Forma&on of the Solar System

Transcription

1 Forma&on of the Solar System

2 Overview We can explain the observed trends in our solar system through the nebular theory The laws of physics (Chapter 4) come into play here. The major dis&nc&on between terrestrial planets and Jovian planets comes from where in the solar nebula they formed The excep&ons arise from collisions and other interac&ons We can find the age of our solar system by studying radioac&ve isotopes in meteorites and rocks.

3 Making a Model A hypothesis for solar system forma&on must explain: PaMerns of mo&on of the orbits The 2 classes of planets Why asteroids and comets Excep&ons It should be predic&ve Does it apply to other solar systems?

4 Two models Close Encounter &dal stream

5 Physics Hot gas will expand due to high pressure, rather than collapsing Gas pressure nt N is gas density T is the gas temperature If the pressure exceeds that of the interplanetary medium, it will expand

6 Close Encounter Nebular Hypothesis Two models

7 Physics Large, cold cloud of gas (D ~ few ly) Collapse begins Gravity pulls cloud together Cloud heats (why?) Cloud Rotates (why?) Disk forms (why?) Sun forms at hot center

8 How do we know this happened? We see disks around young stars

9 Proplyd : protoplanetary disk

10 Edge- on Disks

11 Debris Disks

12 Planet Forma&on Planet forma&on in flamened disks, dictated by conserva&on of angular momentum, explains the shape of our Solar System

13 Frac&on of Stars with Disks Hernandez et al 2007, ApJ 662, 1067

14 Elemental Abundances Sun Mass Frac+on Earth Mass Frac+on H 0.74 Fe 0.32 He 0.24 O 0.30 O Si 0.15 C Mg 0.14 Ne S Fe Ni N Ca Si Al Mg Cr 0.005

15 Planet Forma&on in a Disk Solar nebula had uniform composi&on Temperature decreases outwards Different materials condense at different T H and He never condense

16 Condensa&on Sequence Temperature (K) Condensate 1500 Fe 2 O 3, FeO, Al 2 O Fe, Ni 1200 Silicates 1000 MgSiO FeS 175 H 2 O 150 NH CH 4 65 Noble gases

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19 Compe&ng Models Accre&on Gas collapse

20 Domina&ng the Orbit Hill radius: r H =a (m p /3M ) 1/3 a: distance from planet to Sun M p : mass of planet M : mass of Sun Planet will sweep up all mamer within r H of its orbit

21 Terrestrial Planets Inside frost line: rock/metal condenses Small size reflects limited material Seed grow via accre&on to make planetesimals Planetesimals grow via gravity to 100s of km Only the largest planetesimals survive fragmenta&on This idea is supported by meteorites metal grains embedded in rock

22 Birth of the Earth Small dust grains collide and s&ck Once grain becomes large enough, gravity takes over Runaway accre&on ensues.

23 Chondrite

24 Jovian Planets Beyond frost line H compounds can condense (ices: CH 4, NH 3, H 2 O) Lots of ice planetesimals grow large Can gravita&onally capture H and He Grow very large Moons form from accre&on disk onto Jovian planets Sub- nebula also has temperature gradient

25 Jupiter as a Miniature Solar System

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27 Core Accre&on or Gravita&onal Instability? Core accre&on difficul&es: Growth of cores takes longer than typical disk life&mes (few Myr) Larger disks or lower opacity can help Turbulence in the disk may help Gravita&onal instability difficul&es: Requires disk to cool Only operates in massive disks and at large orbital distances (> 100 AU) May not be able to explain the higher metallici&es in giant planets Gravita&onal instability works quickly young (1 Myr old) stars should already have gas planets

28 End of Planet Forma&on Solar wind / radia&on pressure blows disk away Gaseous phase ~ 10 million yr Strong magne&c field transfers angular momentum outward Supported by observa&ons of young stars

