Sustaining volcanism on Enceladus

Edwin KiteUniversity of Chicago

February 18, 2015

~200 kg/s water ice erupting from 250 km-radius 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

surface

usually frozen vs. sustained melt pool?

intermittent vs. steady?

duty cycle? timescale?

habitability

astrobiology

Understanding the sustainability of water eruptions on Enceladus is important

iceshell

Understanding the sustainability of water eruptions on Enceladus is important

Mass loss and vent temperature forcings for tectonics

Throttles supply of oxygen to Titan’s reducing atmosphere

Sets frost shrouding of small “cueball” moons of Saturn

Clues to anomalously variable densities of mid-sized Saturnian moons

Analog for Europa (Jupiter moon) and Miranda (Uranus moon)

Open questions

Source:What is the water source forEnceladus’ eruptions?

A salty ocean is connected to the surface, but exposing ocean water to spaceraises energy balance problems.

Plumbing system:How can conduits between ocean and surface avoid freezing shut? (w/Allan Rubin, Princeton U.)

Tidal heating is the only plausible candidate, but location of heating ispoorly constrained.

Engine:What powers Enceladus volcanism?

Probing tectonicsVolcanism

Geology ?Tectonic mode:

Seismicity

Geology Geomorphology

Gravity

EarthEnceladus

10 km

Open questions

Source:What is the water source forEnceladus’ eruptions?

A salty ocean is connected to the surface, but exposing ocean water to spaceraises 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?

(4.6±0.2) GW excess thermal emission from surface fractures (~10 KW/m length; all four tiger stripes erupt as “curtains”)

Spencer & Nimmo AREPS 2013Porco et al. Astron. J. 2014

South polar projection

Hotspots up to 200KNo liquid water at surfaceLatent heat represented by plumes < 1 GW

to Saturn

130 km

Enceladus is small for active interior-driven volcanism

endogenicvolcanism

Radiogenic

Tidal

Chemical

Maximum equilibrium (constant-eccentricity) heat production: 1.1 GW(due to 2:1 Enceladus:Dione resonance)

Steady release of accretional heat <0.1 GWAl-26 heating may be significant early on

Serpentinization: <0.1 GW (assuming complete serpentinization; 2.4 x 105 J/kg srp)

<0.32 GW assuming CI chondritic composition

Only tidal heating can provide the required heat

Falls short(some alsofail the “Mimas test”)

Heat source

Possible

Secularcooling

Joule heating

Recent impact

<0.05 GW (Hand et al. JGR 2011)

Improbable

Long-lived tidally-powered volcanism requires both resonant boosting of eccentricity and a dissipative interior

Murray & Dermott, 2000

Dione:EnceladusPeriod: 1:2Mass: 10:1

Saturn’s apparent movementviewed from Enceladus (e = 0.005)

Tides on Enceladus raised by Saturn (1.3 day period)

Enceladus

Meyer & WisdomIcarus 2007

Dione

Resonances appear insufficient to sustain current eruptions over GyrD

istan

ce fr

om S

atur

n (S

atur

n ra

dii)

Large moons tidally disrupted(rings of Saturn)

<1.1 GW

Tethys

Mimas

Fastest possible Mimas recession rate(?)

Tides raised by the moons on Saturn cause Saturn to spin slower and moons to move outwards

2:1

Is there a limit-cycle workaround for the weakness of steady-state tidal heating?

Unfortunately, detailed simulations show only very-small-amplitude limit cycles.

Spencer & Nimmo 2013

Location of dissipation unclear; ice shell orocean?

Can intermittent ice-shell overturn act as a thermal capacitor?

O’Neill & Nimmo, Nature Geoscience, 2010

Unfortunately, this result may be a model-resolution artifact.Assumption: volcanism is a passive tracer of ice-shell tectonics

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 – very likely intermittent.

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.

Tidal energy budget rules out steady volcanism over GyrConsidering timescales < 106 yr simplifies the problem

Oceanfreezes:

Orbitcircularizes:

Regionalocean spreads:

Crater retention age of South Polar Terrain < 0.5 x 106 yr

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 – very likely intermittent - but location of heating is poorly constrained.

Engine:What powers Enceladus volcanism?

Sodium and nano-silica tell us that the source is a subsurface ocean, not clathrates or sublimation

Turbulent dissipation within tiger stripes can sustain the phase curve of Enceladus’ eruptions.

