Search for Life in the Universe

Search for Life in the Universe Chapter 4 The Habitability of Earth (Part 1) 02/01/12 AST 248, Spring 2007 1

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Outline (1) • Geology and Life• Reconstructing the History of Earth and Life• The Hadean Earth and the dawn of life 02/01/12 AST 248, Spring 2007 2

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Geology and Life • Earth among other inner worlds in the Solar System – Moon and Mercury: too smal– Venus: hot (runaway greenhouse effect)– Mars: smal → cold, thin atmosphere • Primary geological factors: – Volcanism: outgassing → atmosphere and oceans – Plate tectonics: stabilize climate– Earth’s magnetic field: shields from solar wind 02/01/12 AST 248, Spring 2007 3

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Rocks • Rock and fossil record: – Need: human history too short, only one mil ionth 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 02/01/12 AST 248, Spring 2007 5

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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 al epochs– Overlap of multiple sites → complete geological sequence 02/01/12 AST 248, Spring 2007 8

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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 02/01/12 AST 248, Spring 2007 12

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Fossils • 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: tel of mobility and structure, e.g., dinosaurs– Only a smal fraction of living organisms leave fossils 02/01/12 AST 248, Spring 2007 16

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Geological Time Scale • Initial qualitative division; radiometric dating came later• Hadean (“hel ish earth”) eon – Hellish conditions in the young Earth (4.5−4.0 byr) • Archaean (“ancient life”) eon – Single-cel organisms: first fossil record (4.0−2.5 byr) • Proterozoic (“earlier life”) eon – Single-cel organisms: fossils visible under a microscope (2.5−0.54 byr) • Phanerozoic (“visible life”) eon – Multi-cel organisms: fossils visible to the naked eye (0.54−0 byr)– Many subdivisions based on fossil record– Usual y rapid transition in the sedimentary layers 02/01/12 AST 248, Spring 2007 18

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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): 4.3−4.4 byr • Oldest known life in a rock layer: 3.85 byr• No good limit on when life started 02/01/12 AST 248, Spring 2007 20

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Accretion (1) • Starting point: solar nebula, col apsing 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 ~ 1−10 km orbiting the Sun → Protoplanets: size ~ few hundred km, col iding violently → Moon/Mars-size objects: ~ 1000 km → Terrestrial planets: ~10,000 km and wel separated 02/01/12 AST 248, Spring 2007 22

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Accretion (2) • Water: due to icy planetessimals in elliptic orbit, originating from Jovian distances • Giant (Jovian) planets: different mechanism, preserving hydrogen and methane 02/01/12 AST 248, Spring 2007 23

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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 02/01/12 AST 248, Spring 2007 24

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Age of the Earth • Oldest rocks: 4.0 byr• Zircons: 4.3−4.4 byr• Moon rocks (Apol o): > 4.4 byr• Meteorites: 4.6 byr (smal fraction a little younger) ⇒ Age of solar system: 4.57±0.02 byr• Lead dating of Earth: Same age ±0.02 byr• Differentiation (because lead sinks): within first 0.1 byr 02/01/12 AST 248, Spring 2007 25

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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 3.0−3.9 byr ago – More precise measurement: cratering dropped off 3.8 byr ago 02/01/12 AST 248, Spring 2007 26

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Outgassing (1) • Source of gas: – Little hydrogen or helium gas captured during formation (unlike giant planets) – Source of water: comets on el iptical orbits → CH and CO 4 2 – 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 (4.3−4.4 byr ago): studies suggest oceans already present (See Scientific American Oct 2005) ⇒ Suggests oceans present by the time Earth was 0.2 byr old 02/01/12 AST 248, Spring 2007 28

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Outgassing (2) • What?: – Outgassing today (volcanoes): H O, CO , N , 2 2 2 H S, SO 2 2 – Early atmosphere: CO and no O 2 2 – Today’s atmosphere: N (78%), O (21%), Ar 2 2 (1%); CO < 0.03% 2 02/01/12 AST 248, Spring 2007 30

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Impact Sterilization • Oceans as early as 0.2 byr after formation• Impact sterilization by large asteroids: – 350−400 km: raise temperature to 2,000 C and vaporize oceans – 150−190 km: vaporize top few hundred meters of the oceans – Last sterilizing impact: 3.9−4.2 byr ago– ~100 asteroids of these sizes still exist, but orbits are stable and not earth-crossing 02/01/12 AST 248, Spring 2007 31

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