An Introduction to Astronomy Part V: The Moon, Our Nearest Neighbor

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An Introduction to AstronomyPart V The Moon,
Our Nearest Neighbor

• Lambert E. Murray, Ph.D.
• Professor of Physics

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We know more about the Moon than any object in
our solar system besides Earth.
Because weve been there!
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On July 20, 1969, Neil Armstrong was the first
to step onto the moons surface as part of Apollo
11.
The Apollo program ended in December 1972 with
Apollo 17 after 6 landings (Apollo 13 excepted)
and a total of 12 astronauts who walked on the
moon s surface.
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Surveyor III and Apollo 12 on the Moon View of
two U.S. spacecraft on the surface of the moon,
taken during the second Apollo 12 extravehicular
activity (EVA-2). The Apollo 12 Lunar Module is
in the background. The Surveyor III spacecraft is
being inspected by an Apollo 12 astronaut in the
foreground. Surveyor III was an unmanned
vehicle that achieved the second successful US
soft landing on the moon. The astronauts
retrieved parts of the earlier spacecraft for
analysis.
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Lunar Boulder at Taurus-Littrow
Scientist-Astronaut Harrison H. Schmitt is shown
here photographed standing next to a huge split
boulder during the third Apollo 17 extravehicular
activity at the Taurus-Littrow landing site on
the Moon. The lunar rover, which transported
Schmitt and mission commander Eugene A. Cernan to
this location from their Lunar Module, is seen in
the background..
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Lunar Surface Features

• Dominant Lunar Surface Features
• Craters
• Maria
• Highlands
• Lunar Regolith (lunar soil which may be 100s
  of feet thick)
• All these features can be explained by the
  effects of massive bombardments of the Moons
  surface early in its lifetime and the subsequent
  continual exposure to intense solar radiations
  which helps to break down and sterilize the
  surface.

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Topographical Features Craters

• Craters come in all sizes many overlapping one
  another.

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Main Topographical Regions of the Moon
Highlands
Maria
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Highlands and Maria Another View
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Major Topographical Regions of the Moon

• The Moon has two topographically distinct
  regions
• Maria (Latin for sea) the darker regions of
  Moon, which are relatively smooth and flat.
• Once thought to be seas, these regions are now
  known to be relatively young regions formed from
  lava flows on the surface of the Moon.
• They are found almost exclusively on the Earths
  side of the Moon.
• Highlands the lighter regions of the Moons
  surface which are mountainous.
• These regions consist of older rock, and are
  extensively cratered.

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Topography and Age

• More the 800 lbs of Moon rocks were returned to
  Earth by Apollo missions.
• Radiological dating of these rocks showed that
• Rocks from the Highlands ranged in age from 3.8
  to 4.3 billion years (the age of the oldest rocks
  in the crust of the Earth)
• Rocks from the Maria were all much younger

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Number of Surface Craters and Age

• We postulate that the relative age of the surface
  of many planets and moons can be determined from
  the number of craters present (and whether they
  overlap)
• Young regions have fewer craters and few
  overlap.
• Older regions have a large number of craters,
  many of which overlap.
• If the planet or moon has little or no atmosphere
  to promote surface changes due to erosion, the
  craters which were formed early in the lifetime
  of the planet or moon may still be preserved.
• The maria on the Moon are regions where lava
  flowed over the surface and destroyed older
  craters.

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Number of Craters and Age II
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Craters on the Far Side
50 mi
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Topographical Features Rays
Rays

• Rays of material ejected from craters during
  impacts can be easily seen on the surface.

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Topographical Features Rilles
Numerous Lunar canyons called Rilles may be seen
on the surface of the Moon. These may have been
formed by lava flows.
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Topographical Features Scarps

• Scarps are cliffs on the surface of the Moon

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Lunar Seismic Studies

• The Apollo missions left 4 nuclear powered
  seismic stations on the moon.
• These instruments revealed that the moon had few
  moonquakes of any significant size (most can be
  correlated with tidal forces from the earth).
• Some may result from continued cooling of the
  lunar interior
• Some are due to meteor impacts
• Several moonquakes were generated by the
  deliberate crashing of a Lunar Module on the
  surface.
• The data derived from these studies are
  consistent with the picture of the moon
  represented on the next slide.