Sourcing water eruptions from an ocean is challenging

Crawford & StevensonIcarus 1988

Kieffer (clathrates)Porco (liquid water)

Can clathrate decomposition within the ice shell explain the plumes?

Postberg et al. Nature 2011

18 km/s (Cassini @Enceladus)

6 km/s (Stardust mission)

aerogel

Dust track (mm long)

>99% of erupted ice grains are salt-rich, ruling out “dry” water source

Enceladus

(Active?) hydrothermal vents required to explain nano-silica observations

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 – very likely intermittent.

Engine:What powers Enceladus volcanism?

Sodium and nano-silica tell us that the source is a subsurface ocean, not clathrates or sublimation

Turbulent dissipation within tiger stripes can sustain the phase curve of Enceladus’ eruptions.

Key constraint #1: energy balance at water table

x

z

watertable

(30±

5) k

m

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, Princeton U.)

Tidal heating is the only plausible candidate – very likely intermittent.

Engine:What powers Enceladus volcanism?

Sodium and nano-silica tell us that the source is a subsurface ocean, not clathrates or sublimation

Turbulent dissipation within tiger stripes can sustain the phase curve of Enceladus’ eruptions.

?

5 x

phaseshift 55°

Key constraint #2: Eruptions are modulated by tides

smaller plume grains

larger plume grains

(period: 1.3 days)

base level

NAS

A/JP

L

How to find tidal stresses at volcanic vents

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

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

An alternative model: 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

Tiger stripes can be approximated as straight, parallel and in-phasePa

ul S

chen

k /

LPI /

USR

A

130 k

m

35 km

Slots interact elastically: usedboundary-element code

By Allan Rubin

Daily cycle of tidal flow of water in slotsWide slots track the tidal forcing Narrow slots lag the tidal forcing by 8 hours

Width change (m

)Volume change (fractional) Volume change

(fractional)

Width change (m)

1.3 days1.3 days

Turbulent dissipation of tidally-pumped vertical flow inside tiger stripes explains power output, phase lag and sustainability of the eruptions

m/s Width ratio

Zero-stress slot half-width (m)

Changing slot w

idth

Long-lived water-filled slots have tectonic consequences

COLD, STRONGICE

WARM, WEAKICE

x

z

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

Slot model explains and links sustainability of volcanism on 10 yr through 106 yr timescales

Gravityconstraint

How does along-crack branching affect power?

Next: testing the slot model

Can slots of the required widths form?

What testable consequences would long-lived tiger stripes have for surface tectonics?

How does changingwater level and conduit width affect flowin vent?

Hei

ght

Summary

Source:What is the water source forEnceladus’ eruptions?

Ocean water is exposed to space,raising energy balance problems

Plumbing:How can conduits between ocean and surface avoid freezing shut? (w/ Allan Rubin, Princeton U.)

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 may explain the phase curve of Enceladus’ eruptions.

Summary

Thanks to Allan Rubin (Princeton U.) for displacement-discontinuity code and numerous discussions, & Robert Tyler, Terry Hurford, Alyssa Rhoden, & Karl Mitchell for additional discussions. Research website: www.climatefutures.com

Paul

Sch

enk

/ LP

I

What powers Enceladus volcanism?

What is the water source forEnceladus’ eruptions?

How can conduits between ocean and surface avoid freezing shut?

Turbulent dissipation within tiger stripes may explain the phase curve of Enceladus’ eruptions.

Sodium and nano-silica require a subsurface ocean source - not clathrates or sublimation.

Tidal heating is the only plausible candidate, the location of heating is poorly constrained,and eruptions must be intermittenton >106 yr timescales.

ENERGY SOURCE

WATER SOURCE

CONNECTION

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:

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

Interacting slots are not overpowered

Width ratio

Displacement-discontinuitycode by Allan Rubin

Changing slot

width

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

Daily tidal cycle of water in tiger stripesChangein slotwidth

Width changedue to water flow from ocean into slot

Width change due to dailytidal forcesfrom Saturn

Width change dueto back-forcefrom elasticdistortion of shell

Coefficientsthat depend ondetailed tigerstripe geometry

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

Zolo

tov,

GRL

200

7

Salt composition matches expectations for hydrothermal leachingAge of interaction unconstrained