How did the Earth Get Here?

Making the Periodic Table of Elements via Stellar Evolution

• Helium Fusion in Stars
• Creating Carbon and Beyond
• Up to Iron

The Relative Abundances of the elements reflect these fusion reactions. Elements divisible by 4 are the most abundant.

To get beyond iron requires Supernova and neutron capture:

• Supernova represent massive stars that go bang
  

  

• Pre-supernova star is heavily layered
• They are very important sites to make the heavy elements
• Elements heavier than iron are built up by neutron capture.

Neutron Capture: Two processes:

• R-process Rapid = capture of a neutron before a neutron to proton decay can occur

• S-process Slow = Neutron capture decays into proton another neutron is captured

• Example of R and S Processes

• Slow Process Sequence for Formation of Colbalt 59
• Rapid Process Sequence for Formation of Colbalt 61

In the solar system (meteorites) most heavy elements are proton-rich indicating S-processed elements, but some R-processing occurred which is of very important consequence to the earth.

But R-processing produced a small bit of Uranium which is extremely important.

• U-235/U-238 = 0.31
• U-235 has 1/2 life of 700 million years
• U-238 has 1/2 life of 4.5 billion years

• U-238 drives plate tectonics!!!

Overall Composition of the Earth:

• ⁵⁶Iron 34.6%
• ¹⁶Oxygen 29.5%
• ²⁸Silicon 15.2%
• ²⁴Magnesium 12.7%
• ⁵⁶Nickel 2.4%
• ³²Sulfur 1.9%

Composition of the Earth's Crust:

• Oxygen 46.6%
• Silicon 27.7%
• Aluminum 8.1%
• Iron 5.0%

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