EART160 Planetary Sciences

1. EART160 Planetary Sciences

2. Updates Rosetta mission high-res, Nov. 12 landing

4. Last week –crusts and impacts • Planetary crustal compositions may be determined by in situ measurements or remote sensing (spectroscopy) • Most planetary crusts are basaltic • Impact velocity will be (at least) escape velocity • Impacts are energetic and make craters • Crater size depends on impactor size, impact velocity, surface gravity • Crater morphology changes with increasing size • Crater size-frequency distribution can be used to date planetary surfaces • Atmospheres and geological processes can affect size-frequency distributions

5. This Week • Volcanism, tectonics and sedimentation • What controls where and when volcanism happens? • What kinds of tectonic features are observed on other planetary bodies, and what do they imply? • How are loads on planetary bodies supported? • What sedimentary features are observed?

6. Volcanism • Volcanism is an important process on most solar system bodies (either now or in the past) • It gives information on the thermal evolution and interior state of the body • It transports heat, volatiles and radioactive materials from the interior to the surface • Volcanic samples can be accurately dated • Volcanism can influence climate

7. Volcanoes Hawaiian shield Sif Mons (Venus) 2km x 300km Note vertical exaggeration! Olympus Mons, Mars

8. Dikes Exhumed dikes (Mars & Earth) Mars image width 3km MOC2-1249 Ship Rock, 0.5km high New Mexico Radiating dike field, Venus Dike Swarms, Mars and Earth

9. Lava tubes and rilles Venus, lava channel? 50km wide image Hadley Rille (Moon) 1.5km wide Io, lava channel? Schenk and Williams 2004

10. Lava flows - Moon

11. Lava flows - Moon Hiesinger and Head (2003)

12. Lava flows - Moon Spectral based identification of mare basalt flows. Hiesinger and Head (2003)

13. Lava flows on the Moon - ages Hiesinger and Head (2003)

14. Lava flow – lunar stratigraphy ~2 meters in height

15. Lava flows on Io and Venus Amirani lava flow, Io • Dark flows are the most recent (still too hot for sulphur to condense on them) • Flows appear relatively thin, suggesting low viscosity 500km 500km Comparably-sized lava flow on Venus (Magellan radar image)

16. Example - Mars Hartmann et al. Nature 1999 Olympus Ascraeus Pavonis Arsia Although the Tharsis rise itself may be ancient, some of the lavas are very young (<20 Myr). We infer this from crater counts (see last lecture). So it is probable that Mars is volcanically active now. How might we test this? (Very) recent methane results? The Tharsis rise contains enormous shield volcanoes. Most of them are about 25km high. What determines this height? What about their slopes?

17. 250km Example - Io • What’s the exit velocity? • How do speeds like this get generated? • Volcanism is basaltic – how do we know? • Resurfacing very rapid, ~ 1cm per year Loki Pele April 1997 July 1999 Sept 1997 Pillan Galileo images of overlapping deposits at Pillan and Pele 400km Pele

18. Why does it happen? • Why are Earth materials anywhere near their melting points?

19. Why does it happen? • Volcanism is in many ways a result of planetary materials: • Being terrible thermal conductors.

20. Why does it happen? Temperature • Material (generally silicates) raised above the melting temperature (solidus) • Increase in temperature (plume e.g. Hawaii) • Decrease in pressure (mid-ocean ridge) • Decrease in solidus temperature (hydration at island arcs) Reduction in pressure Increase in temperature Depth Normal temperature profile liquidus solidus Reduction in solidus • Partial melting of (ultramafic) peridotite mantle produces (mafic) basaltic magma • More felsic magma (e.g. andesite) requires additional processes e.g. fractional crystallization

21. a Eruptions • Magma is often less dense than surrounding rock (why?) • So it ascends (to the level of neutral buoyancy) • For low-viscosity lavas, dissolved volatiles can escape as they exsolve; this results in gentle (effusive) eruptions • More viscous lavas tend to erupt explosively • We can determine maximum volcano height: h What is the depth to the melting zone on Mars? Why might this zone be deeper than on Earth? d rc rm

22. Cooling timescale hot • Conductive cooling timescale depends on thickness of object and its thermal diffusivity k cold • Thermal diffusivity is a measure of how conductive a material is, and is measured in m2s-1 • Typical value for rock/ice is 10-6 m2s-1 Temp. d • Characteristic cooling timescale t ~ d2/k • How long does it take a meter thick lava flow to cool? • How long does it take the Earth to cool?

