Lecture 2 - Formation of the solar system and the planets.

Review of discussion questions:

• 1) Why do the inner and outer planets have different bulk compositions, and describe what differences you might expect in bulk compositions of the inner planets given the scenario for nebular formation of the planets given in your text?
• 2) What determines the bulk composition of the nebular gas cloud?
• 3) Velikovsky in Worlds in Collision proposed that Venus was an interloper, a comet that was captured into its present orbit (yielding the earth some atmosphere and rather wild times on the way). How might this idea be tested if the model for solar system evolution described in your text is anywhere near correct? Put another way - is this idea consistent or inconsistent with the model for solar system evolution given in your text?

Elaboration on the planetary differentiation: what are processes of differentiation?

• rheid gravity differentiation? Can work for larger denser bodies (subducting slabs or denser meteorites), but not for individual mineral grains. What would have been range of densities of accreting meteoritic material?
• magmatic gravity differentiation? Partial melting and Bowen's Reaction series, and magmatic ascent.
• convection? This would tend to stir and homogenize instead of segregate?
• ascent of fluids and gases along fractures? Importance of fluid-filled fractures.
• temporal evolution of materials accreted? Remember that as cloud cools then get different precipitates with much of the heavier material precipitating as higher T.

When did these processes occur and what is their product?

• atmosphere, hydrosphere, crust, mantle, core
• magmatic gravity differentiation still going on, to what degree?

All these processes dependent on heat and T distribution. What are sources of heat for accreting and differentiating planetary bodies?

• gravitational potential energy
• radiogenic heat
• heat of collision
• solar energy
• tidal energy
• heat of reaction

Sources of radiogenic heat?

• isotopes of K, Th, U major present day contributors.
• Archean heat production 3-4 times todays.
• Emiliani - planetary formation rapid enough would incorporate 26Al (half life = 720,000) and 60Fe (1.5 Ma half life) that this would be significant in very early history of earth.

Importance of size in differentiation process?

Did early meteorites go thud or splat?

Discussion question for next time: What processes can modify meteoritic material after condensation from the nebulaic gases?