Formation of the Earth's Atmosphere and Oceans

• Formed via acretion of grains in a 4-step process
• 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 gases in its interior

The original atmosphere of the Earth, Venus and Mars consisted of Hydrogen and Helium. Those light elements, however, were heated to escape velocity by solar radiation and thus this atmosphere escaped. The current atmospheres of the Earth, Venus and Mars are therefore secondary and evolved from other processes.

Overview of Atmospheric Compositions:

• Venus Atmosphere is almost entirely Carbon Dioxide. Atmospheric pressure is 90 times that of the Earth. There is a lot of Carbon Dioxide present.
• Earth Atmosphere is essentially 75% Nitrogen and 25% Oxygen. Where did the Carbon Dioxide go?
• Mars Atmosphere is almost entirely Carbon Dioxide but there is not much of it now. Atmospheric pressure is about 1% that of the Earth's.

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 planetesimals and cometisimals. The influence of Jupiter on the orbits of leftover cometisimals may well be key.

Remember this? See solar system formation lecture
(Gomes et al. 2005 Nature V435, p 466-469)

Tidal forces from a newly formed moon may also be key in promoting first stages of development of life as well by churning up primordial oceans. Recall, that the moon was much closer in the past than now due to tidal forces exerted. Hence the earth may need both Jupiter and the Moon for the atmosphere/ocean/life.

Formation of Planetary Atmospheres:

• Condensation of water vapor is crucial.
• On this planet it was too cold for water vapor to condense. Hence the atmosphere is all Carbon Dioxide
• On this planet it was too hot for water vapor to condense. Hence the atmosphere is all Carbon Dioxide
• On Earth it was just right.

Steps for building atmosphere

• First atmosphere of He and H evaporated away.
• Planetesimals crashing to Earth and Volcanos released volatiles: N₂ gas, H₂O, CO₂, S₂.
• Temperature is just right for water to condense, this is key to removing CO₂ gas from atmosphere by water combining with CO₂ creating carbonic acid. This combines with rocks forming limestone and dolomite:
  
  • CO₂ + H₂O H₂CO₃ (acid raid)
  • H₂CO₃ + Ca⁺⁺ CaCO₃ (Limestone)
  • H₂CO₃ + CaMg(CO₃)₂ dolomites
• N₂ is present in the atmosphere and chemically inert. This is good since our present atmosphere consists of 78% nitrogen gas.
• No Oxygen gas - need life to make it!