Search for Life in the Universe

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Search for Life in the Universe

• Chapter 4
• The Habitability of Earth
• (Part 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

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Geology and Life

• Earth among other inner worlds in the Solar
  System
• Moon and Mercury too small
• Venus hot (runaway greenhouse effect)
• Mars small ? cold, thin atmosphere
• Primary geological factors
• Volcanism outgassing ? atmosphere and oceans
• Plate tectonics stabilize climate
• Earths magnetic field shields from solar wind

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Rocks

• 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

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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 all epochs
• Overlap of multiple sites ? complete geological
  sequence

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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

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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 tell of mobility and structure, e.g.,
  dinosaurs
• Only a small fraction of living organisms leave
  fossils

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Geological Time Scale

• Initial qualitative division radiometric dating
  came later
• Hadean (hellish earth) eon
• Hellish conditions in the young Earth (4.5?4.0
  byr)
• Archaean (ancient life) eon
• Single-cell organisms first fossil record
  (4.0?2.5 byr)
• Proterozoic (earlier life) eon
• Single-cell organisms fossils visible under a
  microscope (2.5?0.54 byr)
• Phanerozoic (visible life) eon
• Multi-cell organisms fossils visible to the
  naked eye (0.54?0 byr)
• Many subdivisions based on fossil record
• Usually rapid transition in the sedimentary layers

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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

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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 1?10 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

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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

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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 Earths structure
• Average density
• Seismic waves

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Age of the Earth

• Oldest rocks 4.0 byr
• Zircons 4.3?4.4 byr
• Moon rocks (Apollo) gt 4.4 byr
• Meteorites 4.6 byr (small fraction a little
  younger)
• Age of solar system 4.570.02 byr
• Lead dating of Earth Same age 0.02 byr
• Differentiation (because lead sinks) within
  first 0.1 byr

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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

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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 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 (4.3?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

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Outgassing (2)

• What?
• Outgassing today (volcanoes) H2O, CO2, N2, H2S,
  SO2
• Early atmosphere CO2 and no O2
• Todays atmosphere N2 (78), O2 (21), Ar (1)
  CO2 lt 0.03

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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