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Search for Life in the Universe Chapter 4 Geological History of the Earth (Part 1) 5/23/2017 AST 248, Fall 2005 1 Outline (1) • Reading the Earth’s History – – – – – Rocks and Fossils Sedimentary Strata & Relative Age Geological Time Scale Radiometric Dating: Absolute Age How Far the Record? • Formation of the Earth – – – – Accretion Differentiation Formation of the Moon Age of the Earth 5/23/2017 AST 248, Fall 2005 2 Outline (2) • Hadean Earth – Heavy Bombardment – Outgassing – Impact Sterilization – Origin of the Continents 5/23/2017 AST 248, Fall 2005 3 Rocks and Fossils (1) • Rock and fossil record: – Need: human history too short, only one millionth of the life of the Earth – Rock record: structure & composition climate – Fossil record: environment at the time and evolution • Rocks: – Igneous: molten rock that cools and solidifies – Sedimentary: gradual compression of sand and silt – Metamorphic: rock changed by high pressure or temperature without melting – Rocks can change from one type to another – Constituents: minerals 5/23/2017 AST 248, Fall 2005 4 Rocks and Fossils (2) • Fossils: – (Almost) only found in sedimentary rock – Minerals replace organic material; rarely, some organic material survives – Coprolites: fossils of petrified excrement – Stomach content: undigested food and/or gastroliths, pebbles used for digestion – Footprints: tell of mobility and structure, e.g., dinosaurs – Only a small fraction of living organisms leave fossils 5/23/2017 AST 248, Fall 2005 5 Sedimentary Strata & Relative Age • (Almost) only source of fossils • Strata differentiation: – Rate of accumulation – Composition – Grain size • Chronological sequence: – Strata laid on top of each other – No single site contains all epochs – Overlap of multiple sites complete geological sequence 5/23/2017 AST 248, Fall 2005 6 Geological Time Scale • Initial qualitative division; radiometric dating came later • Hadean (“hellish earth”) eon – Hellish conditions in the young Earth (4.54.0 byr) • Archaean (“ancient life”) eon – Single-cell organisms: first fossil record (4.02.5 byr) • Proterozoic (“earlier life”) eon – Single-cell organisms: fossils visible under a microscope (2.50.54 byr) • Phanerozoic (“visible life”) eon – Multi-cell organisms: fossils visible to the naked eye (0.540 byr) – Many subdivisions based on fossil record – Usually rapid transition in the sedimentary layers 5/23/2017 AST 248, Fall 2005 7 Radiometric Dating: Absolute Age • Radioactive decay – Elements have multiple isotopes (different # of neutrons for the same # of protons) – Some isotopes are unstable and decay: • Alpha decay: emit a helium nucleus • Beta decay: emit an electron • Electron capture: absorb an electron – Decay is probabilistic: • Half life: characteristic lifetime • Exponential decay • Geologically useful: half-life thousands to billions of years • Radioactive dating: – Igneous (avoid metamorphic) rocks: determine last solidification – Need to know initial composition, e.g., no inert argon – Check for consistent dates from different isotopes 5/23/2017 AST 248, Fall 2005 8 5/23/2017 AST 248, Fall 2005 9 How Far the Record? • • • • Oldest dated rocks: 4.0 byr Earlier rocks: no radiometric dating Oldest fossils: 3.5 byr Zircons (zirconium silicates, tiny grains embedded in sedimentary rocks): 4.34.4 byr • Oldest known life in a rock layer: 3.85 byr • No good limit on when life started 5/23/2017 AST 248, Fall 2005 10 Accretion (1) • Starting point: solar nebula, collapsing cloud of gas + dust Protoplanetary disk: Sun, surrounded by disk of gas + dust Condensing solid particles: – Terrestrial distance: metals and rocks – Jovian distance: also ices Accretion: solids merge, first randomly, then aided by self-gravity Planetessimals: size ~ 110 km orbiting