Saturn’s Rings: …..some highlights 3D structure of rings: thickness, gravity wakes Evolution of moonlets in & near the rings: embedded objects F ring region Composition: a higher dimension: role of meteoroid bombardment

15km/s V rel <<< 15km/s “Classical” ring model

Dynamically expected ring model: Densely packed ring due to inelastic collisions Gentle inelastic collisions and weak gravity between particles give the rings the quality of a viscous fluid Viscosity: local collisional, nonlocal collisional, gravitational 30m thick Different powerlaw size distributions

Cassini VIMS - low resolution, but large wavelength (albedo) range Coherent backscattering fits better; classical model not supported Nelson et al 2006; Hapke et al 2006 LPSC Phase angle I/F Width of peak Wavelength (microns) Low albedo High albedo The Opposition Effect: Shadow hiding between particles at low volume density? or Coherent backscattering from particle surfaces? 1.5  Mishchenko & Dlugach 1992; Akkermans et al 1988; Hapke 2000

Salo and Karjalainen 2003; cf also Dones et al 1989, 1993; Salo et al 2005 Photometric modeling of non-classical (closely packed) layers (Monte Carlo modeling) classicaldense

Stellar occultations by the rings: Colwell et al submitted Movie by H. Salo azimuthal view angle determines opacity and elevation angle gives wake thickness 300m Self-gravity wakes VKVK

Self-gravity wake properties in the A ring Colwell et al GRL 2006 Total optical depth; Gap optical depth Local, unresolved mixture of high-tau waves and low-tau gaps will affect ring I/F and must be included in photometric models Wake height/width

ISS approach color composite A B C C D F D

wavelength, microns wavelength (microns) R IRTF HST model Saturn’s entire B ring Groundbased reflectance spectrum: water ice bands and reddish material C ring & CD are darker & less red; ‘polluted’ by meteoroid bombardment? F. Poulet et al. (2003) 98% water ice, 2% carbon, 1% tholins

Meteoroid Bombardment and Ballistic Transport Cook & Franklin 1970; Morfill 1983; Ip 1983; Durisen et al 1984, 1989, 1992, 1996; Cuzzi & Estrada 1998 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 10 8 yr

wavelength, microns wavelength (microns) R IRTF HST model Saturn’s entire B ring New Cassini VIMS results: correlation between spectral properties with high radial resolution (SOI)

Nicholson et al 2006; Icarus submitted(?)  slope  slope ice band depths CBA

Outer A ring Multiple strands Prometheus, Pandora, other new objects F ring “propellor objects” Encke and Keeler gaps; containing Pan and Daphnis

Moonlets within the rings 320 km wide 30 km wide Pan in Encke gap: complex edges Daphnis ( r=3.5km ) in Keeler gap

“Propellor” disturbances by 100 m diameter objects in the A ring ISS SOI images; 50 m/pxl ! Seiss et al GRL 2005 Tiscareno et al Nature 2006 unlit face of the rings

A ring “propellors” (Tiscareno et al Nature 2006) Zebker et al (Voyager) main ring particles Fully populated? N(>R) km -2

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 R R (Smoluchowski 1978,1979; Weidenschilling et al 1984; Longaretti 1989) Canup & Esposito 1995 Accretion allowed “tidally corrected” classical force balance:  c ~ CM p /a 3 or a c ~ ( CM p /  ) 1/3 R

Critical density for growth at distance a:  c =9M p /4  a 3  0.15 in A ring  c ≈27M p /4  a 3  CE95) or 9M p /4  a 3  b  c (Weiss et al 2006)  Smoluch., Weid. et al, Longaretti

….. and the survey says (Porco et al 2006 LPSC) Critical density for growth at distance a: 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.  c =9M p /4  a 3  0.15 in A ring  c ≈27M p /4  a 3  CE95) or 9M p /4  a 3  b  c (Weiss et al 2006)  Smoluch., Weid. et al, Longaretti

The F ring: tinsel on a massive moonlet belt? VGR-1 RSS Marouf et al 1986  

1200km Data: Pioneer 11 magnetospheric e,p Simpson et al 1980 The F ring “moonlet belt” Unlike G ring in out Van Allen 1982 Conjectures :

F Van Allen 1982, Cuzzi & Burns km Data: Pioneer 11 magnetospheric e,p ?? Simpson et al 1980 F ring itself a collisional product ( g; every yrs) Transient clumps of cm-size rubble released in collisions between members of a 2000 km wide belt of moonlets; mass g (Prometheus + Pandora) Conjectures : The F ring “moonlet belt” Unlike G ring in out

