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
Close Encounter Nebular Hypothesis Two models
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 Fe 2 O 3, FeO, Al 2 O 3 1300 Fe, Ni 1200 Silicates 1000 MgSiO 3 680 FeS 175 H 2 O 150 NH 3 120 CH 4 65 Noble gases
Compe&ng Models Accre&on Gas collapse
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
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: 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
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 < 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. 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
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
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
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