Geodynamics of icy satellites

Transcription

1 Geodynamics of icy satellites Francis Nimmo (U. C. Santa Cruz) With help from: Bruce Bills, Eric Gaidos, Bernd Giese, Michael Manga, Isamu Matsuyama, Bill Moore, McCall Mullen, Bob Pappalardo, Louise Prockter, James Roberts, Paul Schenk, John Spencer, Chris Zhang

2 Talk Outline Why should anyone care about icy moons? Common processes: Tidal heat production Shell thickening Reorientation Shear heating Conclusions and the Future... Enceladus Titan PIA07787

3 3600 km Icy Moons

4 Why do we care about icy moons? Many of them! (large N is good) They provide lessons elsewhere (silicate planets, Kuiper Belt, super-ganymedes...) They record a lot of history, and exhibit a surprising diversity They are astrobiologically important (lots of liquid water beneath the ice) They have complex behaviour (e.g. thermal-orbital coupling)

5 Similarities to Silicate Bodies Interior structures are similar (except for thick surface layer of ice) Ice in thick shells can undergo phase changes due to high pressure Ice may also convect in thick shells 670 km scarp Near-surface ice is cold and rigid and will deform in a brittle fashion Close-up of Miranda rift, showing large fault scarp (~5km high)

6 Differences to silicate bodies Ice is less dense than water subsurface oceans, melt is hard to erupt Ice is weaker (less rigid) than rock and near melting has a viscosity ~10 6 times lower Major source of energy and deformation is tides, not radioactive decay or accretion Interior structure places constraints on mode of formation (caveat emptor) Tidal dissipation means that orbital evolution and thermal evolution are inextricably linked Thermal evolution can be non-monotonic

7 Common Processes Large N means allows identification of universally important processes predictions Provide constraints on interiors and evolution of icy bodies Processes to examine: Tidal heat production Shell thickening Reorientation Shear heating Combination of modelling and observations

8 1. (Tidal) heat production Why do we care? Constraints on orbital evolution Constraints on interior structure How do we measure it? Direct approach (Io, Enceladus) Eccentricity evolution (Meyer and Wisdom) Indirect approaches: Crater relaxation (Dombard and McKinnon) Flexure Keck AO 3.5 mm 2002 Spencer et al. 2006

9 Flexure Near-surface of satellite is cold, rigid and elastic Depth to which elastic behaviour extends depends on thermal gradient (heat flux) Stresses will cause flexure of this elastic plate Wavelength of deformation gives the elastic thickness T e Elastic thickness can be used to infer heat flux fault Elastic plate T e Flexural wavelength Brown & Phillips 1999

10 Ganymede Topography Two episodes of deformation furrows (old; below) and grooves (younger) Topography courtesy Bernd Giese, DLR Horiz. Resolution 200m, vertical resolution 20-40m.

11 Furrow Profiles Zu North Zu South Middle T e = km What heat flux does this imply? Lakhmu

12 Heat flux Heat flux range approx mwm -2 (at the time of deformation)

13 eccentricity Resonance Passages? Ganymede may have passed through resonances Passage could explain grooved terrain heat pulse Surface observations constrain orbital evolution Showman & Malhotra (1997) Symbols are for different resonances Current e= mwm Heat flux mwm -2 J dissipation decreases G dissipation increases (Q/k) G /(Q/k) J J dissipation increases G dissipation decreases

14 Europa Elastic thickness constrains ice shell thickness Nimmo & Manga, submitted Shell thickness has changed through time?

15 Summary We can use observations (topography) to infer elastic thickness and hence heat flux (or ice shell thickness) Heating is usually dominated by tides (e 2 ) Surface observations can constrain orbital evolution of satellites

16 2. Reorientation Planetary bodies have equatorial bulge due to rotation Adding a load perturbs the moments of inertia and leads to reorientation The ( fossil ) part of the bulge which does not relax opposes any reorientation Why do we care? Clue to internal structure and behaviour May invalidate some commonly-used approaches

17 Reorientation Examples Pappalardo et al Mars Tharsis Bulge (roughly equatorial) Miranda coronae Also Earth!

18 Enceladus hot spot and geysers Spencer et al. Science 2006 Porco et al. Science 2006 Regional heat flow (~100 mwm -2 ) and geysers at South Pole

19 Reorientation? Density consistent with ice shell & silicate mantle IR data and plume could indicate subsurface warm, low density region (diapir) Region of low density (mass deficit) causes polewards reorientation

20 Reorientation Theory Matsuyama et al. (2006) tan n Qsin 2 L Qcos 2 L L Initial load colatitude Size of load compared to fossil bulge Orientation of load relative to tidal axis (n=1 to n=4) The effective load Q Q R G ( k f 2 20 k 2 ) Degree-2 potential anomaly Rotation rate Change in Love number (size of fossil bulge) G 20 and (k 2f -k 2 ) depend on internal structure (rigidity)

21 Application to Enceladus S pole of Enceladus has high heat fluxes and deformation Could a subsurface diapir have caused reorientation? Equator Pole Equator Mass excess Pole Rigid lid Weak lid Low density diapir Mass deficit Low density diapir Mass deficit Ice shell Ice shell To get polewards motion, a relatively rigid lid is needed

