Sustaining eruptions on EnceladusEdwin Kite (U.Chicago), Allan Rubin (Princeton)
Geysers on Enceladus draw water from a subsurfaceocean, but the sustainability of conduits linking ocean and surface is not understood
Today, use a model of tidally flexed slots thatpuncture the ice shell to interpret the tidalphase curve of the eruptions, the surprisingpersistence of fissure eruptions, and the total power output of the tiger stripe terrain.
ocean
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?
Meyer & Wisdom 2007, O’Neill & Nimmo 2008, Behounkova et al. 2012
Kite & Rubin, in prep.
conduit = crack?Prevailing view. e.g. Hurford et al. 2007, Nimmo et al. 2007, Olgin et al. 2011, Smith-Konter & Pappalardo 2008
water source = 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
?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. accepted)
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 slotsWide 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.
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 power to 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.
Testable predictions
• Endogenic thermal emission will be absent between tiger stripes. • There should be no correlation between the magnitude of emission and local tiger-stripe
orientation, a prediction that distinguishes the slot model from all crack models. • The slot model predicts a smooth distribution of thermal emission, in contrast to the
spotty emission near jets that would be expected if flow is concentrated in pipes. • The pattern of spatial variability should be steady, in contrast to bursty hypotheses (e.g.
Matson et al. 2012).Vapor flux should covary with ice-grain flux.
For late 2015 / early 2016 Cassini flybys:Kite & Rubin, in prep.
For numerical experiments:• How does changing water level and conduit width affect gas-dynamic flow in vent?
(Ingersoll & Pankine, 2010)• What testable consequences would long-lived tiger stripes have for “ropy terrain” in
between tiger stripes?(Barr & Pruess, 2010)
Conclusions
Paul
Sch
enk
/ LP
I
• Turbulent dissipation may explain the phase curve of Enceladus 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).
• Testing habitability on Enceladus (or Europa) ultimately requires access to ocean materials - easier if turbulent dissipation props 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
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.
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
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
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
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
Top Related