Exploring the Lunar Surface

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Module 9 The Moon in Close-up
Activity 1 Exploring the LunarSurface
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Summary
In this Activity, we will investigate (a) Moon
missions, (b) the Moons vital statistics,
and (c) cratering.
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(a) Moon Missions

• The Moon is the only other Solar System body
  whichhas been visited by a manned mission from
  Earth.
• Grainy images of the Apollo landings on the Moon
  are part of history now ...

Buzz Aldrin descending fromthe Apollo 11 lunar
module
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... however in 1969 the firstlanding, and
views of Earthrise from the Moon, had a
profound effect on the way many people think
about theEarth and our place in the Solar
System.
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• As the Earth and Moon share a common orbit around
  the Sun, sending a spacecraft to the Moon is not
  particularly difficult. (We will see in later
  Activities thatsending spacecraft to visit
  bodies orbiting closer or further from the Sun
  is significantly more difficult.)

To send a spacecraft plus human occupants from
Earth to the Moon and back required a
significant payload. Aspacecraft that massive
would have been dangerouslycumbersome to
maneuver for landing and takeoff on the Moon.
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• Instead the main spacecraft - the Command Module
  -was used to travel between the Earth and Moon
  ...

Command module, Apollo 11,photographed from the
LunarModule
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• connected to a light Lunar Module built for low
  weightand high maneuverability in the low lunar
  gravity andlack of an atmosphere.

Lunar ascent module, Apollo 11,photographed from
the Command Module
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• The six Apollo missions which successfully landed
  on the Moon brought back 380 kg of soil and rock
  samples, and visited a range of sites, from flat
  low-lying plains to craters, the edge of a lunar
  mountain range, and a huge channel carved in the
  Moons surface by an ancient lava flow.

Apollo 16 lunar rover leavestracks on the Moon
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• The USSRs unmanned Luna missions also visited
  the Moon between 1965 and 1976, with 5 missions
  achieving soft landings on the Moon.

Luna 9, the first soft lander on the Moon
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• Luna missions between1972 and 1976 drilled for
  rock samples and then brought them back to Earth.
  
• Luna 17 21 were equipped with robotic lunar
  rovers, Lunokhod 1 2, which were controlled
  from Earth. Lunokhod 1 toured Mare Imbrium for 11
  months Lunokhod 2 covered 27 km of the Moons
  surface in 4 months.

Artists conception ofLunokhod 1
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• To find out more about the USA and USSR missions
  to the Moon, visit the History page of the Lunar
  Prospector site at the following Internet site
• http//lunar.arc.nasa.gov/history/index.html

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One advantage of using unmanned missions like the
Luna series is the simplicity and cost savings
due to the reduced payload because of the
absence of a crew or life-support system.
An advantage of using manned missions like Apollo
isflexibility - for example, astronauts can
assess a landing site and change plans to suit
circumstances more easily than can a robotic
mission. Although costly, the importance of
manned Apollo missions from the public relations
point of view for both NASA and the USA should
not be underestimated.
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We will revisit these issues when we consider the
MarsPathfinder mission, planned to be the first
of a new generation of unmanned landers.

• For now, we will investigate the main facts of
  what we know (and are still discovering) about
  the Moon and its evolution - largely as a result
  of space missions to the Moon over the last 30
  years, both manned and unmanned.

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(b) The Moons vital statistics

• In the last Activity we investigated the space
  missions which have given us a close-up view of
  Luna, our Moon.

In this Activity well summarize someof the
basic information which we have learnt about
the Moon.
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• When we look at the properties of other planets
  and natural satellites in our Solar System, we
  will use our Earth - the Solar System we know
  best - as a reference point.

For this purpose, we use the symbol ? to refer
to Earth,so that, for example, in the following
tables M? means the mass of the Earth, and D?
means the diameter of the Earth
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Moon
Earth
mantle
core
mantle
crust
crust
Core
M 0.01 M?
M M?
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• The Moon is much less dense than the Earth, which
  means that its core must be very small.

Results from Lunar Prospector experiments
confirmed the Moons small core in 1999,
indicating that the core is less than 4 of the
Moons total mass. The Earths core, for
comparison, is about 30 of the Earths total
mass.
The different densities also have implications
when welook at models for the Earth and Moons
formation, aswe will see later.
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Earth
Moon
Av. Distancefrom Sun
1 AU
1 AU
Length of Year
1 y ?
1 y ?
Length of solar day
29.5 d?
1 d?
Inclinationof axis to ecliptic
1.3
23.5
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• Look back at the previous table, and decide
  whether you would expect the Moon to have seasons.

