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Exploring the Solar System
Lecture 3:
Structure of the Moon
Professor Paul Sellin
Department of Physics
University of Surrey
Guildford UK
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Lecture 3
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Overview
 Physical properties of the Moon
 Rotation and phases of the Moon
 The Moon’s orbit
 Observation of the Moon’s features from Earth
 Formation of the Moon
 Crater formation and the lunar regolith
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Harrison Schmidt walking on the lunar surface – December 1972. Photographed
by Eugene Cernan, Apollo 17. This was the last manned mission to the Moon.
The Moon has only been visited by 12 people. The large boulder in the image is
one of the most recent surface features on the Moon – it tumbled down a
mountainside to its present location a mere 800,000 year ago.
Unlike Earth, the surface of the Moon has remained essentially unaltered for
billions of years.
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Guiding Questions
What key findings resulted from the manned
exploration of the Moon during the Apollo
program in the 1970s?
Does the Moon’s interior have a similar structure to
the interior of the Earth?
How do Moon rocks compare to rocks found on the
Earth?
How did the Moon form?
The image shows the Earth and Moon to scale, taken
from the Galileo spacecraft
Both objects are at the same distance of 6.2 million
kilometres from the spacecraft
The moon shows no atmosphere, no oceans, and no
evidence of plate tectonics
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Size of the Moon
Compared to planetary distances, the Moon is very close – just 384,400 km
The angular diameter is only 1/2°  the diameter of the moon is relatively small
The orbital motion of the Moon around the Earth cannot be considered as simple rotation of an
object around a fixed point:
 mass of the Moon is 1.23% of the mass of the Earth
 both objects orbit around a fixed centre of mass of the Earth-Moon system – this is inside
the volume of the Earth. The centre of mass moves in an elliptical orbit around the Sun:
The actual value
for the diameter
of the moon is
3476 km, 27% of
the Earth’s
diameter
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Phases of the Moon
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The figure shows the Moon at 8 positions in its orbit, plus photographs of what
the moon look like at each position as seen from Earth. The changes in phase
occur because light from the Sun illuminates one half of the Moon, and as the
Moon orbits the Earth we see varying amounts of the Moon’s illuminated half.
It takes 29.5 days (synodic period) to go through a complete cycle of phases.
Note that the phases of the moon are not due to the shadow of the Earth falling on
the Moon’s surface – this is a popular misconception. When this occurs we get as
lunar eclipse.
Notice that the Moon does rotate around its axis – see next slide
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Rotation of the Moon
The phase of the Moon is always changing, but the Moon always keeps the same hemisphere
towards the Earth  the Moon rotates about its axis with the same period as its orbit around
the Earth:
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The Dark Side of the Moon
“There is no dark side of the moon… matter of fact it’s all dark”
So said the final line of Pink Floyd’s famous 1973 concept album ‘Dark Side of the Moon’
Pink Floyd, clockwise from the top: Roger
Waters, Nick Mason, Dave Gilmour, Rick Wright.
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The real ‘dark side of the moon’ – as seen by Apollo 16
Locked in synchronous rotation, the
Moon always presents its well-known
near side to Earth.
But from lunar orbit, Apollo astronauts
also observed the Moon's far side.
This sharp picture from Apollo 16's
mapping camera shows the eastern
edge of the familiar near side (left) and
the heavily cratered far side of the
Moon.
Surprisingly, the rough and battered
surface of the far side looks very
different from the near side which is
covered with smooth dark lunar maria.
The likely explanation is that the far side
crust is thicker, making it harder for
molten material from the interior to flow
to the surface and form the smooth
maria.
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Both sides of the Moon
Photo: NASA, Apollo 16, 1972
Photo: our back garden, ~1960
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Tidal forces (1)*
In the Earth-Moon system, tidal forces are the gravitational forces between these two bodies.
 The near and far sides of the Moon will experience different gravitational force, due to the
varying distances
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Tidal forces (2)*
The tidal force experienced by the Moon, due to Earth, is proportional to R3, such that:
Problem: calculate the tidal force for two 1kg rocks on either side of the Moon’s surface
G = 6.67x10-11 N m2/kg2
M = 6x1024 kg
d = 3476 km
R = 384,400 km
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Tidal forces (3)*
Now calculate the gravitational force experienced by a 1kg mass on the Moon’s surface:
Thus the tidal force on the lunar surface is much less than the gravitational force
 But the tidal force due to Earth deforms the Moon’s surface, producing tidal bulges around
the equatorial region
 These tidal forces produce a torque on the Moon which keeps the rotation locked into
synchronisation with the orbit around the Earth
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The Sidereal and Synodic Months
 The sidereal month is
the time the Moon takes to
complete one revolution of
the Earth with respect to the
background stars = 27.32
days
 Because the Earth is
moving along its orbit, the
Moon must travel through
more than 360° to get from
one new moon position to
the next
  the synodic month is
the time from one new moon
to the next. This is longer
than the sidereal month, =
29.53 days
 This is the period of the
new moon and full moon
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The time for a complete lunar day (the time for the Moon to rotate once on its
axis) is about 4 weeks. Because the Moon’s rotation is synchronous, it takes the
same time for complete lunar orbit.
