PLATO - 4. The terrestrial planets, planetology

Transcription

1 PLATO - 4 The terrestrial planets, planetology 1

2 Disk Formation Why does the contracting cloud form a disk?! As matter rotates faster and faster, it feels more and more centrifugal force, resisting gravity! Centrifugal force eventually balances gravity - Kepler orbit! " Centrifugal force is always away from rotation axis " Gravity always points to center " Net force: matter is pulled towards mid-plane 2

3 link Star & planet formation 3

4 Formation of the Sun This explains:! Uniform sense of rotation! The fact that planets orbit in a disk What about the formation of the Sun?! Center of rotation: natural place for matter to collect! The proto-sun formed from the lowest angular momentum material that sank to the center of the disk.! Friction moves disk material inward, adding mass to the Sun! This process is called accretion 4

5 Phase Transitions Melting (solid!liquid) Evaporation (liquid!gas) Sublimation (solid!gas) Freezing (liquid!solid) Condensation (gas!liquid) Deposition (gas!solid) 5

6 Composition Element:! Sun! Earth! Condensation Temp.! Hydrogen! 71.1%! %! 180K (H2O) Helium! 27.4%! 2x10-8 %! 3K Oxygen! 0.65%! 30.1%! 1300K (Silicates), 180K (H2O) Carbon! 0.25%! 0.045%! 80K (CH4) This must Iron! 0.14%! 32.1%! 1400K have been the composition of the cloud the solar system formed from Neon! 0.12%! 4x10-10 %! 9K Nitrogen! 0.08%! %! 130K (NH3) Magnesium! 0.07%! 13.9%! 1300K (Silicates) Silicon! 0.06%! 15.1%! 1300K (Silicates) Sulfur! 0.04%! 2.9%! 700K (FeS) 6

7 Composition Element:! Sun! Earth! Condensation Temp.! Iron! 0.14%! 32.1%! 1400K Oxygen! 0.65%! 30.1%! 1300K (Silicates), 180K (H2O) Magnesium! 0.07%! 13.9%! 1300K (Silicates) Silicon! 0.06%! 15.1%! 1300K (Silicates) Sulfur! 0.04%! 2.9%! 700K (FeS) Hydrogen! 71.1%! %! 180K (H2O) Nitrogen! 0.08%! %! 130K (NH3) Carbon! 0.25%! 0.045%! 80K (CH4) Helium! 27.4%! 2x10-8 %! 3K Neon! 0.12%! 4x10-10 %! 9K 7

8 Composition Earth s composition is closely related to the condensation temperature of matter. What is the link?! Planets must form in the outer disk, away from center! Data show: Planets form from elements that easily condense/ freeze into solids (dust particles, ice crystals) Planets are formed from dust or ice particles that collide, stick together and grow bigger and bigger 8

9 Composition Why didn t all disk matter condense?! Radiation from proto-sun heated the gas.! Temperature was highest closest to the Sun! Close in, only silicates and iron condensed to rock! Further out, water and ammonia condensed to ice Copyright The McGraw-Hill Companies Inc. 9

10 Composition Copyright The McGraw-Hill Companies Inc. Permission required for reproduction or display 10

11 Hierarchical Planet Formation 1. Planet seeds form by collisions of microscopic dust (=rock) and ice particles in the outer parts of the proto-solar disk 2. Colliding particles stick together, making bigger particles. We call these bigger particles planetesimals 11

12 Hierarchical Planet Formation 1. Planet seeds form by collisions of microscopic dust (=rock) and ice particles in the outer parts of the proto-solar disk 2. Colliding particles stick together, making bigger particles. We call these bigger particles planetesimals 12

13 Hierarchical Planet Formation 1. Planet seeds form by collisions of microscopic dust (=rock) and ice particles in the outer parts of the proto-solar disk 2. Colliding particles stick together, making bigger particles. We call these bigger particles planetesimals An artist s impression of planetesimals in the early Solar System 13

14 Hierarchical Planet Formation 1. Planet seeds form by collisions of microscopic dust (=rock) and ice particles in the outer parts of the proto-solar disk 2. Colliding particles stick together, making bigger particles. We call these bigger particles planetesimals 3. Bigger planetesimals sweep up more particles than smaller ones 4. Some planetesimals grow massive enough to attract other particles by gravity (gravitational focusing) Without gravitational focusing focusing 14

