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).