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.

~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

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

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?

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) = ?

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)

? 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

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 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

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

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

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: 1:2 Mass: 10:1

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(?)

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.

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

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.

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 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.

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

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) 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

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.

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

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.

Key constraint #2: Eruptions are modulated by tides NASA larger plume grains smaller plume grains 5 x phase shift 55° ? Stacking observations from 2005-2012 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 Furthest distance from Saturn tiger stripes = time-averaged shape eccentricity tide only thin-shell approximation k2 appropriate for global ocean

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

Crack models are falsified by eruptions at periapse

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

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)

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

Extensional normal stress (Pa) 1 tiger stripe

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

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

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?

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.

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.

Supplementary slides

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.

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

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!

inferred from gravity Rudolph & Manga, Icarus 2009

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

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