Europa Robert Pappalardo Laboratory for Atmospheric and Space Physics and the NASA Astrobiology Institute University of Colorado at Boulder Europa's Ocean: Overview

• What is the evidence for an ocean within Europa? • Is Europa active today, and has activity changed through time? • What are Europa’s sources of free energy and biogenic elements? • How might the interior and surface of Europa communicate? Geology of Europa: Overview

• Interior constraints:  Gravity  Magnetometry • Geology:  Stress mechanisms  Ridge origin(s)  Bands  Lenticulae & convection  Chaos  Surface composition  Impact structures  Changes through time • The future: JIMO Europa's Interior: Gravity Data Axial moment of inertia from Doppler gravity data:

 Ic = 0.346 ± 0.005

 H2O-rich crust: ~80 - 170 km thick.

[Anderson et al., 1998] Europa's Bimodal Geology ridged plains mottled terrain Europa's Eccentric Orbit

85 hr orbit • Eccentric orbit (e = 0.01). • Solid body tide ~30 m if ice shell is decoupled by ocean. • Libration (constant rotation rate; variable orbital speed). • Complexly varying diurnal stress field with maximum stress ~0.1 MPa. • Tidal bulge torque promotes nonsynchronous rotation. • Deformation dissipates

energy: tidal heating. torque min.

not to scale potential min. Nonsynchronous Rotation Stress

• Nonsynchronous rotation stress pattern backrotated ~25° provides the best match to global lineament patterns. • Nonsynchronous rotation is facilitated if ice shell is decoupled from rocky mantle.  Suggests decoupling by global ocean. • Stresses ~0.1 MPa per degree of shell rotation. Nonsynchronous Rotation Stress • Assume brittle failure perpendicular to tensile stress ticks.

Compression Tension Compression Tension

0.5 MPa Arcuate Cracks from Nonsynchronous Rotation Stress

Compression Tension Compression Tension Global Lineament Fit to 25° Backrotated NSR

(backrotated west 25°) "Diurnal" Stresses

Compression Tension Cycloidal Fracture Form from Diurnal Stress

Tension Tension Cycloidal Ridges

• Cycloids explained by time-varying diurnal stresses. • Ocean necessary for sufficient tidal amplitude and stress.

[Hoppa, Tufts, et al., 1999] Adding Diurnal and 1° Nonsynchronous Stress

Compression Tension Adding Diurnal and 5° Nonsynchronous Stress

Compression Tension Adding Diurnal and 2° Nonsynchronous Stress

Compression Tension Compression Tension Adding Diurnal and 5° Nonsynchronous Stress

Compression Tension Compression Tension Agenor Linea Fracture Formation on Europa

• Global lineaments best matched by combined diurnal and NSR stress. Fracture form varies with NSR stress: • 0° NSR  cycloidal (~0 - 2° NSR)  wavy (~5° NSR)  arcuate (>10° NSR) • Propagation governed by available stress vs. ice strength (here 70 kPa). 5° NSR • NSR compressional zones allow fracture propagation to terminate. • Addition of NSR stress shifts preferred longitude of crack formation. 10° NSR Morphological Transitions

• Continuum from fractures to ridges to bands. Ridges Ridge Formation Models

• Several candidate ridge formation models proposed:  None universally accepted. • Shear heating along fracture plains is morphologically and geophysically viable:  Diurnal strike-slip motion heats ice, inducing uplift. Bands

• Separation & spreading of Europa’s icy lithosphere.

• Dark bands brighten with age.

[Sullivan et al., 1998] Bands • Icy spreading center analog:  Axial trough, hummocky zone, flanking ridges and troughs. Ridged Plains

• Oldest surviving units.

• Variety of ridge and trough morphologies.

• Individual structures and in sets.

• Morphologies of some sets imply normal faulting or folding. Regional-Scale Folds

• Astypalaea region and elsewhere. • Wavelengths ~20-25 km. • Folding above warmer ductile ice. Surface Composition

• NIMS: Non-ice material has shallow asymmetrical bands.

