Structure of the Earth
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.
The NEAR Mission
The Near Earth Asteroid Rendezvous Mission
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
Sources of Internal Heat
• Accretionary Heat
1) Gravity attracts
planetesimal to the protoearth
2) Planetesimals accelerate
on their journey, gaining
kinetic energy (KE=1/2mv2)
3) They strike the proto-earth
at high speed
Proto-earth
4) Their kinetic energy is
converted to thermal energy
(HEAT)
Sources of Internal Heat
• Accretionary Heat
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
Sources of Internal Heat
• Radioactive decay
– Short-lived Isotopes
 26Mg + Energy + … (t1/2 = 0.72 x 106 yrs)
129I  129Xe + Energy + … (t
6
1/2 = 16 x 10 yrs)
26Al
– Long-lived Isotopes
40K
 40Ar + Energy + … (t1/2 = 1270 x 106 yrs)
232Th (t
6
1/2 = 1400 x 10 yrs)
235U (t
6
1/2 = 704 x 10 yrs)
238U (t
6
1/2 = 4470 x 10 yrs)
The Differentiated Earth
The earth differentiated into layers by density:
High
Least
SiDense
Low Fe
1) Crust
2) Upper Mantle
1) Lithospheric
2) Asthenospheric
3) Lower Mantle
4) Outer Core
Most
5) Inner Core
Low
SiDense
High Fe
Because different minerals have
different composition and
densities, physical partitioning of
the earth led to:
chemical differentiation
The Differentiated Earth
Whole Earth Density
Surface Rocks
~5.5 g/cm3
2.2 - 2.5 g/cm3
Core: Nearly
pure Fe/Ni
Mantle: Fe/Mg
rich, Si/Al poor
Crust: Si/Al rich,
Na/K/Ca rich
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
The Crust
Continental Crust
• 35 - 40 km
• Less Dense
Oceanic Crust
• 7 - 10 km
•More Dense
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
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
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!
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
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
The Atmosphere
Early Atm.
N2
CO2
H2O
H2S
HCN
…others
Present Atm.
N2 (78%)
O2 (21%)
Ar (1%)
CO2 (0.04%)
H2O (varies)
…others
Where’s the H and He?
The importance of life to the
development of the planet