Transcript The Earth - USU Department of Physics

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!”
Basic Earth Properties
• 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…
What else
has to do
with this
bulge???
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 any
surfaces
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 an
established fact.
Venus’ atmosphere of CO2, Methane, Ammonia, and other acidic components
have turned its surface into a 1000 °F nightmare!
• “Global Warming”
– Based on temperature measurements
– Skewed data: only certain parts of whole timeseries 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 know
that the Earth’s
magnetic field
completely changes
its 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