• Candidate materials:  Hydrated sulfates salts

(epsomite: MgSO4 • 7H2O).  Hydrated sulfuric acid

(H2SO4 • nH2O).

 Hydronium (H3O) or

hydrogen peroxide (H2O2). • Sulfur chains could explain red visible color.

• Mg, Na sulfates consistent with chondritic starting material. Surface Composition

• Spectral matches to NIMS results are non-unique...

Dalton [2002] Ductile Ice Deformation

• Does the ice shell convect? • Can convection drive resurfacing? • Implications for surface-ocean transport and communication? • Implications for ice thermal structure? • Implications for exploration techniques? Europa’s Geology: Lenticulae

• Morphologically related pits, spots, & domes. • Commonly ~10 km diameter & regularly spaced. • Consistent with diapiric origin related to convection. Convection in Europa's Ice Shell

• Tidal heating greatest in "hot" ductile ice, near shell's base. • Conductive models have suggested ~20-30 km thick ice.  Assumes tidal heating in ice shell & radiogenic heating in mantle. • Ice shell can convect if 10s km thick and tidally strained.  Convective models imply >20 km thick ice.

~ 100 K

~ 260 K Conditions for Ice Convection

• Convection can occur if Ra > Racr(λ,δT) where "g#$TD3 Ra = %&(T,',d) ρ = density g = gravitational acceleration α = thermal expansion coefficient ∆T = (Tbot – Ttop) D = thickness of ice shell κ = thermal diffusivity η = viscosity;! a function of temperature (T), stress (σ), grain size (d) Ductile Deformation

"˙ = strain rate * A n % Q ( # = stress "˙ = # exp'$ * p d = grain size d & RT ) T = temperature A constant Effective viscosity: = Q* = activation energy # " = n = stress exponent ˙ $ p = grain size exponent Measured in lab experiments

• If n=1, viscosity is independent o!f stress/strain rate (Newtonian). • If n≠1, viscosity depends on stress/strain rate (non-Newtonian). Non-Newtonian ice is softer at greater strain rate!

!

! Ductile Ice Deformation

)1 • Goldsby & Kohlstedt # 1 1 & (2001): ice deforms by "˙ = "˙ + "˙ + % + ( total diff disl % "˙ "˙ ( • Grain-size sensitive $ gbs bs ' creep (non-Newtonian) • Basal slip • Grain boundary sliding ! • Volume diffusion (Newtonian) • Dislocation creep (non-Newtonian) . • Cannot invert εtot for effective viscosity ηtot Ice Deformation Maps

• Ice deformation mechanism depends on T, σ, d. • Large grain size d:  Dislocation creep dominates (n = 4). n=4 • Small grain size d: n=1.8  Diffusional flow n=2.4 contributes (n = 1) n=1.0 at low σ, high T. Composite Ice Viscosity

1 1 1 #1 = + + ("gbs + "bs) "total "diff "disl

where: ! & Q* ) " = # (d) $ (1%n) exp( + i i ( RT+ ' *

• Approximate ηtot as sum ! of constituent viscosities (cf. van den Berg et al., 1995). Numerical Convection Model

1 1 1 #1 • ηtot depends on T, σ, d: = + + ("gbs + "bs) "total "diff "disl • Circulation time scale is ~105 yr. • Plumes cannot easily penetrate cold stagnant lid to extrude. ! Initiation of Non-Newtonian Convection in Icy Satellites • Icy satellite convection probably initiates from conductive state. • Non-Newtonian η → ∞ if not perturbed: tidal strain required? • Critical shell thickness for convection is grain-size dependent.

convection δT=3 K no convection

δT=30 K n=1.8-2.4 n=1 GSS Creep (GBS + Basal Slip) Newtonian Volume n=4 Diffusion composite rheology Dislocation Creep

grain size = 1mm Compositional Buoyancy of Plumes

• Impurities imply temperature-induced compositional segregation. • Segregation of impurities with low eutectic temperatures (chlorides or H2SO4 • nH2O) could drive plumes upward.

