Homework #4

1. Homework #4 Homework #4 is due Tuesday, October 16.

2. Notes & Reminders Quiz #2 will be the last 15 minutes of class on Tuesday, October 16 (any math problems will not require calculators). - will cover material through Thursday’s lecture Our Moon landing/hoax and Flat Earth debate will be Thursday, October 18.

3. Inside frost line: small metal/rock planets form – the terrestrial planets Outside the frost line: large bodies form – the Jovians ➢ The gas giants are thus “miniature solar systems” with their own accretion disk ➢ Moons form about them from disks of dust/gas ➢ Later, asteroids/comets etc. can be captured to complement the moon systems

4. Moons of jovian planets form in miniature disks  look like miniature Solar Systems.

5. Jupiter system formed like the Solar System in miniature.

6. Is the Nebular Theory a Good Theory? • Good theories not only explain current observations but they also predict what should be found next. • The nebular theory predicts: • 1) we should see IR emission from warm nebulae • cores where stars are forming ✔ • 2) we should see flat gas/dust disks ✔ • 3) first generation stars should have no • terrestrial planets – TBD • 4) no gas giants very near star in other solar systems X

7. Protoplanetary disks in Orion star-forming region.

8. Disks around Other Stars AU Microscopii HD141569A • Observations of disks around other stars support the nebular hypothesis.

9. Disks around Other Stars HL Tauri

10. Is the Nebular Theory a Good Theory? • Good theories not only explain current observations but they also predict what should be found next. • The nebular theory predicts: • 1) we should see IR emission from warm nebulae • cores where stars are forming ✔ • 2) we should see flat gas/dust disks ✔ • 3) first generation stars should have no • terrestrial planets – TBD • 4) no gas giants very near star in other solar systems X

11. What ended the era of planet formation?

12. Most of the nebula material (mostly H and He) never condenses or accretes. It is blown out of the young solar system by a strong solar wind — outflowing protons and electrons from the Sun  Jovian planets stop growing key point

13. Where did asteroids and comets come from?

14. Asteroids and Comets • Leftovers from the accretion process • Rocky asteroids inside frost line • Icy/rocky comets outside frost line

15. Role of Jovian Planet Gravity Planetessimals near Jupiter get thrown out/in by gravitational interaction; Growth of Mars is limited?! Growth of an outer neighbor to Mars is prevented. Beyond Neptune material stays in place  Kuiper belt (most in plane of the Solar System) From amongst the Jovian planets throw-out is dramatic – wholesale clearance  Oort cloud (spherical spatial distribution)

17. How do we explain “exceptions to the rules”?

18. Period of Heavy Bombardment • Leftover planetesimals bombarded other objects in the late stages of solar system formation.

19. Period of Heavy Bombardment • What of planetesimals that get thrown into inner Solar System?  collisions • Heavy bombardment within the first few 100s million years after Solar System formation. • Impact craters and basins on Moon/ Mercury.

20. Origin of Earth’s Water • Water may have come to Earth by way of icy planetesimals (comets).

21. Captured Moons • Unusual moons of some planets may be captured planetesimals.

22. How do we explain the existence of our Moon?

23. Failed Moon Formation Ideas • Moon “spun off” from Earth’s crust – requires too large initial spin of Earth. X • Earth gravitationally captures a pre-formed Moon – energy/momentum does not work unless Earth had a very extended, thick atmosphere early. X • Earth-Moon system formed simultaneously from same nebular material – does not explain Moon’s much smaller iron core/lower density. X

24. Giant Impact – Current Leading Theory

25. Odd Rotation • Giant impacts might also explain the different rotation axes of some planets (Venus, Uranus).

26. When did the planets form? • Radiometric dating tells us that oldest moon rocks are 4.4 billion years old.  little younger than Earth • Oldest meteorites are 4.55 billion years old. • Planets probably formed 4.5 billion years ago.

27. How does radioactivity reveal an object’s age?

28. Radioactive Decay • Some isotopes decay into other nuclei. • A half-lifeis the time for half the nuclei in a substance to decay. • Decay mediated by the weak, EM force.

29. Age Estimation Via Radioactive Decay 14C has a half life of ~5700 years  not suitable for cosmological dating  common misconception that age of Earth determined from 14C dating 40K has a half life of 1.3 billion years  decays to 40Ar - do not expect any 40Ar in newly-formed rock, therefore, all 40Ar found today must have decayed from 40K 238U decays to 206Pb (lead) with a half-life of4.5 billion years  get consistent ages with 40K-40Ar studies.

30. Important Point! Radiometric dating gives us time since rock crystallized.  Resets the clock once rock is re-melted and re-solidified

31. Chapter 9Planetary Geology:Earth and the Other Terrestrial Worlds

32. What are terrestrial planets like on the inside? Seismology gives us clues about the interiors of planets.

33. Earth’s Interior • Core: highest density; 58Ni and 56Fe • Mantle: moderate density; silicon, oxygen, etc. • Crust: lowest density; granite, basalt, etc. Temperature increases as you go deeper down.

34. Earth’s Core The Earth’s core is divided into the inner solid core, and an outer, liquid (molten) core. Question: If the temperature increases with increasing depth beneath the surface, why is the outer core liquid, while the inner core is solid?

35. Earth’s Core The Earth’s core is divided into the inner solid core, and an outer, liquid (molten) core. Question: If the temperature increases with increasing depth beneath the surface, why is the outer core liquid, while the inner core is solid? Answer: Higher pressure of outer layers compresses inner core, making it solid despite having a higher temperature.

36. Differentiation • Gravity pulls high-density material to center. • Lower-density material rises to surface. • Material ends up separated by density. • Differentiation happened when planet was still liquid.

37. Strength of Rock • Rock stretches when pulled slowly but breaks when pulled rapidly. • The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere. • Bodies over 300-500 km in diameter will become spherical in ~1 billion years by slow, slow deformation of rock by gravity.

38. Lithosphere • A planet’s outer layer of cool, rigid rock is called the lithosphere (crust + part of mantle). • It “floats” on the warmer, softer rock that lies beneath. • Thin lithospheres can crack easily  easier for volcanic eruptions • Thick lithospheres do not allow magma/molten rock through easily.

39. Seismic Waves • Vibrations that travel through Earth’s interior tell us what Earth is like on the inside. • P-waves(primary or pressure waves): longitudinal, faster, travel through air, liquid, gas • S-waves (secondary or sideways waves): side-to-side, slower, only move through solids

40. How do we know what’s inside a planet? • P waves go through Earth’s core, but S waves do not. • We conclude that Earth’s core must have a liquid outer layer.

41. Terrestrial Planet Interiors • Applying what we have learned about Earth’s interior to other planets tells us what their interiors are probably like, mainly from their average densities. The presence (or absence) or magnetic fields also yields clues.