Astronomy 330

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Astronomy 330

• Lecture 6

Image courtesy of the Image Analysis Laboratory,
NASA Johnson Space Center.
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Astronomy 330 The Earth as a Planet

• We will try to study the Earth as a planet, its
  the closest planet to us !
• We have only recently appreciated the fact that
  the Earth is a planet, subject to similar forces
  and processes as the other planets
• We will not, so much study the details of the
  Earth, but rather we will step back and try to
  focus on the big picture
• How is the Earth similar and how is it different
  from the other planets ?

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Astronomy 330

• How did the Earth form ?
• What is its composition ?
• What is its internal structure ?
• What sources of energy drive its geologic
  activity ?
• Why does the Earth have liquid H2O and an N2, O2
  atmosphere ?
• Why do the oceans and atmosphere circulate and
  how ?
• Why is there life ?

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Astronomy 330 The broad view from space

• The Earth has a unique surface appearance
• H2O, only planet with abundant liquid H2O on its
  surface (also ice, clouds common)
• Continents separate oceans
• Chemistry of atmosphere - N2 (78), O2(21), CO2
  (trace), water vapor (this is NOT seen on any
  other planets, the Oxygen is locked up in rocks
  or is in CO2 on Mars and Venus). This is related
  to the presence of life.

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Astronomy 330

• There is enough water to cover the Earth to a
  depth of 3 km.
• 1/3 of the Earths surface is above water - the
  continents
• Highest point on Earth is 9 km above sea level
• Lowest point on Earth is 11 km below sea level

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Astronomy 330

• Indications of geologic activity
• Hot springs and volcanoes
• young mountain ranges (sharp ridges)
• Earth has a magnetic field
• Traps charged particles from the Sun (radiation
  belts), provides a way of studying interior
• Generated by dynamo action in Earths
  interiormust be a fluid
• Also implies Earths interior is differentiated
  (iron core)

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Astronomy 330

• The Earth is very active geologically relative to
  other planets (e.g. the Moon and Mercury show
  little current activity).
• The Earth is close to being in equilibrium and is
  not evolving from one completely different state
  to anotherThe Earth has complicated cycles which
  regulate the flow energy and mass

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Astronomy 330 The Earths Cycles (Examples)

• Water Cycle
• Evaporation - Condensation - Rain - Rivers,
  Streams flow into ocean - repeat cycle
• Cycling of Earths Crust
• Volcanoes - erosion - mountains built -
  subduction
• Chemical Cycles
• O2 (plants) - CO2 (animals, planets, volcanoes,
  fossil fuels)

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Astronomy 330 Interior of Earth

• Difficult to study, bore holes have only gone to
  15 km (less than 025 the radius of the Earth).
• How do we know anything ?
• The Earth has a high density, 5.5 gm/cm3, so the
  Earth must have heavy elements in its core
• Earthquakes and seismology. Seismic waves
  traveling through the Earths reflect and refract
  differently under different conditions. They are
  detected at different places on the Earths
  surface at different times. This gives us
  information about densities at different depths

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Astronomy 330 Seismology

• 2 types of waves propagate through the Earth.
• P waves (Pressure), oscillations of wave are in
  the direction of the waves motion, pass through
  liquids and solids
• S waves (Shear), oscillations of waves are
  perpendicular to the direction of the waves
  motion, pass only through solids

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Astronomy 330 The Earths core is partly liquid
Core
P and S waves detected
Mantle
Only P waves detected
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Astronomy 330

• The velocities of the waves depend on density, so
  by timing their arrival at different points on
  Earth we can get detailed information about
  density structure of Earth (idea of
  helioseismology similar)
• Chemical analysis and physical properties of
  surface rocks tells us about internal composition
  since some surface rocks are the result of the
  flow of material from deep in the Earth ( 200
  km)kimberlite (diamonds)

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Astronomy 330

• Existence of magnetic field indicates that an
  electrically conducting, turbulent fluid is
  present deep in the Earth

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Astronomy 330 Regions of the Earth

• Magnetosphere - Region of charged particles (from
  upper atmosphere (200 km) to boundary w. Solar
  wind (100,000 km)
• Atmosphere - layer of gas from surface to 200 km
• Hydrosphere - the oceans
• Crust 10 - 30 km thick
• Mantle - solid but plastic rock down to 2,900 km,
  2/3 of Earths mass
• Core - dense, metallicouter (liquid), inner
  (solid)

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Astronomy 330 Structure of the Earths Interior
Upper Mantle
Lower Mantle (inside of 700 km)
Ocean Crust
Continents
Outer Core, liquid (inside of 2900 km)
Inner Core, solid (inside of 5,100 km)
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Astronomy 330

• Fast convection in the outer, liquid core gives
  rise to the magnetic field of the Earth (and thus
  the magnetosphere)
• Convection in the Mantle gives rise to motion of
  crustal plates (plate techtonics) and to
  formation of mountains, recycling of crustal
  material

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Astronomy 330 The Crust

• Upper, most solid layer of the Earth
• Formed mostly of igneous rock which has come from
  the mantle
• Thin, roughly 6 km thick on average, composed
  mostly of basalt
• Similar in composition to the Moon and some
  meteoritic basalts
• Relatively young, solidification ages of rocks is
  less than 200 million years

