Saturn’s Rings: …..some highlights 3D structure of rings: thickness, gravity wakes Composition: a higher dimension: role of meteoroid bombardment Evolution of moonlets in & near the rings: embedded objects F ring region
Vrel <<< 15km/s 15km/s “Classical” ring model
Dynamically expected ring model: Densely packed ring due to inelastic collisions 30m thick Different powerlaw size distributions Gentle inelastic collisions and weak gravity between particles give the rings the quality of a viscous fluid Viscosity: local collisional, nonlocal collisional, gravitational
Mishchenko & Dlugach 1992; Akkermans et al 1988; Hapke 2000 The Opposition Effect: 1.5 I/F Phase angle Shadow hiding between particles at low volume density? or Coherent backscattering from particle surfaces? Nelson et al 2006; Hapke et al 2006 LPSC Low albedo Width of peak Cassini VIMS - low resolution, but large wavelength (albedo) range High albedo Coherent backscattering fits better; classical model not supported Wavelength (microns)
Photometric modeling of non-classical (closely packed) layers (Monte Carlo modeling) classical dense Salo and Karjalainen 2003; cf also Dones et al 1989, 1993; Salo et al 2005
Self-gravity wakes 2 1 Colwell et al submitted Stellar occultations by the rings: VK azimuthal view angle determines opacity and elevation angle gives wake thickness 300m Movie by H. Salo 1 2
0.5 0.4 0.3 Wake height/width 0.2 0.1 Self-gravity wake properties in the A ring Total optical depth; Gap optical depth Colwell et al GRL 2006 Local, unresolved mixture of high-tau waves and low-tau gaps will affect ring I/F and must be included in photometric models
D C B C D A ISS approach color composite F
Saturn’s entire B ring HST IRTF model R wavelength (microns) 98% water ice, 2% carbon, 1% tholins F. Poulet et al. (2003) Groundbased reflectance spectrum: water ice bands and reddish material C ring & CD are darker & less red; ‘polluted’ by meteoroid bombardment? wavelength, microns
Meteoroid Bombardment and Ballistic Transport Main rings intercept roughly their own mass in the age of the solar system; large uncertainties in mass flux! Rings get “polluted”; ejecta moves material around Structural and compositional evolution; model ages ~ few 108 yr Cook & Franklin 1970; Morfill 1983; Ip 1983; Durisen et al 1984, 1989, 1992, 1996; Cuzzi & Estrada 1998
Saturn’s entire B ring HST IRTF model R wavelength (microns) New Cassini VIMS results: correlation between spectral properties with high radial resolution (SOI) wavelength, microns
C B A 0.3 - 0.5 slope ice band depths 0.6 - 0.9 slope Nicholson et al 2006; Icarus submitted(?)
“propellor objects” Multiple strands Prometheus, Pandora, other new objects Encke and Keeler gaps; containing Pan and Daphnis F ring Outer A ring
Moonlets within the rings 30 km wide Daphnis ( r=3.5km ) in Keeler gap Pan in Encke gap: complex edges 320 km wide
“Propellor” disturbances by 100 m diameter objects in the A ring Seiss et al GRL 2005 ISS SOI images; 50 m/pxl ! unlit face of the rings Tiscareno et al Nature 2006
main ring particles Fully populated? N(>R) km-2 A ring “propellors” Zebker et al (Voyager) (Tiscareno et al Nature 2006)
So what are these things..? Primordial shards of the ring creation event? Home-grown by accretion of local ring material? Need to understand accretion within the Roche limit RR (Smoluchowski 1978,1979; Weidenschilling et al 1984; Longaretti 1989) classical force balance: c ~ CMp/a3 or ac ~ (CMp/r)1/3 Accretion allowed RR “tidally corrected” Canup & Esposito 1995
Critical density for growth at distance a: c=9Mp/4a30.15 in A ring Smoluch., Weid. et al, Longaretti c≈27Mp/4a3CE95)or9Mp/4a3bbbc (Weiss et al 2006)
Critical density for growth at distance a: c=9Mp/4a30.15 in A ring Smoluch., Weid. et al, Longaretti c≈27Mp/4a3CE95)or9Mp/4a3bbbc (Weiss et al 2006) ….. and the survey says (Porco et al 2006 LPSC) Moonlets in and near the rings obey accretion-limited critical density, may be dense shards buried in local ring material. Implication is that “propellor” objects are similar, just smaller.