the Sun Protoplanets: size ~ few hundred km, colliding violently Moon/Mars-size objects: ~ 1000 km Terrestrial planets: ~10,000 km and well separated 5/23/2017 AST 248, Fall 2005 11 Accretion (2) • Water: due to icy planetessimals in elliptic orbit, originating from Jovian distances • Giant (Jovian) planets: different mechanism, preserving hydrogen and methane 5/23/2017 AST 248, Fall 2005 12 Differentiation • Differentiation: heavier material sinking toward the center • Heat sources to melt the Earth: – Kinetic energy of accreting planetessimals – Friction during differentiation – Radioactive decay, more earlier on, but continuing until now • Resultant layers: – Core: iron, nickel • Solid at the center (because of high density) • Molten on the outside – Mantle: silicates – Crust: lowest density • Evidence for Earth’s structure: – Average density – Seismic waves 5/23/2017 AST 248, Fall 2005 13 Formation of the Moon • Earth’s Moon: – Unique among terrestrial planets – Affects life: • Tides • Life cycles: e.g., menstruation • Composition: – Smaller proportion of volatiles: water, gold and lead – Lower density, similar to that of mantle lacks iron • Impact theory: – Moon formed by a Mars-sized object colliding with the Earth: • Splash off mantle-like material • Lose volatiles – Simulations: support the hypothesis – Evidence: only circumstantial 5/23/2017 AST 248, Fall 2005 14 5/23/2017 AST 248, Fall 2005 15 Age of the Earth • • • • Oldest rocks: 4.0 byr Zircons: 4.34.4 byr Moon rocks (Apollo): > 4.4 byr Meteorites: 4.6 byr (small fraction a little younger) Age of solar system: 4.55±0.02 byr • Lead dating of Earth: Same age ±0.02 byr • Differentiation (because lead sinks): within first 0.1 byr 5/23/2017 AST 248, Fall 2005 16 Heavy Bombardment • Continued bombardment after terrestrial planets were formed • Solid surface impact crater • Earth: craters obliterated by erosion • Moon: – Rocky regions: older than 4 byr are densely covered by craters – Maria (huge craters later covered by lava): few craters – Radioactive dating: maria formed 3.03.9 byr ago – More precise measurement: cratering dropped off 3.8 byr ago 5/23/2017 AST 248, Fall 2005 17 5/23/2017 AST 248, Fall 2005 18 Outgassing (1) • Source of gas: – Little hydrogen or helium gas captured during formation (unlike giant planets) – Source of water: comets on elliptical orbits CH4 and CO2 – Atmosphere: gases escape from molten rock, or through the volcanic vents of the crust – Oceans: water vapor condensing as rain • When?: – Isotopic ratios suggest major early release, before crust formed – Zircons (4.34.4 byr ago): studies suggest oceans already present (See Scientific American, Oct 2005) Suggests oceans present by the time Earth was 0.2 byr old 5/23/2017 AST 248, Fall 2005 19 Outgassing (2) • What?: – Outgassing today (volcanoes): H2O, CO2, N2, H2S, SO2 – Early atmosphere: CO2 and no O2 – Today’s atmosphere: N2 (78%), O2 (21%), Ar (1%); CO2 < 0.03% 5/23/2017 AST 248, Fall 2005 20 Impact Sterilization • Oceans as early as 0.2 byr after formation • Impact sterilization by large asteroids: – 350400 km: raise temperature to 2,000 C and vaporize oceans – 150190 km: vaporize top few hundred meters of the oceans – Last sterilizing impact: 3.94.2 byr ago – ~100 asteroids of these sizes still exist, but orbits are stable and not earth-crossing 5/23/2017 AST 248, Fall 2005 21 Origin of the Continents • Seafloor crust (and volcanoes): – Basalt: high-density igneous rock – 510 km thick – Radiometric dating: < 0.2 byr old • Continental crust: – – – – Granite: lower-density igneous rock 2070 km thick Radiometric dating: up to 4.0 byr old Floats like an iceberg: higher and deeper • Plate tectonics: – Recycles seafloor crust – Continually add to continental crust 5/23/2017 AST 248, Fall 2005 22 5/23/2017 AST 248, Fall 2005 23
