Plumes on Enceladus: The Ocean Debate Nick Schneider, LASP, U. Colorado

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1 Plumes on Enceladus: The Ocean Debate Nick Schneider, LASP, U. Colorado Based on No sodium in the vapour plumes of Enceladus, by Schneider, Burger, Schaller, Brown, Johnson, Kargel, Dougherty & Achilleos, Nature 459, (25 June 2009), doi: /nature08070 Art by Michael Carroll

2 Credit: NASA / JPL / SSI / mosaic by Emily Lakdawalla

3 Credit: NASA / JPL / SSI / animation by Gordan Ugarkovic

4 Outline for the Talk I. How Salty is the Material Escaping Enceladus? II. How Well Do Plume Models Explain Ejecta Composition?

5 Key Observations of Enceladus Plumes Mostly escaping Mostly gaseous, ~0.5% particles by mass Escaping gases: Mostly CO 2, NH 3 Miscellaneous organics Sodium <10 ppm Escaping particles: mostly low-salt small fraction salty (1% salinity)

6 Searching for Minor Species Why sodium? Cosmically abundant 2nd, 3rd most common ejecta from Europa, Io Extraordinarily detectable - in atomic form Tracer of refractories Potential astrobiological significance Associated with oceans or aquifers A major electrolyte in terrestrial biology

7 Ocean Modeling: +rock +time=salt Figure from Postberg et al. Zolotov et al. 07

8 Internal Structure of Enceladus: Ocean? Plume composition is a strong constraint on interior structure and chemistry Salt-rich particles strongly suggest presence of liquid, perhaps from a salty ocean, near the source (Postberg et al. 2009) Non--soluble gases (e.g. N 2 ) may indicate clathrates (Keiffer et al. 2006) Where do the organics come from?

9 Sodium in the Solar System Io, Schneider et al. Europa, Brown et al. Moon, Potter et al. Mercury, Wilson et al.

10 Conclusive Proof of Sodium s Unsurpassed Visibility

11 Sodium - the most easily detected element? Io Na escape ~20kg/sec 1-few% mixing ratio ~2-hour lifetime ~200kiloRayeighs peak Europa Na escape ~0.1kg/sec ~1% mixing ratio Few-hour lifetime ~10kiloRayleighs peak [Brown & Hill 96]

12 Minor Species Io Europa Enceladus Escape rate ~10 28 SO 2 /sec ~5x10 26 O/sec ~10 28 H 2 O/sec Sodium escape Sodium fraction ~10 26 Na/sec 20 kg/sec ~3x10 24 Na/sec ~0.1 kg/sec 1-few % ~<1%?? Sodium origin Volcanic NaCl? Oceans???

13 The Search for Enceladus Sodium Keck and Anglo-Australian Telescope Observations Deep searches in the E-ring and at Enceladus Feb & March 2007 Spatially-resolved spectra Resolving power 50, ,000 Enceladus in slit E-ring in slit

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15 Enceladus Sodium model Matt Burger, GSFC Sources Considered Atomic Na in plume Sodium-bearing molecules Na released from E-ring Matt Burger, GSFC Rings edge Enceladus orbit Dominant processes: Photoionization Photodissociation Electron impact ionization Resonant scattering Radiation Impact on rings

16 Molecule/ Particle escape Model mixing ratio=1x10-5 Na/H 2 O Actual upper limit=7x10-6 Na/H 2 O (=7ppm)

17 Atomic escape Model mixing ratio=2x10-6 Na/ H 2 O Actual upper limit=4x10-7 Na/ H 2 O (=0.4ppm)

18 Observational Conclusions The sodium content of the vapor plumes is <7ppm if sodium is released in molecular or particulate form, and <0.4ppm if released in atomic form Enceladus has the lowest sodium escape rate among small bodies with escaping atmospheres These upper limits are consistent with the small fraction of salty-bearing particles observed in the E-ring by Cassini s CDA experiment.

19 II. How Well Do Plume Models Explain Ejecta Composition?

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21 Plume Vent Models Near-Surface Geysers Shallow Vents Tectonic Melt Ice Sublimation Deep Misty Caverns Violent boiling Brine pocket Low-salinity melt Warm, fractured ice

22 1. Near-Surface Geysers Near-Surface Geysers Violent boiling Shallow Vents Tectonic Melt Ice Sublimation The boiling liquid can produce a cloud of gas and ice if some of the liquid is carried along with the vapor and freezes as it expands out of the vent [Porco et al. 2006] Source of : ocean? Melt? Brine pocket Deep Misty Caverns Low-salinity melt Warm, fractured ice

23 1. Near-Surface Geysers Near-Surface Geysers Shallow Vents Strengths Tectonic Melt Terrestrial analog physics well stood Ice Sublimation Deep Misty Caverns Weaknesses Violent boiling Sodium vapor and/or very salty particles would be easily observed Warm, fractured ice Brine pocket Plumes cannot come from complete vaporization of ocean including its dissolved salts, as might occur from Low-salinity a near-surface geyser boiling melt into a vacuum.