22 Enceladus Results Initial load latitude 45 o 1 1 tan 2 Qsin 2 L n Qcos 2 L Nimmo & Pappalardo Nature 2006 Lithospheric thickness > ~1 km (for ice) (consistent with estimates of T e based on topography) Reorientation can be large if density contrast is large

23 Tests Gravity (~ few mgal at s/c altitude) Craters (leading/trailing asymmetry) Tectonics (Matsuyama, yesterday) Fossil terrains? Normal faults 88 o reorientation Thrust faults Strike-slip faults

24 Effect of impact basins (Melosh, 1975) Rotational bulge 400km Rhea, from Schenk and Moore 2007 Total relief ~10 km Most icy satellites have deep present-day impact basins (presumably uncompensated) Such a hole will promote polewards reorientation The amount of reorientation depends on the relative sizes of fossil bulge and load

25 b c Effect of tides Triaxial ellipsoid (not oblate spheroid) Reorientation around tidal (a) axis is easy Reorientation around b a axis is hard (tidal axis) Matsuyama and Nimmo (JGR, in press) Four equations, four unknowns Given present day load location and size, can solve for initial load and tidal/rotational axis location (or vice versa)

26 Generic Results Large basins reorient more Slow rotators reorient more Nimmo and Matsuyama 2007

27 Specific Results R (km) P (days) j D (km) h (km) s (MPa) Dg (mgal) J 2,hyd Rhea o o Charon (15 o ) (310) (2) 20 o Reorientation Stress Gravity anomaly Significant stresses (~0.1 MPa) can result Global tectonic patterns should be visible Gravity perturbation comparable to that expected for a fluid (hydrostatic) body J 2 /

28 Hydrostatic assumption Hydrostatic assumption means only sources of gravity are rotation and tides Degree-2 gravity gives internal structure if hydrostatic assumption is correct Other sources of l=2 gravity anomalies (e.g. big craters) will make body non-hydrostatic and internal structure determination will be wrong Rhea is non-hydrostatic (Mackenzie et al., submitted) Is Callisto really undifferentiated?

29 Summary Reorientation can take place if big craters or diapirs are present Global tectonic stresses are generated Big uncompensated craters will affect global gravity field and may invalidate hydrostatic assumption Internal structure models must be viewed with caution!

30 3. Shear Heating Stresses due to tides are time-varying Varying stresses can lead to strike-slip motion Strike-slip motion leads to shear heating (e.g. San Andreas fault, Earth) Nimmo and Gaidos JGR 2002 Europa strike-slip faults

31 So what? Different from normal mechanism of tidal heating Likely important in formation of geological features Possible mechanism for transfer of material from interior to exterior (astrobiology) Potentially detectable at present day

32 Shear heating and double ridges Nimmo and Gaidos JGR 2002, Prockter et al. GRL 2005, Han and Showman LPSC 2007

33 Shear heating at Enceladus? Geyser vapour flux could be due to shear heating (if a subsurface ocean is present) Relative temperatures Nimmo et al. Nature 2007 Spencer et al.science 2006

34 Geysers at Europa? Shear heating on Europa could also produce vapour plumes Plumes would be harder to see Difficult to distinguish from sputtered species Future mission? 70 km Graphic courtesy Bob Pappalardo

35 4. Ice shell thickening Many satellites have (or had) subsurface oceans If the ice shell thickens, surface extension results (stresses are large, several MPa) This could explain why almost all satellites show much more extension than compression Thickening ice shell also leads to ocean pressurization and possibly cryovolcanism (Manga and Wang 2007) ice water Nimmo (2004)

36 Summary/Lessons Large N allows us to identify universal processes: Shell thickening -> extension (if ocean present) Shear heating -> vapour production Reorientation -> global stress patterns These processes constrain histories and interiors Internal structure and orbital evolution are complicated and intertwined but we can constrain them with surface observations (e.g. heat flux) Where is the field going?

37 Future Missions Cassini (Saturn) Dawn (Ceres) New Horizons (Pluto) Outer Solar System Flagship?

38 1. Geological mapping Timing (stratigraphy) is crucial to understanding evolution (thermal & orbital) of these bodies Topography allows determination of heat flux Absolute timing (crater fluxes) is also important Galileo antenna failure made these approaches very difficult at Galilean satellites Cassini (and future missions) should do a much better job

39 2. Thermal-orbital coupling Interpreting geological observations requires coupling of thermal and orbital evolution Orbital dynamicists and geophysicists have to talk to each other! Hussmann and Spohn (2002)

40 3. Interior structure We may have to throw away the hydrostatic assumption What else can we use? Bills and Nimmo, submitted

41 Conclusions Large N allows us to identify universal processes: Shell thickening -> extension (if ocean present) Shear heating -> vapour production Reorientation -> global stress patterns These processes constrain histories and interiors Internal structure and orbital evolution are complicated and intertwined but we can constrain them with surface observations (e.g. heat flux) Beware the hydrostatic assumption!