Geol 2000 Mon. Sep. 09, 2013 Today: Collect homework Hand out new homework Exam Friday Sept. 20 Review session THIS Friday Sept. 13 10AM? Geol. 216? (Discuss with class if this time works for students. If it does -- need to check if room is available.) I'll be out of town next week: Come to review session or see me THIS WEEK with any questions. Office Hours M & W 2:10-3:00 PM Fri. 11:00-11:50 AM Carrick Eggleston begins lectures on Wednesday Today: Formation of the Solar System (I dropped a few slides from the Solar System Lecture I posted late last week. This shorter one is now available on-line.) 1
Formation of the Solar System Formation of the Solar System Overview patterns in the solar system Orbital patters: Solar nebula model Compositional patterns: Equilibrium Condensation Model Equilibrium Condensation Model Reason for Terrestrial/Jovian planet difference Tests using detailed composition of meteorites Varying levels of activity on small vs. big objects 2
Solar System Origin Patterns in the Solar System, explained by Solar Nebula Model Equilibrium Condensation Model Optional Reference on reserve (Geology Library and E-Reserve) For Planetary Geology (G4460) class Wood: The Solar System Ch 1: An Overview of the Solar System Ch 6: Asteroids and Meteorites Ch 8: The Solar Nebula and Planet Formation 3
Relative Size of the Planets From Christiansen & Hamblin Fig. 1.3 4
Division into inner (terrestrial) and outer (Jovian) planets From: Wood Fig. 1.3 5
Patterns in Planetary Composition Terrestrial Planets Relatively small Made primarily of rocky material: Si, O, Fe, Mg perhaps with Fe cores (Note for earth H2O is only a very small fraction of the total) Jovian Planets Relatively large Atmospheres made of H2, He, with traces of CH4, NH3, H2O,... Surrounded by satellites covered with frozen H2O Within terrestrial planets inner ones tend to have higher densities (when corrected for compression due to gravity) Planet Mercury Venus Earth Mars (Moon) Density (gm/cm3) 5.44 5.24 5.50 3.94 3.36 Uncompressed Density (gm/cm3) 5.30 3.96 4.07 3.73 3.40 6
Patterns in the solar system Composition of planets Inner terrestrial planets: rocky worlds Outer Jovian planets: H, He rich gas giants More subtle trends within groups Higher density in close to sun for terrestrial planets Higher density out farther from sun (less H, He) for Jovian planets Motion of planets All in ~ circular, low eccentricity, prograde orbits All regular satellite systems of Jovian planets similar to this Planetary orbits regularly spaced Rotation less regular, but most prograde and low inclination Patterns in activity Large terrestrial planets more geologically active Small bodies less active and will preserve more of original conditions 7
Solar nebula explains patterns Disk like orbits explained by solar nebula Original proposals by Kant, Laplace and others As cloud of gas contracts to form star, it spins faster and faster Spinning material tends to flatten into disk Some of material makes it in to form star Other material left behind in disk surrounding star Composition explained by condensation model As disk material cools dust condenses Expect different composition dust at different temperatures temperature falls off as you get farther from sun For rocky or icy planets only the solid is used gas is lost For gas giant planets (for reasons we ll see) gravity of planet is strong enough to retain gas Activity explained by size Big planets retain heat and stay active, small ones cool quickly and die 8
Protoplanetary Disks our text: Horizons, by Seeds From Horizons, by Seeds Hubble images of proplyds in the Orion nebula Seen in silhouette against glowing gas clouds Central star just visible shining through dust at long (red) wavelengths 9
Equilibrium Condensation Model Start with material of solar composition material (H, He, C, N, O, Ne, Mg, Si, S, Fe...) Material starts out hot enough that everything is a gas May not be exactly true but is simplest starting point As gas cools, different chemicals condense First high temperature chemicals, then intermediate ones, then ices Solids begin to stick together or accrete snowflakes snowballs Once large enough gravity pulls solids together into planetesimals planetesimals grow with size At some point wind from sun expels all the gas from the system Only the solid planetesimals remain to build planets Composition depends on temperature at that point (in time and space) Gas can only remain if trapped in the gravity of a large enough planet 10
How can we test the condensation model? Bulk composition of major planets But geologic activity has reprocessed material so only elemental abundances are preserved Samples of primitive material surviving from early history of solar system Need to find places where there has been no geologic activity That means look for small bodies which cooled quickly Asteroids (and meteorites which come from them) Comets 11
