ESS 298: OUTER SOLAR SYSTEM

1. ESS 298: OUTER SOLAR SYSTEM Francis Nimmo Io against Jupiter, Hubble image, July 1997

2. Galilean Satellites • Last week • Io – volcanically active, tidally-deformed • Callisto – inactive, heavily cratered, undifferentiated (?) • This week • Europa • Ganymede Callisto Europa Ganymede Io

3. Europa

4. Outline - Europa • Basic characteristics • Tectonic features • Is there really an ocean? • How thick is the ice shell? • Summary

5. What is it like? • Cold ( ~120K on average) • Rough – heavily tectonized • Young – surface age ~60 Myrs • Icy, plus reddish “non-ice” component, possibly salts? • Trailing side darker and redder, probably due preferential implantation of S from Io • Interesting – it has an ocean, maybe within a few km of the surface, and possibly occasionally reaching the surface

6. Basic parameters • Note higher eccentricity and greater degree of mass concentration than Io

7. Interior Structure • Similar to Io, but with a layer of ice (~100 km) on top • Magnetometer data strongly suggest ocean at least a few km thick (see later) • Thickness of solid ice shell not well known (see later) • Ice/water layer thinner than on Callisto – why? Ice shell Ocean ~120km Silicate mantle

8. Surface Observations • Only lightly cratered (surface age ~60 Myr) • Surface heavily deformed ~100km lenticulae bands ridges chaos

9. Tectonic features after Extensional features (bands) from Tufts et al. 2000 Strike-slip features from Hoppa et al. 1999 50km before

10. Strike-slip faults • How do they form? A consequence of the way tidal stresses rotate over one diurnal cycle (Tufts et al. 1999). Vertical (map) view Friction prevents block motion Tidal stresses • This ratcheting effect can lead to large net displacements • Strike-slip motion will lead to shear heating if sufficiently rapid (Gaidos and Nimmo 2000) • This may have consequences for double ridge topography and local elastic thicknesses

11. Bands (1) 20km from Sullivan et al., Nature (1998)

12. Bands (2) Nimmo et al., Icarus 2003 double ridge band • Bands appear to be elevated with respect to their surroundings. Why? • Thermal differences? Implies recent activity • Variations in shell thickness? Also implies recent activity (why?) • Compositional/porosity variations?

13. Compression (?) • Rarely observed. Why not? • Is it hidden somewhere? E.g. double ridges. • Major unanswered question The only example of unambiguously documented compressional features on Europa to date Prockter and Pappalardo, Science 2000

14. Endogenic Contamination (?) • Double ridges appear to start “red” and become fainter with age • Redness is “non-ice” contaminant (salts?) • Correlation with structural features strongly suggests that non-ice component is endogenic • E.g. is there dirtier ice beneath a clean surface? Voyager false-colour image 1260 km across

15. 15 km Cryovolcanism? • Little evidence for cryovolcanic activity • Why is this not very surprising? • Density structure – ice less dense than water • How might water be erupted? Possible flooding by cryo-lavas

16. Stratigraphy(?) • Establishing a stratigraphic column is useful (why?) • But it’s hard to do – mappers tend to define tectonic units, not geological ones; no absolute ages, only cross-cutting relationships • There is a lot of dispute over identifying old features (e.g. chaos regions) – do they exist or not? Figueredo and Greeley (2004) • There are two results that appear robust: • Very few craters are tectonized or otherwise modified • Chaos regions are generally the youngest features • So what? We’ll come back to these observations.

17. Sources of Stress • Surface deformation and young surface age suggest geologically active. What are the sources of stress? • Diurnal tides – amplitude 30m, max. stress~0.1 MPa, period 3.5 days, global • Non-synchronous rotation (NSR) – stress ~ EHq/R (about 0.1 MPa per degree), period T.B.D., global • Polar wander – similar to NSR, global • Despinning/Differentiation – minor effect at present day, global • Convection (thermal/compositional) – stress ~0.01-0.1 MPa, period ~ kyrs, local • Impacts – transient stresses GPa, instantaneous, local • Oceanic currents? • Others??

