Activity: The Earth as a model planet

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Activity The Earth as a model planet
Home Planet the Earth
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Summary

• In this Activity, we will investigate
• (a) Why astronomers study the Earth, and
• (b)The structure of the Earth.

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(a) Why astronomers study the Earth ...
Look again on the slide of the Apollo view of
the Earth rising over the Moon (title page),
taken on the Apollo 8 mission on 22 Dec 1968.
Apollo pictures like this one presented for the
first time direct visual imagery of Earth as one
celestial body among others in our Solar System,
rather than as an all-encompassing world.

NASA Press release
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• Now that space missions travel further than our
  Moon, we have even more graphic pictures of our
  Earth as a planet - here photographed with the
  Moon, at a distance of about 6 million
  kilometres, by the Galileo spacecraft on Dec 22
  1992.

NASA Press release
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• The Earth is a planet, and as such is studied by
  astronomers as well geologists.

Modern astronomy and geology study comparative
planetology - comparing planets with each other
to find similarities, which in turn might suggest
theories to explain their formation evolution.
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• Astronomers study the Earth because it is the
  planet about which we know the most. Earth acts
  as a model planet with which to compare the
  properties of other planets.

For example, if you study the other planets in
the Solar System, you will notice that
astronomers mostly quote vital statistics of
other planets in terms of Earth. (For example,
the mass of Mars is easier to conceptualize if we
say that it is approx. 11 that of Earth, rather
than that it has a mass of approx 642 400 000
000 000 000 000 000 kg!)
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• If you think that Earth is too familiar a topic
  to be of interest, wait till you see it from an
  astronomers point of view - it may surprise you!

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(b) The structure of the Earth

• The Earth is affected by

• geological influences from within
• e.g. volcanic outflows

• biological influences on the surface
• e.g. production of oxygen by plants

• astronomical influences from outside
• e.g. tidal forces between the Earth Moon.

First we will investigate the Earths overall
internal structure
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The average density of the Earth can be estimated
by measuring its gravitational attraction on
satellites (including our natural satellite, the
Moon). It turns out that the average density of
the Earth is about 5.5 times the density of
water.

• The density of rocks on the surface of the Earth
  is only approx. half this value - therefore at
  least some of the interior of the Earth must be
  very dense (otherwise the average density would
  not be so high).
• To determine the structure of the Earth,
  geologists study the way earthquake waves travel
  through its interior (seismology).

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• The large-scale model seismologists have come up
  with for the Earths internal structure looks
  basically like this

crust
mantle
liquid outer core
solid inner core
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• The inner cores radius is approx. 20 of that
  of the whole Earth, with a density of approx. 4.6
  times that of the Earths crust. The inner core
  is hot (temperatures up to nearly 5000 C),
  metallic, and rotates very slightly faster than
  the rest of the Earth.

solid inner core
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• The outer core extends out to almost one-third of
  the Earths radius, with density gradually
  decreasing until it drops at its outer surface to
  approx. 1.6 times that of the Earths crust.

Seismological evidence suggests that the outer
core is hot, liquid and metallic, but its exact
composition is not known for certain.
liquid outer core
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The mantle extends out almost to the surface of
the Earth.

• Made up of solid silicate minerals, its density
  gradually decreases until it drops at its outer
  surface to only slightly more than that of the
  Earths crust.

mantle
Though cooler than the core,(temperatures from
approx. 2 200 C down to 1 200 C), the mantle is
still hot enough to undergo plastic flow - that
is, move in convective currents like those in
water heated on a stove.
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• By comparison, the crust of the Earth is only
  approx. 35 km thick under the continents (and
  approx. 5 km thick under the oceans), but
  together with the atmosphere it supports all the
  Earths known lifeforms, including us.

crust
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• The convective currents in the Earths mantle are
  driven by the considerable temperature difference
  between the hot core (approaching 5000 C) and the
  relatively cool crust. The molten rock in the
  mantle is called magma.

