Sustaining volcanism on Enceladus Edwin Kite University of Chicago February 18, 2015.
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Transcript of Sustaining volcanism on Enceladus Edwin Kite University of Chicago February 18, 2015.
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