13-Nov The Archaean: Divergence of Earth, Venus, Mars

atmospheres; comets and water; greenhouse, Mars atmosphere, mantles and convection, faint sun

• Evidence of life in Archaean (3.5 BY; algae, colonies) -> implies earlier origin!
• End of Hadean - giant impacts (same time as mare forming impacts on moon) serious implications for primitive life - oceans vaporized several times?
• Building the continents
  • the first "cratons" - continental shields
  • andesite volcanism; granite batholiths
  • plate tectonics, continental building - rapid in Archean
    • addition of crust at margins of continents/subduction zones (?)
  • Divergence between Earth, Venus, Mars - role of water
    • Andesite low density material - stable "floating" platforms
    • Granite a later development - resists subduction
    • Carbonate-Silicate cycle - stable feedback loop (T, rain, carbonates)
    • Water lubricates/catalyzes plate tectonics - recycled CO2
  • Venus: basalt "continents", no differentiation or plate tectonics
  • Mars - shield volcanoes, same
• Faint early sun, early climates - solar evolution
  • Evidence for early liquid water (sedimentary rocks, fossils)
  • Early strong greenhouse effect on Earth - much more CO2 - controversy
    • absorption of thermal infrared by gases
  • Carbonate-Silicate cycle - stable feedback loop (T, rain, carbonates)
  • Divergent paths for Earth, Mars, Venus
    • Role of condensation and atmospheric loss in greenhouse
    • Mars lost heat, CO2 quickly; CO2 clouds limit heat trapping
      • also lost more atmosphere to impacts early on; died young
    • Venus Greenhouse runaway - water lost from top of atmosphere
    • lack of plate tectonics and recycling in both cases