Tiscareno (1/22)
Dynamics of Planetary Rings Ma4hew S. Tiscareno, Cornell University Astro 6570, 12 Nov 2013 Tiscareno (2/22)
Planetary Rings as Astrophysical Disks •  The rings around the giant planets are the most easily accessible astrophysical disk systems •  We can’t go to other stars, or back in Nme to solar system origin, but we can study Saturn’s rings up close •  We can elucidate many of the processes that have been inferred to sculpt other, more remote disk systems Whirlpool Galaxy
Saturn
Beta Pictoris Protoplanetary Disk
Tiscareno (3/22)
SIMILARITIES: RINGS & CIRCUMSTELLAR DISKS Both systems orbit dominant central masses. Fla4ened disks (vorb>> vrandom) in which gravity (plus gas pressure) balances centrifugal acceleraNon. Viscous torques cause radial spreading of disks. Embedded and exterior bodies, thru density waves, influence radial migraNon of the disk and of the bodies themselves. GravitaNonal instabiliNes: Global for C-­‐S disks, local for rings. Tiscareno (4/22)
DIFFERENCES: RINGS • 
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C-­‐S DISKS Scale: 108 m vs. 1013 m Mass raNo (disk to central body): 10-­‐8 vs. 10-­‐2 RelaNve Thickness: 10-­‐7 vs. 10-­‐1 Collisions: Common vs. rare. Dynamical Age: 1012 revs vs. 104-­‐108 revs (less for galaxies) AccreNon frustrated by Ndes for rings. Exterior perturbers significant for rings. The Cassini-­‐Huygens Mission 15 October 1997 Cape Canaveral 1 July 2004 Saturn Tiscareno (5/22)
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Character of Saturn s Rings •  Water ice with minor contaminaNon •  Truncated power-­‐law size distribuNon between cm and m, very few larger objects •  Typical opNcal depths ~ 0.1 -­‐ 5 •  The understood structure is set by Typical image resoluNon = 1-­‐10 km embedded and external moons OccultaNons resolve @ 10-­‐100 m. F Ring A Ring Keeler Cassini Div. Encke B Ring Rings of Jupiter Tiscareno (7/22)
•  All four giant planets have rings, but of different characters –  Jupiter, a tenuous homogeneous dusty disk conNnuously generated by debris thrown off by small moons –  Nested “tuna can” geometry –  Belts of large objects in the Main ring Low Phase
High Phase
Rings of Uranus •  All four giant planets have rings, but of different characters –  Uranus, a series of dense but very narrow strands (truly “rings” in plural) –  Intricate dust lanes glimpsed in the sole high-­‐phase Voyager image –  “PorNa group” of 8 moons packed between 2.31 and 2.99 RU can be regarded as an accreNon-­‐
dominated extension of the disk Tiscareno (8/22)
Rings of Neptune •  All four giant planets have rings, but of different characters –  Neptune has narrow rings, much like Uranus, BUT –  Much dusNer, opNcal depth not as high AND –  Longitudinally confined arcs in the Adams ring •  Do not follow simple model •  Change over decades Voyager 2
Dumas et al 1999, Nature
Tiscareno (9/22)
Rings of Saturn •  All four giant planets have rings, but of different characters –  Saturn a true disk, both dense and broad –  Also contains dense narrow rings –  Also contains narrow dusty rings –  Also contains diffuse dusty rings Tiscareno (10/22)
Other Ring Systems? •  Mars and Pluto may have undetected dusty rings (Showalter et al 2006, P&SS; Steffl and Stern 2007, AJ)
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•  Rhea does not have rings –  Detected EM signature can’t be explained by non-­‐detected rings (Tiscareno et al 2010, GRL) •  Exoplanets may have rings –  Hot Jupiters probably don’t (SchlichFng and Change 2011, ApJ) –  Studies have idenNfied what they’d look like in transits (Barnes and Fortney 2004, ApJ) –  Fomalhaut B may reflect excess visible light Kalas et al 2008, Science
Tiscareno et al 2010, GRL
Does Rhea Have Rings? •  Charged-­‐parNcle absorpNons have been interpreted as three sharp rings of Rhea, along with a background cloud, (Jones et al. 2008, Science) •  Ring locaNons are 2.1, 2.4, and 2.7 RRhea, or possibly less •  Background cloud fills Rhea s Hill sphere (~8 RRhea) Tiscareno (12/22)
Tiscareno (13/22)
Does Rhea Have Rings?
