science unit 9.1

• Hadean Eon


  The period of Earth's history when it first formed, about 4.6 billion years ago
• Planets in our solar system

  • Sun
  • Mercury
  • Venus
  • Earth
  • Mars
• Accretion Disk


  Flattened astronomical objects made of rapidly rotating gas which slowly spirals onto a central gravitating body
• Nebular Hypothesis


  The most widely accepted model to explain the formation and evolution of planetary systems, first developed by Immanuel Kant in 1755
• Planets most likely, and most often, form from gas and dust orbiting the Sun which clump up together due to gravity to form planets (or other bodies)
• Atmospheres on Earth


  • First atmosphere (mainly H and He, no oxygen)
  • Second atmosphere (gases from volcanoes like CH4, NH3, H2O)
  • Third atmosphere (CO2, H2O, N2 from chemical reactions)
  • Fourth atmosphere (current, N2, O2, Ar, trace gases)
• First atmosphere


  • Gravity too low to hold on to gases heavier than H + He
  • No magnetic field to deflect solar winds
• Second atmosphere


  • Formed as Earth's crust cooled and volcanoes formed
  • Magnetic field began to form, deflecting solar winds/cosmic rays
• Third atmosphere


  • Methane began to form CO2, and ammonia led to the formation of N2
  • Concentration of gases was about 100x more than today
  • Greenhouse effect helped prevent freezing on Earth's surface
• Oceans began to form about 3.8 billion years ago
• CO2 dissolves into oceans easily, forming carbonates and turning into rocks, an example of the slow carbon cycle
• Leftover gas is nitrogen (N2) - now a main component of the atmosphere. Not very reactive (triple bonds!), so largely remains in its elemental form
• Stromatolites


  • Calcareous mounds built up of layers of lime-secreting cyanobacteria and trapped sediment, found in Precambrian rocks as the earliest known fossils
• Evidence of photosynthesis can be seen in stromatolites
• Evidence of oxygen build-up can be seen in banded iron formations in rock formations
• Most of the economically viable iron deposits in the US are in the midwest, explaining the auto industry's location
• Great Oxygenation Event


  Also called the "Oxygen Catastrophe", when organisms, specifically cyanobacteria, in the oceans evolved to perform photosynthesis, leading to a high concentration of oxygen in the atmosphere
• Although entire species went extinct, the Great Oxygenation Event also allowed organisms that survived to develop a use for oxygen through cellular respiration
• Earliest traces of photosynthesis are 3.5 billion years ago in cyanobacteria
• Eukaryotes capable of cellular respiration are seen 2.5 billion years ago
• During the Proterozoic Eon, photosynthesis becomes more efficient, oxygen concentration increases, and the ozone layer can form, allowing microorganisms to exist outside of the oceans
• Purple Earth Hypothesis

  The hypothesis that the earliest photosynthetic life forms on Early Earth were based on the simpler molecule retinal, which reflects purplish light, rather than the more complex chlorophyll, which reflects green light
• Permanent gases


  Gases with relatively long residence times in the atmosphere, like N2 and O2
• Variable gases


  Gases with relatively short residence times in the atmosphere, like H2O and CO2
• Water can exist in all 3 states of matter, and the energy released from water changing states is a major driver of atmospheric circulation and other weather systems
• O3 protects Earth from harmful UV rays
• CH4 has been increasing in concentration recently, contributing to global climate change
• Ice Age


  A long interval of time (106 to 107 years) when global temperatures are relatively cold and large areas of Earth are covered by continental ice sheets and alpine glaciers
• effects of ice ages

  • Change in atmospheric composition
  • Change in Earth's orbit (Milankovitch cycles)
  • Motion of tectonic plates
  • Change in solar output