Transcript Powerpoint
EART 160: Planetary Science Last Time • Solar System Formation – Nebular Theory – Jeans Collapse – Runaway and Oligarchic Growth – Distribution of solar system materials – Planetary composition, structure – Late-stage accretion Today • Paper Discussions – Asphaug et al. (2006) ? – Thommes et al. (1999) • Origin of the Moon • Planetary Migration – Uranus and Neptune – Extrasolar Planets: “Hot Jupiters” • Meteorites? The Nebular Theory Explains: All planets’ orbits and Sun’s rotation in a single plane. Prograde orbits of all planets Planetary orbits nearly circular Angular momentum distribution Some meteorites contain unique inclusions Correlation of planetary composition with solar distance. • Meteorites different from terrestrial and lunar rocks • Spacing of the planets Giant impacts on all planetary bodies Prograde rotation, low obliquity of most planets Similar rotation periods for many planets Spherical distribution of comets Satellite systems of giant planets Gas Drag Circular Orbits • Gas supported by pressure, orbits slower than Keplerian • Gas slows down solids (but can carry dust) • Dust settles toward center of nebula • Gas drag damps random motions – Drag is strongest when planet is close – High velocities damped more – Velocities become more regular • Circular orbits • Once r > 1 km, gas drag inefficient • Technique used to circularize spacecraft orbits – Aerobraking Low v, r Weak damping High v, High r Strong damping Formation of the Moon • Co-accretion (sibling) – and formed together from Solar Nebula • Capture (spouse) – made a close pass to , captured into orbit • Fission (child) – Fast-spinning , a blob tore away • Apollo mission to determine which one is real. None of Them! • similar to ’s mantle. Depleted in Fe, siderophiles, volatiles. – Cannot form from same assemblage • O, Si-isotopes in and rocks IDENTICAL. – Meteorites all different – Implies common origin of the silicates. • Angular Momentum of - too small for fission. – -orbit not in equatorial plane. – Implies different trajectories Requirements • Explain Angular Momentum of System • Explain Metal depletion of Moon • Initially different orbits • Silicates mixed • Earth’s core untouched • Giant Impact! – Parasite-host relationship? – Genetic Engineering Experiment? – Other bad relationship analogy? Giant Impact Hypothesis Mars-sized Planetesimal Proto-Earth Asphaug et al., 2001 • Oblique impact, rotation increases – 5 hour day! • Impactor destroyed, Mantle stripped away • Cores merge, silicates form accretion disk • Some silicates fall back onto planet • Rest forms the Moon Canup and Asphaug, 2001 – At 12 R Migration • Do planets have to stay where they formed? • Why are Uranus and Neptune so small? • Extrasolar gas giants have TIGHT orbits! – Hot, hot, hot! WAY inside “frost line” Bwa ha ha! Um, guys? ! Cheese it! Gas Giant Formation • Beyond frost line, planets accrete rock AND ice • Grow to 10-15 M • Accrete Gas • Uranus and Neptune have little gas – Failed cores – BUT nebula too sparse that far out to even get cores! • Standard formation model doesn’t work! •Four 15 M cores between 4 and 10 AU. •Jupiter forms where nebula is the densest, gets big. •All three other cores scatter off Jupiter, flung outward •Saturn still close enough to accrete a bunch of gas. •What happens to Joop? Conservation of Angular Momentum! Thommes et al., 1999 Kuiper Belt Formation Early in solar system Ejected planetesimals (Oort cloud/Scattered Disk Objects) “Hot” population J S U Initial edge of planetesimal swarm N 18 AU Present day J 30 AU “Hot” population Planetesimals transiently pushed out by Neptune 2:1 resonance S U 48 AU N Neptune 3:2 Neptune stops at original edge resonance (Pluto) See Gomes, Icarus 2003 and Levison & Morbidelli Nature 2003 “Cold” population 2:1 Neptune resonance Hot Jupiters • Less than 0.05 AU from star • Problems with forming in situ HD209458b – Not enough material – No ice, gas at all! – Atmosphere gets stripped away? Image Courtesy ESA/ Alfred Vidal-Madjar / NASA Inward Migration • Type I: Dynamical Friction – Small Planets drive spiral density waves in disk – Outer wave imparts torque, planet loses L. – Moves inward. • Type II: Coevolution – Growing planet clears a gap in the disk – “Relay station” for L-transport – Moves L outward, planet and gap move inward Movie courtesy Phil Armitage http://jilawww.colorado.edu/~pja/planet_migration.html Consequences • Hot Jupiters probably were Regular Jupiters that got Type II Migration • Giant moves in – What does Conservation of Angular Momentum say? – Terrestrial Planets move out. Wayyyy