ages in Solar System

• 238U -> 206Pb half-life 4.6 billion y
• 40K -> 40Ar 1.3 billion y

• Age is time from when rock formed in the Solar System
  • can’t tell age of atoms themselves
  • Oldest rocks
    • Earth ~ 3.9 billion y (Greenland)
    • Moon ~ 4.5 billion y (Apollo Moon rocks from Highlands)
    • meteorites ~ 4.6 b.y. (rocks falling to Earth from space)
    • Age of Solar System 4.6 b.y.
      • Agrees with solar models
        • Sun 5 +/ 1 billion y old
      • Secondary age indicator -- impact cratering Meteor, Mars
        • density of craters depends on age of surface and bombardment rate
          • oldest (Highlands) regions of Moon very heavily cratered
            • much more so than younger lowlands (maria) Moon image
            • implies cratering rate extremely high during first billion y (“heavy bombardment”) cratering vs age

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