18. N.S.R. observations • Has it happened? • We can predict stress orientations and fault styles (e.g. strike-slip vs. normal) for different amounts of NSR • Many authors have compared these predictions with data • Unfortunately, there is no consensus. Also, Sarid et al. (2004) showed that backwards NSR would fit the data just as well! Geissler et al. Nature 1998 • Observations from successive missions have found no evidence for NSR, putting a lower bound of ~104 years on NSR rotation timescale (Hoppa et al. 1999)

19. 100 km 40 km From Carr et al., Nature, 1998 Is there really an ocean? • An important question, esp. for astrobiology • Geophysical models of ice shell structure allowed an ocean, but didn’t require one • Geological observations suggested an ocean, but couldn’t rule out features being created by ductile ice undergoing solid-state creep

20. Magnetometer data • Europa experiences a changing magnetic field due to Jupiter’s rotation (see Week 2) • The result is an induced magnetic field, similar in magnitude to that expected if Europa were a perfect conductor • We can only constrain the product of the conductivity and conductor thickness • A shallow, salty ocean more than a few km thick fits the observations and is a plausible model • Other possibilities are either much less plausible (e.g. a graphite layer) or don’t work (e.g. the core is too small and too deep)

21. Summary • Magnetometer data indicate that Europa is very conductive • The best explanation for this conductivity is a salty ocean > a few km thick • Should we be surprised? • Tidal heating is a good way of keeping things hot (c.f. Io) • Convection is rather inefficient at getting rid of heat (especially in non-Newtonian materials) • Oceans are probably quite common in icy satellites (both Ganymede and Callisto appear to have them) • An important effect of the ocean is to decouple the interior from the shell . . .

22. How thick is the ice shell? • Let’s assume there is an ocean, sitting beneath a solid ice shell • The thickness of this shell is uncertain by at least an order of magnitude (as you’ll see) • Why do we care about the shell thickness? • Astrobiology – nutrient transport depends on how thick the shell is • Fundamental feature of the satellite • Important for design of future missions

23. How do we estimate the ice shell thickness? • Fracture propagation / cycloids (1-2 km) • Iceberg models (~2 km) • Flexural features (1-20 km) • Impact craters (> 12 km) • Convective features (> ~20 km) • Tidal dissipation calculations (20-50 km)

24. 110 K Temp. “lid” Depth convection conduction ~270K Temperature and Heat Flow • Thin shells will be conductive, thick shells may convect (see later) • Relevant heat fluxes: • Chondritic mantle = 0.14 TW • 20km conductive shell = 0.7 TW • 2km conductive shell = 7 TW • Tidal stresses will generate heat in ice shell and (potentially) in the silicate mantle • In equilibrium, the heat generated within the satellite is balanced by the heat transported across the ice shell

25. Hussmann et al. 2002 convection conduction Equbm. Shell thickness Tidal Dissipation (1) • Balancing heat generation against heat transport • Why does convective heat transport decrease as shell thickness increases? • Obtain equilibrium shell thickness 20-50 km • Major uncertainties in rheology and (particularly) amount of heat being generated in silicate mantle • What would happen if Europa’s mantle was like Io’s?

