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

Supplementary slides

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

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

Zolo

tov,

GRL

200

7

Salt composition matches expectations for hydrothermal leachingAge of interaction unconstrained