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 MissionThe 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

Proto-earth

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)

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

26Al 26Mg + Energy + … (t1/2 = 0.72 x 106 yrs)129I 129Xe + Energy + … (t1/2 = 16 x 106 yrs)

– Long-lived Isotopes40K 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)

The Differentiated EarthThe earth differentiated into layers by density:

1) Crust

2) Upper Mantle

1) Lithospheric

2) Asthenospheric

3) Lower Mantle

4) Outer Core

5) Inner Core

Least Dense

Most Dense

Because different minerals have different composition and densities, physical partitioning of the earth led to:

chemical differentiation

High Si

High FeLow Si

Low Fe

The Differentiated Earth

Whole Earth Density

~5.5 g/cm3

Surface Rocks

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 EarthThe geotherm is the description of how the temperature of the earth increases with depth.

Near the surface (to 8 km depth):

2-3 °C/100 m depth

Heat loss by conduction!

Pure conduction geotherm

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

Present Atm.N2 (78%)O2 (21%)Ar (1%)CO2 (0.04%)H2O (varies)…others

Early Atm.N2

CO2

H2OH2SHCN…others

Where’s the H and He?

The importance of life to the development of the planet