Origin of Solar System

Nebular Hypothesis

• Rotating dust cloud (nebulae)
• Rotation causes flattening
• Gravity causes contraction
• Rotation increases
• Material accumulates in center--protosun
• Compression cause T to increase to 10⁶ °C­fusion begins
• Great explosion
• Origin of planets
• Gases condense
• Gravity causes them to coalesce into planetesimals
• Planetesimals coalesce and contract into planets
• The planets
• Terrestrial or inner planets ­ Mercury, Venus, Earth, Mars ­ loss of volatiles (H, He, H₂O) by solar wind ­ made of rock
• Jovian planets (4 of the 5 outer planets) ­ Jupiter, Saturn, Neptune, Uranus ­ mostly volatiles (H, He)
• Pluto ­ anomalous composition--rock with frozen water and methane

Differentiation of Earth

• Homogenous planetesimal
• Earth heats up
• Accretion and compression (T -> 1000°C)
• Radioactive decay (T -> 2000°C)
• Iron melts--migrates to center
• Frictional heating as iron migrates
• Light materials float--crust
• Intermediate materials remain--mantle

Differentiation of Earth Continents, Oceans, and Atmosphere

• Continental crust forms from differentiation of primal crust
• Oceans and atmosphere
• Two hypotheses
• internal: degassing of Earth's interior (volcanic gases)
• external: comet impacts add water CO₂, and other gases
• Early atmosphere rich in H₂, H₂O, N₂, CO₂; poor in O₂
• Photosynthesis begins: CO₂ + H₂O --> organic matter + O₂