How did the Earth Get Here?

The relative abundances of elements in the Galaxy reflects the stellar processing that was discussed in the Last Lecture . The overall composition of the Earth is similar to this.

Overall Composition of the Earth:

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

Composition of the Earth's Crust:

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

Formation of the Earth by accretion:

Initial solar nebula consists of mixtures of grains (rock) and ices. The initial ratio is about 90% ices and 10% grains

The sun is on so there is a temperature gradient in this mixture:

In the inner part of the solar system, only things which exist as a solid at high temperature are available (so how come there is so much water on the earth? -- answer later)

So in the inner part of the solar system you can only make a rocky planet via acretion of grains.

In the outer part of the solar system, ices can exist so you can make larger planets out of the more abundant ices

Jupiter (mostly H and He) formed in a manner similar to the Sun, that is not by accretion.

Jupiter has a large mass and perturbs orbits of objects near them. There were lots of these objects scattered between Jupiter and Pluto.

Jupiter redirected some of this cometary material into the inner solar system and most of the earth's water was delivered through comet bombardment (therefore would we be here without Jupiter?)

Steps in the accretion process:

• Step 1: accretion of cm sized particles
• Step 2: Physical Collision on km scale
• Step 3: Gravitational accretion on 10-100 km scale
• Step 4: Molten protoplanet from the heat of accretion

• Final step is differentiation of the earth: Light objects float; heavy objects sink.
• Iron-Nickel Core (magnetic field) and oxygen-silicon crust

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