F Van Allen 1982, Cuzzi & Burns km Data: Pioneer 11 magnetospheric e,p ?? Simpson et al 1980 F ring itself a collisional product ( g; every yrs) Transient clumps of cm-size rubble released in collisions between members of a 2000 km wide belt of moonlets; mass g (Prometheus + Pandora) Conjectures : Pandora & Prometheus themselves now known to be chaotic (French et al 2003, Goldreich & Rappaport 2003 a.b) Scargle et al 1993 DPS: Entire F region is probably chaotic (higher order Pandora & Prometheus resonances) Some Uranian ringmoons appear chaotic; may collide in 0.5Myr (Duncan & Lissauer 1997; Showalter & Lissauer 2006; Showalter this mtg) Subsequent developments: HST and Cassini observations of large clumps in F region-> The F ring “moonlet belt” Unlike G ring in out

McGhee et al 2001 Icarus 152, 282 Arcs 60,000 km long;  ~ 10 -3

Transient object 2004S6? Porco et al 2005, Spitale et al 2006 AJ PIA07558 PIA km   km

S6’ 1500km Pr Pa S6 C M M M M M A A A A A A A A A A S3/4 JH F ring core 33 S6’’ A collection of F region features…. I/F,  Distance from Saturn

UVIS F ring (Tiscareno et al Nature 2006) Zebker et al (Voyager) Esposito et al 2006, submitted Actual moonlets in the F ring region A ring “propellors” main ring particles N(>R) km -2 Spitale et al 2006

UVIS F ring (Tiscareno et al Nature 2006) Zebker et al (Voyager) Esposito et al 2006, submitted Optically thin moonlet belt still quite massive ( g) CB88 A ring “propellors” main ring particles N(>R) km -2 Spitale et al 2006

F ring (Tiscareno et al Nature 2006) Zebker et al (Voyager) Esposito et al 2006, submitted Evolution of the F ring (strand) itself: ? A ring “propellors” main ring particles Barbara & Esposito 2004 Esposito et al 2006 (km -2 in a narrow ringlet) 3x10 20 g N(>R) km -2 Spitale et al 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 km wide F region may be full of chaotically moving moonlets. Evolutionary modeling should allow for this. Large distributed mass may have dynamical implications.

Structural evolution creates familiar structures in << T ss; “ramps” seen at inner B & A edges Compositional evolution creates global compositional variations: smooth color/composition profiles across abrupt ring boundaries Simultaneously, Durisen et al 1996 Implication: rings started as icy-organic material and became polluted by dark, neutrally-colored material. Meteoroid Bombardment and Ballistic Transport Estrada et al 2003 May be able to age-date the rings this way; several unknowns

Scargle et al 1993: F region chaos ( higher order resonances)? 2000km

SOI RPWS experiment detects “tones” from meteoroids hitting the rings (?) Gurnett et al Fall AGU 2004

Mass flux - the big unknown!

Transient objects? “2004S6”.. Or not? Charnoz et al Science 2006 X Spitale et al Moonlet belt collision? Meteoroid impact? Cuzzi & Burns 1988, Showalter 1998, Poulet et al 2000, Barbara & Esposito 2002

See Mitchell et al Science 2006

F Cuzzi & Burns Icarus km Pioneer 11 data L  c ~ 10(1km/L)  c ~ 10 -3, L~20000? ?? Van Allen 1982 F ring itself a collisional product (more rare; yrs) Transient clumps of cm-size rubble released in collisions between members of a 2000 km wide belt of km moonlets and total mass of about 1021 g (Prometheus). Conjectures: The F ring “moonlet belt”

Particle sizes from radio occultation Blue = small; red = large, white = very opaque

Stellar Occultation Variation Summary UVIS Colwell et al GRL 2006 cassini

Smoluchowski 1978, 1979; Canup & Esposito 1995; Karjalainen & Salo 2004 Icarus

The Opposition Effect: Shadow hiding and low volume density? or Coherent backscattering?

Ring microstructure 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? 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 Evidence exists for transient clumps/arcs of length ~ km HST and Cassini, in addition to Pioneer 11 microsignatures Prediction: Cassini will see more and larger clumps and arcs Speculation: F ring itself only one of the larger, more recent events Speculation: might g (or more!) have an influence on nearby orbits? Collisional belt modeling must be redone with induced chaotic e’s