Homework #4 has been posted, due Tuesday, Oct. 13, 11 pm

1. Homework #4 has been posted, due Tuesday, Oct. 13, 11 pm

2. Building the Planets. I COLLAPSE OF PROTOSTELLAR CLOUD INTO A ROTATING DISK Composition of disk: • 98%hydrogen and helium • 2%heavier elements (carbon, nitrogen, oxygen, silicon, iron, etc.). Most of this was in gaseous form!

3. Building the Planets. II There was a range of temperatures in the proto-solar disk, decreasing outwards Condensation: the formation of solid or liquid particles from a cloud of gas (from gas to solid or liquid phase) Different kinds of planets and satellites were formed out of different condensates

4. Building the Planets. III Accretion Accretion is growing by colliding and sticking The growing objects formed by accretion – planetesimals (“pieces of planets”) Small planetesimals came in a variety of shapes, reflected in many small asteroids Largeplanetesimals (>100 km across) became spherical due to the force of gravity

5. In the inner solar system(interior to the frost line), planetesimals grew by accretion into the Terrestrial planets. In the outer solar system(exterior to the frost line), accretion was not the final mechanism for planet building – nebular capture followed once accretion of planetesimals built a sufficiently massive protoplanet.

6. Building the Planets. IV. Nebular Capture Nebular capture – growth of icy planetesimals by capturing larger amounts of hydrogen and helium. Led to the formation of the Jovian planets Numerous moons were formed by the same processes that formed the proto-planetary disk Condensation and accretion created “mini-solar systems” around each Jovian planet

7. Building the Planets. V. Expulsion of remaining gas

8. The Solar wind is a flow of charged particles ejected by the Sun in all directions. It was stronger when the Sun was young. The wind swept out a lot of the remaining gas

9. Building the Planets. VI. Period of Massive Bombardment

10. Planetesimals remaining after the clearing of the solar nebula became comets and asteroids Rocky leftovers became asteroids Icy leftovers became comets Many of them impacted on objects within the solar system during first few 100 million years (period of massive bombardment - creation of ubiquitous craters).

11. Brief Summary

12. To aid in our search for life and suitable environments, we will be examining various timescales of importance, e.g., • How old is the Earth • How long did it take for the Earth to develop an oxygen atmosphere • How long did it take for life to form on Earth • Many others…

13. How do we determine ages like these? Radioactive dating

15. Recall that some isotopes of an element are unstable and will decay into another element. Parent: the atoms of the original unstable isotope Daughter: atoms of the element that results from the decay of the parent Parent decays into daughter

16. Half-Life: the Time it Takes for Half of the Original (Parent) Atoms in a Sample to Decay to a "Daughter" Product

17. Parent Daughter Half Change in... Carbon-14 Nitrogen-14 5730 years Uranium-235 Lead-207 704 million years Uranium-238 Lead-206 4,470 million years Potassium-40 Argon-40 1,280 million years Thorium-232 Lead-208 14,010 million years Rubidium-87 Strontium-87 48,800 million years Examples of radioactive isotopes useful in dating

22. Life depends critically on environment. We will examine how life-friendly environments can form in the universe. Fundamentals: TemperatureLiquids (particularly H2O) Sources of EnergyChemical environmentRadiation environment

23. What determines the environments of terrestrial-like planets? A look at:(much of what follows also has applications to Jovian moons). interiors surfaces atmospheres

24. What accounts for interior, surface, and atmospheric structures?

25. Terrestrial planets are mostly made of rocky materials (with some metals) that can deform and flow. Likewise, the larger moons of the Jovian planets are made largely of icy materials (with some rocks and metals) that can deform and flow. The ability to deform and flow leads every object exceeding approximately 500 km in diameter to become spherical under the influence of gravity.

26. Early in their existence, the Terrestrial planets and the large moons had an extended period when they were mostly molten. The heating that led to this condition was caused by impacts, where the kinetic energy of the impacting material was converted to thermal energy. Today, the interiors of planets are heated mainly by radioactive decay.