Then go to the next slide to see if you are
right...
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• The seasons on Earth are due to the sizeable
  (23.5) tilt of the Earths rotation axis.

23.5

plane of the ecliptic
(To remind yourself of why theaxis tilt causes
seasons on Earth,revisit the Activity on Earths
Seasons)
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1.3
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The tilt of the Moons axis is very small - only
1.3 to the ecliptic - so we would not expect the
Moon to experience significant seasons, and
indeed it does not.

Note that the plane of the Moons orbit around
the Earth is tilted 5 to the plane of the
ecliptic, but it is the tilt of the rotation axis
to the ecliptic that determines whether a planet
(or satellite) will experience seasons.
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Sun-Earth-Moon system
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Earth
Moon
av. albedo
0.39
0.10
Note the Moons low albedo. The Moon looks
bright in our sky not because it is highly
reflective, but rather because it is so close to
us. The albedo of the Moon (like the Earth)
varies depending on the terrain, varying from
5-10 for the maria to between 12-16 for the
highlands.
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Earth
Moon
av. albedo
0.39
0.10
accelerationdue to gravity
g?
0.17g?
The Moons low mass and density means that the
effectsof gravity are much weaker on the Moon
than on Earth.
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Earth
Moon
av. albedo
0.39
0.10
accelerationdue to gravity
g?
0.17g?
78 N2, 21 O2 0.03 CO2, 2 H2O
Almostnone
atmosphere
Because of the low lunar gravity, gases can
easily escape.The Moon has only the slightest
trace of an atmosphere made up from gases baked
out (outgassed) from the rocky surface, with a
pressure of about 1/100 000 000 000 000 th of p
?, atmospheric pressure on Earth!
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Earth
Moon
av. albedo
0.39
0.10
accelerationdue to gravity
g?
0.17g?
78 N2, 21 O2 0.03 CO2, 2 H2O
Almostnone
atmosphere
surface temperature
-170 C ? 130 C
- 50 C ? 50 C
The insulating effect of our atmosphere helps to
smooth out temperature variations here on
Earth - with essentially no atmosphere,
temperature variations on the Moon are much more
extreme.
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Earth
Moon
av. albedo
0.39
0.10
accelerationdue to gravity
g?
0.17g?
Almostnone
78 N2, 21 O2 0.03 CO2, 2 H2O
atmosphere
surface temperature
-170 C ? 130 C
- 50 C ? 50 C
Heavy cratering, ancientvolcanoes lava flows
Cratering largely obliteratedby active surface
(volcanoes plate tectonics), weatheringand
biological activity
surfacegeology
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(c) Cratering
Heavy cratering, mostly caused by impacts with
Solar System debris over the history of the Moon,
forms the most prominent feature of the Moons
surface.
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Surface features on the Moon likecraters stand
out best when viewednear the terminator - the
boundary between sunlightand shadow on the Moon.
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The largest craters are named after philosophers,
mathematicians, and scientists.
For example, the crater Copernicus dominates a
regioncalled the Ocean of Storms.
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Many craters, including Copernicus, exhibit
patternscalled rays, caused by debris thrown out
by the initial impact. This debris, called
ejecta, can in turn create secondary craters
when it falls back to the surface.
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The low surface gravity of the Moon means that
some ejecta can escape entirely
- some meteorites found on Earth turn out to have
beenejected long ago from the Moon.
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• When a meteorite strikes, it penetrates into the
  lunar surface, deforming the surface layers and
  vaporizing itself and surrounding rock. The
  crater starts to form.

Surface material (ejecta) is thrown out by the
shock wave, and the outer edges of the crater
collapse and overturn.
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The surface may partly rebound to form a central
peak in the crater,
and the walls of the crater may partlyfall back
in to create a terraced effect,as can be seenin
this crater from the far side of the Moon.
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Modelling the formation of a crater by a
meteorite impact
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• Two of the largest impact craters, Mare Imbrium
  and Mare Orientale, are the results of impacts so
  large (it has been estimated that Mare Imbrium
  was caused by a meteorite 65 km in diameter)
  that seismic shock wavescaused bythe impacts
  would have travelled right around the Moon.

