1. Sustaining volcanism on Enceladus
Edwin Kite
Princeton University
November 14, 2014
Thank you X for that introduction! Hello everybody, I’m very happy to be here!
You’re seeing water-ice plumes rising into the polar night of Enceladus; as they reach a few miles above the surface they catch the sunlight. These eruptions have been sustained for at least the 9 years that the Cassini orbiter has been monitoring them.

2. ~200 kg/s water ice erupting from 250 km-diameter Enceladus sustains a >102 yr old ring around Saturn
And they must have been erupting for longer than that! Zooming out so that Enceladus is a tiny black disk we see that some plume material escapes Enceladus to forms a ring around Saturn.
Streamers indicate magnetospheric interactions
Micron-size water ice grains are shredded in an ion beam
Sputtered on decadal timescales into small grains that drift quickly out of orbit
So must be replenished, so the ring is an activity marker

3. Cryo-volcanism on Enceladus has deep tectonic roots
ancient,
cratered
South polar terrain:
no craters
And they must have been erupting for longer than that! It’s possible for Active geysers to be powered by sunlight – geysers on Mars and on Netprune’s moon Triton– but that’s not the case on Enceladus; there’s a one-to-one correspondence between the eruptions and tectonic resurfacing, and the eruptions are unaffected by local season. The whole south polar region is tectonized and geologically young, and the plumes only come from four “tiger stripes”. Suggests a Single underlying mechanism or tectonic mode accounting for both the volcanism and the resurfacing. And here is one suggestion. The surface is nearly pure water ice but the moon’s density requires a rocky core, so one possibility is convective resurfacing above an ocean sandwiched between the rock core and the ice shell, opening cracks for volcanism and causing stresses that fold and fracture the region.
4 continuously-
active “tiger stripes”
Density = 1.6 g/cc

4. Understanding the sustainability of water eruptions on Enceladus is important
surface
astrobiology
intermittent vs. steady?
ice
shell
duty cycle? timescale?
water source
habitability
So Enceladus is an exceptionally active moon, so perhaps Are we looking at a special time?
And we can think of this in terms of the water source – the cryo-magma chamber - and in terms of the volcanic conduit. ….
We care ….
I think that’s the most important, but there are others …
usually frozen vs.
sustained melt pool?

5. Understanding the sustainability of water eruptions on Enceladus is important
Mass loss and vent temperature forcings for tectonics
Analog for Europa (Jupiter moon) and Miranda (Uranus moon)
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
Sets viability of a plume-sampling mission
p (origin of life | long-lived ocean) = ?

6. 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.
Given the importance … it’s surprising how basic the open questions about the lifetime of volcanism on Enceladus are. Today I’m going to talk about three …
Plumbing system:
How can conduits between ocean and surface avoid freezing shut? (w/ Allan Rubin)

7. ? Probing tectonics Earth Enceladus Volcanism Tectonic mode:
Seismicity ?
Geology Geomorphology
Geology
Driven to plate tectonics by the evidence; you could give someone a hundred years in a room with the rubber handbook a whiteboard a pocket calculator and they’d never dream up plate tectonics. At it’s heart it’s an empirical kinematic synthesis, and this successful idea is refined and strengthened by modeling. Comparably many modes of planet-scale tectonics as recently-resurfaced worlds we have visited;
we go to the Moon and we add magma oceans to our vocabulary; we go to Io and are reminded about vertical heat-pipe tectonics as a mode of planet cooling. and this makes us look at Archean earth with new eyes. –
On Enceladus we have less to go on; there’s no seismology. The geologic mapping of the South Polar region is a Rorschach test, and all our gravity comes from just three close flybys
- So we have less to go on than for Earth. But we have two key advantages with the volcanism: we can sample plume composition directly, thanks to the mass spectrometers on Cassini guided by the superb and/or fearless navigators at JPL, and the volcanism has a regular response to a Saturn’s tides, which “ping” the system for free.
10 km
Gravity

8. 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)

9. (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
South polar projection, direction to Saturn is *down* because the moon is tidally locked in a 1:1 spin orbit resonance.
Close-up view of tiger stripes [Dimensions].
Plumes are only a signpost
Physical interpretation is that water vapor is condensing on the walls of the crack in the shallow subsurface
Of course this just pushes the energy supply problem one level down.
to Saturn
Spencer & Nimmo AREPS 2013
Porco et al. Astron. J. 2014
Hotspots up to 200K
No liquid water at surface
Latent heat represented by plumes < 1 GW