24 2. Shallow Vents (-filled cracks) Near-Surface Geysers Violent boiling Shallow Vents Tectonic Melt Ice Sublimation Deep Misty Caverns Evaporation occurs near equilibrium at the interface of small vents Particles can come from the liquid or spalled from the vent walls Preferential evaporation of may enrich Brine non-volatiles in the vent pocket Low-salinity melt Warm, fractured ice

25 2. Shallow Vents (-filled cracks) Near-Surface Geysers Shallow Vents Tectonic Melt Strengths Ice Sublimation Deep Misty Caverns Could explain vapor & salty particles Violent boiling Weaknesses Preferential evaporation of H 2 O in the vent plumping may lead to salt clogging Low-salinity melt Warm, fractured ice Brine pocket choking off the vent? ejecting too-salty particles? Difficult to provide heat flux

26 3. Tectonic Melt Near-Surface Geysers Violent boiling Shallow Vents Tectonic Melt Ice Sublimation Deep Misty Caverns Heating along fractures caused by tidal flexing (Nimmo et al. 2007, Hurford, DPS 09) Melt could collect in cracks Preferential evaporation of H Brine 2 O could pocket enrich vent to ~1% salinity Low-salinity melt Warm, fractured ice

27 3. Tectonic Melt Near-Surface Geysers Violent boiling Shallow Vents Tectonic Melt Low-salinity melt Ice Sublimation Strengths Warm, fractured ice Brine pocket Deep Misty Caverns Heating mechanism common to most scenarios Explains vapor, salty particles, and potentially other gases Weaknesses Not modeled well enough to evaluate

28 4. Ice & Clathrate Sublimation Near-Surface Geysers Violent boiling Shallow Vents Tectonic Melt Icy crust is cracked and warmed by tectonic activity Sublimation and clathrate decomposition release cometary gases Particles are entrained in the flow Kieffer et al., 2006 Ice Sublimation Brine pocket Deep Misty Caverns Low-salinity melt Warm, fractured ice

29 4. Ice & Clathrate Sublimation Near-Surface Geysers Strengths: Shallow Vents Tectonic Melt Naturally explains low-sodium vapor, comet-like gases (Waite et al. 2009) Ice Sublimation Deep Misty Caverns Weaknesses: Source Violent of salt-bearing particles boiling is an ad hoc assumption and may not match observations What composition & conditions match INMS results? Low-salinity melt Cold temperatures may not yield high enough vent velocity Warm, fractured ice Brine pocket

30 5. Deep Misty Caverns Near-Surface Geysers Cavern actually within crust Violent boiling above ocean Shallow Evaporation Tectonic occurs at the Vents Melt interface of a large liquid/ vapor reservoir many km below the surface. Ice Sublimation Equilibrium evaporation explains low-sodium vapor Flow out the vent is slow, like a small leak in a balloon Droplets entrained in the flow freeze on ejection Warm, Low-salinity fractured melt (read online Supplementary Material!); ice Postberg et al., Nature 2009 Schmidt et al. Nature 2008 Brine pocket Deep Misty Caverns

31 5. Deep Misty Caverns Near-Surface Geysers Violent boiling Shallow Evaporation Tectonic occurs at the Vents Melt interface of a large liquid/ vapor reservoir many km below the surface. Ice Sublimation Equilibrium evaporation explains low-sodium vapor Flow out the vent is slow, like a small leak in a balloon Droplets entrained in the flow freeze on ejection Warm, Low-salinity fractured melt (read online Supplementary Material!); ice Postberg et al., Nature 2009 Schmidt et al. Nature 2008 Brine pocket Deep Misty Caverns

32 5. Deep Misty Caverns Near-Surface Geysers Violent boiling Shallow Vents Strengths Tectonic Melt Model contains most physically rigorous components Ice Sublimation Explains low-sodium vapor Explains salty particle entrainment Explains vent velocities Brine pocket Deep Misty Caverns Low-salinity melt Warm, fractured ice

33 5. Deep Misty Caverns Near-Surface Geysers Violent boiling Shallow Vents Tectonic Melt Weaknesses/Concerns Low-salinity melt Ice Sublimation Some model components* based on ad hoc assumptions Are km-scaled caverns realistic? What process makes droplets? *Do droplets evolve as brownian motion carries them for km s? *What makes salt-poor particles? Warm, fractured ice Brine pocket *Can clathrate decomposition make escaping gases observed by Cassini s INMS? Do these processes affect D/H? Deep Misty Caverns

34 The Scorecard So Far Model Observable Near-surface ocean geyser Ocean-filled cracks Freshfilled cracks Ice/Clathrate vaporization Deep caverns Low-salt vapor Salty particles Other gases Grade X X X F X? X? X D?? B X? B? B

35 Plume Vent Models Near-Surface Geysers Shallow Vents Tectonic Melt Ice Sublimation Deep Misty Caverns Ruled Out Unlikely Possible? Possible? Possible? Violent boiling Brine pocket Low-salinity melt Warm, fractured ice

36 The Bottom Line: Don t Jump on Any Bandwagons A wide variety of observational constraints & physical limitations argue against near-surface geysers and deep ocean-filled cracks Several viable scenarios (and hybrids) remain: sublimation/clathrate decomposition, melt evaporation, deep misty caverns More rigorous modeling needed to bring alternative scenarios to same level as deep misty caverns scenario.

37 The Big Debate: Oceans, Puddles, Ice or??? Salt, Fresh or Ice?