Forma&on of the Solar System 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. 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? Two models • Close Encounter – &dal stream 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 • Close Encounter • Nebular Hypothesis 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 Debris Disks Planet Forma&on Planet forma&on in flaMened disks, dictated by conserva&on of angular momentum, explains the shape of our Solar System Frac&on of Stars with Disks Hernandez et al 2007, ApJ 662, 1067 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 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 Condensa&on Sequence Temperature (K)
Condensate 1500
Fe2O3, FeO, Al2O3
1300
Fe, Ni
1200
Silicates
1000
MgSiO3
680
FeS
175
H 2O
150
NH3
120
CH4
65
Noble gases
Compe&ng Models • Accre&on • Gas collapse Domina&ng the Orbit Hill radius: rH=a (mp/3M⊙)1/3 • a: distance from planet to Sun • Mp: mass of planet • M⊙: mass of Sun Planet will sweep up all maMer 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 100s of km • Only the largest planetesimals survive fragmenta&on • 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 gravita&onally capture H and He • Grow very large • Moons form from accre&on disk onto Jovian planets • Sub-­‐nebula also has temperature gradient Jupiter as a Miniature Solar System 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 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 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 – 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 Craters Comparison of ~ 30 km craters on different bodies. Names and loca&ons : • 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 Na&onal 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 • 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 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) 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. The Asteroid Belt • Why not another terrestrial planet? • Total mass ~ 1 lunar mass • Perturba&ons by Jupiter Kirkwood Gaps 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. 40K becomes 40Ar – Parent isotope: 40K – Daughter isotope: 40Ar • Half-­‐life: &me it takes for ½ of parent nuclei to decay Radiometric Da&ng • 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 • 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 Radiometric Da&ng. II. Useful radioisotopes C14 → N14: t1/2 = 5730 years Al26 → Mg26: t1/2 = 717,000 years K40 → Ar40: t1/2 =1.25 billion years U238 → Pb206: t1/2 = 4.47 billion years Rb87 → Sr87: t1/2 = 49.4 billion years • 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