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 >10 2 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 ice shell

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 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.) Tidal heating is the only plausible candidate, but location of heating is poorly constrained. Engine: What powers Enceladus volcanism?

Probing tectonics Volcanism Geology ? ? Tectonic mode: Seismicity Geology Geomorphology Gravity EarthEnceladus 10 km

Open questions 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) Tidal heating is the only plausible candidate, but location of heating is poorly 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 2013 Porco et al. Astron. J South polar projection Hotspots up to 200K No liquid water at surface Latent heat represented by plumes < 1 GW to Saturn 130 km

Enceladus is small for active interior-driven volcanism endogenic volcanism

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 GW Al-26 heating may be significant early on Serpentinization: <0.1 GW (assuming complete serpentinization; 2.4 x 10 5 J/kg srp) <0.32 GW assuming CI chondritic composition Only tidal heating can provide the required heat Falls short (some also fail the “Mimas test”) Heat source Possible Secular cooling 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:Enceladus Period: 1:2 Mass: 10:1 Saturn’s apparent movement viewed from Enceladus (e = 0.005) Tides on Enceladus raised by Saturn (1.3 day period)

Enceladus Meyer & Wisdom Icarus 2007 Dione Resonances appear insufficient to sustain current eruptions over Gyr Distance from Saturn (Saturn radii)  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 or ocean?

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 for Enceladus’ 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 Gyr Considering timescales < 10 6 yr simplifies the problem Ocean freezes: Orbit circularizes: Regional ocean spreads: Crater retention age of South Polar Terrain < 0.5 x 10 6 yr

Open questions Source: What is the water source for Enceladus’ 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 & Stevenson Icarus 1988

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

Postberg et al. Nature km/s 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 for Enceladus’ 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 water table (30±5) km

Open questions Source: What is the water source for Enceladus’ 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 phase shift 55° Key constraint #2: Eruptions are modulated by tides smaller plume grains larger plume grains (period: 1.3 days) base level NASA/JPL

How to find tidal stresses at volcanic vents Closest distance to SaturnFurthest distance from Saturn Enceladus period = 1.3 days Enceladus orbital eccentricity = Tidal stress amplitude ~ 1 bar tiger stripes = time-averaged shape eccentricity tide only thin-shell approximation k 2 appropriate for global ocean Looking down on South Pole

Crack models are falsified by eruptions at periapse

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

An alternative model: Melted-back slot Compression Tension ocean supersonic plume water level falls water 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-phase Paul Schenk / LPI / USRA 130 km 35 km Slots interact elastically: used boundary-element code By Allan Rubin

Daily cycle of tidal flow of water in slots Wide slots track tidal forcingNarrow slots lag tidal forcing by 8 hours width change (m) volume change (fractional) volume change (fractional) width change (m) 1.3 days forcing

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 width

Long-lived water-filled slots have tectonic consequences COLD, STRONG ICE WARM, WEAK ICE x z

Highly simplified ice flow: assuming cools the ice shell Initial temperature profile (conductive) Steady-state temperature 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 10 6 yr timescales Gravity constraint Observed power Inferred power Predicted Myr-average power output matches observed phase lag slot aperture varies by >1.5

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 changing water level and conduit width affect flow in vent? Height

Summary Source: What is the water source for Enceladus’ 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 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 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: Paul Schenk / LPI What powers Enceladus volcanism? What is the water source for Enceladus’ 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 intermittent on >10 6 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 constraint of shell thickness:

Open questions Source: What is the water source for Enceladus’ 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 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 Tidal heating is the only plausible candidate 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 Changing slot width

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) 130 km 35 km

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

1 tiger stripe Extensional normal stress (Pa)

Rudolph & Manga, Icarus 2009 inferred from gravity

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

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