sustaining cryovolcanic eruptions on enceladus edwin kite (university of chicago)
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Sustaining cryovolcanic eruptions on EnceladusEdwin Kite (University of Chicago)
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~200 kg/s water ice erupting from 250 km-diameter Enceladus sustains a >102 yr old ring around Saturn.
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ancient,cratered
South polar terrain:
no craters
4 continuously-active “tiger stripes”
Cryo-volcanism on Enceladus has deep tectonic roots.
Density = 1.6 g/cc
water source = subsurface oceanPostberg et al. 2009, 2011; Hu et al. 2015Iess et al. 2014; Porco et al. 2014; Waite et al. 2009; Nimmo & Spencer 2013
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(4.4±0.2) GW excess thermal emission from surface fractures (“tiger stripes”)
Spencer & Nimmo AREPS 2013Porco et al. Astron. J. 2014
South polar view of Enceladus
Hotspots up to 200KNo liquid water at surfaceLatent heat represented by plumes < 1 GW
to Saturn
Close-up of one “tiger stripe”
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water source
surface
habitability
astrobiology
Today: How are curtain eruptions sustained on 101-106 yr timescales?
iceshell
Parkinson et al. 2008
Tsou et al. 2012McKay et al. 2014
Not addressed today: How is the ocean sustained on 107 – 1010 yr timescales? e.g. Meyer & Wisdom 2007, Tobie et al. 2008, O’Neill & Nimmo 2008, Behounkova et al. 2012
Kite & Rubin, in prep.
Prevailing view: conduit = cracke.g. Hurford et al. 2007, Nimmo et al. 2007, Olgin et al. 2011, Smith-Konter & Pappalardo 2008
Water ultimately sourced from a sub-ice oceane.g. Postberg et al. 2009, 2011; Hu et al. 2015; Iess et al. 2014; Porco et al. 2014; Waite et al. 2009; Nimmo & Spencer 2013; Zolotov, 2007.
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Probing tectonicsVolcanism
Geology
Tectonic mode:Seismicity
Gravity
Earth
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Probing tectonicsVolcanism
??Tectonic mode:
Seismicity
Gravity
EarthEnceladus
10 km
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?phase shift 55°
(Q ~ 1)
Challenge for the prevailing view: Understanding tidal modulation of eruptions(all four tiger stripes erupt as “curtains” throughout orbit; Spitale et al. Nature 2015)
smaller plume grains
larger plume grains
(period: 1.3 days)
Crack models are hard to reconcile with curtain eruptions maintained throughout orbit
NAS
A/JP
L/H
edm
anet
al.
2013
Hur
ford
LPS
C 20
15
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Challenge for the prevailing view: energy balance at water table
x
z
watertable
(35±
5) k
mIe
ss e
t al.
2014
Kite & Rubin, in prep.
(4.4±0.2) GWHowett et al. 2014
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Alternative: Melted-back slotCompression Tension
ocean ocean
supersonic plumesupersonic plume
water level fallswater level rises
x
z
Attractive properties:•Slot width lags tidal cycle•Slot does not close •Turbulent dissipation heats slot•Pumping disrupts ice formation
Kite & Rubin, in prep.
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Daily tidal cycle of water in tiger stripes
Changein slotwidth
Width changedue to water flow from ocean into slot
Width change due to tides from Saturn
Width change dueto back-forcefrom ice shell elasticity
Slots interactelastically: useboundary-element method
Kite & Rubin, in prep.
Tiger stripes can be approximated as straight, parallel and in-phase
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Daily cycle of tidal flow of water in slots
Wide slots track tidal forcing Narrow slots lag tidal forcing by 8 hours
width change (m
)volume change (fractional) volume change
(fractional)
width change (m)
1.3 days1.3 days
forc
ing
forc
ing
5m wideat zero stress
½ m wide at zero stress
Kite & Rubin, in prep.
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Turbulent dissipation of tidally-pumped vertical flow inside tiger stripes explains power output, phase lag and sustainability of the eruptions
Colored disks = diurnal width ratio
Increasing slot w
idth
Observedpower
Wide slots
Narrowslots
Observedphase lag
Changing“active slot” length
Kite & Rubin, in prep.
Best-fit: 1-2 m wide
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Long-lived water-filled slots have tectonic consequences
COLD, STRONGICE
WARM, WEAKICE
x
z
Kite & Rubin, in prep.
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Long-lived water-filled slots have tectonic consequences
COLD, STRONGICE
WARM, WEAKICE
x
z
Kite & Rubin, in prep.
Tectonic feedback between subsidence and melt-back buffers Enceladus output to 1700 kg/s × Lvap = 5 GW (no tuning, no free
parameters)
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Summary: Slot model explains and links sustainability of volcanism on 10 yr - 106 yr timescales
Gravity(Iess et al.2014)
Observedpower
Predicted Myr-averagepower output
matches observed phase lag slot aperture varies by >1.5
No tuningNo free parameters
Kite & Rubin, in prep.
melting + vaporization of ice
flowing from ice shell in
to base of slots.
cooling of the ice shell due
to subsidence of cold ice
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Testable predictions
1. Endogenic thermal emission should be absent between tiger stripes. 2. No correlation between emission and local tiger-stripe orientation 3. Smooth distribution of thermal emission.