Astronomers define two types of lunar month, depending on whether the Moon’s
motion is measured relative to the stars or to the Sun:
Sideral month – time for one complete orbit of the Earth, with respect to the
stars (‘true’ orbital period)
Synodic month, or lunar month – time for the Moon to complete one cycle of
phases (eg. full moon to full moon), and hence return to the same position
relative to the Sun.
The synodic month is longer by about 2.2 days because the Earth is orbiting the
Sun whilst the Moon goes through its phases. Thus the Moon must travel more
than 360° around its orbit in order to complete a cycle of phases.
Lecture 3
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The Moon’s Orbit
The Moon’s orbit around the Earth is elliptical, not circular:
 The Moon is at perigee when it is nearest the Earth
 The Moon is at apogee when it is farthest from the Earth
 The line connecting these 2 points passes through the Earth and is called the line of
apsides
Average centre-centre distance of the Earth and
Moon is 384,400 km
Minimum distance is 356,410 km and maximum
distance is 406,697 km
Average speed of the Moon along its orbit is 1.02
km/s
As seen from the Earth the Moon moves eastwards
through the constellations from one day to the next:
• the Moon’s daily eastern progress averages 13.2°
(360°divided by 27.32 days in the sidereal month)
• in 1 hour the moon moves slightly more than 1/2°
which is a bit larger than its own diameter
• moonrise is about 50 minutes later from one day to
the next
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Inclination of the Moon’s orbit and the Line of Nodes
The image shows the Moon’s orbit around
the Earth (yellow) and part of the Earth’s
orbit around the Sun (red)
The plane of the Moon’s orbit (brown) is
tilted by ~5° wrt the plane of the Earth’s
orbit (blue, plane of the ecliptic)
The 2 planes intersect along the line of
nodes
Lunar Eclipse
An eclipse can only occur when the Sun and Moon are both very near to or on the line of nodes
Only then can the Sun, Moon and Earth all lie along a straight line, required for the Moon’s shadow
to fall on the Earth
 A solar eclipse occurs if the Moon is on the line of nodes at new moon
 A lunar eclipse occurs if the Moon is on the line of nodes at full moon
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Conditions for the Eclipses
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An eclipse requires that the Sun, Earth and Moon all fall on the same line.
A lunar eclipse occurs when the Moon passes through the Earth’s shadow, ie.
when the Earth is between the Sun and the Moon. At this position the surface of
the Moon facing the Earth would normally be fully illuminated, but is instead lit
by a dim glow.
A solar eclipse occurs when the Earth passes through the Moon’s shadow. As
seen from the Earth, the Moon moves in front of the Sun. For a solar eclipse to
occur the Moon must be between the Earth and the Sun and so can only occur at a
new moon.
A similar effect is seen by the (rare) transit of Mercury and Venus across the Sun.
Calculate the angle subtended by the Sun at Earth:
D(km) 
 (arc sec) d (km)
206265
D(km)  206265
d (km)
Earth-Sun distance: d = 1AU = 1.49x108 km 11
5 1.43  10
Diameter of the Sun: D
6.96x10
 (=arc
sec) km
 962  0.26
8
1.49  10
 (arc sec) 
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Total solar eclipses on the Earth 1997 - 2020
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More complex features of the Moon’s orbit
Gravitational pull from the Sun further complicates the orbit of the Moon around the Earth, ie.
the orientation of the Moon’s orbital plane constantly changes:
 the plane of the Moon’s orbit always maintains
a 5° tilt to the plane of the ecliptic
 the orientation of the orbit’s plane changes
slowly, causing the line of nodes to slowly move
westward with time
 it takes 18.61 yrs for the line of nodes to
complete one full rotation
The orientation of the Moon’s elliptical orbit within its plane
also changes due to the Sun
 the line of apsides moves slowly within the Moon’s orbital
plane, called the rotation of the Moon’s orbit
 while the line of nodes is moving westward, the Sun’s gravity
causes the line of apsides to move east
 it takes 8.85 yrs for one full rotation of the line of apsides
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Libration of the Moon (1)
The Moon’s orbital period (sidereal month) is
equal to the period of rotation of the Moon
However due to the elliptical orbit of the Moon
the 2 motions are not completely synchronised
From the figure:
 in ¼ of the sidereal period (6.83 days) the
Moon only travels 84° but has rotated by 90°
 when the Moon is at point 2, observers on
the Earth can see 6° further west on the
Moon’s surface than when at point 1
 similarly when at point 4 observers can see
6° further east on the Moon’s surface
The Moon appears to rock back and forth in
an east-west direction by 6°. This is called
libration in longitude
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If the Moon’s orbit was a perfect circle then the orbital motion and the rotation of
the moon would be completely synchronised, eg. during the time for the Moon to
complete 90° of its orbit it would also have rotated by exactly 90°.
