Astonishing Astronomy 101With Doctor Bones (Don R. Mueller,

Ph.D.)

EducatorEntertainer

JU

G G LE

RPLANETARY

Scientist

ScienceExplorer

Chapter 7 – EarthThe Blue Planet

The Composition of the Earth

• The Earth’s surface is composed mostly of silicon and oxygen:– These two combine to

form silicates• More difficult to

determine is what is inside the Earth:– Average density is 5.5

kg/liter (5.5 mg/mL)– Surface density is

closer to 3 kg/liter– Interior must be made

of denser matter.• Earthquakes help us

examine the interior.

Composition of the Earth’s CrustChemical Element (Symbol)

% of Element in Crust by Mass

% of Element in Crust by Number

Oxygen (O) 46% 60%

Silicon (Si) 28% 21%

Aluminum (Al) 8% 6%

Iron (Fe) 6% 2%

Calcium (Ca) 4% 2%

Magnesium (Mg) 2% 2%

Sodium (Na) 2% 2%

Potassium (K) 2% 1%

Titanium (Ti) 0.6% 0.2%

Hydrogen (H) 0.1% 3%

Others (combined) <1% <1%

S and P Waves – Sonogram for the Earth• Earthquakes generate

two kinds of waves that help us examine the interior of the Earth:

• P waves (pressure waves) act like sound waves and travel through rock and liquid equally well.

• S waves (shear waves) are like the waves on a guitar string (moving side-to-side) and only travel through rock.

S-Wave shadows reveal the core• When an earthquake

occurs, it sends out S- and P-waves.

• P waves travel through liquid and rock and are detected everywhere on the planet.

• S waves do not travel through liquid.

• The liquid core of the Earth casts a shadow in S-waves, revealing itself.

• Seismic wave data gives us a way to learn about our planet's core.

The Interior of the Earth

The Earth has four layers:1) Crust: outermost layer, composed of mostly silicates.2) Mantle: Region of hot, but not quite molten rock.3) Liquid Core: Dense liquid mixture of iron, nickel, maybe

sulfur.4) Solid Inner Core: Probably composed of iron and nickel.

The Earth’s Hot Interior• The impact of

planetesimals heated the early Earth to high temperatures: Causing differentiation and radioactive decay.

• The Earth’s core has a temperature of around 6500 K, as hot as the surface of the Sun.

• The Earth’s interior has remained hot due to its surface area to volume ratio.

Conduction

• Heat transfer by conduction occurs in:

• Solids• Liquids and• Gases.

Materials with the highest heat conductivities are metals.

Metals are good conductors of heat because they have free electrons.

Convection

• Mantle material heated near the hot core rises and cooler material near the top of the mantle sinks.

• This process is called convection and drives the dynamics of the Earth’s surface.

• Heat transfer by convection occurs only in gases and liquids.

Dynamics at the Earth’s Surface

• As the hot mantle rises, it pushes aside the Earth’s crust along cracks known as rifts.

• This rifting pushes apart portions of the Earth’s crust, called continental plates.

• The continental plates float atop of the mantle, moving slowly (~2 cm/year) across the surface of the planet (plate tectonics).

• As plates are pushed together, one plate can slide under another.– This is called subduction.– Buckling in the crust due to this

process produces mountain ranges.

• Plates can also slide alongside each other.– This is a source of earthquakes.

Plate Tectonics: Over millions of years, the continental plates drifted apart in some areas and came together in other areas.

Plate Boundaries: Easy to find: just look for earthquakes and volcanoes.

Please insert figure 35.13Plate Boundaries:

The Earth’s Magnetic Field

The Earth’s magnetic field– Similar to the field of

a bar magnet.– Strongest at the

poles.• The Earth’s magnetic field is

generated by currents flowing in the molten iron core.– Magnetic dynamo.

• All magnetic fields originate in moving electric charges.

• The Earth’s magnetic field protects the surface from harmful cosmic rays and energetic particles from the Sun.

• A typical value for the Earth’s magnetic

field near sea level is 3 x 10- 5 Tesla (T)

Atmosphere and Hydrosphere

• The Earth is surrounded by an envelope of gas: atmosphere– Mass of the atmosphere is

only 10-6 of the total mass of the Earth.