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Comparison of Near and Far Sides of Moon

• The assymetrical nature of the moon (the fact
  that the crust is thinner on the earth side than
  the farside) is believed to explain the major
  difference in appearances between the farside
  (first observed by a soviet spacecraft) and the
  nearside.
• The farside has almost no maria regions and is
  very heavily cratered.
• These differences can be clearly seen in the
  Clementine images shown on the next slide.

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Rotation of the Moon

• The Moon passes through a complete phase (from
  full moon to full moon) in a synodic month (29 ½
  days). A sidereal month is 27 days, 7 hrs, 43
  min.
• During this time the same side of the Moon is
  always facing Earth. This means that a point on
  the surface of the Moon is in daylight for about
  15 days, where the temperature reaches about 130
  degrees C, and in darkness for about 15 days
  where the temperature can reach 110 degrees C.

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The Origin of the Moon I

• Before the Apollo landings, there were three
  principle theories about the origin of the Moon
• Capture The Moon was formed far from Earth and
  later captured by Earths gravity
• Fission The Moon was separated from the
  material that formed the Earth.
• Co-accretion The Moon was formed at about the
  same time and in the same place as the Earth (the
  double-planet hypothesis).

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The Origin of the Moon II

• The results from the Apollo missions indicate
  problems with all three of these theories.
• The model now favored is the giant impact model
• A large body impacted the Earth and material from
  the outer layers (the mantle) of the Earth
  accreted to form the Moon. This would explain
  the similarity in composition between the moon
  and the earths outer crust. Both are quite
  deficient in heavier materials.

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Computer Simulation of Moons Formation
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Ocean Tides

• One of the most obvious influences of the moon on
  the earth is its connection with the tides.
• Knowledge of tidal occurrences was very important
  in the development of the maritime age when wind
  driven ships were the source of commerce.
• The first mechanical computers built, were
  built in order to predict the occurrences of
  tides at English harbors.

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The Cause of the Tides
The tides result from the DIFFERENTIAL
gravitational force the Moon exerts on the Earth
and the oceans on its surface.
The Difference in strength between these forces
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The Characteristics of Tides

• Two high tides and two low tides occur each day.
• The high tide leads the moon slightly due to
  friction between the water and the surface.
• Tides are larger at full moon and new moon
  stages.

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The Suns Influence on Tides

• While the Sun exerts a larger total gravitational
  force on the Earth than the Moon does, the
  differential force (the force on the surface
  minus the force on the center) is smaller than
  the Moons.
• The combination of the effect of the Moon and Sun
  give rise to spring tides and neap tides
• When the Suns forces and the Moons forces act
  together, one gets the highest possible tides.
• When these forces act at right angles to one
  another, one gets the lowest tides.

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Tidal forces of Sun Moon act together.
Spring Tides
Tidal forces of Sun Moon oppose each other.
Neap Tides
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Variations in Times and Heights of Tides

• Thus, we expect the tides at any one location on
  the Earth occur at roughly 12 hour intervals due
  to the rotation of the earth. However, the
  actual timing of tides at a given location, and
  the severity of the tides is strongly influenced
  by the local conditions at the coastline.
• Also, the severity of the tides are influenced
  greatly by the relative alignment of the Sun and
  Moon (the spring and neap tides), but the
  strength of the tides also depends on the
  relative distance to the Moon, and the Moons
  orbit is slightly elliptical.

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Tidal Effects on theEarth-Moon System

• These tidal forces (which also occur in earths
  solid crust) result in a gradual slowdown of the
  earths rotational speed of about .002
  seconds/century.
• By Newtons 3rd Law, the Earth also exerts a
  force on the Moon which accelerates the moon
  increasing its orbital speed, and consequently
  increasing its orbital radius (Keplers 3rd Law).
  The Earth-Moon distance increases about 3 cm per
  year.
• Current technology allows the direct measurement
  of these two effects!

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Water on the Moon

• News Release NASA finds Water on Moon
  http//www.space.com/common/media/show/player.php?
  show_id37ep1

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End of Part V