The F ring: tinsel on a massive moonlet belt? l= VGR-1 RSS Marouf et al 1986 l=
The F ring “moonlet belt” Conjectures: Van Allen 1982 Data: Pioneer 11 magnetospheric e,p in Unlike G ring out 1200km Simpson et al 1980
The F ring “moonlet belt” Conjectures: Van Allen 1982, Cuzzi & Burns 1988 Data: Pioneer 11 magnetospheric e,p Transient clumps of cm-size rubble released in collisions between members of a 2000 km wide belt of moonlets; mass 1020 - 1021 g (Prometheus + Pandora) in F ring itself a collisional product (1015-16 g; every 102-103 yrs) ?? F Unlike G ring out 1200km Simpson et al 1980
The F ring “moonlet belt” Conjectures: Van Allen 1982, Cuzzi & Burns 1988 Data: Pioneer 11 magnetospheric e,p Transient clumps of cm-size rubble released in collisions between members of a 2000 km wide belt of moonlets; mass 1020 - 1021 g (Prometheus + Pandora) in F ring itself a collisional product (1015-16 g; every 102-103 yrs) ?? Subsequent developments: F Unlike G ring Scargle et al 1993 DPS: Entire F region isprobably chaotic (higher order Pandora & Prometheus resonances) out Pandora & Prometheus themselves now known to be chaotic (French et al 2003, Goldreich & Rappaport 2003 a.b) Some Uranian ringmoons appear chaotic; may collide in 0.5Myr (Duncan & Lissauer 1997; Showalter & Lissauer 2006; Showalter this mtg) 1200km HST and Cassini observations of large clumps in F region-> Simpson et al 1980
Arcs 60,000 km long; ~ 10-3 McGhee et al 2001 Icarus 152, 282
10000 km ~ 10-2 Transient object 2004S6? Porco et al 2005, Spitale et al 2006 AJ 1000 km PIA07558 PIA07716
C M M M M A A A M A A A A A A A A collection of F region features…. 1500km F ring core I/F, 3 JH 143141 S6 S3/4 S6’ S6’’ Pr Pa Distance from Saturn
Actual moonlets in the F ring region main ring particles UVIS N(>R) km-2 F ring A ring “propellors” Spitale et al 2006 Esposito et al 2006, submitted Zebker et al (Voyager) (Tiscareno et al Nature 2006)
Optically thin moonlet belt still quite massive (1020-1021g) main ring particles UVIS N(>R) km-2 F ring A ring “propellors” Spitale et al 2006 CB88 Esposito et al 2006, submitted Zebker et al (Voyager) (Tiscareno et al Nature 2006)
Evolution of the F ring (strand) itself: ? main ring particles Barbara & Esposito 2004 Esposito et al 2006 (km-2 in a narrow ringlet) 3x1020g N(>R) km-2 F ring A ring “propellors” Spitale et al 2006 Esposito et al 2006, submitted Zebker et al (Voyager) (Tiscareno et al Nature 2006)
Summary Gaining good 3D understanding of micro-ring structure: key implications for ring photometry, particle albedo. “Classical” photometric models are obsolete for A, B rings. Ring composition varies in slow and systematic ways across abrupt mass boundaries; ring composition = intrinsic icy-organic with added ‘cometary’ pollution? Need a better estimate of mass flux to age-date the rings. Embedded and nearby ringmoons seem to obey accretion- limited densities; any primordial shards are deeply buried. Size distribution for 10m < r < few km is telling us something. 1500-km wide F region may be full of chaotically moving moonlets. Evolutionary modeling should allow for this. Large distributed mass may have dynamical implications.
Meteoroid Bombardment and Ballistic Transport Structural evolution creates familiar structures in << Tss; “ramps” seen at inner B & A edges Durisen et al 1996 Simultaneously, Compositional evolution creates global compositional variations: smooth color/composition profiles across abrupt ring boundaries Estrada et al 2003 Implication: rings started as icy-organic material and became polluted by dark, neutrally-colored material. May be able to age-date the rings this way; several unknowns
Scargle et al 1993: F region chaos ( higher order resonances)? 2000km
RPWS experiment detects “tones” from meteoroids hitting the rings (?) SOI Gurnett et al Fall AGU 2004
Transient objects? “2004S6”.. Or not? Spitale et al Moonlet belt collision? Meteoroid impact? Cuzzi & Burns 1988, Showalter 1998, Poulet et al 2000, Barbara & Esposito 2002 Charnoz et al Science 2006 X
The F ring “moonlet belt” Conjectures: Cuzzi & Burns Icarus 1988 Pioneer 11 data Transient clumps of cm-size rubble released in collisions between members of a 2000 km wide belt of 0.3-3 km moonlets and total mass of about 1021 g (Prometheus) . Van Allen 1982 F ring itself a collisional product (more rare; 102-103 yrs) ?? F c ~ 10(1km/L) L 1200km c ~ 10-3, L~20000?
Particle sizes from radio occultation Blue = small; red = large, white = very opaque
UVIS Stellar Occultation Variation Summary cassini Colwell et al GRL 2006
135000 140000 142000 145000 Smoluchowski 1978, 1979; Canup & Esposito 1995; Karjalainen & Salo 2004 Icarus
The Opposition Effect: Shadow hiding and low volume density? or Coherent backscattering?
Ringmicrostructure A ring wakes: gaining full 3D picture: what does it mean? B ring wakes? C ring and CD ? Colwell et al, Sremcevic et al this mtg Ring thermal & RSS observations Leyrat et al, Marouf et al this mtg Implications: Photometric models must treat dense layers: Deau et al , Dones et al, Weiss et al, Chambers et al More complications for photometric modeling: wakes: high & low optical depth in same pixel!
Implications for shards / propellors: Accretion in the rings is possible but limited by needed compaction to keep rho > rho_crit Objects can keep growing AT rho_crit until they open a gap around themselves. Propellor objects, perhaps, or maybe larger (likely size dist) Can’t learn about primordial material from studying these, on surface, too bad, but lots of intriguing parallels to protoplanetary evolution and time-dependent ring structure come to mind Next treat a related but different problem
“Narrow, stranded” F ring: the tail, not the dog? Evidence exists for transient clumps/arcs of length ~ 103 - 105 km HST and Cassini, in addition to Pioneer 11 microsignatures Prediction: Cassini will see more and larger clumps and arcs Objects between Pandora & Prometheus lead chaotic lives Orbits become eccentric; collisions occur in 1500km wide zone Resulting debris clumps spread & are swept up by other objects Collisional belt modeling must be redone with induced chaotic e’s Speculation: F ring itself only one of the larger, more recent events Speculation: might 1021g (or more!) have an influence on nearby orbits?