Kite & Rubin, in prep.
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Limitations and caveats• Initiation of ocean-to-surface conduits on ice
moons remains hard to explain (Crawford & Stevenson 1988). – may be related to ice-shell disruption at high orbital eccentricity:
such disruption could have created partially-water-filled conduits with a wide variety of apertures, and evaporative losses caused by tiger stripe activity would ensure that only the most dissipative conduits (width 1-2m) endure to the present day.
• Slot stability is less of a concern. – along-slot stirring is rapid relative to freeze-shut and
flow-shut.
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Conclusions.
Paul
Sch
enk
/ LP
I
• Turbulent dissipation may explain Enceladus’ phase curve and the maintenance of fissure eruptions over geological timescales.
• With long-lived slots, Myr-average power is buffered by tectonic feedback to ~5 GW, equal to observed power (suggesting long-term stability).
- Geysers are not a passive tracer of tectonics – they can drive tectonics.
• Testing habitability on Enceladus (or Europa) ultimately requires access to ocean materials - easier if turbulent dissipation maintains active fissures open for Kyr. ≫
We thank Terry Hurford, Karl Mitchell, Alyssa Rhoden, Joseph Spitale, Robert Tyler, and Steve Vance.
http://geosci.uchicago.edu/~kite
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Supplementary slides
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Porco et al.AJ, 2014
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Tectonic feedback between subsidence and meltback buffers the South Polar terrain to 5 GW
Equilibrium power output
5 km
100 km
Ice shell thickness:
Gravity constraintof shell thickness:
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Highly simplified ice flow: assuming
cools the ice shell
Initial temperatureprofile (conductive)Steady-statetemperature profile(Subsidence-perturbed)
Initial ice inflow rate
Steady-state ice inflow rate
Tectonic feedback between subsidence and meltback buffers South Polar terrain power to 5 GW
Kite & Rubin, in prep.
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Open questions
Source:What is the water source forEnceladus’ eruptions?
Ocean water is exposed to space,raising energy balance problems
Plumbing system:How can conduits between ocean and surface avoid freezing shut? (w/ Allan Rubin)
Tidal heating is the only plausible candidate, but location of heating ispoorly constrained.
Engine:What powers Enceladus volcanism?
Sodium and nano-silica tell us that the source is a subsurface ocean, not clathrates or sublimation
Tidal heating is the only plausible candidate
Turbulent dissipation within tiger stripes can sustain the phase curve of Enceladus’ eruptions.
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Mey
er &
Wis
dom
, Ica
rus,
200
8
History of the Enceladus:Dione resonance
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Clathrate-source and solid-sublimation explanations face insuperable challenges
• Salt• Nano-silica• I:G ratio• Thermal
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Pipe model and slot model
Option: Array of unresolved pipesArea changes by 10^-4
Option: Each tiger stripe is one slotArea changes by order unity under 1-bar pressure cycle
Apertures < 10 m (not to scale)
130 k
m
35 km
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1 tiger stripe
Extensional normal stress (Pa)
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Rudolph & Manga,Icarus 2009
inferred from gravity
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Models of tidal modulation
Nimmo et al., Astron. J. 2014
supersonicascent time
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Crack models are falsified by eruptions at periapse
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All existing models fail!Model A Model B Model C
Porc
o et
al.
Astr
on. J
. 201
4
Should erupt
Shouldnot
erupt
Should erupt
Shouldnot
erupt
Should erupt
Shouldnot
erupt
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Closest distance to Saturn Furthest distance from Saturn
Enceladus period = 1.3 daysEnceladus orbital eccentricity = 0.0047Tidal stress amplitude ~ 1 bar
tiger stripes
=time-averaged shape
eccentricity tide onlythin-shell approximationk2 appropriatefor global ocean
Looking down on South Pole Looking down on South Pole
Crack models are hard to reconcile with curtain eruptions at periapse
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Paul
Sch
enk
/ LP
I / U
SRA
130 k
m
35 km
Slots interact elastically: useddisplacement-discontinuity method
Kite & Rubin, in prep.
Crouch & Starfield, 1983Rubin & Pollard, 1988
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(4.4±0.2) GW excess thermal emission from surface fractures
Spencer & Nimmo AREPS 2013Spitale et al., accepted
Porco et al. Astron. J. 2014Abramov & Spencer 2009
South polar projection
Hotspots up to 200KNo liquid water at surfaceLatent heat represented by plumes < 1 GW
to Saturn
130 km
Howett et al. 2014
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Zolo
tov,
GRL
200
7
Salt composition matches expectations for hydrothermal leachingAge of interaction unconstrained