Sustaining cryovolcanic eruptions on Enceladus Edwin Kite (University of Chicago)

~ 200 kg/s water ice erupting from 250 km-diameter 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 = subsurface ocean Postberg et al. 2009, 2011; Hu et al Iess et al. 2014; Porco et al. 2014; Waite et al. 2009; Nimmo & Spencer 2013

(4.4±0.2) GW excess thermal emission from surface fractures ( “tiger stripes”) Spencer & Nimmo AREPS 2013 Porco et al. Astron. J South polar view of Enceladus Hotspots up to 200K No liquid water at surface Latent heat represented by plumes < 1 GW to Saturn Close-up of one “tiger stripe”

water source surface habitability astrobiology Today: How are curtain eruptions sustained on yr timescales? ice shell Parkinson et al Tsou et al McKay et al Not addressed today: How is the ocean sustained on 10 7 – yr timescales? e.g. Meyer & Wisdom 2007, Tobie et al. 2008, O’Neill & Nimmo 2008, Behounkova et al Kite & Rubin, in prep. Prevailing view: conduit = crack e.g. Hurford et al. 2007, Nimmo et al. 2007, Olgin et al. 2011, Smith-Konter & Pappalardo 2008 Water ultimately sourced from a sub-ice ocean e.g. Postberg et al. 2009, 2011; Hu et al. 2015; Iess et al. 2014; Porco et al. 2014; Waite et al. 2009; Nimmo & Spencer 2013; Zolotov, 2007.

Probing tectonics Volcanism Geology Tectonic mode: Seismicity Gravity Earth

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

? phase shift 55° (Q ~ 1) Challenge for the prevailing view: Understanding tidal modulation of eruptions (all four tiger stripes erupt as “curtains” throughout orbit; Spitale et al. Nature 2015) smaller plume grains larger plume grains (period: 1.3 days) Crack models are hard to reconcile with curtain eruptions maintained throughout orbit NASA/JPL/Hedmanet al Hurford LPSC 2015

Challenge for the prevailing view: energy balance at water table x z water table (35±5) km Iess et al Kite & Rubin, in prep. (4.4±0.2) GW Howett et al. 2014

Alternative: 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 Kite & Rubin, in prep.

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 tides from Saturn Width change due to back-force from ice shell elasticity Slots interact elastically: use boundary- element method Kite & Rubin, in prep. Tiger stripes can be approximated as straight, parallel and in-phase

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 5m wide at zero stress ½ m wide at zero stress Kite & Rubin, in prep.

T urbulent dissipation of tidally-pumped vertical flow inside tiger stripes explains power output, phase lag and sustainability of the eruptions Colored disks = diurnal width ratio Increasing slot width Observed power Wide slots Narrow slots Observed phase lag Changing “active slot” length Kite & Rubin, in prep. Best-fit: 1-2 m wide

Long-lived water-filled slots have tectonic consequences COLD, STRONG ICE WARM, WEAK ICE x z Kite & Rubin, in prep.

Long-lived water-filled slots have tectonic consequences COLD, STRONG ICE WARM, WEAK ICE x z Kite & Rubin, in prep. Tectonic feedback between subsidence and melt-back buffers Enceladus output to 1700 kg/s × L vap = 5 GW (no tuning, no free parameters)

Summary: Slot model explains and links sustainability of volcanism on 10 yr yr timescales Gravity (Iess et al. 2014) Observed power Predicted Myr-average power output matches observed phase lag slot aperture varies by >1.5 No tuning No free parameters Kite & Rubin, in prep. melting + vaporization of ice flowing from ice shell into base of slots. cooling of the ice shell due to subsidence of cold ice

Testable predictions 1.Endogenic thermal emission should be absent between tiger stripes. 2.No correlation between emission and local tiger-stripe orientation 3.Smooth distribution of thermal emission. Kite & Rubin, in prep.

Limitations and caveats Initiation of ocean-to-surface conduits on ice moons remains hard to explain (Crawford & Stevenson 1988). – may be related to ice-shell disruption at high orbital eccentricity: such disruption could have created partially-water-filled conduits with a wide variety of apertures, and evaporative losses caused by tiger stripe activity would ensure that only the most dissipative conduits (width 1-2m) endure to the present day. Slot stability is less of a concern. – along-slot stirring is rapid relative to freeze-shut and flow-shut.

Conclusions. Paul Schenk / LPI Turbulent dissipation may explain Enceladus’ phase curve and the maintenance of fissure eruptions over geological timescales. With long-lived slots, Myr-average power is buffered by tectonic feedback to ~5 GW, equal to observed power (suggesting long-term stability). - Geysers are not a passive tracer of tectonics – they can drive tectonics. Testing habitability on Enceladus (or Europa) ultimately requires access to ocean materials - easier if turbulent dissipation maintains active fissures open for ≫ Kyr. We thank Terry Hurford, Karl Mitchell, Alyssa Rhoden, Joseph Spitale, Robert Tyler, and Steve Vance.

Supplementary slides

Porco et al. AJ, 2014

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:

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 Kite & Rubin, in prep.

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

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

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

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

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 hard to reconcile with curtain eruptions at periapse

Paul Schenk / LPI / USRA 130 km 35 km Slots interact elastically: used displacement-discontinuity method Kite & Rubin, in prep. Crouch & Starfield, 1983 Rubin & Pollard, 1988

(4.4±0.2) GW excess thermal emission from surface fractures Spencer & Nimmo AREPS 2013 Spitale et al., accepted Porco et al. Astron. J Abramov & Spencer 2009 South polar projection Hotspots up to 200K No liquid water at surface Latent heat represented by plumes < 1 GW to Saturn 130 km Howett et al. 2014

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