compositional buoyancy D: dominates

[Pappalardo & Barr, 2004] Europa's Geology: Chaos

• Disruption of surface into mobile plates and intervening lumpy matrix. Chaos Models

• Melt-through model:  Ice shell thins and melts above oceanic megaplumes.  But: requires huge heat flux, and ice flow may prevent thinning. • Diapirism model:  Ice convection partially melts salty ice causing in situ degradation.  But plumes may cool too quickly to partially melt shallow ice. Chaos: Tilted Blocks? Chaos Topography

• High-standing chaos topography favors diapiric model.  Refrozen water is not expected to stand above surroundings. • Local highs correlate with depletion of chaos blocks.  Consistent with in situ thermal disruption.

[Schenk & Pappalardo, 2004] Impact Structures

• Paucity of large impacts suggest ~50 Myr surface age. • Central peak craters show “relaxed” topography. • Multi-ringed structures may have penetrated ~20 km thick ice. Pwyll Tyre Europa's “Thick Shell” Geology Sources of Free Energy and Biogenic Elements

• Radiation chemistry on H2O creates oxidants:

 H2O2 is detected.  HCHO is predicted. 40  K decay ⇒ O2, H2. • Impacts are source of biogenic elements:  ~1012 kg C in 4.5 Byr.  C≡N, C-H at Ganymede & Callisto. • Hydrothermal activity at rocky mantle ⇒ nutrients? • Chances of life & detection improve if ocean & surface communicate. “Thin Shell” Alternative Model

• Diurnal opening of fractures to water permits photosynthetic life.

• Easy and rapid access of oxidants to ocean.

• Easy surface sampling of life.

[Greenberg et al.] Trends in Geological Evolution Trends in Geological Evolution

• Mapping suggests geological changes:  Transition from ridged plains to chaos.  Overall waning activity. • Odd for a surface only ~50 Myr old… • Has Europa's activity monotonically decreased or is it cyclical?

Pappalardo et al., 1999 Greeley et al., 2000; Figueredo & Greeley, 2004 Cyclical Geological Activity?

• Tidal heating and orbital evolution of the 3 resonant Galilean satellites are linked through internal structure. • Possible cyclical tidal heating and geological activity. Coupled orbits Europa ice thickness

[Hussmann et al., 2004] Equilibrium Configurations: Is Europa's Rocky Mantle Hot?

• Moon-like: Meager heat supply (radiogenic only). • Io-like: Extreme tidal heating partially melts rock.

cooling by heating convection outpaces outpaces cooling by heating convection

[W.B. Moore, 2004] Europa: Summary

• Europa probably has a ~100 km deep ocean beneath ≥20 km thick icy shell.

• Plausible sources of free energy and biogenic elements exist.

• Organisms (dormant or dead) might exist within warm, partially melted ice.

• Geological processes permit surface-ocean communication:  Lenticulae and chaos are good candidates.  Convection circulates lower shell in ~ 105 yr.  Ridges might not have communicated.

• Europa activity may be non-steady-state.

• We need to return for more observations! Europa Geophysical Explorer

• Assess tidal effects to confirm the presence of a current global subsurface ocean.

• Characterize the properties of the ice shell and describe three-dimensional distribution of liquid water.

• Elucidate the formation of surface features and seek sites of current or recent activity.

• Identify and map surface composition with emphasis on compounds of astrobiological interest.

• Prepare for a future lander mission. Europa Pathfinder Concept

Covers Patch Antenna

69.72 cm

Viewed from Patch Antenna

ISO View

15.24 cm segment Antenna

Side View Instrument port Lander Structure Jupiter Icy Moons Orbiter

• Magnetometer data implies briny oceans within all three icy Galilean satellites.

• A spacecraft mission to the icy Galilean satellites is fundamental to understanding their processes and potential habitability.

• Altimetry plus gravity from orbit would test for oceans within all three icy satellites.

• Ground-penetrating radar could detect warm ice zones and intra-shell brines.

• Seismology is the only definitive means of determining Europa's ice shell thickness. Jupiter Icy Moons Orbiter (JIMO)