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Module 9: The Moon in Close-up
Activity 1:
Exploring
the Lunar
Surface
Summary:
In this Activity, we will investigate
(a) Moon missions,
(b) the Moon’s vital statistics, and
(c) cratering.
(a) Moon Missions
The Moon is the only other Solar System body which
has 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 from
the Apollo 11 lunar module
... however in 1969 the first
landing, and
views of
Earthrise
from the Moon,
had a profound
effect on the
way many
people think
about the
Earth and our
place in the
Solar System.
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 that
sending 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. A
spacecraft that massive would have been dangerously
cumbersome to maneuver for landing and takeoff on the
Moon.
Instead the main spacecraft - the Command Module was used to travel between the Earth and Moon ...
Command module, Apollo 11,
photographed from the Lunar
Module
… connected to a light Lunar Module built for low weight
and high maneuverability in the low lunar gravity and
lack of an atmosphere.
Lunar ascent module, Apollo 11,
photographed from the
Command Module
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 Moon’s surface by an ancient
lava flow.
Apollo 16 lunar rover leaves
tracks on the Moon
The USSR’s 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
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 Moon’s surface in 4
months.
Artist’s conception of
Lunokhod 1
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
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 is
flexibility - 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.
We will revisit these issues when we consider the Mars
Pathfinder 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.
(b) The Moon’s 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 we’ll summarize some
of the basic information which
we have learnt about
the Moon.
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:
Moon
mantle
Earth
core
mantle
crust
crust
Core
0.27 D
M = 0.01 M
D
M = M
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 Moon’s small core in 1999, indicating that the core
is less than 4% of the Moon’s total mass. The Earth’s
core, for comparison, is about 30% of the Earth’s total
mass.
The different densities also have implications when we
look at models for the Earth and Moons’ formation, as
we will see later.
Moon
Earth
1 AU
1 AU
Length of
“Year”
1y
1y
Length of
solar day
29.5 d
Av. Distance
from Sun
Inclination
of axis to
ecliptic
1.3°
1 d
23.5°
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...
23.5°
The seasons on Earth are due
to the sizeable (23.5°) tilt of
the Earth’s rotation axis.
plane of the ecliptic
(To remind yourself of why the
axis tilt causes seasons on Earth,
revisit the Activity on Earth’s Seasons)
1.3°
23
The tilt of the Moon’s 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 Moon’s 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.
23
Sun-Earth-Moon system
av. albedo
Moon
Earth
0.10
0.39
Note the Moon’s 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.*
* these will be discussed in the next Activity.
av. albedo
acceleration
due to gravity
Moon
Earth
0.10
0.39
0.17g
g
The Moon’s low mass and density means that the effects
of gravity are much weaker on the Moon than on Earth.
av. albedo
Moon
Earth
0.10
0.39
acceleration
due to gravity
0.17g
atmosphere
78% N2, 21% O2
Almost
none 0.03% CO2, ~2% H2O
g
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!
av. albedo
Moon
Earth
0.10
0.39
acceleration
due to gravity
0.17g
atmosphere
78% N2, 21% O2
Almost
none 0.03% CO2, ~2% H2O
surface
temperature
-170 °C
 +130 °C
g
- 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.
av. albedo
Moon
Earth
0.10
0.39
acceleration
due to gravity
0.17g
atmosphere
Almost
78% N2, 21% O2
none 0.03% CO2, ~2% H2O
surface
temperature
surface
geology
-170 °C
 +130 °C
Heavy cratering,
ancient
volcanoes & lava
flows
g
- 50 °C
 + 50 °C
Cratering largely obliterated
by active surface (volcanoes
& plate tectonics), weathering
and biological activity
(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 Moon’s surface.
Surface features on the Moon like
craters stand out best when viewed
near the terminator
- the boundary between sunlight
and shadow on the Moon.
The largest craters are named after philosophers,
mathematicians, and scientists.
For example,
the crater
Copernicus
dominates a
region
called the
Ocean of
Storms.
Many craters, including Copernicus, exhibit patterns
called 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.
The low surface gravity of the Moon means that some
ejecta can escape entirely
- some meteorites found on Earth turn out to have been
ejected long ago from the Moon.
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.
The surface may
partly rebound to
form a central
peak in the crater,
and the walls of
the crater may
partly
fall back in to
create a
terraced effect,
as can be seen
in this crater
from the far side
of the Moon.
Modelling the formation of a crater by a
meteorite impact:
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 waves
caused by
the impacts
would have
travelled right
around the
Moon.
This image, centred on
Mare Orientale, has
been image
enhanced to
bring out
fine details.
In particular, the locations on the Moon directly opposite
these 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.
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 don’t see microcratering on Earth?)
The bedrock of the Moon’s surface is
covered by the regolith, a layer of loose
soil and rocks produced by meteorite
impacts over the long history of the Moon.
Footprint left in the Moon’s regolith
The regolith absorbs most of the light which falls on it,
which explains the Moon’s low albedo (0.10).
Much of the rocks on the
Moon’s surface are made up
of fragments of older rocks
which have been broken
up by meteorite impacts,
then fused together by heat
and pressure of subsequent
impacts. These rocks are
called breccias.
Rate of Cratering
Geologists have used radioactive dating to estimate the
ages of rocks collected from the Moon’s 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 of
cratering at their original locations, geologists have
concluded that the Moon underwent intense bombardment
from at least 4.6 billion years ago (which is when its surface
solidified into a crust), until 3.8 billion years ago.
Cratering rate
The first 0.8 billion years of the Moon’s
history was dominated by heavy
bombardment, including some giant
impacts - perhaps by planetesimals.
By 3 billion years ago, the cratering
rate had dropped to about
1 millionth of its initial value.
4
3
2
1
0
Time before the present (billions of years)
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 Sun’s strong
solar wind, the rate of cratering dropped quickly to its
present relatively low value.
In the next Activity we will investigate other prominent
features of the lunar surface and its evolution.
Image Credits
NASA:
Three-filter color image of the Moon (Galileo)
http://nssdc.gsfc.nasa.gov/image/planetary/moon/gal_moon_color.jpg
Apollo 17 astronaut Harrison Schmitt standing next to boulder at Taurus-Littrow
during third EVA
http://nssdc.gsfc.nasa.gov/image/planetary/moon/apollo17_schmitt_boulder.jpg
Apollo 11, Buzz Aldrin descending LM ladder
http://images.jsc.nasa.gov/images/pao/AS11/10075261.jpg
Apollo 11, Earth rising over Moon’s surface
http://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.jsc.nasa.gov/images/pao/AS11/10075285.jpg
Luna 9
http://antwrp.gsfc.nasa.gov/apod/image/luna_9_unk.gif
Image Credits
NASA:
View of the Mid-Pacific Ocean
http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_mid-pacific.jpg
Lunar cratering, Clementine mission
http://nssdc.gsfc.nasa.gov/image/planetary/moon/clem_strtrk.jpg
Taurus-Littrow
http://pds.jpl.nasa.gov/planets/welcome/thumb/taurus.gif
Copernicus Crater
http://images.jsc.nasa.gov/images/pao/AS17/10075987.gif
Mare Imbrium & Orientale Basin
http://images.jsc.nasa.gov/images/pao/STS34/10063796.gif
Cratering on the Moon's far side
http://images.jsc.nasa.gov/images/pao/AS11/10075255.gif
Highland breccia
http://pds.jpl.nasa.gov/planets/welcome/thumb/breccia.gif
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