However because the orbit is elliptical, Kepler’s 2nd law tells us that the Moon
moves fasted at perigee and slowest at apogee. The average time for the orbit (the
period) is the same as for a perfectly circular orbit, but the Moon relatively lags
or leads against its rotation by a small amount.
Lecture 3
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Libration of the Moon (2)
The axis of the Moon’s north and south
poles (axis of rotation) is not exactly
perpendicular to the plane of the Moon’s
orbit
 the axis of the Moon is tipped by ~7°
relative to the plane of the orbit
  we can see 7° over the Moon’s north
pole when at point 1, and over the south
pole when at point 2
 this causes a ‘vertical’ nodding motion of
the Moon, or libration in latitude
The 5° angle between the Moon’s orbital plane and the ecliptic means that the Moon’s poles have
a net rotation of 2° from the ecliptic
This is much less than the 23.5° rotation of the Earth’s poles from the ecliptic, and as a result the
Moon does not have seasons
 astronomers predict that there may be regions at either pole on the Moon where the Sun’s light
never shines, eg. at the bottom of craters. The temperature in these areas could be as low as 50K
(-223°C), and are potential regions for the existence water ice
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The nature of the Moon
The Moon is a relatively small object, with a diameter 27% that of Earth.
 The Moon has no atmosphere, no global magnetic field, and no liquid water
 The average surface temperature is 130°C (day time) and -180°C (night time)
 The surface of the Moon has remained unaltered for billions of years
The most favoured theory of formation of the
Moon was a collision between Earth and another
planet (similar in size to Mars) 4.56 billion years
ago:
 the interior of the Moon originally contained
lava, and evidence of volcanic activity can be
seen on the Moon’s surface
 the Moon contains a small iron-rich core which
is now solid, approximately 700km in diameter
 ‘Moon-quakes’ occur with significantly less
severity than on the Earth (eg. Richter scale up to
1.5).
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Internal structure of the Moon
Like Earth, the Moon has a crust, a mantle and a core. The partially liquid iron-rich core is
roughly 700 km in diameter and contains ~3% of the total lunar mass. In contrast, Earth’s core
contains 32% of its total mass
 the Moon has no magnetic field,
hence the core is at least partially
solidified
 the lithosphere is relatively thick
(800km) compared to Earth (50100km). This is the limit of the solid
part of the Moon
 the lunar asthenosphere
extends right down to the core, and
is composed of heavier elements.
This layer is plastic and can flow to
some extent
 the lunar crust is ~50km thick in
the near side, and ~100km thick on
the far side. In contrast the Earth’s
crust can be only 5km thick
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The formation of the Moon
The Moon probably formed from debris cast into space when a huge proto-planet struck the
early Earth
This is called the collisional-ejection theory - that the Earth was struck by a Mars-sized
object and that debris from this collision coalesced to form the Moon
This theory successfully explains most properties of the Moon:
 The Moon was molten
in its early stages, and the
anorthositic crust solidified
from low-density magma
that floated to the lunar
surface
 The mare basins were
created later by the impact
of planetesimals and filled
with lava from the lunar
interior
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The image shows a simulation of the effect of this type of giant collision. The
energy released during the collision produces a huge plume of vaporised rock that
squirts out from the point of impact. This is the ejected material which cools to
form the Moon.
The collisional ejection theory is consistent with the low iron content of the
Moon:
• rock vaporised by the impact is depleted of volatile elements and water. If the
collision with the Earth occurred after ‘chemical differentiation’ has occurred,
then the Earth’s iron would have already moved to its core.
• consequently little iron would be ejected, to form the Moon. This explains the
Moon’s low density and small size of its iron core.
• the South Pole Aitken basin on the moon is a very deep crater produced by an
ancient impact. Measurements from the Clementine spacecraft show a very low
iron concentration (~10%) at the bottom of this crater 12km deep, compared to
20-30% iron seen at a comparable depth on Earth.
