Sustaining volcanism on Enceladus
Edwin Kite
University of Chicago
February 18, 2015
~200
kg/s water ice erupting from 250 km-radius Enceladus
sustains a >102 yr old ring around Saturn
Cryo-volcanism on Enceladus has deep tectonic roots
ancient,
cratered
4 continuouslyactive “tiger stripes”
Density = 1.6 g/cc
Understanding the sustainability of
water eruptions on Enceladus is important
surface
astrobiology
ice
shell
water source
intermittent vs. steady?
duty cycle? timescale?
habitability
usually frozen vs.
sustained melt pool?
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
Engine:
What powers Enceladus
volcanism?
Tidal heating is the only plausible
candidate, but location of heating is
poorly constrained.
Source:
What is the water source for
Enceladus’ eruptions?
A salty ocean is connected to the surface,
but exposing ocean water to space
raises energy balance problems.
Plumbing system:
How can conduits between ocean
and surface avoid freezing shut?
(w/Allan Rubin, Princeton U.)
Probing tectonics
Volcanism
Enceladus
Earth
Tectonic mode:
Seismicity
Geology Geomorphology
Geology
10 km
Gravity
?
Open questions
Engine:
What powers Enceladus
volcanism?
Tidal heating is the only plausible
candidate, but location of heating is
poorly constrained.
Source:
What is the water source for
Enceladus’ eruptions?
A salty ocean is connected to the surface,
but exposing ocean water to space
raises energy balance problems
Plumbing system:
How can conduits between ocean
and surface avoid freezing shut?
(w/ Allan Rubin)
(4.6±0.2) GW excess thermal emission from surface fractures
(~10 KW/m length; all four tiger stripes erupt as “curtains”)
South polar projection
to Saturn
Porco et al. Astron. J. 2014
Spencer & Nimmo AREPS 2013
Hotspots up to 200K
No liquid water at surface
Latent heat represented by plumes < 1 GW
Enceladus is small for active interior-driven volcanism
endogenic
volcanism
Only tidal heating can provide the required heat
Heat source
Tidal
Radiogenic
Chemical
Maximum equilibrium (constant-eccentricity)
heat production: 1.1 GW
(due to 2:1 Enceladus:Dione resonance)
Possible
<0.32 GW assuming CI chondritic composition
Serpentinization: <0.1 GW
(assuming complete serpentinization; 2.4 x 105 J/kg srp)
Secular
cooling
Steady release of accretional heat <0.1 GW
Al-26 heating may be significant early on
Joule
heating
<0.05 GW (Hand et al. JGR 2011)
Recent
impact
Improbable
Falls short
(some also
fail the
“Mimas test”)
Long-lived tidally-powered volcanism requires both resonant
boosting of eccentricity and a dissipative interior
Saturn’s apparent movement
viewed from Enceladus (e = 0.005)
Tides on Enceladus raised by Saturn (1.3 day period)
Murray &
Dermott, 2000
Dione:Enceladus
Period:
1:2
Mass:
10:1
Distance from Saturn (Saturn radii) 
Resonances appear insufficient to sustain current eruptions over Gyr
Meyer & Wisdom
Icarus 2007
Tides raised by the moons on Saturn cause
Saturn to spin slower and moons to move outwards
Dione
2:1
Tethys
Enceladus
Mimas
Large moons tidally disrupted
(rings of Saturn)
Fastest possible Mimas
recession rate(?)
<1.1 GW
Is there a limit-cycle workaround for the weakness of
steady-state tidal heating?
Location of dissipation
unclear; ice shell or
ocean?
Spencer & Nimmo 2013
Unfortunately, detailed simulations show only very-small-amplitude limit cycles.
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
Engine:
What powers Enceladus
volcanism?
Tidal heating is the only plausible
candidate – very likely intermittent.
Tidal heating is the only
plausible candidate
Source:
What is the water source for
Enceladus’ eruptions?
Sodium and nano-silica tell us that
the source is a subsurface ocean,
not clathrates or sublimation
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)
Turbulent dissipation within tiger
stripes can sustain the phase
curve of Enceladus’ eruptions.
Tidal energy budget rules out steady volcanism over Gyr
Considering timescales < 106 yr simplifies the problem
Crater retention age of South Polar Terrain < 0.5 x 106 yr
Ocean
freezes:
Orbit
circularizes:
Regional
ocean spreads:
Open questions
Engine:
What powers Enceladus
volcanism?
Tidal heating is the only plausible
candidate – very likely intermittent - but
location of heating is poorly constrained.
Source:
What is the water source for
Enceladus’ eruptions?
Sodium and nano-silica tell us that
the source is a subsurface ocean,
not clathrates or sublimation
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)
Turbulent dissipation within tiger
stripes can sustain the phase
curve of Enceladus’ eruptions.
Sourcing water eruptions from an ocean is challenging
Crawford & Stevenson
Icarus 1988
Can clathrate decomposition within the ice shell explain the plumes?
Porco (liquid water)
Kieffer (clathrates)
>99% of erupted ice grains are salt-rich, ruling out “dry” water source
18 km/s (Cassini @Enceladus)
6 km/s (Stardust mission)
Dust track (mm long)
Postberg et al.
Nature 2011
aerogel
(Active?) hydrothermal vents required to explain
nano-silica observations
Open questions
Engine:
What powers Enceladus
volcanism?
Tidal heating is the only plausible
candidate – very likely intermittent.
Source:
What is the water source for
Enceladus’ eruptions?
