Terrestrial Planetary Geology: Basic Processes & Earth

1. Terrestrial Planetary Geology:Basic Processes & Earth

2. What are terrestrial planets like on the inside? Planets (inc. Moon) to scale, with typical surface features

3. Earth’s Interior • Core: Highest density; nickel and iron • Mantle: Moderate density; silicon, oxygen, etc. • Crust: Lowest density; granite, basalt, etc. • Other terrestrial planets have similar layers

4. Terrestrial Planet Interiors • Applying what we have learned about Earth’s interior to other planets tells us what their interiors are probably like

5. Differentiation • Gravity pulls high-density material to center • Lower-density material rises to surface • Material ends up separated by density • This generates heat inside planet

6. Lithosphere • A planet’s outer layer of cool, rigid rock is called the lithosphere: crust + outer mantle • It “floats” on the warmer, softer rock that lies beneath: most of the mantle is “plastic”-- the rock slowly deforms

7. Strength of Rock • Rock stretches when pulled slowly but breaks when pulled rapidly--like Silly Putty but MUCH slower • The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere if bigger than about 300 km in diameter • Rapid shear, like an earthquake or impact breaks rock

8. Seismic Waves Let UsKnow What’s Inside a Planet • P (primary) waves push matter back and forth (longitudinal like sound waves) • S (secondary) waves shake matter side to side (transverse, like water waves)

9. SEISMIC WAVES REVEAL INTERIOR SEISMOGRAPHS detect EARTHQUAKES, VOLCANOS, and EXPLOSIONS at varied distances. • Long surface (L) waves travel fastest, but are not very useful as they don't probe the interior. • P-waves, PRIMARY, (push-pull waves) are COMPRESSIONAL, LONGITUDINAL waves. Propagate through liquids as well as solids. VP = function of (compressibility; composition, T, P) • S-waves, SECONDARY, (side-slip) are SHEAR, TRANSVERSE waves. CANNOT propagate through liquids (OUTER core). VS = (a new) function of (rigidity; composition, T, P) • We have some seismic measurements for the Moon too: seismometers left by Apollo astronauts

10. Seismic Wave Paths Both S and P waves can be detected from earthquakes on the same side of the earth, • ONLY P waves are detected on the opposite side of the earth “shadow zone”: region where no S (absorbed) & only weak P (refracted) waves are detected. Tells us the outer core of the earth is LIQUID

11. Seismographic measurements have found for Earth • Boundaries between: inner/outer core; outer core/mantle; composition changes in mantle (600 km); oil and natural gas deposits in crust. • Seismic measurements can even detect excess rotation of earth’s inner core, upwellings and sideways motions in the mantle. • MODERN SEISMOLOGY CAN GIVE A 3-D PICTURE, CHANGING IN TIME, OF THE EARTH'S INTERIOR! --- Tomography • There is a good analogy with 3-D images of people's interiors from MRI or CAT scans.

12. Thought Question What is necessary for differentiation to occur in a planet? a) It must have metal androck in it b) It must be a mix of materials of different density c) Material inside must be able to flow d) All of the above e) b and c

13. Thought Question What is necessary for differentiation to occur in a planet? a) It must have metal androck in it b) It must be a mix of materials of different density c) Material inside must be able to flow d) All of the above e) b and c

14. Causes of Geologic Activity • Heating of Interior • Accretion and differentiation when planets were young • Radioactive decay is most important heat source today

15. Cooling of Interior • Convection transports heat as hot material rises and cool material falls (outer core; inner mantle) • Conduction transfers heat from hot material to cool material (lithosphere) • Radiation sends energy into space (surface & atmosphere)

16. Role of Size is Dominant • Smaller worlds cool off faster and harden earlier • Moon and Mercury are now geologically “dead” • Mars lasted longer; Venus may still be active • Earth is VERY active

17. Surface Area to Volume Ratio Gives Cooling Time • Heat content depends on volume • Loss of heat through radiation depends on surface area • Time to cool depends on surface area divided by volume • Larger objects have smaller ratio and cool more slowly

18. Why do some planetary interiors create magnetic fields? Iron filings follow the magnetic field lines of a bar magnet

19. Sources of Magnetic Fields • Motions of charged particles create magnetic fields • Electromagnets via currents in coil of wire (usually amplified by magnetic material, like iron) • Permanent magnets: electron spins act as currents in iron or nickel

