Astronomy 330

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Astronomy 330

• Lecture 7

The Earth and Moon as seen by Voyager 1
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Astronomy 330 Volcanoes

• Driven by the heat rising through the mantel
• Largest and rarest are vast flows of lava which
  create flat plains (Lunar maria)
• Eruptions through fissures in the crust are more
  common

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Astronomy 330 Volcanoes

• Shield volcanoes - Shield shaped, caused by
  repeated eruptions of fluid lavas (e.g. Mauna Loa
  volcano in Hawaii)
• Composite volcanoes - Steep sided, cone shaped,
  created by violent eruptions which build and
  destroy the cone (e.g. Mt. St. Helens, Mt. Fuji),
  formed by the fall-back of plumes of viscous lava

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Astronomy 330 Other Geologic Activity

• Glaciers, erosion by water
• Causes sedimentation, material washed off
  continents and deposited in the oceans

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Astronomy 330 Craters

• Where are they on Earth ? The Moon has many
  craters, the Earth has few.
• Earth has been cleaned of craters by erosion,
  volcanism, and mountain building through plate
  tectonics
• Can see evidence of large impacts from space
• Note We will discuss craters in more detail
  later, but FYI cratering events were more common
  in that past in the young solar system, the Earth
  is just as susceptible to cratering as other
  planets and their satellites

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Astronomy 330 Craters

• Meteors hit the Earth daily, the atmosphere
  protects the Earth and small craters do not form
• Roughly one meteor every 100 years is large
  enough to hit the Earth and make a crater
• Large cratering events are theorized to cause
  mass extinctions

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Astronomy 330 Craters

• Images from Earth Impact Database, 2003.
  lthttp//www.unb.ca/passc/ImpactDatabase/gt
  (Accessed 08/02/2005)

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Astronomy 330 Craters

• Images from Earth Impact Database, 2003.
  lthttp//www.unb.ca/passc/ImpactDatabase/gt
  (Accessed 08/02/2005)

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Astronomy 330 Plate Tectonics

• Theory first proposed by Alfred Wegener
• Rocks on either side of the Atlantic are very
  similar
• Continents fit together like the pieces of a
  jig-saw puzzle (e.g. Africa and South America)
• Continents must move, Africa and Eurasia are
  separating from North and South America, there
  should be volcanic activity in the central
  Atlantic where plates are pulling apartIceland

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Astronomy 330 Plate Tectonics

• Observation of mid-Atlantic ridge showed
  volcanoes formed at plate boundaries are forced
  apart, lava flows in to form new crust
• Tectonic - Any forces which stress a planet and
  how the crust of the planet responds
• Plate tectonics appears to only operate on the
  Earth
• Lithosphere - the upper layer of the Earth which
  comprises the continental plates, the crust plus
  the upper mantel.

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Astronomy 330 Plate tectonics

• Convection in the mantle drives plate tectonics
• Detected by measurements of magnetic orientation
  of rocks on the floor of the Atlantic
• Plates move a few centimeters a year
• Has been measured directly using GPS

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Astronomy 330 Plate Tectonics
Image from http//pubs.usgs.gov/publications/text
/dynamic.html
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Astronomy 330 Plate Tectonics

• At the boundaries between plates several
  behaviors are possible
• Spreading creates a rift zone (e.g. in mid
  Atlantic)
• Subduction zone - when one plate dives under
  another into the mantel (eastern Pacific)
• Collision zone - results in folded mountains
  (e.g. Himalayas, Appalachians)
• Fault zones - plates grind past one another (e.g.
  California)

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Astronomy 330 The Cycle of Plate Tectonics and
Erosion

• Sediments erode off of continents into oceans (2
  km thick in some places)
• Crust destroyed in rift zones (the max. age of
  any oceanic crust is 100 million years)
• Volcanoes create new crust by returning mantle
  material to the surface

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Astronomy 330 Heat Transfer

• Ultimately the geologic activity of the Earth is
  driven by the transfer of heat from the interior
  to the surface
• Conduction and convection are the most important
  in the Earth (Recall convection and radiation in
  the Sun).
• Volcanic activity is a sign of convection in the
  interior of planet
• Plate tectonics is driven by convection in the
  mantle

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Astronomy 330 Continental Crust

• Composed mostly of granites
• Lighter and less dense materialfloats on top of
  the mantle
• The continents slowly grow, but are not destroyed
  as quickly as the oceanic crust. Material not
  eroded as quickly as material is added.
• Over time the continents have been pushed great
  distances over the surface of the Earth. At one
  point they were all bunched together with Africa
  at the South Pole.

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Astronomy 330 Continents

• Formation of continents is aided by the collision
  of plates, mountain building, metamorphic rocks
  (marble) are created here

Sedimentary Rock
Igneous Rock
A typical mountain
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Astronomy 330 The Hydrosphere

• Covers 70 of the Earths surface
• Comprises all oceans, lakes, rivers
• Presence makes Earth unique among the planets

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Astronomy 330 Ocean Composition

• Mostly H2O plus NaCl and other soluble salts
• Soluble salts make up 3.5 of the mass of the
  oceans - chlorine, sodium, magnesium, sulfur,
  calcium, and potassium
• Inflow from weathering of rocks
• Outflow - precipitation into sediments (e.g.
  calcium is incorporated in limestones, also into
  the shells of marine creatures)

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Astronomy 330 Ocean Composition

• Gases dissolve in the Ocean as well
• Most important are O2 and CO2
• O2 content in the oceans is less than in the
  atmosphere
• There is 60 times more CO2 in the oceans than in
  the atmospherethe oceans act as an effective
  sink for CO2 in the atmosphere

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Astronomy 330 Ocean Temperature

• The Oceans are an immense heat reservoir
• The average Temp. is about 4 degrees C
• It is 30 degree C at the equator near the surface
• Freezing or lower at the polar caps
• 1 km deep the temp is constant
• Water is heated near rift regions, volcanic vents
  recently discovered, new forms of life which
  thrive without the Sun. Is there a similar
  environment on Europa ?

