variations at 1200 km • Evidence for diffusive separation at 900 km • Mesopause temperature 113K at 615km • Heavier hydrocarbons peak at 600 -750 km • Discontinuous aurora and haze layers seen

19. UVIS H2 band data on Saturn

20. Saturn in atomic and molecular hydrogen emission

21. What is the source of the dark material on Iapetus? • Does Phoebe material contaminate the LH of Iapetus? • At visible wavelengths, Iapetus’s spectrum is redder than Phoebe’s • What do the UV data contribute to this mystery?

22. Iapetus

23. Iapetus compared with Phoebe

24. Iapetus: Light/Dark Ratio Looks a lot like H2O or CO2!

25. Iapetus reflectance: dark and light terrains • Iapetus dark material is dark because of very little frost (H2O and • CO2), NOT because of a lot of dark material

26. Summary:Iapetus vs. Phoebe • What can we say about dark material on Iapetus? • It does not look like Phoebe • Phoebe material is too H2O-rich to match Iapetus dark spectrum, although a good match for bright material! • We need very little H2O to model Iapetus dark material • Though if Phoebe material (or any other material) coats Iapetus, H2O might be lost in the impact process • So our results do not discriminate between endogenic and exogenic models for Iapetus dark material • Loss of volatiles might be associated with either process

27. UVIS RESULTS FOR MAIN RINGS • We summed all SOI spectra from same distance to produce radial profile and color rendition • UV spectra show water abundance increasing outward to a peak in outer A ring • Rings A and B more icy than ring C and Cassini Division, consistent with VIMS • More structure than seen in Voyager and HST images of A ring

30. A Ring Particle Properties

31. RING HISTORY • Large scale variation consistent with meteoritic pollution of initially pure ice • Variations over scales of 1000 - 3000 km too narrow to be explained by ballistic transport over the age of rings • One or more 20-km moonlets shattered in last 10 -100 million years could do it • Same future result from disrupting Pan

32. COLLISIONAL CASCADE FROM MOONS TO RINGS • Big moons are the source for small moons • Small moons are the source of rings • Largest fragments shepherd the ring particles • Rings and moons spread together, linked by resonances • Small moons caught in resonances with larger moons: this slows linked evolution

33. COLLISIONAL CASCADE

34. EXAMPLE: F RING • After parent body disruption, F ring reaches steady state where accretion and knockoff balance (Barbara and Esposito 2002) • The ring material not re-collected is equivalent to ~6km moon; about 50 parent bodies coexist… • Exponential decay would say half would be gone in 300 my. • Considering re-accretion, loss of parents is linear: as smaller particles ground down, they are replaced from parent bodies. The ring lifetime is indefinitely extended

36. MARKOV MODEL CONCLUSIONS • Although individual rings and moons are ephemeral, ring/moon systems persist • Ring systems go through a long quasi-static stage where their optical depth and number of parent bodies slowly declines • Below some threshold, recycling declines and the rings are rapidly lost

37. Ring Plane Radius (km)

38. Ring Plane Radius (km)

39. Ring Plane Radius (km)

40. Bending Waves in Saturn’s Rings 5:3 DW 5:3 BW

41. Density Waves

42. Wavelet Power Spectrum Estimation for Wave Dispersion Provides local surface mass density. Period (km) Ring Plane Radius (km)

44. Titan 1:0 Ringlet Inner Edge Titan 1:0 Inner Edge 7.2 m res. 29 m resolution

45. Sharp Edges in the Rings

49. Encke Gap Pan Wakes Density Waves

50. UVIS data are consistent with active ring/moon recycling • Oxygen fluctuations require replacement of E ring grains from parent bodies • Radial spectral variations in A ring require multiple reservoirs and recent release of purer ice • 20 km embedded moonlets have right lifetimes and mass, given estimated diffusion rates