A. Origin and Structure of the Earth

• Astronomers - have acquired the first direct evidence that gravitational waves rippled through our infant universe during an explosive period of growth called inflation
• Heliocentric system


  Sun-centered system of planetary motions
• Geocentric system


  Earth-centered system of planetary motions
• Copernicus - placed the planets in circular paths around the sun and put the Earth as the sixth planet with its satellite, the moon

  
• Nebular Hypothesis


  The solar system developed with the observed regularities in its motions

  
• Planetesimal Theory


  The planetary system was formed from materials removed from the sun by tidal action caused by a passing star

  
• Dust Cloud Theory


  The nebula was assumed to have a composition mainly of hydrogen and helium, like the sun, with only 1% of heavier elements

  
• Protoplanet Hypothesis


  The original nebula was so massive that on further contraction and flattening, it broke into separate clouds or protoplanets

  
• Terrestrial planets


  Mercury, Venus, Earth, Mars
• Gas giants


  Jupiter, Saturn, Uranus, Neptune
• Crust


  Brittle outermost layer of the planet, variable thickness from 5-10 km in ocean basins to 25-70 km in continents
• Mantle


  Dense layer of molten silicate rocks, in perpetual convection motion due to core heating, varies in physical characteristics with depth
• Core


  Hottest layer, source of Earth's magnetic field, has liquid outer core and solid inner core
• Lithosphere


  Rocky crust of the Earth, including uppermost mantle and crust
• Hydrosphere


  All the waters that circulate on Earth, including oceans, seas, rivers, lakes, and atmospheric moisture
• Biosphere


  All living organisms, from smallest bacteria to largest whale, including plants, animals, and single-celled organisms
• Plate tectonics - is useful but not imperative for life, as volcanism can also provide fresh supplies of molecules needed for life
• Earth - remains the only known planet to host life, due to a unique combination of factors, but continued monitoring of alien worlds may find other planets that share these attributes or discover different ways that life can blossom