1. EART 160: Planetary Science

2. Last Time HW 2 due Today –How are people doing? Planetary Surfaces –Volcanism –Magma –Volcanic Features on Planets

3. Today HW 2 due Today (for reals) Planetary Surfaces –Tectonics –Stress and Strain –Faults and Folds

4. Tectonics Global tectonic patterns give us information about a planet’s thermal evolution Abundance and style of tectonic features tell us how much, and in what manner, the planet is being deformed –How active is it? Some tectonic patterns arise because of local loading (e.g. by volcanoes)

5. Modes of Deformation Extension Compression Shear

6. Extensional Tectonics 37km diameter Pappalardo & Collins 2005 Diam. appx 40km Craters on Ganymede Valles Marineris, Mars (~8km deep) Crater on Venus

7. Extension Extension accommodated by normal faulting L0L0 L   Stretching factor: Fault blocks rotate as extension proceeds Typical normal faults start with dips of 60 o and lock up when dips=30 o, giving stretching factor = 1.7 Stretching factor also controls amount of subsidence that happens during extension

8. A Martian Rift Valley Hauber and Kronberg, JGR Planets, 2001 Looks similar to terrestrial continental rifts. Not been heavily studied, but may provide useful insights into crustal properties.

9. Graben Systems across) 15km Steep scarp Relay ramp? Flat floor Canyonlands graben, Utah, 2km across Graben, Ganymede

10. Bands (Europa) Sullivan et al., Nature (1998) 20km What drives the extension?

11. Compression Accomodated by reverse (or thrust) faulting Typical reverse faults start with dips of 30 o L0L0 L 

12. Rarely Seen on Icy Satellites Prockter and Pappalardo, Science 2000 The only example of unambiguously documented compressional features on Europa to date Lots of Extension, No compression. How can this be?

13. Wrinkle Ridges and Lobate Scarps Probably thrust faults at depth (see cartoon) Found on Mars, Moon, Mercury, Venus Possibly related to global contraction due to cooling? Spacing may be controlled by crustal structure Tate et al. LPSC 33, 2003 50km Krieger crater, Moon 25 km Mars, MOC wide-angle

14. Io compressional tectonics Burial leads to large compressive stresses due to change in radius Stresses easily large enough to initiate faulting Additional compressive stresses may arise from reheating the base of the crust RR After McKinnon et al., Geology 2001 Low-angle thrust faulting is probably responsible for many of the mountain ranges seen on Io Schenk and Bulmer, Science 1998 550 km 10km stereo

15. Horizontal Movement Strike-slip Motion Right-Lateral Left-Lateral

16. Relatively rare (only seen on Earth & Europa) Associated with plate tectonic-like behaviour Europa, oblique strike-slip (image width 170km)

17. Plate Tectonics Dominant style of tectonics on Earth Unknown elsewhere Early Plate Tectonics on Mars? –(Sleep 1994, Nimmo & Stevenson, 2000) Image Credit: USGS

18. Stress Stresses are forces per unit area that are transmitted through a material. –Stresses transmitted perpendicular to a surface are normal stresses –Stresses transmitted parallel to a surface are shear stresses  zz  xz

19. Strain Stress in an elastic solid results in strain, or deformation of the solid –Normal strain is the change in length compared to the original length –Shear strain is the change in angle due to deformation. xx  x‘ wxwx   xx =  x’/  x  xz = -½ (  1 –  2 ) 

20. Stress and Strain in Solids Stress and Strain are Tensors (think of it as a vector of vectors) Maximum direction of principal stress controls style of fault.  xx  yy  zz

21. Rheology Rheology is the study of the deformation and flow of matter under the influence of an applied stress If the deformation (or strain,  ) follows the stress, , the material is elastic –Returns to original state when stress is released If the strain rate, d  /dt follows the stress, , the material is viscous

22.   ElasticViscous yy Plastic behavior E is the Young’s Modulus, or elasticity Analagous to the spring constant in Hooke’s Law  is the viscosity Rarely a straight line plot in reality 00 Failure Strength Yield Strength Brittle Behavior

23. Deformation You can think of Young’s modulus (units: Pa) as the stress  required to cause a strain of 100% Typical values for geological materials are –E = 100 GPa (rocks) and 10 GPa (ice) –  = 10 21 Pa s (rocks) 10 14 Pa s (ice) HIGHLY Temperature, Pressure, Stress-dependent Elastic deformation is reversible; but if strains get too large, material undergoes fracture (irreversible) Material may be both elastic AND viscous, depending on the time-scale. We’ll talk more about this next week – Planetary Interiors.