– Mostly nitrogen (~78%)• More than 70% of the Earth’s

surface is covered by water, its hydrosphere

• The earth's atmosphere helps to keep the surface warm by absorbing a large fraction of the outgoing infrared radiation. The so-called Greenhouse Effect.

Atmospheric Pressure• The Earth’s atmosphere

presses down on the Earth’s surface and everything on it.

• This weight is called atmospheric pressure.

• At sea level, atmospheric pressure is called one atmosphere or 1 atm.

• 1 atm = 760 mm Hg = 760 torr

Atmospheric Layers

• The atmosphere of the Earth is layered:– Troposphere

• Lowermost layer, extending to around 12 km upward.

• Temperature decreases rapidly with altitude.

• Most clouds are here.– Stratosphere

• Extends from the tropopause to about 50 km upward.

• Temperature increases with altitude.

• Ozone layer resides here.

Atmospheric Layers

Ionosphere• Located above 80 km or so.• Most atoms are ionized – one or

more electrons have been removed from each atom.

• These ionized particles reflect AM radio signals back down to Earth.

• Radio communication is possible around the curvature of the Earth because of the ionosphere.

Auroras: Aurora Australis “Southern lights”

Aurora Borealis “Northern lights”

–The Aurora Borealis (Northern Lights) are located in the ionosphere

• Electrical currents flowing through the atmosphere.

• Created by energetic particles colliding and exciting ionospheric particles.

The Shaping Effects of Water

• Liquid water covers 70% of the Earth’s surface.

• Flow of water cuts through rock via erosion, carving river channels and canyons.

• Water also acts as storage for carbon dioxide, a greenhouse gas.

• It also can change the chemical makeup of rocks, making them melt at lower temperatures.

The Atmosphere, Light and Global Warming

• Our atmosphere blocks certain wavelengths of light, keeping them from reaching the surface.

• Ultraviolet light is absorbed by ozone.

• Carbon dioxide also absorbs infrared radiation, mostly from the surface.

• It re-radiates the IR photons, effectively trapping them.

• Carbon dioxide is a greenhouse gas and contributes to the warming of the surface.

The Good EarthKeep the Earth clean

It’s not Uranus!!!

The Greenhouse Effect

The Coriolis Effect: http://www.youtube.com/watch?v=mcPs_OdQOYU

• You stand at the North Pole and throw a rock at high velocity toward the equator. Because the earth rotates under the rock, the rock’s path appears curved.

• This phenomenon is known as the Coriolis Effect.

• The effect of the Coriolis force is the apparent deflection of the path an object takes as it moves in a rotating coordinate system. Although the object does not actually deviate from its path, it appears to do so because of the Coriolis Effect.

The Coriolis Effect and Weather: The Coriolis force figures prominently in studies of the dynamics of the atmosphere and hydrosphere. In this case, affecting prevailing winds and the rotation of storms in the atmosphere and affecting the rotation of oceanic currents within the hydrosphere.

• The Coriolis Effect.

• Moving parcels of air are pushed to the right in the northern hemisphere and to the left in the southern hemisphere.

• Moving air follows a

curved path, giving rise to cyclonic storms, trade winds and such.

The Origin of Tides

• The Moon exerts a gravitational force on the Earth, stretching it outward.

• The water responds to this pull by flowing towards the source of the force, creating tidal bulges both beneath the Moon and on the opposite side of the Earth.

High and Low Tides As the Earth rotates beneath the Moon, the surface of the Earth experiences high and low tides.

The Size of the Tidal Force• Consider 1 kg of water at the average distance between the

Earth’s and Moon’s centers: 3.84 × 108 m. The Moon’s mass is 7.35 × 1022 kg and the gravitational force of the Moon on this kilogram of water will be:

• A 1 kg amount of water on the side of the Earth nearest the Moon (Earth’s radius, R) , feels a slightly larger force:

Newtons 103.33

m) 10(3.84kg 1kg 107.35/kgmnewton 106.67

dmMGF

5

28

222211

2M

Newtons 1044.3

m) 106.37-m 10(3.84kg 1kg 1035.7/kgmnewton 1067.6

)(5

268

222211

2,

RdmMGF nearM

The difference is the Tidal force: 1.1 10-6 N

The Sun creates tides, too.

• When the Sun and Moon line up, high tides, called spring tides form. When the Sun and Moon are at right angles to each other, their tidal forces work against each other and smaller neap tides result.

Tidal Braking