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Lecture 4

GEO 330 Lecture Notes - Lecture 4: Planetary Differentiation, Planetesimal, Goldschmidt Classification

Course Code
GEO 330
Timothy Glotch

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Origin of Planets and Planetary Layering
Collapse of molecular cloud of dust and gas into rotating disc over a period of about 10,000
years - solar nebula
Dust particles collide and stick ultimately achieving km-scale within about 100,000 years -
Collision of planetesimals give rise to planetary embryos
Sun "ignition" results in solar winds that sweep out dust, ice, and gas beyond "snow line" where
giants planets form
Process rapid - < million years
Has significant effect on chemistry
Formation of "snow line"
Formation of Solar System
Current consensus theory of planet formation is planetesimal hypothesis
During accretion, planetary embryos form and sweep up smaller planetesimals - perhaps a few
hundred embryos at most
Example of current controversy is degree to which accreting planetesimals were themselves
differentiated into core-mantle-crust
Lithophile - form minerals found in rocky material (e.g., oxides, silicates) and concentrate in
mantle and crust
Siderophile - tend to form metallic phases (e.g., Fe-Ni) and concentrate in metal cores
Chalcophile - tend to form sulfide minerals and expected to partition into sulfur-bearing
Atmophile - tend to form gas phases (e.g., H2O, O2, CO2); term also used to refer to gas
phases that condense at very low temperature; partition into atmospheres/hydrospheres
Elements have differing geochemical affinity which governs their "partitioning" behavior when
solids crystallize from melts or condense from very high temperature gas:
In detail, partitioning behavior is influenced by pressure, temperature, and composition
Element Partitioning
Moderately volatile elements (Tcond = 400 - 1100K) variably depleted in terrestrial planets and
some meteorites (i.e., inner solar nebula)
Well-illustrated by ratios of moderately volatile to refractory elements with same partitioning
behavior, e.g. - K/U
Depletion occurred later in solar system development after Sun mostly formed, likely due to
intense solar activity
Volatile Depletion
Near end of processes, only large bodies remain resulting in giant impacts - type that formed the
Giant impact energy commonly resulted in whole or nearly whole planet melting - Magma Oceans
Formation of Planets - 2
Composition of nebula varied radially from Sun (illustrated by colors in diagram below), likely to
be considerable mixing throughout inner solar system
Expect variations in planetary composition depending on final proportions
Formation of Planets - 3
Formation and Internal Structure
Tuesday, January 30, 2018
1:57 PM
GEO 330 Page 1
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