29 Resul&ng Solar System Inside Frost Line: small rocky planets Outside Frost Line: large gaseous planets

30 The Debris Solar wind removed gas Small planetesimals remained Asteroids: remaining rocky planetesimals Between Mars and Jupiter planet forma&on inhibited Ini&ally lots of planetesimals Most crashed into inner planets or were ejected Comets: remaining icy planetesimals Ini&ally all throughout outer solar system

31 Craters Comparison of ~ 30 km craters on different bodies. Names and loca&ons : Golubkhina (Venus), 60.30N, E; Kepler (Moon), 8.10N, 38.10W; (Mars), 20.80S, 53.60E; (Ganymede), 29.80S, W. Image Credit: Image of Ganymede Crater contributed by Paul Schenk (Lunar and Planetary Ins&tute). Image of Mars crater obtained from the Mars Mul&- Scale Map, Calvin Hamilton (Los Alamos Na&onal Laboratory). Images of lunar and venusian craters from Robert Herrick (Lunar and Planetary Ins&tute).

32 Barringer Crater, Arizona Diameter: 1.2 km. Depth: 170m. Rim: 45m. Age: 50,000 years. Impactor: 50m Fe- Ni meteor

33 Origin of the Moon Moon too large to be captured by Earth Composi&on different than Earth Moon has lower density (less iron/nickel) Could not have formed in same place/&me as Earth Giant Impact Many large planetesimals lerover during SS forma&on Collision between proto- Earth and Mars- sized object Possible outcomes Change in axial &lt Change in rota&on rate Complete destruc&on outer layers of Earth blown off

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37 Support for Collisional Lunar Origin Moon's composi&on matches outer layers of Earth Moon has deficit of vola&les: all were vaporized Other large impacts: Pluto/Charon Mercury Uranus Venus? (perhaps more complicated)

38 A Model Series of papers in Nature in 2005 Solar system: Sun, planets, debris disk of planetesimals Planets accrete or scamer planetesimals Angular momentum exchange causes planetary migra&on Jupiter moves inward, Saturn, Uranus, and Neptune move outward 1:2 orbital resonance between Jupiter and Saturn reached Kick in eccentrici&es, destabili&za&on of orbits Uranus and Neptune scamered outward, switch posi&ons Small bodies move inward Interac&ons explain current orbital radii and eccentrici&es 1:2 resonance explains late heavy bombardment period Ini&al geometry can give resonance (and hence scamering) at right &me. Asteroids will also be perturbed at this &me.

39 The Asteroid Belt Why not another terrestrial planet? Total mass ~ 1 lunar mass Perturba&ons by Jupiter Kirkwood Gaps

40 Age of the Solar System Radiometric da&ng: measure solidifica&on age Look at propor&ons of isotopes and atoms Radioac&ve decay: Breaking apart or change (p into n) of nucleus E.g. 40 K becomes 40 Ar Parent isotope: 40 K Daughter isotope: 40 Ar Half- life: &me it takes for ½ of parent nuclei to decay

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42 Radiometric Da&ng Rock forms with 40 K but no 40 Ar Any 40 Ar you find in the rock is due to radioac&ve decay Remains trapped in the rock unless heated Ra&o tells you age Moon rock aging uses U and Pb (note: different chemical proper&es + understanding minerals) Result: moon rocks ~ 4.4 billion years old

43 Radiometric Da&ng. II. Useful radioisotopes C 14 N 14 : t 1/2 = 5730 years Al 26 Mg 26 : t 1/2 = 717,000 years K 40 Ar 40 : t 1/2 =1.25 billion years U 238 Pb 206 : t 1/2 = 4.47 billion years Rb 87 Sr 87 : t 1/2 = 49.4 billion years

44 Radiometric da&ng solidifica&on age Earth rock age < SS age (surface reshaping) Moon rock age < SS age (impact) Meteorite ages work Have not melted or vaporized since SS forma&on Age ~ 4.55 billion years Age consistent with solar evolu&on theory