10. Enceladus is small for active interior-driven volcanism
Mimas – same radius, roughly the same composition
endogenic
volcanism

11. Only tidal heating can provide the required heat
Heat source
Tidal
Maximum equilibrium (constant-eccentricity)
heat production: 1.1 GW
(due to 2:1 Enceladus:Dione resonance)
Possible
Radiogenic
<0.32 GW assuming CI chondritic composition
Chemical
Serpentinization: <0.1 GW
(assuming complete serpentinization; 2.4 x 105 J/kg srp)
Falls short
(some also
fail the
“Mimas test”)
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

12. 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
Sync lock, but eccentric orbit. 1 day period, tidal stresses are of order 1 bar.
Increase the eccentricity and you increase the amplitude of the torques and thus the potential for tidal heating
Need a dissipative interior as well.
Tidal dissipation reduces the eccentricity, about 1 Myr for Enceladus at observed power output.
Need something to boost the eccentricity, and we have it!
But ultimately the eccentricity of both moons will be reduced
Dione:Enceladus
Period: :2
Mass: 10:1

13. Resonances appear insufficient to sustain current eruptions over Gyr
Meyer & Wisdom
Icarus 2007
Tides on Saturn raised by moons cause
Saturn to spin slower and moons to move outwards
<1.1 GW
Dione
Distance from Saturn (Saturn radii) 
2:1
Tethys
Enceladus
Mimas
So far we’ve been talking about the tides raised by the planet on the moons, but the moons also raise tides on the planet
This is not a completely cut-and-dried argument; there is an “out.”
Large moons tidally disrupted
(rings of Saturn)
Fastest possible Mimas
recession rate(?)

14. Is there a limit-cycle workaround for the weakness of steady-state tidal heating?
Location of dissipation
unclear; ice shell or
ocean?
Only dealing with a factor of a few
Spencer & Nimmo 2013
Unfortunately, detailed simulations show only very-small-amplitude limit cycles.

15. Can intermittent ice-shell overturn act as a thermal capacitor?
Similar to Archean / Early Proterozoic Earth?
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

16. 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
Plumbing system:
How can conduits between ocean and surface avoid freezing shut? (w/ Allan Rubin)
Ocean water is exposed to space,
raising energy balance problems
Turbulent dissipation within tiger stripes can sustain the phase curve of Enceladus’ eruptions.

17. 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:

18. 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
Plumbing system:
How can conduits between ocean and surface avoid freezing shut? (w/ Allan Rubin)
Ocean water is exposed to space,
raising energy balance problems
Turbulent dissipation within tiger stripes can sustain the phase curve of Enceladus’ eruptions.

19. Sourcing water eruptions from an ocean is challenging
Crawford & Stevenson
Icarus 1988
Water is denser than ice!
Crack must form
Water must rise to surface without freezing

20. Can clathrate decomposition within the ice shell explain the plumes?
Porco (liquid water)
Kieffer (clathrates)

21. >99% of erupted ice grains are salt-rich, ruling out “dry” water source
18 km/s
Enceladus
6 km/s (Stardust mission)
Postberg et al.
Nature 2011
Dust track (mm long)
Salt composition matches expectations for hydrothermal water-rock interaction
aerogel
(Active?) hydrothermal vents required to explain nano-silica observations

22. 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
Plumbing system:
How can conduits between ocean and surface avoid freezing shut? (w/ Allan Rubin)
Ocean water is exposed to space,
raising energy balance problems
Turbulent dissipation within tiger stripes can sustain the phase curve of Enceladus’ eruptions.

23. Key constraint #1: energy balance at water table
~40 km
Convection is insufficient!
Latent heat would swiftly clog cracks! (200 kg/s out of vents, 2000 kg/s for observed emission, 20,000 kg/s for latent heat!  z x

24. 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
Plumbing system:
How can conduits between ocean and surface avoid freezing shut? (w/ Allan Rubin)
Ocean water is exposed to space,
raising energy balance problems
Turbulent dissipation within tiger stripes can sustain the phase curve of Enceladus’ eruptions.

