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Cassini Observes the ActiveSouth Pole of Enceladus

Porco et al. 2006

Presented by Tanya Harrison

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Where is Enceladus?

Image Source:http://photojournal.jpl.nasa.gov/catalog/PIA03550

http://photojournal.jpl.nasa.gov/catalog/PIA03550http://photojournal.jpl.nasa.gov/catalog/PIA03550

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How Big is Enceladus?

Image Source: BBC

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Enceladus

Distance from Saturn: 238,040 km

Radius: 247 km

Mass: 7*1019 kg

Eccentricity: 0.00452

Albedo: 0.99

Inclination: 0

Mean Surface Temp: 75 K

Atmosphere:

91% H2O Vapour

4% N2

3.2% CO2

1.7% CH4

Source: saturn.jpl.nasa.gov

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Topography Dichotomy

laps.fsl.noaa.gov/albers/sos/sos.html

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Y-ShapedDiscontinuity

GroovedBandsAdjacent to aTiger Stripe

Transition ofY-Shaped

Discontinuityto N-STrendingFractures

(Porco et al. 2006)

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Tiger Stripes

saturn.jpl.nasa.gov

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Tiger Stripes

~130 km long, 2 km wide, 500 m deep

~35 km apart, striking 45 away from Saturn direction

No craters in region >1 km 500,000 yrs old or younger

Global snow cover absent here young terrain orthermal processing is occurring

Spectra indicate ~100 m ice grains

Porco et al. 2006

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South Pole Geyser

Emanating from at least 17separate vents along thefour Tiger Stripes (Kieffer et al. 2006)

Brightness temperatureimplies T = 145 K 14 K

3-6 GW thermalemission! (Spencer et al. 2006)

Escape rate ~5-10*1027molecules/sec (10-7-10-6kg/s/m2) (Spencer et al. 2006, Kieffer et al. 2006)

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South Pole Geyser

H2O ice mass in plume:3*10-6 kg/m2 (Porco et al. 2006)

Plume composition:(Waite Jr. et al. 2006)

91 3% H2O

3.2 0.6% CO2

4 1% N2 or CO

1.6 0.4% CH4

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Plume Generation Theories

Clathrate reservoirhypothesis

No differentiationneeded

Pure H2O plume from

interior heating Differentiation needed

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Clathrate Reservoir Hypothesis

Clathrate reservoircapped by a leakyH2O/CO2 ice cap

Fractures self-sealingdue to refreezing ofH2O If crack propagates to

surface, exposingclathrates to vacuum,violent decompositioncan occur

(Kieffer et al. 2006)

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Clathrate Reservoir Hypothesis

No ammonia detected Thought to be present to lower

H2O melting temperature and

increase buoyancy to aid inresurfacing

Doesnt explain the presenceof catalytic trace elements inthe plume

The possible N2 in the plumecould also be CO; cant bedistinguished spectrally

Cracks sealing over time canexplain high variability inplume emissions

Explains possible presence ofN2 in the plume if NH4 isconverting to N2 beforereaching the surface

Pros Cons

(Kieffer et al. 2006)

(Waite Jr. et al. 2006)

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Pure H2O Plume Hypothesis

photojournal.jpl.nasa.gov/catalog/PIA07799

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Rate of H2O loss: S = nhv n= 1.5*1016/cm2 (observed during flyby), plume

length L = 80 km

Assume ncorresponds to vapour in vapour pressureequilibrium w/ a warm ice source T ~ 145 K (lowerlimit)v= 41,200 cm/sS = 5*1027 molecules/s

Or, if plume length is taken from the horizontaldimension of the plume (~175 km):

T ~ 180Kv= 46,000 cm/sS = 1*1028 molecules/s ~350 kg/s

Pure H2O Plume Hypothesis

(Hansen et al. 2006)

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Pure H2O Plume: Trace Elements

Presence of C3H8, C2H2, and HCN implycatalytic reactions in a very hot

environment(Matson et al. 2007)

Propane acids aid in formation of higher orderhydrocarbons, which facilitate acetyleneformation reactions

