Structure of the Earth Gravity reshapes the protoEarth

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=1/2 mv 2) 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 ® 26 Mg + Energy + … (t 1/2 = 0. 72 x 106 yrs) 129 I ® 129 Xe + Energy + … (t 6 1/2 = 16 x 10 yrs) 26 Al – Long-lived Isotopes 40 K ® 40 Ar + Energy + … (t 1/2 = 1270 x 106 yrs) 232 Th (t 6 1/2 = 1400 x 10 yrs) 235 U (t 6 1/2 = 704 x 10 yrs) 238 U (t 6 1/2 = 4470 x 10 yrs)

The Differentiated Earth The earth differentiated into layers by density: High Least. Si. Dense Low Fe 1) Crust 2) Upper Mantle 1) Lithospheric 2) Asthenospheric 3) Lower Mantle 4) Outer Core Most 5) Inner Core Low Si. Dense 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/cm 3 2. 2 - 2. 5 g/cm 3 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. N 2 CO 2 H 2 O H 2 S HCN …others Present Atm. N 2 (78%) O 2 (21%) Ar (1%) CO 2 (0. 04%) H 2 O (varies) …others Where’s the H and He?

The importance of life to the development of the planet