•  Recent work found equatorial markings on Rhea (Schenk et al. 2011, Icarus) –  Would such a ring be stable? –  Would debris fall narrowly on the equator? –  Would it make markings of this color? IR/Grn/UV Composite
IR/UV Ratio
IR/Grn Ratio
Schenk et al. 2011, Icarus
Rhea Has No Rings •  If Jones et al. (2008) calculaNons are accepted, our observaNons require –  Broad cloud parNcle size > 1 mm –  Narrow ring parNcle size > 8 m •  Does not rule out broad cloud •  Narrow rings implausible because erosion would create smaller parNcles, which we would see •  However, calculaNons ignore electron penetraNon depth! Tiscareno (14/22)
Rhea Has No Rings •  For parNcles larger than the penetraNon depth, electron absorpNon goes like surface area, not mass (volume) •  When we properly account for this, our observaNons clearly rule out circum-­‐Rhea material massive enough to have caused the MIMI signatures •  Unknown magnetospheric processes at Rhea! Tiscareno (15/22)
What are rings? •  Assemblage of many parNcles in similar orbits –  Why does it not accrete? Answer: Roche limit •  VerNcally fla4ened –  That is, vorbit >> vverNcal –  Why not a spherical cloud? Answer: Gravity of fla4ened planet Tiscareno (16/22)
Roche limit •  Tide: The difference in force on opposite sides of an object •  What kind of object can hold itself together with its own gravity? 1/3
! 3M $
aRoche = #
&
" 2!" %
•  Depends only on object’s density! 3M
! Roche =
2" a 3
•  In reality, material strength can further help hold objects together Tiscareno (17/22)
Roche CriNcal Density Tiscareno (18/22)
•  Objects need ρ > ρR to be held together by gravity 3M P
•  Dense seeds accrete fluffy mantle unNl !R = 3
ρ ≈ ρR (object “fills its Roche zone”) "a
•  At ring’s outer edge: –  Transient parNcles have ρ > ρR –  OR material for making rings is not abundant •  At Saturn and Uranus, many small moons just outside the main rings –  Boundary between accreNon-­‐dominated and disrupNon-­‐dominated regions AccreNon in the Rings Atlas
Tiscareno (19/22)
Pan ~ 15 km Density: 0.4 g/cm3 41 x 36 x 20 km Density: 0.4 g/cm3 •  Ring-­‐moons have low densiNes, odd shapes •  Dense cores accrete porous mantle unNl they fill the zones dominated by their gravity Rings by Type Tiscareno (20/22)
•  Dense broad disks –  Saturn (A, B, C, Cassini Div.) •  Dense narrow rings –  Saturn (Titan, Maxwell, Bond, Huygens, “Strange”, Herschel, Jeffreys, Laplace) –  Uranus (6, 5, 4, α, β, η, γ, δ, ε) •  Narrow dusty rings –  Saturn (F, “Charming”, Encke ringlets) –  Uranus (λ) –  Neptune (Le Verrier, Arago, Adams) •  Diffuse dusty rings – 
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Jupiter (Halo, Main, Gossamer) Saturn (D, E, G, Roche Div., J/E, Me/Pa/An, Phoebe) Uranus (ζ, ν, μ) Neptune (Galle, Lassell) Gravity of a fla4ened planet •  The giant planets are fast rotators (10 to 17 hours) •  Centrifugal force causes them to be fla4ened –  Gravity strongest in the mid-­‐plane –  Inclined orbits precess about the mid-­‐plane GM
GMR 2
V =!
+ J2
P2 (cos! )
3
r
r
•  Collisions among ring parNcles most likely in planet mid-­‐plane Tiscareno (21/22)
Gravity of a fla4ened planet •  So the planar qualiNes of rings are more Nghtly regulated than those of circumstellar disks –  Even though angular momentum origin is same in both cases –  Reoriented central body can take rings with it (e.g., Uranus) –  Fla4ening of the disk is much more thorough (10-­‐7 rather than 10-­‐1) •  More finely detailed structure –  Why? Toomre criNcal wavelength determines size of local disturbances Lcrit
2! G" # M disk &
=
"%
( Ldisk
2
!
$ M central '
–  Mass raNo is 10-­‐8 (rings), 10-­‐2 (c-­‐s disks) Tiscareno (22/22)