26. Ridge formation model, Greenberg et al. 1998 Fractures / Cycloids • Diurnal (3.5 day) tidal stresses on Europa are small ~30 kPa • This is only sufficient to open fractures to depths of ~40m • If features like cycloids or ridges require fracture penetration to an ocean, and if the diurnal stresses are the relevant ones, the shell must be no more than a few km thick Cycloid model, Hoppa et al. 1999 No-one has yet come up with a convincing alternative explanation for cycloids

27. Icebergs • “Icebergs” and the edges of chaos regions stand a few 100 m higher than the matrix • Implies a background shell thickness of 1-2km if the icebergs really are floating • Do chaos regions really involve liquid water? • Rotation and translation of blocks suggest a liquid matrix h iceberg water tc tc ~ 10 h 40 km From Carr et al., Nature, 1998

28. Flexural models • Wavelength of deformation gives rigidity of ice (can be converted to elastic thickness Te – see later) • Rigidity can be converted to shell thickness (assuming a conductive temperature structure): tc~ 5 Te • Te varies significantly. Is this due to local effects, or variations in shell properties over time?

29. Brittle-ductile transition depth? Depth Effect of ocean beneath shell? Diameter Crater Observations Schenk, Nature 2002 • Depth-diameter curves for Europa craters show sharp transitions • Very largest craters show anomalously shallow depths • The characteristic crater size may imply a shell thickness > 19km • Small numbers, uncertainties in depth estimates • Do we really understand cratering mechanics well enough?

30. Cratering Models • Europan impact craters show central peaks • This suggests that the ice shell is not breached during impact • Implied thickness of shell is > 12 km water ice Water reaches the surface during impacts into thin shells but not into thicker ones Turtle and Ivanov, LPSC 2002

31. 150km Convection • Certain surface features look convective • Convection only occurs if ice shell thickness exceeds a critical value: Ra = r g aDT tc3 / k h • Critical Ra ~ 1000, so tc > ~20km • Considerable uncertainties in ice rheology e.g. grain size unknown • Convective stresses are small, ~0.01 MPa. Problem? • Are the features really convective?

32. Summary • None of the current shell thickness estimates are entirely satisfactory • Thin (~2 km) shells are supported by some observations and would require an Io-like interior • Thick (~20 km shells) are more plausible if little dissipation occurs in the interior • Variations in shell thickness in space or time may account for some of the scatter • E.g. if double ridges are active, local Te estimates may not reflect background shell thickness • Is the ice shell necessarily in steady state . . . ?

33. Steady state? • Surface is much younger than satellite age – steady state or catastrophic resurfacing? • Almost no impact craters are tectonized (Figueredo and Greeley Icarus 2004), suggesting a (relatively) sudden change in resurfacing rate • Stratigraphic relations suggest change in tectonic activity e.g. chaos regions are all young • Theoretical models of coupled orbital-thermal evolution suggest that large changes in heating rate can occur on timescales comparable to Europa’s surface age

34. Stresses • How do we estimate stresses? • Dome-like topography of ~100m ~0.1 MPa • Brittle failure at few km depth ~ 1 MPa • Flexural deformation ~ few MPa (depends on Te) • What sources of stress are available? • Thermal convection ~ 0.01 MPa • Diurnal stresses ~0.1 MPa • Compositional convection ~0.1 MPa (?) • Non-synchronous rotation ~0.1 MPa per degree • Why is the surface deformation dominantly extensional?

35. Summary • Most likely source of stress is NSR, although observational evidence for it is weak • Present-day dissipation in ice shell is sufficient to maintain thick (~20km shell) • Thinner (~2km shell) requires dissipation in silicate interior • Some evidence for time-dependence of heating and shell thickness • How might future missions resolve the shell thickness debate?

36. Ganymede

37. Outline - Ganymede • Basic parameters • Tectonics and deformation history • Topography and heat flux • Summary Ganymede Europa

38. Basic Parameters • Slightly larger than Mercury! • Most centrally-concentrated object measured in the solar system

39. Interior Structure • Similar to Callisto (or Io with an 800km thick ice lid) • Ocean inferred from magnetometer data, probably at I-III boundary • MoI and density data suggest iron core, probably liquid because of dynamo (see next slide) Remember all these structures are non-unique: the ones shown assume plausible but not necessarily correct densities.

