PLATO - 7. The outer solar system. Tethis eclipsed by Titan; Cassini (NASA)

Transcription

1 PLATO - 7 The outer solar system Tethis eclipsed by Titan; Cassini (NASA) 1

2 Titan (Saturn s largest moon) Cold temperature weather Thick Nitrogen atmosphere Similar atmospheric pressure to Earth Liquid methane rivers and lakes? Huygens probe landed in 2005 Icy rocks on surface Signs of erosion 2

3 Titan (Saturn s largest moon) Huygens panorama (NASA, 2005) 3

4 Enceladus 4

5 Enceladus An ice world Young, smooth surface Much like Europa? 5

6 Enceladus An ice world Young, smooth surface Much like Europa? Water vapor from surface! Possibly liquid ocean 6

7 Enceladus An ice world Young, smooth surface Much like Europa? Water vapor from surface! Possibly liquid ocean Possible responsible for E-ring Cassini image (NASA) 7

8 Enceladus An ice world Young, smooth surface Much like Europa? Water vapor from surface! Possibly liquid ocean Possible responsible for E-ring Synchronous orbit Tidal heating still possible Combined with radioactive heating? 8

9 Neptune s moon Triton Bigger than Pluto Retrograde elliptical orbit Captured? Atmosphere: Massive Cold (far out in solar system) Eruptions? 9

10 Pluto 10

11 Pluto Discovery 1930 (Clyde Tombaugh) Based on erroneous claims of perturbations to Uranus & Neptune orbits Properties: Earth masses High eccentricity (elliptical orbit), crosses Neptune orbit In stable 3:2 resonance with Neptune (hiding from Neptune) Density: 2 kg/liter 11

12 The Pluto Controversy 2006, International Astronomical Union, General Assembly: A "planet" is defined as a celestial body that A) Is in orbit around the Sun B) Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape C) Has cleared the neighborhood around its orbit. All Terrestrial and Jovian planets satisfy these criteria Objects that satisfy (A) and (B) called dwarf planets 12

13 Trans-Neptunian Objects Name Diameter Mass Semi-major axis Orbital Eccentricity Eris 2600 km kg 67.8 AU 0.44 Pluto 2310 km kg 39.4 AU FY9 ~1600 km? 45.8 AU 0.16 Sedna ~1500 km? 526. AU EL61 ~ km kg 43.3 AU 0.19 Charon 1210 km kg 39.4 AU satellite of Pluto Quaoar ~1000 km? 43.6 AU Orcus ~950 km kg 39.4 AU

14 Trans-Neptunian Objects TNOs: Unkown, but large number of Kuyper Belt objects Crossing orbits (not cleared ) Small, icy objects Some are spherical (e.g., Eris) TNOs that are dwarf planets are called Plutoids Some TNOs have similar orbits to Pluto TNOs in 2:3 resonance with Neptune are called Plutinos 14

15 Comets Comet Hale-Bopp; (Rob Jones) 15

16 Comets Comet West 16

17 Comets Orbital periods From ~ 100 years to millions of years Objects from outer Solar System Kuyper Belt (TNOs) Short period comets Roughly in plane of ecliptic, short periods Oort Cloud (out to 100,000 AU) Long period comets Random orientations, long periods 17

18 Comet Origins Oort cloud or Kuyper belt objects Kicked onto elliptical orbits by gravitational encounters 18

19 Comet Structure Core: ~ 1-10 km snow ball Coma: ~ 10 5 km gas & dust cloud Halley s Comet Comet C/2001 Q4 Envelope: ~ 10 6 km Hydrogen cloud Tail: ~10 8 km gas a Comet Hale-Bopp 19

20 Comet Structure 20

21 Comet Probes Deep impact: Crashed probe into comet Tempel 1 Stardust: Collected samples from comet Wild 2 Returned them to Earth Contains crystalline material Must have formed at high temperature Early transport of matter out from inner Solar system to Kuyper belt? 21

22 Comet Tails Gas and dust supply: Comets spend most of their time far from Sun When they approach, they heat up Ice sublimates (goes from solid to gas) Core releases gas and dust cloud But why the tail? Recall: Tail always points away from the Sun Radiation pressure Solar wind 22

23 Comet Tails Radiation pressure: Photons have energy and momentum When a photon is absorbed, its momentum lives on... Both gas & dust particle pushed away from Sun Solar wind: Energetic particles and magnetic fields Streaming away from Sun Pushes only gas particles 23

24 Tails Pin the tail on the comet: Where is the gaseous tail going to point at the location of the comet along its path? 24

25 A Tale of Two Tails Why are there two tails? Gas particles Are locked to the Solar Wind magnetic field They move straight out from the Sun Dust grains Get pushed out only by radiation pressure This is like orbiting a star of lower mass (weaker gravity) Dust particles go into trailing Keplerian orbits (slower, further out) 25

26 Comet Tails 26

27 Meteor Showers Comets leave behind pebbles/dust grains In orbit around Sun When dust orbits cross Earth s obit Comet bits rain down Meteor shower Most prominent: Perseids (August 13) 27