15 Hierarchical Planet Formation 1. Planet seeds form by collisions of microscopic dust (=rock) and ice particles in the outer parts of the proto-solar disk 2. Colliding particles stick together, making bigger particles. We call these bigger particles planetesimals 3. Bigger planetesimals sweep up more particles than smaller ones 4. Some planetesimals grow massive enough to attract other particles by gravity (gravitational focusing) 5. The more particles they attract, the more massive they become, the more they attract, the more... runaway growth! 6. Planets continue to collide and grow until all material is either used up or pushed out of the Solar System by the Solar Wind 15

16 Hierarchical Planet Formation 16

17 Question: Which planets would you expect to grow faster? A) Inner planets B) Outer planets C) They should all grow at the same rate 17

18 Inner vs. Outer Planets Why are the outer planets more massive?! They grew faster because they could accumulate not just rock, but also ice particles! Hydrogen and Oxygen were very common, making water ice a good food source for a growing planet! This explains the large water content of objects in the outer solar system 18

19 Inner vs. Outer Planets Why are outer planets mostly Hydrogen and Helium?! Thermal velocity Escape velocity 3kT 2GM v thermal = v escape = m particle R! Hydrogen and Helium particles are very light and thus very fast. They are not bound to small (terrestrial) planets.! Massive cores in the outer solar system had much higher escape velocities and were able to hold on to H and He. Massive outer planets could attract H and He gas from the disk and hold on to it, making them even more massive 19

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21 Gap Formation 1. Planet seeds form by collisions of microscopic Planet dust (=rock) and ice particles in the outer parts of the proto-solar clears a gapdisk 2. Colliding particles stick together, making bigger particles. We call these bigger particles planetesimals 3. Bigger planetesimals sweep up more particles than smaller ones 4. Some planetesimals grow massive enough to attract other particles by gravity The more particles they attract, the more massive they become, Phil Armitage the more they attract, the more... runaway growth! Gas giants suck up all the matter surrounding them, eventually starving themselves of any more mass to add. Growth stops. 21

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23 The Age of the Solar System When did the Solar System form? Best direct evidence: radiometric dating.! Many atomic nuclei are unstable to fission (splitting).! This happens randomly Example: Uranium 238, which occurs naturally in rock, decays into Lead via a complicated decay chain.! Half life of 238 U: 4.5 billion years.! That means, on average, after 4.5 billion years, half of original 238 U has decayed into Lead and is now gone. 23

24 The Uranium Clock Newly formed Zircon (ZrSiO4) crystals contain some Uranium instead of Zr, but no Lead Photo: R. Lavinsky 24

25 The Uranium Clock Newly formed Zircon (ZrSiO4) crystals contain some Uranium instead of Zr, but no Lead 238 U and 235 U decay into stable 206 Pb and 237 Pb, respectively Measure ratio Pb/U Determine age of Zircon crystal: age = yrs ln 206 Pb 238 U +1 25

26 The Age of the Solar System Many other kinds of clocks available, e.g.:! 40 Potassium" 40 Argon (half life: 1.3 billion years)! 14 Carbon" 14 Nitrogen (half life: 5730 years) All available age measurements of the oldest rocks, lunar samples, meteorites:! The solar system is about 4.5 billion years old 26

27 Geology Learn about planet formation and evolution from studying the crusts of terrestrial planets 27

28 Earth s Surface Mostly: Water 28

29 Earth s Surface Canyons: Erosion by water 29

30 Earth s Surface Limestone (Calcium Carbonate): Fossil deposits 30

31 Earth s Surface Volcanism 31

32 Earth s Surface Mountain ranges: Plate tectonics 32

33 Earth s Surface Asteroid impacts craters (rare) 33

34 Earth s Surface Lithosphere: Crustal plates, about 20-70km thick! Volcanism! Plate tectonics: Plates move ~ cm / year " Mountain ranges " Earth quakes! Very few impact craters visible on Earth Mostly covered by water (up to 11km deep)! Erosion! Deposits 34

35 Question: Which of these surfaces is youngest? A) Mercury B) Mars C) Earth 35

36 Geological Activity Impact craters! Are mostly old: planetesimal impact Geological processes: " Tectonic activity " Volcanism " Erosion! Act as erasers " Planetary equivalent of face lift " No craters = young surface 36