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Astronomy 330 The Crust

• The Oceanic crust covers 55 of the Earths
  surface, formed from magma rising along rifts
  (spreading centers) where crustal plates are
  separating
• The crust sinks back into the mantle at
  subduction zones

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Astronomy 330

• Continental Crust is thicker, older (some rocks
  billions of yrs old) and less dense than the
  oceanic crust.
• Continental Crust covers 45 of Earths surface
  and makes up 0.3 of Earths mass
• Continental Crust is made mostly of granites
  (igneous rocks, different composition than
  basalts, solidified beneath surface under great
  pressure). Also contains sedimentary and
  metamorphic rocks
• Continental Crust is 20 km to 70 km thick and
  floats on top of the mantle

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Astronomy 330 The Mantle

• Region below crust, composed of high density
  material
• Heated by its own radioactive elements plus heat
  from core
• Rocks under these conditions are plastic, can be
  deformed without breaking and they flow
• Divided into the Upper, Middle and Lower Mantle

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Astronomy 330 The Upper Mantle

• Rigid down to 100 km
• The upper mantle and the crust together are
  called the lithosphere
• Composition changes from crust to upper mantle
• Lithosphere is not strongly attached to lower
  mantle and slides over it.

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Astronomy 330 The Middle Mantle

• Otherwise known at the convective mantle
• Extends down to 700 km below Earths surface
• Heat transported by convection and is what also
  drives plate tectonics, builds mountains, and
  creates volcanoes

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Astronomy 330 The Lower Mantle

• Extends to 2,900 km below earths surface
• Convection not present here
• Material altered (phase change) by intense
  pressure and has a higher density

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Astronomy 330 The Core

• The beginning of the core is marked by a sharp
  change in composition from the mantle
• Primarily iron (with some nickel and sulfer)
• At the core/mantle boundary temperature is 4,500
  K and pressure is 1.3 million bars (1 bar
  atmosphere pressure)
• Iron is liquid under these conditions
• Cores radius is 3,500 km which is greater than
  the radius of Mercury
• 1/3 of the mass of the Earth is in its core

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Astronomy 330 The Core

• At 1,200 km from the center the pressure goes to
  3.2 x 106 bars and iron makes a phase change to a
  solid state
• At the Earths center the pressure is 4x106 bars
• The liquid outer core convects and along with the
  Earths rotation becomes turbulent
• Turbulence gives rise to a dynamo effect and the
  Earths magnetic field is generated.
• The Earths magnetic field reverses polarity
  periodically and even wanders, has become weaker
  over the last few decades

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Astronomy 330 Sources of Earths Heat

• Primordial (heat left over from formation of the
  Earth) (also called accretional heating)
• Tidal heating (more on this later, not important
  for Earth as a source of heat)
• Radiogenic heating - radioactive decay of
  unstable isotopes (235U, 238U, 232U, 40K), most
  heating occurs in crust and mantle
• Roughly 1/2 of Earths heat is primordial and 1/2
  is radiogenic

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Astronomy 330 Heat Transfer in the Earth

• Conduction
• Convection
• Advection - movement of hot material from one
  place to another, movement caused by another
  process other than from buoyancy
• Heat transfer leads to phenomena such as
  differentiation, mantle convection, plate
  tectonics, and volcanism

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Astronomy 330 How the Earth Changes

• The Earth changes, e.g. erosion in rivers and
  streams (small), e.g. volcanoes, floods,.(big)
• Geologic history of the Earth extends over 4
  billion years.
• Strata laid down in successive layers, but tend
  to be mixed up, making interpretation difficult

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Astronomy 330 Mantle Convection
From http//pubs.usgs.gov/publications/text/unans
wered.html
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Astronomy 330

• Stratigraphy is the study of deducing the
  relative sequence of the layers of rock in the
  Earths crust (aided by fossils of planets and
  animals found in the rocks)
• Radioactive dating - when a substance solidifies
  it has a know amount of different elements and
  their isotopes. Some isotopes are radioactive
  and give off neutrons and decay into other
  isotopes. By measuring the relative amount of
  different isotopes in a rock and knowing the rate
  of decay of the isotopes (the half life) we can
  date the rock.

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Astronomy 330 The Geologic Record

• Precambrian (before life, no fossils), from
  Earths formation to 590 million years ago.
• Phanerozoic - 590 million years ago to present

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Astronomy 330 Finer divisions of Geologic time

• Eras
• Archean - formation to 2,500 million years ago
• Proterozoic - to 590 million years ago
• Paleozoic - 248 million years ago, one celled
  organisms
• Mesozoic - 65 million years ago, fish, early
  reptiles, land plants
• Cenozoic - Present

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Astronomy 330

• Oldest rocks on Earth are 3.8 billion years old
• Most geology we see is relatively young
• Hawaii is only a few million years old
• The Grand Canyon is about 10 million years old

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Astronomy 330 Sideling Hill

• Rocks are roughly 330 to 340 million years old
  (The Appalachians are very old)
• Produced when continental crust folded in
  collision with Africa (Atlas Mountains)

Photo by Paul Breeding,1988
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Astronomy 330 Reading

• Read Chapter 9 of Morrison Owen