A. Homogeneous vs. heretogeneous accretion

1. Homogeneous accretion asserts that the accumulation of planetisemals initially formed a well-mixed Earth that began to undergo differentiation as gravitational collapse resulted in the release of heat. Convection and escape of heat through surface begins, but as Earth gets too big, convection cannot handle heat buildup and partial melting occurs. (Partial melting is required to generate the core without completely melting the mantle). Fe-rich core forms, differentiation now begins.

Hetrogenous accretion theory is the idea that the internal layering of Earth began in time with denser, Fe-rich materials to which were added lighter, silicates that compose the mantle and crust.

B. all dated primordial material comes in at 4.6 Ga

• oldest lunar dates are 4.6
• meteorite ages converge at 4.6 Ga. (The link is a quick discussion about the primordial nature of meteroites.)
• oldest Earth samples are 4.2 to 4.3 (zircons from the Yilgarn Block)

C. Origin of moon

1. Clues to Moon’s origins
   • Mass Moon: Mass Earth is very high, much more so that any other set in the solar system with the possible exception of Pluto
   • High degree of angular momentum retained in the Earth-Moon system
   • Volatile depletion - Moon is even more depleted that Eathly volcanic rocks which implies that the formation of Moon involved high temperatures which drove off more volatile elements.
   • Fe depletion could imply a previously differentiated source (i.e. Earth’s mantle)
2. Simultaneous accretion theories: cannot account for momentum
3. CUrrent favorite: Collisional ejecta from a Mars-sized body collision during the late stages of planetesimal infall

D. Accretion phase ends with the trailing off the the meteoritic bombardment seen at 4.0 Ga on the lunar surface.

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