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This image, centred onMare Orientale, hasbeen
imageenhanced tobring out fine details.
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In particular, the locations on the Moon directly
oppositethese two huge impact craters are
regions called jumbled terrain - areas which
could have been disturbed by the concentrated
shock waves shaking the surface up and down by up
to 10 m.
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• Not all craters are huge they extend in size all
  the way down to microcraters in the surface of
  rocks - tiny crater pits which are less than 1 mm
  across.

(Why do you think we dont see microcratering on
Earth?)
The regolith absorbs most of the light which
falls on it, which explains the Moons low albedo
(0.10).
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Much of the rocks on the Moons surface are made
upof fragments of older rocks which have been
brokenup by meteorite impacts, then fused
together by heatand pressure of subsequent
impacts. These rocks are called breccias.
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Rate of Cratering

• Geologists have used radioactive dating to
  estimate the ages of rocks collected from the
  Moons surface during the Apollo program.

All are igneous (formed from lava). The oldest
rocks collected are 4.4 billion years old - the
youngest 3.1 billion years old.
By comparing the ages of the rocks with the
amount ofcratering at their original locations,
geologists have concluded that the Moon
underwent intense bombardmentfrom at least 4.6
billion years ago (which is when its
surfacesolidified into a crust), until 3.8
billion years ago.
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• The first 0.8 billion years of the Moonshistory
  was dominated by heavy bombardment, including
  some giant impacts - perhaps by planetesimals.

By 3 billion years ago, the crateringrate had
dropped to about 1 millionth of its initial
value.
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• Presumably the intense early bombardment was due
  to planetesimals and smaller debris in the early
  Solar System as it emerged from the Solar Nebula.

As the debris was gradually swept up into planet
making, deflected by gravitational interactions
with the large planets like Jupiter and ejected
into the outer Solar System, or gradually driven
outwards by the young Suns strong solar wind,
the rate of cratering dropped quickly to its
present relatively low value.
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• In the next Activity we will investigate other
  prominent features of the lunar surface and its
  evolution.

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Image Credits
NASA Three-filter color image of the Moon
(Galileo)http//nssdc.gsfc.nasa.gov/image/planeta
ry/moon/gal_moon_color.jpg Apollo 17 astronaut
Harrison Schmitt standing next to boulder at
Taurus-Littrow during third EVAhttp//nssdc.gsfc.
nasa.gov/image/planetary/moon/apollo17_schmitt_bou
lder.jpg Apollo 11, Buzz Aldrin descending LM
ladderhttp//images.jsc.nasa.gov/images/pao/AS11/
10075261.jpg Apollo 11, Earth rising over Moons
surfacehttp//images.jsc.nasa.gov/images/pao/AS11
/10075248.jpg Apollo 11, Command/Service modules
photographed from Lunar Module in
orbit,http//images.jsc.nasa.gov/images/pao/AS11/
10075257.jpg Apollo 11, Lunar Module ascent stage
photographed from Command Module,http//images.js
c.nasa.gov/images/pao/AS11/10075285.jpg Luna
9http//antwrp.gsfc.nasa.gov/apod/image/luna_9_un
k.gif
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Image Credits

• NASA
• View of the Mid-Pacific Oceanhttp//nssdc.gsfc.na
  sa.gov/image/planetary/earth/gal_mid-pacific.jpg
• Lunar cratering, Clementine missionhttp//nssdc.g
  sfc.nasa.gov/image/planetary/moon/clem_strtrk.jpg
• Taurus-Littrowhttp//pds.jpl.nasa.gov/planets/wel
  come/thumb/taurus.gif
• Copernicus Craterhttp//images.jsc.nasa.gov/image
  s/pao/AS17/10075987.gif
• Mare Imbrium Orientale Basinhttp//images.jsc.n
  asa.gov/images/pao/STS34/10063796.gif
• Cratering on the Moon's far sidehttp//images.jsc
  .nasa.gov/images/pao/AS11/10075255.gif
• Highland brecciahttp//pds.jpl.nasa.gov/planets/w
  elcome/thumb/breccia.gif

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• Now return to the Module home page, and read more
  about exploring the lunar surface in the Textbook
  Readings.

Hit the Esc key (escape) to return to the Module
9 Home Page
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