25. Key constraint #2: Eruptions are modulated by tides
NASA
larger plume grains
smaller plume grains
5 x
phase
shift 55° ?
Stacking observations from
base level
(period: 1.3 days)

26. How to find tidal stresses at volcanic vents
Enceladus period = 1.3 days
Enceladus orbital eccentricity =
Tidal stress amplitude ~ 1 bar
Closest distance to Saturn
Furthest distance from Saturn
tiger
stripes =
time-averaged shape
eccentricity tide only
thin-shell approximation
k2 appropriate
for global ocean

27. All existing models fail!
Model B
Model C
Model A
Porco et al. Astron. J. 2014
Should
not
erupt
Should
not
erupt
Should
not
erupt
Should
erupt
Should
erupt
Should
erupt

28. Crack models are falsified by eruptions at periapse

29. An alternative model: Melted-back slot
Compression
Tension
supersonic plume
supersonic plume
water level rises
water level falls z
ocean
ocean x
Attractive properties:
Slot width lags tidal cycle
Slot does not close
Turbulent dissipation heats slot
Pumping disrupts ice formation

30. Pipe model and slot model
Option: Each tiger stripe is one slot
Area changes by order unity under 1-bar
pressure cycle
Option: Array of unresolved pipes
Area changes by 10^-4
35 km
130 km
Apertures < 10 m (not to scale)

31. Alternative: a slot model
Paul Schenk / LPI / USRA
35 km
130 km

32. Extensional normal stress (Pa)
1 tiger stripe

33. Non-interacting slots are overpowered
m/s
Width ratio
Changing slot width
Each dot corresponds to a different solution.
Zero-stress slot width (m)

34. Interacting slots are not overpowered
Width ratio
Changing slot width
Displacement-discontinuity
code by Allan Rubin

35. Next: testing the slot model
Can slots of the required widths form?
How does along-crack branching affect power?
Height
How does changing
water level and conduit width affect flow
in vent?
What testable consequences would long-lived tiger stripes have for surface tectonics?

36. Summary Engine: What powers Enceladus volcanism?
Tidal heating is the only plausible candidate, but location of heating is
poorly constrained.
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
Plumbing:
How can conduits between ocean and surface avoid freezing shut? (w/ Allan Rubin)
Ocean water is exposed to space,
raising energy balance problems
Turbulent dissipation within tiger stripes may explain the phase curve of Enceladus’ eruptions.

37. Summary Paul Schenk / LPI ENERGY SOURCE
Tidal heating is the only plausible candidate, the location of
heating is poorly constrained,
and eruptions must be intermittent
on >106 yr timescales.
Paul Schenk / LPI
What powers Enceladus volcanism?
WATER SOURCE
Sodium and nano-silica require a subsurface ocean source - not clathrates or sublimation.
What is the water source for
Enceladus’ eruptions?
CONNECTION
Turbulent dissipation within tiger stripes may explain the phase curve of Enceladus’ eruptions.
How can conduits between ocean and surface avoid freezing shut?
Thanks to Allan Rubin for displacement-discontinuity code
and numerous discussions, to Robert Tyler, Terry Hurford, Alyssa Rhoden, Karl Mitchell for additional discussions, and to Princeton University Geo. & Astro. Departments for funding.

38. Supplementary slides

39. Open questions Engine: What powers Enceladus volcanism?
Tidal heating is the only plausible candidate, but location of heating is
poorly constrained.
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
Plumbing system:
How can conduits between ocean and surface avoid freezing shut? (w/ Allan Rubin)
Ocean water is exposed to space,
raising energy balance problems
Turbulent dissipation within tiger stripes can sustain the phase curve of Enceladus’ eruptions.

41. History of the Enceladus:Dione resonance
Meyer & Wisdom, Icarus, 2008

42. Clathrate-source and solid-sublimation explanations face insuperable challenges
Salt
Nano-silica
I:G ratio
Thermal
This difficulty of explaining through-shell cracks led to an understandable search for an intra-shell source for the water vapor and ice eruptions … but these all fail!

43. inferred from gravity
Rudolph & Manga,
Icarus 2009

44. Models of tidal modulation
supersonic
ascent time
Nimmo et al., Astron. J. 2014

45. Salt composition matches expectations for hydrothermal leaching
Age of interaction unconstrained
Zolotov, GRL 2007