Survey of Astronomy
Our Barren Moon
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Chapter 7
Chapter 7
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Survey of Astronomy
Our Barren Moon
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Chapter 7
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Survey of Astronomy
Our Barren Moon
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The seismograph placed on the Moon by the
Astronauts of the Apollo Program
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Does the moon
rotate?
What if the
Moon did not
rotate?
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Synchronous Rotation
One rotation for one
revolution
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Lunar Phases as
described by
Aristotle
We only see
the lighted
portion of the
Moon
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Surface Features
Craters on the Moon and the
Earth
Impact
Maria, Basins, Rays
Impact and volcanism
Rills and Domes
Volcanism
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Craters,
Maria and
Basins are all
due to
meteorite
impact
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Moon has large
dark flat areas,
due to impact
and lava flow,
called maria or
basins (early
observers
thought they
were oceans or
seas)
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Craters have round bottoms, flat
bottoms and flat bottoms with a hump
depending on the energy deposited.
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Far side of Moon
has some large
craters, but no
large maria. The
reason for this
difference is still
not fully
understood.
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Meteoroid strikes Moon, ejecting material;
explosion ejects more material; leaving
crater, lips (rim) and ejecta
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Regolith: The surface of the
moon is heavily cratered. One of
the results of the infalling bodies
is to pulverize the surface, thus
creating a layer debris, much of it
is very fine dust. The surfaced is
layered with debris on top of
pulverized rock.
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Most lunar craters date to at least 3.9
billion years ago; much less
bombardment since then.
Craters are typically about 10 times as
wide as the meteoroid creating them, and
twice as deep as they are wide.
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Early Intense
Bombardment
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Moon is still being
bombarded by very
small objects called
“micrometeorites”
which trends to
round the edges of
craters and leave a
layer of dust.
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Meteorites also hit Earth; this crater is in
Arizona
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The great “lakes “ of liquid rock
that filled the large craters are
greatest evidence of vulcanism
on the moon. Vulcanism ceased
when the moon cooled.
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This is an edge of
a mare. The sooth
appearance is due
to the lava that
flowed up through
cracks , smoothed
out then cooled.
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Hadley Rill seems
to be an extended
lava tube whose
roof has fallen in.
There are other
caved in lava tubes
but they are much
shorter,
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Field of Domes
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Moon’s density is relatively low, and it has
no magnetic field – cannot have sizeable
iron/nickel core. Due to cooling over time
the crust has thickened.
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Air molecules have high speeds due to thermal
motion. If the average molecular speed is well below
the escape velocity, few molecules will escape.
Escape becomes more probable:
• for lighter molecules (higher speed for same kinetic
energy)
• at higher temperatures
• for smaller planets (escape speed is less)
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Why Air Sticks Around
Chapter 7
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Early theories of the origin of the Moon
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The currant, post Apollo, theory is the
Collision Ejection Theory. Two large
planetismals collide. Their crusts splash,
their interiors merge. The merged
interiors become the Earth. The splashed
crustal material becomes the Moon.
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This theory accounts for the difference
in density of the Moon and Earth, the
reason for the high percentage of iron
in the Earth, and the strange orbital
placement.
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The early history of
the moon includes
the Early Intense
Bombardment
which is the last
step in the
accretion process.
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Time
before
present:
4.6 billion
yr (?)
3.9 billion
yr
Event:
Formation of Moon;
heavy bombardment
Bombardment much less
intense; lunar volcanism
fills maria
3.2 billion Volcanic activity ceases
yr
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End Chapter 8
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