Material Available Solar composition ranked by mass Element % by # H 91.0 He 8.9 O 0.07 C 0.03 Ne 0.01 N 0.008 Fe 0.003 Si 0.003 Mg 0.003 S 0.002 % by mass 70.9 27.4 0.8 0.3 0.2 0.1 0.1 0.07 0.06 0.04 H abundant, but no H compounds condense till relatively low temperature He, Ne are noble gasses don't condense at all H2O, CH4, NH3 only condense in outer solar system Fe, Si, Mg, +O compounds dominate inner solar system (i.e. Fe core, silicate rocks) 12
Reason for Jovian Planets Solar composition ranked by mass Element % by # H 91.0 He 8.9 O 0.07 C 0.03 Ne 0.01 N 0.008 Fe 0.003 Si 0.003 Mg 0.003 S 0.002 % by mass 70.9 27.4 0.8 0.3 0.2 0.1 0.1 0.07 0.06 0.04 Because you cannot condense O by itself (but only in compounds also containing Si, Mg, Fe), you don t have much material available for making terrestrial planets. You are limited by the low abundance of Si, Mg, Fe: Terrestrial planets are relatively small Once solid H2O becomes available you have lots more material Starting at Jupiter you can make a big enough core from solid H2O that you can gravitationally hold onto the H and He gas 13
Meteorites as samples of asteroids Meteorites (which survived) are not associated with comets They seem to come from the asteroid belt 14
Common Classification of Meteorites Simplest classification is Stones Stony-Irons Irons But there are two very different types of stones Stones that look like ordinary igneous rocks Stones that are strange assemblages of very primitive components Better classification based on how altered they are then with subdivisions based on details of composition 15
Types of Meteorites Undifferentiated Ordinary Chondrite Carbonaceous Chondrite Differentiated Stony-iron achondrite (igneous) Iron unsliced chondrite 16
Differentiated Asteroids/Meteorites Stones and Metals from differentiated planetesimals? S = mantles or crust M = cores Stony-Irons from interface Cooling rates from chemical details in the crystal patterns From the Astro 1050 text Horizons, by Seeds 17
Types of Meteorites Many fine divisions, we ll only care about the main ones Chondrites are undifferentiated Carbonaceous chondrites are the most primitive of the chondrites 18
Abundance of elements 19
Chondrites: Undifferentiated Chondrules: Olivine and Pyroxene CAI (Calcium Aluminum rich Inclusions) Matrix of phyllosilicates, FeO, Fe, FeS, C compounds 20
Reminder Bowen reaction series Olivine one early condensate Anorthite = CaAl rich feldspar another early condensate Cool a basaltic magma and ask which minerals crystallize, and in what order Liquid Solid equilibrium for basaltic composition 21
Equilibrium Condensation Model Start at top (high T) with everything a gas As temperature drops, solids begin to form As temperatures continue to drop, more gaseous components condense, reacting with existing solids to create new ones more stable at lower T Similar to Bowen Reaction Series but more complex Ca Plag Na Plag Mg Olivine Fe Olivine Olivine, Pyroxene Sheet silicates Asteroids 22
Components within primitive meteorites From Norton (2002) The Cambridge Encyclopedia of Meteorites Chondrules: Olivine and Pyroxene CAI (Calcium Aluminum rich Inclusions) Matrix of phyllosilicates, FeO, Fe, FeS, C compounds 23
Crystalline structure in chondrules Spherical shape shows solidification from liquid droplet Radial pyroxene structure shows rapid crystallization from nucleation point From McSween (1999) Meteorites and their parent planets 24
Primitive Meteorite Components and the Condensation Sequence Components seen in primitive meteorites are minerals predicted from various stages of the condensation sequence The fact they appear together demonstrates that equilibrium model is at best an approximation From McSween (1999) Meteorites and their parent planets 25
A more realistic view of solar nebula Condensation model provides paradigm Deviations from model show more complex history Material not all heated to enough to vaporize Not uniformly mixed Different stage condensates partially separated and sometimes modified or sorted Further modified after accretion in planetesimals From McSween (1999) Meteorites and their parent planets 26
Isotopic Anomalies in CAI inclusions Same type of diagram as Rb-Sr isochron, but for 26Al 26Mg except stable 27Al, not unstable 26Al plotted on X axis Al was live even though halflife is only 0.75 My 26 Lots of other isotopic anomalies 27
Solar System Timeline Timeline determined using short-lived isotopes, and very accurate Pb-Pb radiometric dates of formation of meteorite components. Wood Fig. 1.12 28
Solar System Summary Orbital Patterns determined by original Solar Nebula modified by planetary migration Chemical Patterns determined by Equilibrium Condensation Model modified by incomplete equilibrium, and migration location of snow line determines where giant planets begin (in other solar systems migration may move gas giants inward Geological Activity determined by size bigger objects remain active longer except in rare cases where tidal heating maintains activity 29