27. Differentiation – the process by which gravity separates materials according to their densities Denser materials sink, less dense material “float” towards top

28. Layering of interiors by density due to differentiation

29. Terrestrial planets and many large moon had an extended period where their interiors were “molten”. During this time, denser material sank towards center of planet while less dense material “floated” towards top

30. Terrestrial planets have metallic cores (which may or may not be molten) & rocky mantles Earth (solid inner, molten outer core) Mercury (solid core) Earth’s interior structure

31. Differentiated Jovian moons have rocky cores & icy mantles Io Europa Ganymeade Callisto

32. Layering by strength (mantle)

33. The Lithosphere… Layer of rigid rock (crust plus upper mantle) that floats on softer (mantle) rock below While interior rock is mostly solid, at high pressures stresses can cause rock to deform and flow (think of silly putty) This is why we have spherical planets/moons

34. The interiors of the terrestrial planets slowly cool as their heat escapes. • Interior cooling gradually makes the lithosphere thicker and moves molten rocks deeper. • Larger planets take longer to cool, and thus: 1)retain molten cores longer 2) have thinner (weaker) lithospheres

35. Geological activity is driven by the thermal energy of the interior of the planet/moon The stronger (thicker) the lithosphere, the less geological activity the planet exhibits. Planets with cooler interiors have thicker lithospheres. lithospheres of the Terrestrial planets:

36. Earth has lots of geological activity today, as does Venus. Mars, Mercury and the Moon have little to no geological activity (today) • This has important repercussions for life: • Outgassing produces atmosphere • Magnetic fields (need molten cores) protect planet surface from high energy particles from a stellar wind.

37. Larger planets stay hot longer. Earth and Venus (larger) have continued to cool over the lifetime of the solar system thin lithosphere, lots of geological activity Mercury, Mars and Moon (smaller) have cooled earlier  thicker lithospheres, little to no geological activity

38. Initially, accretion provided the dominant source of heating. Very early in a terrestrial planet’s life, it is largely molten (differentiation takes place). Today, the high temperatures inside the planets are due to residual heat of formation and radioactive decay heating.

39. Stresses in the lithosphere lead to “geological activity” (e.g., volcanoes, mountains, earthquakes, rifts, …) and, through outgassing, leads to the formation and maintenance of atmospheres. Cooling of planetary interiors (energy transported from the planetary interior to the surface) creates these stresses Convection is the main cooling process for planets with warm interiors.

40. Convection- the transfer of thermal energy in which hot material expands and rises while cooler material contracts and falls (e.g., boiling water).

41. Convection is the main cooling process for planets with warm interiors.

43. Side effect of hot interiors - global planetary magnetic fields • Requirements: • Interior region of electrically conducting fluid (e.g., molten iron, salty water) • Convection in this fluid layer • “rapid” rotation of planet/moon

44. Earth fits requirements Venus rotates too slowly Mercury, Mars & the Moon lack molten metallic cores Sun has strong field

45. Planetary Surfaces 4 major processes affect planetary surfaces: Impact cratering – from collisions with asteroids and comets Volcanism – eruption of molten rocks Tectonics – disruption of a planet's surface by internal stresses Erosion – wearing down or building up geological feature by wind, water, ice, etc.

46. Impact Cratering: The most common geological process shaping the surfaces of rigid objects in the solar system (Terrestrial planets, moon, asteroids)

47. Volcanism Volcanoes help erase impact craters

48. Volcanic outgassing: source of atmospheres and water

49. Erosion: the breakdown and transport of rocks and soil by an atmosphere. • Wind, rain, rivers, glaciers contribute to erosion. • Erosion can build new formations: sand dunes, river deltas, deep valleys). • Erosion is significant only on planets with substantial atmospheres.

50. Tectonics:refers to the action of internal forces and stresses on the lithosphere to create surface features. Tectonics can only occur on planets or moons with convection in the mantle Earth & Venus Jupiter’s moons Europa & Ganymede?