Today

1. Today Today: Review of Parts 4 & 5 of the text (Weeks 8-12) Cover the last of the material Next week Assignment covering Weeks 8-13 due Projects also due next week Class summary Any student presentations

2. Review: Weeks 8-12 History of our Planetary system Planets Other than Our Own in our Solar System Habitability of places other than Earth Finding Planets outside of the Solar System Visiting or Communicating?

3. Our Solar System Orbits and Gravity Planetary System Formation

4. Orbits Planets are falling towards Sun due to gravitational acceleration Moving toward the side fast enough that they miss Moving too fast – escape entirely, leave Sun Move too slowly – fall into Sun Same with satellites circling Earth, or Sun orbiting in our galaxy, or...

5. Gravity Gravity acts between all massive objects Gravitational force is equal on both objects If orbiting, both objects move, not just one, since both are being acted on by gravity Both orbit the center of mass of the system Equal mass objects; center of mass is at the center of the two objects

6. Gravity If one body is more massive, then gravitational force is increased Center of mass tilts towards more massive body Forces still equal Equal force on lighter body moves it more than the same force on the heavier body Lighter object moves larger distance than heavier object

7. Gravity Force of gravity also increases as objects get nearer Inverse Square Law (same as light)

8. Orbits Kepler's Laws: (EMPERICAL) Planets travel in ellipses, with sun at one focus of ellipse Area swept out by radius is equal over any equal amount of time Square of the planet's period (the `year' for that planet) proportional to the distance to the sun cubed. P2 ~ a3

9. Planets Almost all planets are in same plane All planets (except Uranus) rotate more or less in the same plane, as does Sun Very suggestive of the idea that planets, Sun formed from a disk, as we discussed before Suggested by Laplace in 1600s. Disk near star is depleted in Hydrogen, Helium by evaporation

10. Planet Formation As disk cools, gas/dust disk can begin condensing Grains form, which themselves agglomerate to larger particles Regions where disk is originally dense condense faster, gravitationally attract more material Process of continued agglomeration can form planets

11. Instability Some processes are naturally stable Burning in main sequence stars Core heats up – outer layers puff up – core cools down Automatically stabilizes itself Ball in a right-side-up bowl Once there's a region of high density in a gas cloud or disk, increase in gravitational attraction to that region... Unstable Ball on an up-side-down bowl

12. Planet Formation Proto-planetary-core starts sweeping out material and planetesimals at its radius Accrete material streams in from just outside or inside its radius There is a limit to this process; if there are planets forming on either side, eventually the gaps collide – no more new material This process of slowly sweeping up and accreting material can take millions of years

13. Mystery: `Hot Jupiters' A Jupiter couldn't form at 1AU; evaporation would prevent such a gas giant from forming Many of the extra-solar planets observed are gas giants at distances ~ 1AU What happened? Two possibilities: Migration Different formation mechanism

14. Planet Formation Migration is possible As planets form and accrete material, they experience a drag force Drag takes energy from planets motion and they fall inwards

15. Planet Formation Fast formation is also possible In sufficiently massive disk, instabilities can occur much faster, and on larger scales Can happen quickly enough that perhaps giants can form near star

16. Our Solar System Other Bodies Mercury The Moon Venus Mars Gas Giants Gas Giant Moons

17. The Moon No atmosphere No geological activity No water -> no erosion Can provide information about formation of solar system that is absent from Earth

18. Mercury Similar to moon Similar size Small, empty, simple Very close to Sun No atmosphere to mediate temperature swings: +750o F in sun -230o F in shade

19. Moon's Cratering Nothing to alter surface Complete history of cratering in Moon's history From predicted cratering rate, one expects that crust of moon formed very quickly in solar system history

20. Possible Moon Formation Scenario

21. Possible Moon Formation Scenario Explains similar Oxygen abundances Very different from meteorites Explains fewer volatiles If Earth's iron core had already settled, impact would have dislodged crust material Heat of impact would have vaporized volatiles

22. Venus Closest to Earth ¾ as far away from Sun as Earth is Very similar to Earth's size, density Covered by thick, opaque clouds

23. Venus Runaway greenhouse effect Hot: very near sun Water begins to evaporate Water vapor is a greenhouse gas! Surface gets hotter, more water evaporation Surface is hundreds of degrees No liquid water

24. Mars Red planet between Earth and Asteroid Belt Half again as far away from Sun as the Earth is Expect it to be ~100o F colder than Earth on average Average too cool for water Peak temps ~ 70o F (but -130 at night!)

