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

Vrel <<< 15km/s

“Classical” ring model

Dynamically expected ring model:

Densely packed ring due to inelastic collisions

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

Wid

th o

f pe

ak

Wavelength (microns)

Lowalbedo

High albedo

The Opposition Effect:

Shadow hiding betweenparticles 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)

classical dense

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Stellar occultationsby the rings:

Colwell et al submitted

Movie by H. Salo

azimuthal view angledetermines opacity

andelevation angle

gives wake thickness

300m

Self-gravity wakes

2

1

21

VK

Self-gravity wake properties in the A ring

Colwell et al GRL 2006

To

tal o

ptic

al d

ep

th; G

ap

op

tica

l dep

th

Local, unresolved mixture of high-tau waves and low-tau gapswill affect ring I/F and must be included in photometric models

Wak

e he

ight

/wid

th

0.5

0.4

0.3

0.2

0.1

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 108 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(?)

0.3 - 0.5 slope

0.6 - 0.9 slope

ice band depths

C B A

Outer A ring

Multiple strandsPrometheus, 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 RR (Smoluchowski 1978,1979; Weidenschilling et al 1984; Longaretti 1989)

Canup & Esposito 1995

Accretion allowed

“tidally corrected”

classical force balance: c ~ CMp/a3 or ac ~ (CMp/)1/3

RR

Critical density for growth at distance a:

c=9Mp/4a30.15 in A ring

c≈27Mp/4a3CE95) or 9Mp/4a3bc (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=9Mp/4a30.15 in A ring

c≈27Mp/4a3CE95) or 9Mp/4a3bc (Weiss et al 2006)

Smoluch., Weid. et al, Longaretti

The F ring: tinsel on a massive moonlet belt?

VGR-1 RSSMarouf et al 1986

1200km

Data: Pioneer 11magnetospheric e,p

Simpson et al 1980

The F ring “moonlet belt”

Unlike G ring

in

out

Van Allen 1982Conjectures:

F

Van Allen 1982, Cuzzi & Burns 1988

1200km

Data: Pioneer 11magnetospheric e,p

??

Simpson et al 1980

F ring itself a collisional product (1015-16 g; every 102-103 yrs)

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)

Conjectures:

The F ring “moonlet belt”

Unlike G ring

in

out

F

Van Allen 1982, Cuzzi & Burns 1988

1200km

Data: Pioneer 11magnetospheric e,p

??

Simpson et al 1980

F ring itself a collisional product (1015-16 g; every 102-103 yrs)

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)

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

PIA07716

10000 km

10-2

1000 km

S6’

1500km

Pr Pa

S6

C

MM

MM

MA

AA

AA

AA

A

AA

S3/4

JH143141

F ring core

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 (1020-1021g)

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 2004Esposito et al 2006

(km-2 in a narrow ringlet)

3x1020g

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.

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.

1914-2006

Structural evolution creates

familiar structures in << Tss;“ramps” seen at inner B & A edges

Compositional evolution createsglobal 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 ringsthis way; several unknowns

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

2000km

QuickTime™ and a decompressor

are needed to see this picture.

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 1988

1200km

Pioneer 11 data

L

c ~ 10(1km/L)

c ~ 10-3,

L~20000?

??

Van Allen 1982

F ring itself a collisional product (more rare; 102-103 yrs)

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) .

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 2006cassini

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?

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 zoneResulting debris clumps spread & are swept up by other objects

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

Speculation: F ring itself only one of the larger, more recent events

Speculation: might 1021g (or more!) have an influence on nearby orbits?

Collisional belt modeling must be redone with induced chaotic e’s