The Earths crust is made up of a number of
separatecontinental oceanic plates, all
floating on the mantle.
The convective currentsin the mantle drag along
regions of the(thin) crust.
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• The crust is thinnest under the oceans, where it
  tendsto be made up of heavy, plastic oceanic
  basalt (solidified lava).

ocean
crust
mantle
Where the convection currents under the oceans
sink down, they drag down regions of crust,
forming deep chasms called midocean trenches.
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Where convection currents well up under the
ocean,
magma (molten rock) from the mantle lifts up the
crust to form oceanic ridges, such as the
mid-Atlantic ridge.
This upwelling of lava pushes the oceanic plates
apart, causing continental drift - at a few
centimetres per year.
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When continental plates collide, they produce
folded mountain chains.
The Himalayan Mountains show intricate
foldingpatterns resulting from the collision of
the Indian Asian continental plates.
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• When a continental plate collides with an oceanic
  plate,

the heavy plastic oceanic basalt tends to slide
under the light, brittle continental granite.
oceanic basalt
continental granite
mantle
The rising crust crumples up into coastal
mountain ranges.
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• In the process, the basalt is likely to heat up
  melt, forming outflows called lava volcanic
  activity - volcanism -

and associated earthquakes.
The Andes mountains in South America are the
result of the Pacific Ocean floor slipping under
the continental plate.
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• This NASA globe shows theboundaries betweensome
  of Earths tectonic plates, withassociated
  volcano earthquake regions.

Andes mountains
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• The outermost visible layer of the Earth is its
  inner atmosphere. Well study it in the next
  Activity.

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• Further out still (though not visible in optical
  images from space) are the upper atmosphere and
  the van Allen belts, regions containing charged
  particles from the solar wind trapped by the
  magnetic field of the Earth.

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Image Credits

• NASA Photo AS08-14-2392 High-oblique view of
  Moons surface showing earth rising above horizon
• http//images.jsc.nasa.gov/images/pao/AS8/10074963
  .jpg
• NASA Photo NUMBER p-41508c Image of the Earth
  and Moon from Galileo
• http//nssdc.gsfc.nasa.gov/image/planetary/earth/g
  al_earth_moon.jpg
• NASA View of Australia
• http//nssdc.gsfc.nasa.gov/image/planetary/earth/g
  al_australia.jpg
• NASA Volcanoes Earthquakes
• http//www.earth.nasa.gov/gallery/Originals/Volcan
  esQuakes.jpg
• NASA The Western Himalayas (from the Shuttle
  Atlantis)
• http//kidsat.jpl.nasa.gov/kidsat/exploration/expl
  orations/ESC.00212656/index.html

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Hit the Esc key (escape) to return to the Index
Page
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Press Releases

• NASA Photo AS08-14-2392 High-oblique view of
  Moons surface showing earth rising above horizon
• http//images.jsc.nasa.gov/images/pao/AS8/10074962
  .htm
• File Name 10074962.jpg Film Type 70mm
  Date Taken 12/22/68 Description
• High-oblique view of the moon's surface showing
  the earth rising above the lunar horizon, looking
  west-southwest, as photographed from the Apollo 8
  spacecraft as it orbited the moon. The center of
  the picture is located at about 105 degrees east
  longitude and 13 degrees south latitude. The
  lunar surface probably has less pronounced color
  than indicated by this print.

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Press Releases

• NASA Photo p-41508c Image of the Earth and Moon
  from Galileo
• http//nssdc.gsfc.nasa.gov/image/planetary/earth/g
  al_earth_moon.jpg
• GALILEO December 22, 1992
  P-41508
• Eight days after its encounter with the Earth,
  the Galileo spacecraft was able to look back and
  capture this remarkable view of the Moon in orbit
  about the Earth, taken from a distance of about
  6.2 million kilometers (3.9 million miles), on
  December 16. The picture was constructed from
  images taken through the violet, red, and
  1.0-micron infrared filters. The Moon is in the
  foreground, moving from left to right. The
  brightly-colored Earth contrasts strongly with
  the Moon, which reflects only about one-third as
  much sunlight as Earth. Contrast and color have
  been computer-enhanced for both objects to
  improve visibility.
• Antarctica is visible through clouds (bottom).
  The Moon's far side is seen the shadowy
  indentation in the dawn terminator is the
  south-Pole/Aitken Basin, one of the largest and
  oldest lunar impact features, extensively studied
  from

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• Galileo during the first Earth flyby in December
  1990.
• The Galileo project, whose primary mission is the
  exploration of the Jupiter system in 1995-97, is
  managed for NASA's Office of Space Science and
  Applications by the Jet Propulsion Laboratory.

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