20. Sources of Magnetic Fields in Planets • A world can have a magnetic field if charged particles are moving inside • 3 requirements: • Molten interior • Convection • Moderately rapid rotation Earth has molten iron outer core Outer planets have metallic hydrogen Stars have ionized H

21. Practice Question Answers • True: a CCD is more linear and preferred over film. • False: Jupiter is 11.2 Earth radii but 318 Earth masses, not about 100. • True: oldest rocks on Earth ~4 Gyr, oldest on moon from Apollo ~4.4 Gyr • False: While most large telescopes are reflectors, they spend most of their time taking spectra, not pictures. • False: The earth’s magnetic field is generated in its liquid outer core. (Mantle is rocky and plastic.) • False: Twice the wavelength means 1/2 the energy: E = hf = hc/ • True: liquids, solids & dense gases give continuum thermal spectrum

22. More practice answers • True:this is stimulated emission • B: 293 K is earth or body temp. and thermal emission peaks in IR • D: 500 atoms after 1 half-life of 30 yrs, 250 after 2, 125 after three half-lives, or 90 years • E: angular momentum conservation means flattening, gravity means condensation, collisions meant extra flattening as vertical energy is lost. • A: getting above atmosphere means less turbulence and less absorption by water vapor • D: LA/LB=(RA/RB)2(TA/TB)4= 22(1/2)4=4/16=1/4

23. What processes shape planetary surfaces? Their surfaces are amazingly different, yet same forces act!

24. Key Processes that Shape Surfaces • Impact cratering • Impacts by asteroids or comets • Volcanism • Eruption of molten rock onto surface • Tectonics • Disruption of a planet’s surface by internal stresses • Erosion • Surface changes made by wind, water, or ice

25. Impact Cratering • Most cratering happened soon after solar system formed: the heavy bombardment era • Craters are about 10 times wider than object that made them • Small craters greatly outnumber large ones • Areas with many craters are old; those with few were “repaved”

26. Impact Craters: Classical Tycho (Moon) Barringer Meteor Crater (Arizona) Rim at edge of shock; rebound makes peak in center

27. Impact Craters on Mars: Evidence for Liquid Water in the Past “standard” crater impact into icy ground eroded crater Cratering History Movie

28. Volcanism • Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface • Molten rock is called lava after it reaches the surface • It solidifies to create volcanoes

29. Lava Properties & Volcano Types Runny lava makes flat lava plains Slightly thicker lava makes broad shield volcanoes Thickest lava makes steep stratovolcanoes

30. Outgassing • Volcanism also releases gases from Earth’s interior into atmosphere: in the past for other terrestrial planets • We’ll talk more about this later

31. Tectonics • Convection of the mantle creates stresses in the crust called tectonic forces • Compression forces make mountain ranges (Appalachian Mts on Earth on left) • Valley can form where crust is pulled apart (Ceraunius Valleys on Mars on right)

32. Plate Tectonics on Earth • Earth’s continents slide around on separate plates of crust • Plate Tectonics Applet • Tectonics: Mantle Convection

33. Erosion • Erosion is a general term for weather-driven processes that break down or transport rock • Processes that cause erosion include • Glaciers • Rivers • Wind (which demands an atmosphere!)

34. Erosion by Water • Colorado River continues to carve Grand Canyon • Land has been uplifted, but river manages to wear it down

35. Erosion by Ice • Glaciers carved the Yosemite Valley • They covered most of the Northern US and Europe in recent ice ages

36. Erosion by Wind • Wind wears away rock and builds up sand dunes • Frequent on Earth,also seen on Mars

37. Erosional Debris • Erosion can create new features by depositing debris • Again, frequent on Earth,but remnants seen on Mars too

38. The Earth as a Planet You gotta love it, it's our pretty home! You gotta know something about it too! (At least to pass this class.)

39. The Earth is (nearly) a Sphere and it Rotates • Proofs (other than Greek): Masts of ships visible before their decks. Cicumnavigation in 1522! Distances corresponding to degree of latitude change; bulges near equator. • Newton realized that a rotating object has extra support perpendicular to its axis: • Re = 6.378136 x 106 m • Rp = 6.356753 x 106 m • ELLIPTICITY = (Re -Rp )/Re = 1/298.3 = 0.0033528. If the earth were “mushier”, the same rotation speed would yield a higher ellipticity.

40. More Proofs of Earth’s Rotation • FOUCAULT PENDULUM --- fixed plane of oscillation, with the Earth rotating underneath it. • Satellites in POLAR ORBIT see different sections every 90 minutes as the Earth rotates under the satellite's FIXED path.