Thermal decomposition of NH3 to N2requires internal T ~500-800 K

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Pure H2O Plume: Trace Elements

Thermal decomposition of ammonia: 2NH3N2+3H2, 2NH3(aq)N2(aq)+3H2(aq)

Methane formation: CO+3H2H2O+CH4, CO2+2H2OCH4+2O2

Acetylene & propane formation: 3C4H20+2H2O2HCCH+2C2H4+2CH4+2CO+

7H2, C2H4HCCH+H2

(Matson et al. 2007)

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Pure H2O Plume: Trace Elements

Theory #1: Liquid H2O cycles through hot rock at theice/rock interface

CO, CO2, and NH3- bearing solutions percolate through the rock,

where geochemical reactions produce N2

Theory #2: Serpentinization of rock by H2O to enhanceCH4 production

Hydrated silicates in the core trap reactants, facilitating reactions

when the temperature in the core increases

Not viable for the long-term as reactants in crack walls wouldbecome depleted over time

(Matson et al. 2006)

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Evidence for Differentiation?

Mathematical model of the observed shapedoesnt fit for a homogenous body, but does fitfor a differentiated body with a large south polar

sea(Collins & Goodman 2007)

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Evidence for Differentiation?

Observed axial values are notexactly what is expected for anundifferentiated, equilibriumobject Differentiation makes bodies

more spherical, making a-c

and b-cmuch smaller thanobserved If chas been independently

modified, then a fit to theobservations is obtained

Produces same maxdeviations from an ellipsoid

reported by Porco et al.(2006), with a depressioncentred at the south pole anda ridge surrounding it at~50S latitude

(Collins & Goodman 2007)

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Maintaining a South Polar Sea

If observed energy input is >20km deep, timescale of meltingoverlying ice is shorter than thetime to conduct heat out thesurface Ocean stays confined after

reaching equilibrium: inputfrom melting of stagnant ice lid= loss at sides of ocean(where liquid refreezes)

Inward flow of liquid is slowerthan melting & refreezingprocess:

sea shape dominated byshape of heating profile

sea is stable as long asheating lasts

(Collins & Goodman 2007)

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Problems & Questions

Where is all the heat coming from?

How did melting start in the first place?

~25 GW required to initiate melting (Squyres et al. 1982)

If comprised of pure H2O, eccentricity must havebeen 5-7x the present value to instigate melting

Clathrates decrease thermal conductivity, but highereccentricity still needed (Squyres et al. 1982)

Why centred at the South Pole?

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Heat Sources

3-7 GW of thermal emission in SPT (Collins & Goodman 2007)-Where is it all coming from?-Mimas Testsince Mimas has an ancient surface & is closer to Saturn

than Enceladus, whatever is heating Enceladus cant be heatingMimas (Squyres et al. 1982)

Image Source: saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2071

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Tidal & Radiogenic Heating

Tidal heating from Saturn: 0.12 GW (Porco et al. 2006)

Radiogenic heating: 0.32 GW if Enceladus rock

fraction is ~0.61(Porco et al. 2006)

Image Source: photojournal.jpl.nasa.gov/jpegMod/PIA08220_modest.jpg

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Tidal Heating & Viscoelastic Models

Equilibrium heating in a homogeneous Maxwell model atthe current eccentricity can be up to 920 GW (Ross & Schubert 1989)

2-layer model w/ a conductive elastic lithosphereoverlying a Maxwell interior & a 3-layer model w/ a liquidH2O-NH4 layer between the lithosphere and the Maxwellcore produce ~4 GW of heating (Ross & Schubert 1989; Meyer & Wisdom, in print) Models require a low conductivity insulating layer

Problem: Doesnt necessarily pass the Mimas Test These models, applied to Mimas, would produce too much heat

to account for the ancient surface seen today (Meyer & Wisdom, in print)

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Heating from Resonance

Enceladus currently in 2:1 resonance with Dione

3:2 primary & 4:1 secondary resonance with Mimas Up to 0.18 GW produced (Porco et al. 2006)

Libration in this secondary resonance dampens eccentricity andmakes the resonance unstable, so it could only lead to a singleepisode of heating

Perhaps formerly in 2:1 resonance with Janus? (Lissauer et al. 1984)

Could produce 4.5 GW, which is in the range currently observed

3:4 resonance with Tethys (Meyer and Wisdom, in press)

Image Source: saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2573

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Where is the Rest of the Heat ComingFrom?