out! – Why did we escape this fate? • Atmosphere stripped off by solar wind? – Chthonian planet? Meteorites • Extraterrestrial matter that falls on Earth – VERY late-stage accretion • Why do we care? • Pristine samples from the early solar system • This is how we know what the early solar system was made of! • Vital source of vitamins and minerals Earth-Crossing Asteroids How old is the solar system? • We date the solar system using the decay of long-lived radioactive nuclides e.g. 238U-206Pb (4.47 Gyr), 235U-207Pb (0.70 Gyr) • These nuclides were formed during the supernova which supplied the elements making up the original nebula • The oldest objects are certain meteorites, which have an age of 4550 Myr B.P. Meteorite isochron (from Albarede, Geochemistry: An Introduction) Radioactive Dating N = N0 e-lt = N0 e-t/t N – number of radionuclides now N0 – number we started with at time 0 t – time since start t– Average lifetime of radionuclide © 1996 Frank Steiger; permission granted for retransmission. t1/2 = t ln(2) Half-life (time for half the material to decay) Short Lived Radioactive Isotopes • Some meteorites once contained live 26Al, which has a half-life of only 0.7 Myr. So these meteorites must have formed within a few Myr of 26Al production (in the supernova). • So the solar system itself is also 4550 Myr old • Decay of 26Al releases a LOT of energy, could have melted, differentiated early-forming planetesimals • CAI (Ca-Al inclusions) formation– oldest recorded event in SS. Early SS timing relative to this. • Was 26Al distributed uniformly? Types of Meteorites Type Description Abundance Iron Fe, Ni. Low temperature inclusions Formed deep within differentiated planetary body Ni content tells us about parent body Stony-Iron Stony Mix of metal and rock. Intermediate depth 4% 1% 95% Achondrites No metal or chondrites, similar to basalts Crustal source? 9% Chondrites Contain glassy chondrules, not remelted Ancient Planetesimals 86% Carbonaceous Low-T (< 500 K) Volatiles, organics Ordinary Higher-T Little volatiles, C 5% 81% Famous Meteorites ALH 84001 Martian Meteorite Once thought to have ET life Willamette Iron Meteorite 15.5 tons Images Courtesy NASA Allende Carbonaceous Chondrite How do you know it’s a meteorite? • Look somewhere without rocks! • Antarctica! • 23000+ of meteorites found in Antarctica Source • We can use the meteorite composition to track it back to a type of asteroid • Classify asteroids into types based on spectrum – M-type asteroids UNKNOWN composition – NOT M-class planets • Sometimes we can determine the parent asteroid – Pallasites DO NOT come from 2 Pallas cm-sized olivine crystals in Fe-Ni matrix Next Time • HW 2 will be posted over the weekend – DUE MONDAY 28 JANUARY • Monday’s a holiday! See you on Wednesday. • • • • Meteorites Asteroids The Late Heavy Bombardment Start on Planetary Interiors? • • • • • Find another isotope of the same element as the daughter that is never a result of radioactive decay (call that isotope ``B'' for below). Isotopes of a given element have the same chemical properties, so a radioactive rock will incorporate the NONradioactively derived proportions of the two isotopes in the same proportion as any nonradioactive rock. Measure the ratio of isotopes A and B in a nonradioactive rock. This ratio, R, will be the primitive (initial) proportion of the two isotopes. Multiply the amount of the non-daughter isotope (isotope B) in the radioactive rock by the ratio of the previous step: (isotope B) × R = initial amount of daughter isotope A that was not the result of decay. Subtract the initial amount of daughter isotope A from the rock sample to get the amount of daughter isotope A that IS due to radioactive decay. That number is also the amount of parent that has decayed (remember the rule #parent + #daughter = constant). Now you can determine the age as you did before. 26Al is a radioactive isotope that decays into 26Mg, a stable isotope, with a half-life of 0.73 million years. Although this is so short that all of it has decayed billions of years ago, its presence at the beginning of the solar system has been conclusively established by the discovery of excesses of its daughter isotope 26Mg in the most primitive solar system objects. If these objects containing 26Al at the time of their formation remained relatively undisturbed (i.e., did not experience high temperatures), the decay product 26Mg was frozen in and today provides a record of the original 26Al. The ratio of 26Mg excess measured now relative to the amount of the stable isotope 27Al yields the original 26Al/27Al ratio.