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Forma^On of the Solar System

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Forma^On of the Solar System Formaon of the Solar System Overview Observaons of trends in our solar system, plus observaons of young stars, yield a coherent picture of the formaon of planetary systems • The laws of physics (Week 3) 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. Making a Model A hypothesis for solar system formaon must explain: • Paerns of mo&on of the orbits • The 2 classes of planets • Asteroids and comets • Excepons • It should be predic&ve Does it apply to other solar systems? Two models 1. Close Encounter – &dal stream (Buffon 1745) 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 Two models 2. Nebular Hypothesis (Kant 1755; LaPlace 1790) 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 How do we know this happened? • We see disks around young stars Proplyd : Protoplanetary Disk Edge-on Disks HL Tau: Environs and ALMA image β Pictoris Debris Disks Planet Formaon Planet formaon in flaened disks, dictated by conservaon of angular momentum, explains the shape of our Solar System Frac&on of Stars with Disks Hernandez et al. 2007, ApJ 662, 1067 Towards a Hypothesis: • Disks are ubiquitous • Disks are a by-product of star formaon • Disks are planar • Disks rotate differen&ally • Disk composi&ons = stellar composi&ons Elemental Abundances Sun Mass Frac/on Earth Mass Frac/on H 0.74 Fe 0.32 He 0.24 O 0.30 O 0.010 Si 0.15 C 0.0046 Mg 0.14 Ne 0.0013 S 0.029 Fe 0.0011 Ni 0.018 N 0.00096 Ca 0.015 Si 0.00065 Al 0.014 Mg 0.00058 Cr 0.005 Planet Formaon in a Disk: Condensaon Sequence •Solar nebula had uniform composion •Temperature decreases outwards •Different materials condense at different T •H and He never condense Condensaon Sequence Temperature (K) Condensate 1500 Fe2O3, FeO, Al2O3 1300 Fe, Ni 1200 Silicates 1000 MgSiO3 680 FeS 175 H2O 150 NH3 120 CH4 65 Noble gases Solar Composi&on at Low Temperature Compeng Models • Accreon • Gas collapse Dominang the Orbit 1/3 Hill radius: rH=a (mp/3M⊙) • a: distance from planet to Sun • mp: mass of planet • M⊙: mass of Sun Planet will sweep up all maer within rH of its orbit 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 102 to 103 km • Only the largest planetesimals survive fragmentaon • This idea is supported by meteorites—metal grains embedded in rock Birth of the Earth • Small dust grains collide and s&ck • Once grain becomes large enough, gravity takes over • Runaway accre&on ensues. Chondrite Jovian Planets • Beyond frost line—H compounds can condense (ices: CH4, NH3, H2O) • Lots of ice—planetesimals grow large • Can gravitaonally capture H and He • Grow very large • Moons form in accre&on disks of Jovian planets • Sub-nebula also has temperature gradient Jupiter as a Miniature Solar System Core Accre&on or Gravitaonal Instability? • Core accre&on difficul&es: – Core growth takes longer than typical disk life&mes (few Myr) – Larger disks or lower opacity can help – Turbulence in the disk may help • Gravitaonal 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 – Gravitaonal instability works quickly—young (1 Myr old) stars should already have gas planets End of Planet Formaon • Solar wind / radiaon pressure blows disk away • Gaseous phase ~ 10 million yr • Strong magne&c field transfers angular momentum outward • Supported by observaons of young stars Frac&on of Stars with Disks Hernandez et al. 2007, ApJ 662, 1067 Resul&ng Solar System Inside Frost Line: small rocky planets Outside Frost Line: large gaseous planets The Debris • Solar wind removed gas – Small planetesimals remained • Asteroids: remaining rocky planetesimals – Planet formaon inhibited between Mars and Jupiter – Ini&ally lots of planetesimals – Most crashed into inner planets or were ejected • Comets: remaining icy planetesimals – Ini&ally all throughout outer solar system • KBOs – accreted too slowly Craters Comparison of ~ 30 km craters on different bodies. Names and locaons : • Golubkhina (Venus), 60.30N, 286.40E; • Kepler (Moon), 8.10N, 38.10W; • (Mars), 20.80S, 53.60E; • (Ganymede), 29.80S, 136.00W. 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 Naonal Laboratory). Images of lunar and venusian craters from Robert Herrick (Lunar and Planetary Ins&tute). Barringer Crater, Arizona Diameter: 1.2 km. Depth: 170m. Rim: 45m. Age: 50,000 years. Impactor: 50m Fe-Ni meteor Origin of the Moon (Luna) Luna is too large to have been 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 formaon – Collision between proto-Earth and Mars-sized object • Possible outcomes – Change in axial &lt – Change in rotaon rate – Complete destruc&on – outer layers of Earth blown off Support for Collisional Lunar Origin • Moon's composi&on matches outer layers of Earth • Moon has deficit of volales: all were vaporized • Other large impacts: – Pluto/Charon ? – Mercury – Uranus – Venus? (perhaps more complicated) The Nice Model Series of papers in Nature in 2005 • Solar system: Sun, planets, debris disk of planetesimals – Planets accrete or scaer planetesimals – Angular momentum exchange causes planetary migraon • Jupiter moves inward, Saturn, Uranus, and Neptune move outward • 1:2 orbital resonance between Jupiter and Saturn reached – Kick in eccentrici&es, destabilizaon of orbits – Uranus and Neptune scaered 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 scaering) at right &me. – Asteroids will also be perturbed at this &me. What is a Resonance? The Asteroid Belt • Why not another terrestrial planet? • Total mass ~ 1 lunar mass • Perturbaons by Jupiter Kirkwood Gaps Age of the Solar System • Radiometric dang: measure solidificaon age – Look at propor&ons of isotopes and atoms • Radioac&ve decay: – Breaking apart or change (p+ into no) of nucleus – E.g. 40K becomes 40Ar – Parent isotope: 40K – Daughter isotope: 40Ar • Half-life: &me it takes for ½ of parent nuclei to decay Radiometric Dang Useful radioisotopes 14 14 C → N: t1/2 = 5730 years 26 26 Al → Mg: t1/2 = 717,000 years 40 40 K → Ar: t1/2 =1.25 billion years 238 206 U → Pb: t1/2 = 4.47 billion years 87 87 Rb → Sr: t1/2 = 49.4 billion years Radiometric Dang • Rock forms with 40K but no 40Ar • Any 40Ar you find in the rock is due to radioac&ve decay • Remains trapped in the rock unless heated • Rao è age • Moon rock dang uses U and Pb (note: different chemical proper&es + understanding minerals) Result: lunar rocks ~ 4.4 billion years old The Age of the Solar System • Radiometric dang ≈ solidificaon age • Earth rock age < SS age (surface reshaping) • Moon rock age < SS age (impact) • Meteorite ages: – Have not melted or vaporized since SS formaon – Age ~ 4.55 billion years • Age consistent with solar evolu&on theory .
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