sustaining cryovolcanic eruptions on enceladus edwin kite (university of chicago)
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Sustaining cryovolcanic eruptions on EnceladusEdwin Kite (University of Chicago)
~200 kg/s water ice erupting from 250 km-diameter Enceladus sustains a >102 yr old ring around Saturn.
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
(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”
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.
Probing tectonicsVolcanism
Geology
Tectonic mode:Seismicity
Gravity
Earth
Probing tectonicsVolcanism
??Tectonic mode:
Seismicity
Gravity
EarthEnceladus
10 km
?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
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
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.
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
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.
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
Long-lived water-filled slots have tectonic consequences
COLD, STRONGICE
WARM, WEAKICE
x
z
Kite & Rubin, in prep.
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)
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
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.
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.
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
Supplementary slides
Porco et al.AJ, 2014
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:
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.
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.
Mey
er &
Wis
dom
, Ica
rus,
200
8
History of the Enceladus:Dione resonance
Clathrate-source and solid-sublimation explanations face insuperable challenges
• Salt• Nano-silica• I:G ratio• Thermal
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
1 tiger stripe
Extensional normal stress (Pa)
Rudolph & Manga,Icarus 2009
inferred from gravity
Models of tidal modulation
Nimmo et al., Astron. J. 2014
supersonicascent time
Crack models are falsified by eruptions at periapse
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
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
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
(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
Zolo
tov,
GRL
200
7
Salt composition matches expectations for hydrothermal leachingAge of interaction unconstrained
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