25. Mars Near asteroid belt Likely more collisions than Earth Large impacts can blow off significant rocky material Meteorites As well as gases (atmosphere)

26. Mars ~1/2 radius of Earth ~1/10 mass ~40% surface gravity Force of a 1 lb weight less than ½ lb on Mars Less gravity holding the atmosphere in place

27. Mars Too little gravity to be able to hold onto a significant atmosphere Atmospheric pressure less than 1% of Earth's

28. Evaporation What causes evaporation of liquid, and what prevents it?

29. Evaporation What causes evaporation of liquid, and what prevents it? Fastest moving water (say) molecules can escape into atmosphere Water molecules in atmosphere can collide into water and become part of the liquid Balance is reached when evaporating water = condensing water

30. Evaporation Can change balance: Little water in atmosphere, evaporation happens faster (Why feel so sticky on a humid day) If air pressure is very low, evaporated water molecules can move very far away from pool of water Fewer around to condense Faster evaporation

31. Evaporation Effect of atmospheric pressure happens on our own planet Reason for `high-altitude cooking instructions' on some boxes Higher altitude -> lower air pressure -> evaporation is easier -> lower boiling point

32. Evaporation Martian atmospheric pressure < 1% of Earth's (Earth's atmosphere at 15 miles / 80,000 ft) Water boiling point is so low that any liquid water evaporates immediately No free water possible on surface

33. Evaporation But water ice DOES exist on Mars: Polar ice caps Mostly (on top) dry ice (frozen CO2) Underneath, visible when CO2 has sublimated, water ice Quite likely some trapped under surface: `permafrost'

34. The Giants • The Giants are sometimes all called `Jovian' planets after Jupiter • After more exploration showed their diversity, this term lost favour

35. The Giants • The giant planets can be weighed very accurately by measuring the speed of their moons. • Much heavier than Earth, but not so heavy considering their size • Densities 600 – 1600 kg/m3, compared with Earth's 5700 kg/m3 • Mostly made of gas/liquids?

36. The Birth of Giants • In outer solar system, cooler • Less evaporative stripping of volatile gases • If sufficiently massive cores form, can keep even volatile gases • These gases will be representative of the very early solar system

37. The Birth of Giants • Since early solar system is largely composed of Hydrogen, so will gas giants • Rocky or Icy or Slushy core • High-hydrogen atmosphere has some similarities to atmosphere in Miller-Urey experiment • Can form lots of organics

38. The Birth of Giants • Large mass -> high pressure, temperature at centre • Temperature at centre of Jupiter ~ 4 times surface of Sun! • Collapse from origin of planet still slowly continuing • Releases heat energy • These planets have a source of heat Jupiter in Infra-red

39. The Birth of Giants • Gas giants emit more heat than they absorb from Sun • At earlier times, would have been much hotter • Moons, which are nearby, heated by their nearby planet • Many of these moons are large (planet-sized) • Moons might be interesting for life? Jupiter in Infra-red

40. The Moons of Giants • Planets large enough that many moons were also formed • Many of them planet sized in their own right • Get heat from planet • Some (Io/Jupiter) effected by planets magnetic field • Atmosphere? (Titan, Saturn) • Water? (Europa, Jupiter)

41. The Moons of Giants • Formation: like planets around sun • Rotating body, disk forms • Moons generally along plane of rotation of planet

42. Gas Giants • Convection is a fundamental process • Happens everywhere • Fluid heated at bottom rises, cools, falls back down • Gas giants have hot centres • Large-scale motions • Mix material

43. Gas Giants • Makes it difficult to imagine life forming • No real surface to live on • Chemicals constantly being mixed around • No originally contained environment (`protocell')

44. Moons • Gas giants have planet-sized moons • At least one (Titan) has a significant atmosphere • Another (Europa) very likely has liquid salty water under a layer of ice

45. Europa • Very suggestive it has a liquid underneath • No cratering • Many fractures, ridges on surface • What would this mean for life? • If some source of energy on inside (geothermal, chemical), very real possibility of some sort of life

46. Titan • Very Cold • Massive, Cold enough to have an atmosphere (1.5 x as dense as ours!) • No oxygen • No liquid water • Hydrogen rich • Interesting organic chemistry • Lakes of hydrocarbons? • Huygens probe 2005

47. How Unique is Earth? • What is special about Earth? • How important/rare are those things? • How many such planets are there likely to be?

48. Earth • Atmosphere • Large surface gravity • Reasonable temperature • Rocky surface • Large moon • Lots of heavy elements

49. How Important/Rare are these? • Heavy elements; • Likely ubiquitous in planets around Pop I stars

50. How Important/Rare are these? • Rocky Surface • Can happen if there is heavy elements (see above) • Probably true of all planets close enough to have liquid water • (But planet migration)