Sodium and nano-silica tell us that
the source is a subsurface ocean,
not clathrates or sublimation
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)
Turbulent dissipation within tiger
stripes can sustain the phase
curve of Enceladus’ eruptions.
Key constraint #1: energy balance at water table
z
(30±5) km
x
water
table
Open questions
Engine:
What powers Enceladus
volcanism?
Tidal heating is the only plausible
candidate – very likely intermittent.
Source:
What is the water source for
Enceladus’ eruptions?
Sodium and nano-silica tell us that
the source is a subsurface ocean,
not clathrates or sublimation
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.)
Turbulent dissipation within tiger
stripes can sustain the phase
curve of Enceladus’ eruptions.
NASA/JPL
Key constraint #2: Eruptions are modulated by tides
larger plume grains
smaller plume grains
5x
phase
shift 55°
?
base level
(period: 1.3 days)
How to find tidal stresses at volcanic vents
Enceladus period = 1.3 days
Enceladus orbital eccentricity = 0.0047
Tidal stress amplitude ~ 1 bar
Closest distance to Saturn
time-averaged shape
Furthest distance from Saturn
=
tiger
stripes
eccentricity tide only
thin-shell approximation
k2 appropriate
for global ocean
Crack models are falsified by eruptions at periapse
All existing models fail!
Model C
Model B
Porco et al. Astron. J. 2014
Model A
Should
not
erupt
Should
erupt
Should
not
erupt
Should
erupt
Should
not
erupt
Should
erupt
An alternative model: Melted-back slot
Compression
supersonic plume
Tension
supersonic plume
water level rises
water level falls
z
ocean
x
Attractive properties:
• Slot width lags tidal cycle
• Slot does not close
• Turbulent dissipation heats slot
• Pumping disrupts ice formation
ocean
Tiger stripes can be approximated as straight, parallel and in-phase
Paul Schenk / LPI / USRA
Slots interact elastically: used
boundary-element code
By Allan Rubin
Daily cycle of tidal flow of water in slots
Wide slots track the tidal forcing
1.3 days
Narrow slots lag the tidal forcing by 8 hours
1.3 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
Changing
slot width
Zero-stress slot half-width (m)
Long-lived water-filled slots have tectonic consequences
COLD, STRONG
ICE
z
x
WARM, WEAK
ICE
Tectonic feedback between subsidence and meltback
buffers South Polar terrain power to 5 GW
Initial temperature
profile (conductive)
Steady-state
temperature profile
(Subsidence-perturbed)
cools the ice shell
Initial ice inflow rate
Steady-state
ice inflow rate
Highly simplified ice flow:
assuming
Slot model explains and links sustainability of
volcanism on 10 yr through 106 yr timescales
Gravity
constraint
Next: testing the slot model
Can slots of the
required widths form?
Height
How does along-crack
branching affect power?
How does changing
water level and conduit
width affect flow
in vent?
What testable consequences
would long-lived tiger stripes
have for surface tectonics?
Summary
Engine:
What powers Enceladus
volcanism?
Tidal heating is the only
plausible candidate
Tidal heating is the only plausible
candidate, but location of heating is
poorly constrained.
Source:
What is the water source for
Enceladus’ eruptions?
Sodium and nano-silica tell us that
the source is a subsurface ocean,
not clathrates or sublimation
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.)
Turbulent dissipation within tiger
stripes may explain the phase
curve of Enceladus’ eruptions.
ENERGY SOURCE
What powers Enceladus
volcanism?
Tidal heating is the only plausible
candidate, the location of
heating is poorly constrained,
and eruptions must be intermittent
on >106 yr timescales.
WATER SOURCE
Sodium and nano-silica require a
What is the water source for subsurface ocean source - not
Enceladus’ eruptions?
clathrates or sublimation.
CONNECTION
How can conduits between
ocean and surface avoid
freezing shut?
Turbulent dissipation within tiger
stripes may explain the phase
curve of Enceladus’ eruptions.
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 Schenk / LPI
Summary
Supplementary slides
Tectonic feedback between subsidence and meltback
buffers the South Polar terrain to 5 GW
Equilibrium
power output
Ice shell thickness:
5 km
100 km
Gravity constraint
of shell thickness:
Open questions
Engine:
What powers Enceladus
volcanism?
Tidal heating is the only
plausible candidate
Tidal heating is the only plausible
candidate, but location of heating is
poorly constrained.
Source:
What is the water source for
Enceladus’ eruptions?
Sodium and nano-silica tell us that
the source is a subsurface ocean,
not clathrates or sublimation
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)
Turbulent dissipation within tiger
stripes can sustain the phase
curve of Enceladus’ eruptions.
Meyer & Wisdom, Icarus, 2008
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-discontinuity
code by Allan Rubin
Pipe model and slot model
Option: Array of unresolved pipes
Area changes by 10^-4
Option: Each tiger stripe is one slot
Area changes by order unity under 1-bar
pressure cycle
Apertures < 10 m (not to scale)
Daily tidal cycle of water in tiger stripes
Change
in slot
width
Width change
due to water
flow from
ocean into slot
Width change
due to daily
tidal forces
from Saturn
Width change due
to back-force
from elastic
distortion of shell
Coefficients
that depend on
detailed tiger
stripe geometry
Extensional normal stress (Pa)
1 tiger stripe
inferred from gravity
Rudolph & Manga,
Icarus 2009
Models of tidal modulation
supersonic
ascent time
Nimmo et al., Astron. J. 2014
Zolotov, GRL 2007
Salt composition matches expectations for hydrothermal leaching
Age of interaction unconstrained