23. Cryovolcanism • Cryovolcanism was predicted on the basis of Voyager images to occur on icy satellites, but it appears to be rare • 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

24. 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 i.e. how active is it? • Some tectonic patterns arise because of local loading (e.g. by volcanoes)

25. 50km 25 km Krieger crater, Moon Wrinkle Ridges and Lobate Scarps • Compressional features, probably thrust faults at depth (see cartoon) • Found on Mars, Moon, Mercury, Venus • Some related to global contraction/ • Spacing may be controlled by crustal structure Mars, MOC wide-angle Tate et al. LPSC 33, 2003

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

27. Mechanisms: Compression Hot mantle Liquid core • 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 Cool mantle Where a is the thermal expansivity (3x10-5 K-1), DT is the temperature change and the strain is the fractional change in radius Solid core

28. The Moon IS shrinking Watters et al. 2010

29. Stress and strain • For many materials, stress is proportional to strain (Hooke’s law); these materials are elastic • Stress required to generate a certain amount of strain depends on Young’s modulusE (large E means rigid) • You can think of Young’s modulus (units: Pa) as the stress s required to cause a strain of 100% • Typical values for geological materials are 100 GPa (rocks) and 10 GPa (ice) • Elastic deformation is reversible; but if strains get too large, material undergoes fracture (irreversible)

30. Flexure and Elasticity • 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 • See EART162 for more details!

31. Flexural Stresses load • 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 Crust Elastic plate Mantle

32. Flexural Parameter (1) rw load Te rm • Consider a load acting on an elastic plate: ~3a rm • The plate has a particular elastic thicknessTe • If the load is narrow, then the width of deformation is controlled by the properties of the plate • The approximate width of deformation a is called the flexural parameter and is given by Here E is Young’s modulus, g is gravity andn is Poisson’s ratio (~0.3)

33. Flexural Parameter (2) • Technically, the first zero crossing xo = 3πα/4, and the forebulge maximum is at xb = πα ~3α. • Therefore, α is less than the width of deformation.

34. Flexural Parameter (3) • If the applied load is much wider than a, then the load cannot be supported elastically and must be supported by buoyancy (isostasy) • If the applied load is much narrower than a, then the width of deformation is given by a • If we can measure a flexural wavelength, that allows us to infer a and thus Te directly. • Inferring Te (elastic thickness) is useful because Te is controlled by a planet’s temperature structure

35. Example 10 km • This is an example of a profile across a rift on Ganymede • An eyeball estimate of 3a would be about 10 km • For ice, we take E=10 GPa, Dr=900 kg m-3, g=1.3 ms-2 Load Distance, km • If 3a=10 km then Te=6.5 km • So we can determine Teremotely • This is useful because Te is ultimately controlled by the temperature structure of the subsurface

36. Te 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 Surf. Temp. • E.g. for ice the base of the elastic layer is at about 190 K • The measured elastic layer thickness is 6.5 km (from previous slide) • So the thermal gradient is 12 K/km • This tells us that the ice shell thickness is 13 km • What’s wrong with these assumptions (convection changes geotherm). 110 K 273 K 190 K 6.5 km Depth elastic viscous Temperature Liquid!

37. Te in the solar system • Remote sensing observations give us Te • Te depends on the composition of the material (e.g. ice, rock) and the temperature structure • If we can measure Te, we can determine the temperature structure (or heat flux) • Typical (approx.) values for solar system objects:

38. Mascons and Compensation • Surprising result of the first lunar orbiters: Lunar gravity anomalies • They were being perturbed by a strong density anomaly, as inferred by small velocity changes

39. How to measure velocity/distance? • Doppler shift of transmitter frequency. • Round trip “time of flight” of packet of information.

40. Deep Space Network Goldstone 34 and 70 meter dishes Madrid 70 meter dish Canberra 70 meter dish

41. Aside: Voyager 1 in interstellar space? • 129 AU (2014) • ~2.6 x 1013 meters • How much transmitter power is received on Earth?

42. Mascons and Compensation • Surprising result of the first lunar orbiters: Lunar gravity anomalies • They were being perturbed by a strong density anomaly, as inferred by small velocity changes But the density anomaly was over larger craters!

43. Mascons and Compensation Expect something like this from a mass deficit Crust Mantle Or, at least, why might you expect NOTHING?

44. Mascons and Compensation Or, at least, why might you expect NOTHING? Crust Level A Mantle Isostasy. “Compensated”

45. Mascons and compensation

46. Mascons and Compensation Maybe it filled with dense basalt that is supported by lithosphere (uncompensated)? Lithostatically supported basalt Crust Level A Mantle

47. Mascons and Compensation Nice idea, but models show the basalt fill is TOO THIN to explain the gravity anomaly Lithostatically supported basalt Crust Level A Mantle

48. Mascons and Compensation Lithostatically supported basalt Crust Level B Level A Mantle The mantle appears to have “overshot” the isostatic level and is superisostatic at level B. This possibly happened during “rebound” of the deep layers of the Moon during the impact, by some kind of temporary weakening mechanism. The mantle then “froze” in place only hours later when the weakening mechanism terminated – otherwise, it would have fallen back down to the isostatic level.

49. Mascons and Compensation Lithostatically supported basalt Crust Level B Level A Mantle Ultimately: We have a load on the lithosphere, due to a combination of basalt fill, and a superisostatic mantle plug.

50. Lunar tectonics lab!