1. Structure of the Earth

2. Gravity reshapes the proto-Earth into a sphere
Gravity reshapes the proto-Earth into a sphere. The interior of the Earth separates into a core and mantle.
Forming the planets from planetesimals: Planetessimals grow by continuous collisions. Gradually, an irregularly shaped proto-Earth develops. The interior heats up and becomes soft.

3. The Near Earth Asteroid Rendezvous Mission
The NEAR Mission
The Near Earth Asteroid Rendezvous Mission

4. Why is the Earth (near) spherical?
Accretion: the gradual addition of new material
When the Earth first accreted, it probably wasn’t spherical
What happened?
HEAT was generated and retained

5. Sources of Internal Heat
1) Gravity attracts planetesimal to the proto-earth
2) Planetesimals accelerate on their journey, gaining kinetic energy (KE=1/2mv2)
3) They strike the proto-earth at high speed
4) Their kinetic energy is converted to thermal energy (HEAT)
Accretionary Heat
Proto-earth

6. Sources of Internal Heat
Accretionary Heat

7. Sources of Internal Heat
Radioactive Decay
The natural disintegration of certain isotopes to form new nuclei
Time for nuclei to decay given by a “half-life”
Radioactive decay is an important source of the Earth’s internal heat

8. Sources of Internal Heat
Radioactive decay
Short-lived Isotopes
26Al ® 26Mg + Energy + … (t1/2 = 0.72 x 106 yrs)
129I ® 129Xe + Energy + … (t1/2 = 16 x 106 yrs)
Long-lived Isotopes
40K ® 40Ar + Energy + … (t1/2 = 1270 x 106 yrs)
232Th (t1/2 = 1400 x 106 yrs)
235U (t1/2 = 704 x 106 yrs)
238U (t1/2 = 4470 x 106 yrs)

9. The Differentiated Earth
The earth differentiated into layers by density:
Crust
Upper Mantle
Lithospheric
Asthenospheric
Lower Mantle
Outer Core
Inner Core
Because different minerals have different composition and densities, physical partitioning of the earth led to:
chemical differentiation
High Si
High Fe
Low Si
Low Fe
Least Dense
Most Dense

10. The Differentiated Earth
Whole Earth Density
~5.5 g/cm3
Surface Rocks
g/cm3
Core: Nearly pure Fe/Ni
Mantle: Fe/Mg rich, Si/Al poor
Crust: Si/Al rich, Na/K/Ca rich

11. Another Source of Internal Heat
Residual heat from the formation of the core
Gravitational Settling
E=GMm/r (gravitational potential energy)
Practically speaking:
A 1-kg ball of iron, settling from the surface to the center of the earth produces enough energy to heat a 10-kg piece of rock (granite) to 750°C, where it would begin to melt.
Heat capacity of granite = 840 J/kg K

12. The Crust Continental Crust 35 - 40 km Less Dense Oceanic Crust
More Dense

13. The Mantle
The asthenosphere may contain a few percent molten rock, but the mantle is by and large solid
Despite this, given time, it will flow

15. Loss of Internal Heat All celestial bodies lose heat
Asteroids > Moon > Mars > Earth
There are three main mechanisms
Conduction
Convection
Radiation
Conduction is the transfer of heat without movement of material

16. Temperatures in the Earth
The geotherm is the description of how the temperature of the earth increases with depth.
Pure conduction geotherm
Near the surface
(to 8 km depth):
2-3 °C/100 m depth
Heat loss by conduction!

17. Convection Heating at the bottom: Increases temperature
Decreases density
Less dense hot water rises…
Displacing the cooler, denser water at the top
Denser, cool water descends…
Where it is heated

18. The Core & The Earth’s Magnetic Field
The core is almost completely Fe/Ni alloy. The outer core is liquid, while the inner core is solid.
Convection of the outer, liquid core gives rise to the Earth’s magnetic field

19. The Atmosphere Early Atm. N2 CO2 H2O H2S HCN …others Present Atm.
H2O (varies)
…others
Where’s the H and He?

20. The importance of life to the development of the planet