The Earth: Cradle of Humankind

Structure

Interior

Magnetic Field

Atmosphere

• Audience member: “The earth is a flat disc, held up on the shell of a giant turtle.”

• Scientist: “Then what’s the turtle standing on?”

• Audience member: “You’re very clever, but it’s turtles all the way down!”

• Roughly spherical, R = 6378 km– Bulges slightly at equator (“oblate spheroid”)

• Mass ~ 6 x 1024 kg

• Composition (in order of decreasing abundance)– Oxygen, Silicon, Aluminum, Iron, Calcium,

Magnesium, Sodium, Potassium, trace heavier elements + some naturally radioactive ones

– Silicates (Silicon compounds like SiO2)

• Average Density ~ 5.5 g/cm3

– About twice density of surface rock…

Basic Earth Properties

What elsehas to dowith thisbulge???

Earth’s Interior

• Because Earth’s outer layers are about twice density of normal rock, conclude that interior must be heavier

• When Earth first formed, was homogeneous mixture of Iron, Nickel, and Silicates– Gravity pulls heavier materials toward center,

leaving lighter silicates near surface• “Differentiation”• Like letting chocolate chip ice cream melt…chips

sink to bottom.

Why the difference in shape?

• Inner core: solid Iron & Nickel

• Outer core: liquid Iron & Nickel

• Mantle: solid, but malleable silicates (magma)

• Crust: solid silicates

How Do We Know This?

• Seismology– Like ultrasound sonograms, interior can be

probed by analyzing sound waves created by earthquakes

• P-waves: compression waves (move through solids AND liquids)

• S-waves: “wiggle” waves (cannot move through liquid core)

Ultrasound: creates high-frequency pressure waves in soft tissues and fluid

Strength and angles of reflection allow a computer to create an image of anysurfaces

Heating in the Core

• Two mainstream theories– Impact heating– Radioactive heating

• Important: the Earth does not generate heat like the Sun does; it has been slowly cooling since its formation.

Impact Heating

• Heat generated by collisions with other, small, rocky bodies…

• Surface eventually cools down, but heat trapped in interior (keeps interior molten)

Radioactive Heating

• Radioactive materials (Uranium, Plutonium, Iridium, etc.)– Can produce large

amounts of heat to melt interior rock…

– Like a nuclear reactor

• Larger objects hold on to heat longer– Baked potato vs. large iron ingot– Really surface/volume ratio

– So heat accumulated during birth of Earth is still stored in the interior

• The heat is NOT being being GENERATED in core

• Slowly losing heat

Plate Tectonics

• Hot, molten material can rise to the surface, where it spreads out underneath the crust

• This motion drags parts of the crust with it, creating the crustal plate motions we see– Subduction creates mountain ranges– Sea-floor spreading– Earthquakes generated by “sticking” plates

• Pressure builds until rock breaks and crust “lurches” forward

– Volcanoes• Frictional heating of subsurface material until it rises to

surface and breaks through• Magma turns into lava flows

Earth’s Atmosphere

• Currently on 3rd atmosphere– First was H & He (basic building blocks)

• Rapidly escaped into space due to high surface temperature and low mass/gravity

– Second was water vapor, CO2, and N• Spewed out by newly-forming volcanoes

– Third is O2, N2, plus trace components• As planet cooled, water vapor condensed into our oceans• Single-celled organisms (algaes & bacteria mostly) consume

CO2 and convert it into O2

– Precursors to modern-day photosynthetic plant-life

• Over last couple billion years, the VAST majority of the CO2 was scrubbed into O2

• Other theories: was our atmosphere delivered here?– Gas release by impacts with planetesimals

(small rocky bodies)• Release Oxygen trapped within rock

– Vaporization of frozen water and gases during comet impacts with infant Earth

• Might “life” have been brought to the newborn planet by comets/asteroids?– Spectra analysis and mass spectrometer

impact fragment analysis show presence of complex & organic molecules

– “Panspermia” : “seeds of life everywhere”– Raises some very interesting questions…

Atmosphere

• Greenhouse Effect– Sunlight reaches surface, converted to heat– Surface re-radiates this heat (IR) , SOME of which is

absorbed by atmosphere (recall atmospheric window)– Atmosphere warmed up by increasing concentrations

of greenhouse gases (CO2, Methane) trapping IR light

– DOES NOT GENERATE HEAT…merely slows down it’s escape into space

• Much like a blanket keeps you warm…– The blanket does not generate heat, just keeps it trapped closer

to your body…

This process is hotly-debated here on Earth, but on Venus it is anestablished fact.

Venus’ atmosphere of CO2, Methane, Ammonia, and other acidic componentshave turned its surface into a 1000 °F nightmare!

• “Global Warming”– Based on temperature measurements– Skewed data: only certain parts of whole time-

series shown– Skewed data: only certain locations shown– Urban island effect – extra heat– Keeling curve (1960-present)

– Concentration of CO2

– Glaciers & polar ice announced to be retreating, but advancing glaciers get no attention

• Global Warming

• A.K.A. “Climate Change”

• The atmosphere is a dynamic entity. It will never reach a nice, quiet, calm, steady state. Climate change happens regardless of human activity. The question is: how much worse can we possibly make it?

• Circulation– Air and ocean currents deflected by rotation of

the Earth– “The Coriolis Effect”

– Responsible for spinning air currents into spiral storm systems that develop into hurricanes

– Also influences Jet Stream & Trade Winds

Magnetic Field

• Like a bar magnet, Earth has roughly dipolar magnetic field– North & South poles

• Example– “Mass” generates “Gravitational Field”

– “Electric Currents” generate “Magnetic Field”• Coil, wrapped around nail, attached to battery

• Where do these currents come from?– Combination of rotation and convection of

molten iron in liquid outer core• Very complex motions of high speed positive &

negative charges

– But we don’t know for sure…this is the only mechanism that we can currently describe that can generate Earth’s field…

Effects of Magnetic Field

• Navigation– Compass needle lines up with Earth’s

magnetic north direction• But poles wander and eventually reverse polarity

• Protection– Deflects high-energy particles in solar wind

that are damaging to living tissues

How do we knowthat the Earth’smagnetic field completely changesits polarity?

• Simulated fly-by of Earth’s magnetic field

• Magnetic Pole Reversal

• Aurorae– p+ & e- are deflected and funneled onto the

poles where they collide with atmospheric gases

• “excites” electrons in Ozone and Nitrogen molecules to higher energy levels

• When they fall back down, they emit light seen as the Northern and Southern Lights

• Van Allen Radiation Belts– Regions of trapped p+ and e- that constantly

spiral around the field lines, moving back and forth between N and S poles

– “Space is radioactive!”• Confirmed during first space flights

NEXT TIME

• The Moon