Perhaps Enceladus formed early enough suchthat 26Al and 60Fe were significant, producing a

highly dissipative, partially molten core(Matson et al. 2007)

Image Source: astro.wsu.edu/worthey/astro/html/im-saturn/art-enceladus.jpg

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Saturns E Ring

Image Source: saturn.jpl.nasa.gov

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Saturns E Ring

Largest in Saturnian system (Spahn et al. 2007) Extends from 3-8 Saturnian radii

Narrow particle diameter range: 0.3-3.0 m (Spahn et al. 2007)

Saturnian system filled w/ neutral products from e- andphotodissociation of H2O (neutral OH and atomic O) (Hansen etal. 2006)

Must be consistently replenished to be sustained due toSaturns magnetospheric plasma (Hansen et al. 2006) Rate of loss ~1 kg/second

Entire ring would be lost in

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Enceladus Geyser---Source of theE Ring?

Density peaks and vertical extent is at a minimum atEnceladus orbit (Spahn et al. 2007)

Highly symmetric dust configuration and narrow range inparticle size suggests liquid or vapour source ratherthan impacts (Spahn et al. 2007, Hansen et al. 2006)

Models created for different particle sources onEnceladus, and best fit is at the region of the TigerStripes (>70S) (Spahn et al. 2007)

If Enceladus is the primary source, its geyser must havebeen geologically active for at least the last 15 years (Hansen etal. 2006)

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Reorientation?

Diapir-induced reorientation

Melt pool-induced reorientation

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Diapir-Induced Reorientation?

Surface deformationcaused by anuncompensated low-

density diapir formedby tidal heating If diapir is in the ice

shell, ice must be >0.5km thick

If diapir is in the silicatecore, there cant be asubsurface ocean

(Nimmo & Pappalardo 2006)

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Melt Pool-Induced Reorientation

Completelycompensatedmelt pool

Perhapscoupled with adiapir in the

silicate core(Collins &

Goodman 2007)

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Conclusions & Future Work

Enceladus South Pole Geyser likely source of

Saturns E Ring

Internal heating is leading to the eruption of thegeyser, but all heat sources not necessarilyaccounted for yet

Detailed gravity modeling required to determinewhether or not Enceladus is differentiated

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Bibliography

Collins, G.C. & Goodman, J.C. A South Polar Sea on Enceladus? LPSC 38, abstract#1504 (2007). Handen et al. Enceladus Water Vapor Plume. Science322, 1422 (2006). Kieffer et al. A Clathrate Reservoir Hypothesis for Enceladus South Polar Plume. Science

314, 1764 (2006). Lissauer et al. Ring Torque on Janus and the Melting of Enceladus. Icarus58, 159-168

(1984).

Matson et al. Enceladus Plume: Compositional Evidence for a Hot Interior. Icarus187,569-573 (2007). Meyer, Jennifer and Wisdom, Jack. Tidal Heating in Enceladus. Icarus, in press (2007) Porco et al. Cassini Observes the Active South Pole of Enceladus. Science311, 1393

(2006). Spahn et al. Cassini Dust Measurements at Enceladus and Implications for the Origin of

the E Ring. Science311, 1416 (2006). Spencer et al. Cassini Encounters Enceladus: Background and the Discovery of a South

Polar Hot Spot. Science311, 1401 (2006). Squyres et al. The Evolution of Enceladus. Icarus53, 319-331 (1983). Roddier et al. Discovery of an Arc of Particles Near Enceladus Orbit: A Possible Key to

the Origin of the E Ring. Icarus136, 50-59 (1998). Waite, Jr. et al. Cassini Ion and Neutral Mass Spectrometer: Enceladus Plume

Composition and Structure. Science311, 1419 (2006).