24. water ice Mechanisms: Extension For icy satellites, one possible explanation for the ubiquitous extension is that they possess floating ice shells which thickened with time (see below) Why should the shell thicken?

25. Mechanisms: Compression Silicate planets frequently exhibit compression (wrinkle ridges etc.) This is probably because the planets have cooled and contracted over time Why do planets start out hot? Further contraction occurs when a liquid core freezes and solidifies Contractional strain given by Hot mantle Liquid core Cool mantle Solid core Where  is the thermal expansivity (3x10 -5 K -1 ),  T is the temperature change and the strain is the fractional change in radius

26. Tectonic Stresses & Byerlee’s law Byerlee’s law says that faults don’t move unless the shear stress exceeds the normal stress times the friction coefficient f For almost all geological materials, f=0.6 (unless the fault is lubricated somehow) fault Shear stress Normal stress In general, the normal stress is simply the overburden pressure: The shear stresses are provided by tectonic effects E.g. to cause a fault 10 km deep on Earth to move requires tectonic stresses of 180 MPa (a lot!) Typical tectonic stresses on Earth are usually 10-100 MPa P =  gh Also applies to atmospheres!

27. P =  gz   nn z Stress on a Fault  n = Normal Stress  = Shear Stress P = Pressure (Lithostatic Stress)  tec

28. Elastic Flexure The near-surface, cold parts of a planet (the lithosphere) behaves elastically This lithosphere can support loads (e.g. volcanoes) We can use observations of how the lithosphere deforms under these loads to assess how thick it is The thickness of the lithosphere tells us about how rapidly temperature increases with depth i.e. it helps us to deduce the thermal structure of the planet The deformation of the elastic lithosphere under loads is called flexure EART162: Planetary Surfaces

29. Flexural Stresses In general, a load will be supported by a combination of elastic stresses and buoyancy forces (due to the different density of crust and mantle) The elastic stresses will be both compressional and extensional (see diagram) Note that in this example the elastic portion includes both crust and mantle Elastic plate Crust Mantle load

30. Flexural Parameter Consider a load acting on an elastic plate: TeTe mm load ww The plate has a particular elastic thickness T e If the load is narrow, then the width of deformation is controlled by the properties of the plate The width of deformation  is called the flexural parameter and is given by  E is Young’s modulus, g is gravity and n is Poisson’s ratio (~0.3)

31. If the applied load is much wider than , then the load cannot be supported elastically and must be supported by buoyancy (isostasy) If the applied load is much narrower than , then the width of deformation is given by  If we can measure a flexural wavelength, that allows us to infer  and thus T e directly. Inferring T e (elastic thickness) is useful because T e is controlled by a planet’s temperature structure 

32. Example This is an example of a profile across a rift on Ganymede An eyeball estimate of  would be about 10 km For ice, we take E=10 GPa,  =900 kg m -3 (there is no overlying ocean), g=1.3 ms -2 Distance, km 10 km If  =10 km then T e =1.5 km A numerical solution gives T e =1.4 km – pretty good! So we can determine T e remotely This is useful because T e is ultimately controlled by the temperature structure of the subsurface

33. T e and temperature structure Cold materials behave elastically Warm materials flow in a viscous fashion This means there is a characteristic temperature (roughly 70% of the melting temperature) which defines the base of the elastic layer 110 K 270 K elastic viscous 190 K E.g. for ice the base of the elastic layer is at about 190 K The measured elastic layer thickness is 1.4 km (from previous slide) So the thermal gradient is 60 K/km This tells us that the (conductive) ice shell thickness is 2.7 km (!) Depth 1.4 km Temperature

34. T e in the solar system Remote sensing observations give us T e T e depends on the composition of the material (e.g. ice, rock) and the temperature structure If we can measure T e, we can determine the temperature structure (or heat flux) Typical (approx.) values for solar system objects: BodyT e (km)dT/dz (K/km) BodyTeTe dT/dz (K/km) Earth (cont.) 3015Venus (450 o C) 3015 Mars (recent) 1005Moon (ancient) 1530 Europa240Ganymed e 240

35. Next Time Paper Discussion –Mars Crust and Mantle (Zuber et al., 2001) –Io Volcanism (Spencer et al., 2007) Planetary Surfaces: Gradation –Fluvial (Water) –Aeolian (Wind) –Glacial (Ice) –Mass Wasting (Gravity) –Sputtering (Charged Particles)