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Astronomy 330 Volcanic Vents
Image from http//dwb.unl.edu/Teacher/NSF/C11/C11L
inks/www.pmel.noaa.gov/vents/PlumeStudies/PlumeStu
dies1.html
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Astronomy 330 Europa
Image from http//solarsystem.nasa.gov/planets/pro
file.cfm?ObjectJup_Europa
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Astronomy 330

• Weather only has an effect on the top few meters
  of the oceans

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Astronomy 330 The Atmosphere

• Composition
• N2 - 78
• O2 - 21
• Ar - 1
• CO2 - 0.036
• Pressure in the atmosphere is the product of
  surface gravity x the mass of the atmosphere
  above the point where you are measuring it, its
  the weight of the column of air above you
• Bodies with lower surface gravities will tend to
  have low surface pressures,if they have an
  atmosphere at all

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Astronomy 330 The Atmosphere

• Oxygen and Carbon Dioxide are very reactive
  substances and participate in complicated cycles
• If the Earth were slightly hotter, CO2 would be
  released from the oceans, greenhouse effect would
  warm the atmosphere even more, the oceans would
  boil away resulting in a massive heavy atmosphere
  (like on Venus)
• If it was slightly colder, all water would
  freeze, creating a snowball Earth (like on Mars
  ?)
• Only within a very narrow temperature range can
  the Earth have an N2 dominated atmosphere w.
  abundant liquid water.

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Astronomy 330 Lifes Impact

• Life has a large impact on the Earths atmosphere
• Without it the Earths atmosphere would be mostly
  CO2 N2 Ar like Venus or Mars (In Earths
  early stage it was like this)
• Plants absorb H2O and CO2 to produce Oxygen
• Analysis of rock compositions shows that a
  transition from a reducing to an oxidizing
  atmosphere occurred about 1-2 billions years ago.

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Astronomy 330 Structure of Earths Atmosphere

• Pressure drops rapidly with height
• Temperature also drops with height
• Water vapor is concentrated near the Earths
  surface and drops with height

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Astronomy 330 Structure of Earths Atmosphere
Height above surface
Ionosphere - forms space, atmosphere boundary,
ions due to solar wind and solar UV, X-rays,
electron Density reaches a maximum at about 160 km
140 km
Thermosphere - here temp begins to increase with
height
120 km
30-50 km
Stratosphere - No convection, constant
temperature, Temp -60 deg. C, No water vapor
10 km
Tropopause - -60 deg C, or 213 K
Troposphere - 90 of atmospheres mass, weather,
convection due to solar heating, Temp steadily
decreases, water cycle (clouds-rain-runoff-evapora
tion)
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Astronomy 330 Weather and Climate

• Weather is the short time-scale changes of the
  atmosphere ( days)
• Climate is the long time-scale evolution (
  years, centuries)
• Energy from the Sun which is absorbed by the
  Earths surface drives convection
• Some energy is reflected into space, some is
  absorbed, effected by snow/ice cover, cloud cover

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Astronomy 330 Climate and Weather
Sun light in
30 reflected (clouds, ice, deserts)
Rest radiated in IR
Energy radiated from surface is absorbed by
atmosphere, and reradiated.
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Astronomy 330 Greenhouse Effect

• Certain gases are very efficient at absorbing
  radiation in the IR, but are NOT opaque to light
  in the visible range
• Greenhouse gases H2O, CO2, CFCs, other
  hydrocarbons
• We would be dead without the greenhouse effect in
  operation !!!!

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Astronomy 330 Atmospheric Circulation

• Driven by heating by Sun (convection) and the
  Earths rotation
• Heat near the equator wants to travel to the
  cold poles of the Earth
• North-South traveling winds are deflected by the
  Coriolis Forces due to the rotation of the Earth

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Astronomy 330 Atmospheric Circulation
Polar cells
Ferell cells
Hadley cells
Sunlight in
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Astronomy 330 Atmospheric Circulation

• Coriolis Forces are a consequence of conservation
  of momentum
• Creates cyclones, counter clockwise in the
  northern hemisphere, clockwise in the southern

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Astronomy 330 The Earths Magnetosphere

• Formed by the interaction of the Solar wind with
  the Earths magnetic field
• We would be dead without it !!! Composed of
• High energy solar wind particles
• Escaping atoms and ions from Earths atmosphere
• Cosmic ray impacts

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Astronomy 330 The Earths Magnetosphere
Image from http//www-istp.gsfc.nasa.gov/Educatio
n/wms2.html
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Astronomy 330 Reading

• Read http//science.hq.nasa.gov/missions/earth-su
  n.html to get some information about current
  science missions involving the Earth and its
  interaction with space.
• Watch the movie at http//www.windows.ucar.edu/cs
  em/