37 Earth s Surface Lithosphere: Crustal plates, about 20-70km thick! Volcanism! Plate tectonics: Plates move ~ cm / year " Mountain ranges " Earth quakes! Very few impact craters visible on Earth Mostly covered by water (up to 11km deep)! Erosion! Mineral and fossil deposits 37

38 Earth s Surface Young, constantly reshaped! Few impact craters! Surface erosion by water erases surface features quickly! Volcanism generates new surface features! Mountain ranges created over millions of years by plate tectonics (compare to Earth s age of 4.5 billion years) The two main effects:! Water! Plate tectonics 38

39 Question What happens when you heat a liquid from below? A) Wave motion B) Rolling motion C) No motion D) Shifting motion 39

40 Convection Hot liquid rises, cold liquid sinks 40

41 Plate Tectonics Continents move ~ 2 cm/yr! Measured with GPS! Explains fossil records Cause: Convection! Mantle heated by core 41

42 Earth s Interior Structure Earth s crustal plates rest on boiling layers of semirigid and liquid molten rock and an iron core.! Plate tectonics recycles crust every few 100 million years Solid crust ~300K Semi-rigid mantle ~1500K Liquid core ~4000K Solid inner core ~6500K 42

43 Seismology Like an ultrasound of the Earth s interior! Only P-waves travel through liquid! S-waves cannot travel through liquid P waves and S waves P waves and S waves P wave only P waves S wave wavelength 43

44 Differentiation Why is all the Iron in the core?! Iron atoms are heavy! Gravity pulls heavy things downward! Iron sinks, rock floats 44

45 Earth s Magnetic Field Fairly strong! Dipole field (it has a North and South pole)! Magnetic axis not aligned with rotation axis! Field reverses every few hundred thousand years Note for future reference:! Earth rotates rapidly! Earth s interior is liquid and exhibits convection 45

46 Venus Earth s evil twin! 0.95 Earth radii! 0.82 Earth masses! 0.72 AU orbit! 735K surface temp.! 90x Earth s pressure! Slow retrograde rotation (243 days)! No water 46

47 Venus Venera 13 view (shortly before the probe disintegrated under Venus intense heat and pressure) 47

48 Venus Topographic map of Venus! Only two continents (Ishtar and Aphrodite) 48

49 Venus: Volcanic Activity Maat Mons Volcano 49

50 Currently Active Volcanos Active within the past 250,000 yrs Idunn Mons (courtesy of Venus Express) 50

51 Venus: Volcanic Activity Pancake domes! Rock blisters Fractures! Cracks in crust 51

52 Venus: Geology Geological activity:! Most geological features volcanic in origin! Little tectonic activity (few mountain ranges)! Few impact craters! No magnetic field Difference to Earth:! Venus interior is hot, but likely less liquid! Because in absence of water, lava is probably more viscous! Slow rotation 52

53 Most Earth-like planet! 0.53 Earth radii Mars CO2 ice caps! 0.1 Earth masses! 1.5 AU orbit (eccentric)! 230K surface temp.! 1% Earth s pressure! 24.6 hr rotation! 25 axis tilt (seasons)! Convincing evidence for water 53

54 Mars NASA rovers! Sojourner! Spirit! Opportunity 54

55 Mars Topography Olympus Mons Valle Marineris Fairly heavily cratered Phoenix Viking 2 Viking 1 Pathfinder Opportunity Spirit Tharsis Bulge Hellas Planitia 55

56 Mars Geology Brooks/Cole Publishing 2001 Evidence for past Volcanism! Olympus Mons, 2.6 x higher than Mauna Kea! Southern hemisphere elevated! Few craters on slopes of Olympus Mons! Volcanos now dormant No mountain ranges! No plate tectonics 56

57 Evidence for Water Mars Odyssey:! Lack of neutrons and gamma rays Hydrogen present in crust Flow features:! Islands Sediments Mars Odyssey (Opportunity) (seen by Mars Global Surveyor)! Erosion into craters! Water channels! Molten ice around craters! Sedimentary rock formations Islands Channel Erosion: Water seepage 57

58 Mars Geology Structure:! Weak magnetic field! Sun produces tidal bulge Mars interior is partially molten Composition:! Higher Sulfur content (condensation temperature 700K) Lower density than Earth That explains why interior is molten 58