28 Question Which direction would you see the asteroid coming from, given the Earth s motion and the asteroid s motion? A) B) Earth C) 28

29 Meteors Meteoroid: Burns up in atmosphere Meteorite: Reaches the ground 29

30 Asteroids Not all particles hitting Earth are small cometary dust grains... Wolf Creek Crater, Australia 30

31 Asteroids ~150,000 catalogued so far Sizes: Densities: Many times more uncounted for yet Few km to 1000 km (Ceres) about 3 kg/liter Loose rock 31

32 Asteroids Most occupy asteroid belt Between Mars & Jupiter Trojan Asteroids Jupiter Trojan Asteroids Found based on Bode s rule Planets spacing follows pattern Mars orbit Probable coincidence Jupiter s orbit Planet missing between Jupiter and Mars Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Ceres was first called a planet 32

33 Asteroids Jupiter has cleared out resonance gaps Small fraction of asteroids are Earth-crossing 33

34 Impacts 34

35 Impacts 0.7 miles Barringer impact crater in Arizona 35

36 Impacts 15 miles Ries, Bavaria (Germany) 36

37 Impacts 40 miles Manicougan Crater (Canada) 37

38 Impacts Energy: A 1 km object has the impact energy of 70 million kilotons of TNT (100 x the destructive power of all nuclear weapons during height of cold war combined). A 10 km object has 1000 x more energy yet! Barringer crater, AZ 38

39 Impacts Tunguska event, 1908, Siberia 5-30 megatons of TNT 39

40 Impacts A 10 km asteroid can lead to mass extinctions 65 Million years ago: Cretaceous (dinosaur) extinction Yukatan Peninsula 180 km Fossil records: Mass extinction coincident with deposition of Iridium all over globe Not usually found in Earth s crust, But common in asteroids 40

41 Extinction rate KT-event today 41

42 Cratering Rate Decreased with time Maria formed after cratering rate declined Late heavy bombardment? Read off from the chart how old a surface is Current rates on Earth: Size Rate: once every Energy (kilotons) < 1 cm 15 minutes 10-6k 5 m 1 yr m 1000 yrs 10,000 1 km ~500 thousand yrs 7.5x km ~10 million yrs k 10 km ~200 million yrs 7.5x10 10 k 42

43 Crater Structure Size ~ x size of impacting object 43

44 Cratering Paintball simulation vs. Tycho (Moon) Rays 44

45 Comet impacts Comet Shoemaker Levy 9 45

46 Exoplanets Direct image of a Jovian planet around a brown dwarf sub-star 46

47 The Exoplanet Quest Solar nebula theory: Other stars should also form planetary disks They should be broadly similar the the Solar System Why is it hard to find planets around other stars? They are very dim They are very close to a much brighter object They have much lower masses than stars They are very small compared to stars We don t know where to look for them a priori 47

48 Successful Methods We have found and/or observed planets using these methods: Radial velocity detection Planetary transit observations (eclipses) Direct imaging Pulsar timing Gravitational lensing Binary timing 48

49 Kepler Orbits Sun moves due to planet s gravity Observe long and accurately enough: Infer masses and periods of planets Without ever having to actually see a planet! Other stars with planets must do the same thing! Look for wobbling stars 49

50 Radial Velocity 50

51 Radial velocity 51

52 Radial Velocity Most successful method Pioneered by Marcy & Butler Finding lots of exoplanets 494 planets to date Goeff Marcy In 414 planetary systems It can measure: Lower limit on planet mass Orbital Periods Semi-major axis Paul Butler 52

53 orbital axis Orbital Inclination i The angle between the orbital axis and the observer i=0 : i=90 : i=60 : No Maximum Some Doppler Doppler Doppler shift shift shift i=90 i=60 i=0 orbital axis orbital axis 53

54 We measure: Radial Velocity Radial velocity vrad of the star (Doppler shift) Orbital period P We know: Mass of star Mstar (from stellar structure theory) Kepler s 3rd law: a 3 planet = GM starp 2 4π 2 54

55 Radial Velocity Example: 51 Pegasi 51 Pegasi P = 4 days, v rad =57ms 1 M star = kg Planet mass: M 51Peg =0.47M Jupiter / sin (i) A lower limit in mass (inclination) 55

56 Planet Mass (Jupiter masses) Planet Demographics What kinds of planets have been found? planet systems total Mostly massive planets Mostly close to star High eccentricities Jupiter-size planets at close distance to star: Called Hot Jupiters Jupiter s orbit average orbital distance (AU) 56

57 Planet Demographics Orbital types: Many have high eccentricity! Much higher than Solar System Many exoplanets: On elliptical orbits 57

58 Bias: The way we design observations influences the results We must be careful to account for this Example: We are much more likely to find objects that produce a large radial velocity signal, i.e., planets that... have a large mass (more gravity) are close to the star (more gravity) Basically: Radial velocity searches can only find hot Jupiters. That does not mean that all exoplanets are hot Jupiters. 58