40. This is where a permanent dipole would plot External field External field Induced field Induced field Permanent field Permanent field From Kivelson et al. Icarus 2002. My is a component sensitive to the inducing field. The measured field has a permanent component of 49 mT. Note that the modeled component reverses polarity as expected for an induction effect. A dynamo? • Two (at least) possible magnetic signatures • An induced component from an ocean • A permanent signal from an internal dynamo • How do we separate them? Look for time reversals

41. Surface Observations • Heavily cratered, bimodal surface: • Old, dark terrain (~4 Gyr) • Younger, bright, grooved terrain (~2 Gyr) • N.B. absolute crater ages are sufficiently uncertain that grooved terrain could be > 4 Gyr old • Lots of tectonic activity • Possible (minor) cryovolcanism 140km

42. Steep scarp 15km 100km Flat floor Relay ramp? Image of dark terrain area showing impact crater cut by extensional faults. Degree of extension is roughly 50%. Oblique impact? Tectonized crater Tectonism Image at bright-dark terrain boundary. Dark terrain is rougher and more cratered. Stereo topography shows margin of bright terrain is depressed. Linear scarp features are probably normal faults (see overlay)

43. More tectonism (Left) Strike-slip motion (Below) Band-like activity (c.f. Europa) (?). Not everyone accepts this interpretation. DeRemer & Pappalardo, LPSC 2003 Head et al. GRL 2002

44. Voyager image Galileo Image 15km Grooves (young) vs. Furrows (old) (Left) Image of grooved terrain in Uruk Sulcus. Note that Voyager picked out longer-wavelength grooves, while Galileo also imaged smaller faults. (Below) Furrows in dark terrain of Galileo Regio. Furrows are probably caused by early giant impacts. Grooves furrows Extensional boudinage Pappalardo et al., Icarus 1998

45. Cryovolcanism • Cryovolcanism was predicted on the basis of Voyager images, but (as with Europa) it appears to be rare • Again, eruption of water (or water-ice slurry) is difficult due to low density of ice This image shows one of the few examples of potential cryovolcanism on Ganymede. The caldera may have been formed by subsidence following eruption of volcanic material, part of which forms the lobate flow (?) within the caldera. The relatively steep sides of the flow suggest a high viscosity substance, possibly an ice-water slurry (?). Caldera rim Lobate flow(?) Schenk et al. Nature 2001

46. Impact Structures 215km Bright terrain (Below) Two examples of palimpsests. These are thought to be large, ancient impact structures that have relaxed over time and now have no topographic expression. Dark terrain Jones et al. .Icarus 2003 (Above) A crater chain, Enki catena, at a bright-dark terrain boundary.

47. Catena Statistics • Models predict strong hemispheric asymmetry in catena distribution (why?) • Observations do not bear this out – evidence for NSR • Distribution of impact craters tells similar story observed modelled Zahnle et al. Icarus 2001

48. Crater Relaxation • An impact crater generates stresses in the ice beneath • These stresses may be supported elastically • But the ice will tend to creep over time, and the crater will relax: • Two sorts of relaxation timescales t: • Half-spacet~h/rgl (l is wavelength, h viscosity) (why?) • Thin layer t~hl2/rgd 3 (d is channel thickness) • The two equations behave very differently in l • Big craters / long wavelengths will sample greater depths, so the mean viscosity will be lower, and relaxation times will be faster than for small craters

49. Flexure again • The fundamental equation governing elastic deflections is • where D is the rigidity, w is the deflection, r is the density of the underlying material and L is a load • For a point load, the wavelength l of the response will be determined primarily by the rigidity: Where does this come from? • This wavelength l is referred to as the flexural parameter • So if we can measure l, we can infer the rigidity directly • How do we measure l? Why do we care?

50. q1 q2 h L L h = 1 1 + tan (q1) tan (q2) Stereo Technique (schematic) horiz. resolution = a few pixels, vert. resoln. ~ 0.2 horiz. resoln.