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%

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

How did the Earth Get to Look Like This?

Key points in the formation of the earth:

• Formed via acretion of grains in a 4-step process ( 1. 2. 3. 4. )
• Original Protoearth was molten
• dense material (molten nickel and iron) flowed to the center
• lighter material (molten silicon) flows to the top
• earth cools and solidifies into basic core, mantle and crust structure
• Overall density of the earth is 5.5 grams per cubic centimeter
• The earth has a lot of trapped gasses in its interior

Two significant questions to now ask:

1. Where did the water on the earth come from?

2. This nearby object has a density of 3.3 grams per cubic centimeter. Is this a problem?

The Origin of the Moon:

Until recently, this was not understood. The moon has long been an anamoly because its mass compared to the earth is 1/80 and there is very large for a planetary satellite

Possibilities for Lunar Origin:

• Formed in place by accretion NO! its composition is wrong
• Formed someplace else where the average density is 3.5? Implies formation near Mars how did it get here?
• Fissioned from the earth's mantle (note the density of the earth's mantle is around 3.5) - but by what mechansism?

The above 3 scenarios for lunar formation are all physically implausible so why is the moon there?

Let's imaging the following sequence:

• Shortly after the formation of the earth a leftover planetisimal roughly the size of Mars had either struck the earth or had a close passage to it.
• Here is the Sequence if there was an impact
  • 1. Liberation of Debris
  • 2. Formation of Debris Ring
  • 3. Moon forms via acretion

• Here is the Sequence if it was a close passage
  • Path of Encounter
  • Deformation of Molten Earth
  • Fission
  • Tidal induced debris ring
  • Moon forms via acretion

Outgassing Early Formation of the Earth's Atmosphere

Present day composition of volcanoe effluents:

• Water Vapor 60%
• Carbon Dioxide 24%
• Sulfur 13%
• Nitrogen 5.7%
• Argon 0.3%
• Chlorine 0.1%

It is likely that there was NOT enough water released via outgassing to account for the present day oceans

Most of the water was likely delivered to the earth after it formed via collisions with left over planetisimals and cometisimals. The influence of Jupiter on the orbits of leftover cometisimals may well be key.

Tidal forces from a newly formed moon may also be key in promoting first stages of development of life as well.

Hence the earth may need both Jupiter and the Moon if it is to ask a question sometime later.

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