Lecture 3
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Geological history of the Moon
To summarise the geological history of the Moon:
 the initial impact with Earth probably occurred about 4.5 billion years ago
the newborn Moon’s surface was probably molten for a long period, due to heat released
from the impact, and the decay of radioactive isotopes
 As the Moon cooled, low density lava floating on the surface solidified to form the current
anorthositic crust
 A period of intense rock bombardment ended about 3.8 billion years ago, during which time
the majority of the craters were formed
 At the end of the crater-making period the Moon was struck by several (~10?) massive
asteroids, perhaps 100 km across. These made the vast mare or impact basins
 From 3.8 to 3.1 billion years heat from the radioactive decay of heavy elements (U, Th)
melted the inside of the Moon, causing floods of molten rock to cover the surface and fill the
impact basins, and forming the basaltic maria
 Very little futher major changes occurred on the Moon’s surface from about 3 billion years
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Geology of the Moon
Lunar rocks reveal a geologic history quite
unlike that of Earth
The anorthositic crust exposed in the
highlands was formed between 4.0 and 4.3
billion years ago. During this period the
Moon’s centre was molten – the less dense
anorthosite rose to the surface
The mare basalts solidified between 3.1 and
3.8 billion years ago
The rate of surface change has decreased
rapidly over time; estimated from the density
of craters and collected rocks
Imapact breccias are common on the moon
– these are composite rocks which have
fused together as a result of meteorite
impacts
The Moon’s surface has undergone very little
change over the past 3 billion years –
evidenced by the low crater density in the
maria
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Lunar geology
Highlands
anorthosite
Iron-rich
mare
basalt
Lunar rocks are totally dry. With the exception of
possible ice at the poles, no evidence has been
found that water ever existed on the moon.
Impact
breccia
All of the lunar rock samples are igneous rocks
formed largely of minerals found in terrestrial
rocks. They differ from terrestrial rocks in being
relatively enriched in the refractory elements
and depleted in the volatile elements
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Mare basalt is a type of balsatic rock quite similar to dark coloured volcanic
rocks on earth, such as lava found in Hawaii and Iceland. On the moon the rock
from the low lying plains is called mare basalt. Mare basalt is rich in heavy
elements such as Fe, Mn and Ti.
The lunar highlands are made up of light-coloured rock called anorthosite. On
Earth anorthosite is only found in very old mountain ranges, and is rich in Si, Ca
and Al. Anorthosite is less dense than basalt, and rose to the Moon’s surface
during the liquid stage of the Moon’s formation. In this way anorthosite formed
the lunar terrae that make up the majority of the Moon’s surface.
Impact breccias are composites of different rock types that were broken apart,
mixed, and fused together by meteoric impact.
Lecture 3
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Moon features observed from Earth
Since the Moon has no atmosphere the surface features can be observed easily, typically
even with binoculars:
 craters due to the many impacts on the Moon by space debris over hundreds of millions of
years. Over 30,000 crates can be observed from Earth, with diameters from 1-100’s km.
 maria, (from ‘mare’, Sea) the dark coloured features which are massive solidified lava flows
 terrae, the light-coloured lunar highlands
The largest craters were named in the 17th
century after famous philosophers,
mathematicians and scientists, hence:
Plato, Aristotle, Pythagoras, Copernicus
and Kepler
Close-up photographs from lunar orbit
have shown millions of craters too small to
be seen from Earth
The picture is a composite image of
photographs taken at the 1st quarter and
3rd quarter when long shadows enhance
the surface features
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Paul Sellin
This composite images shows the near and far sides of the moon, taken from
Galileo.
The Earth-facing side of the Moon is on the right, composed mainly of large dark
flat maria.
On the left is the lunar far side, composed almost entirely of light-coloured lunar
highlands.
Mare Orientale lies on the boundary between the near and far sides.
The broad dark region at the lower left is the South Pole Aitken basin, an impact
scar 2100 km across and 12 km deep
Lecture 3
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Impact craters
Craters are caused by impacts of meteoric material, also seen on Earth and the rocky planets
 The circular crater is caused by the shock wave from the impact – irrespective of the direction
or angle of the incoming trajectory
 Many craters also have a central peak, characteristic of a high speed impact
Planets which are geologically active tend to erase evidence of craters, eg by volcanic activity or
motion of tectonic plates. This requires an interior which is at least partially molten. Furthermore
the existence of an atmosphere causes erosion which further reduces surface features.
Smaller planets lose their internal heat most quickly, and so are less geologically active, eg.
Mercury has a heavily cratered surface, whereas as Mars still has intense volcanos
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A crater on (left) the Moon
the Earth, (right) Mars
Paul Sellin
In 1824 German astronomer Franz Gruithuisen proposed that lunar craters were
the result of impacts. A major problem raised at the time was why craters were all
circular when meteorites would have a range of shapes, and would impact the
Moon at a wide range of angles. A century later it was realised that the circular
form of a crater was made by the shockwave of the impact, which will be circular
for all types of trajectory. In a similar way the craters made by artillery shells are
almost always circular.
Lecture 3
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Meteor impact on Earth
Meteor Crater in Arizona is a well-known terrestrial example of a crater formed by impact of a
large meteor. The crater is 1.2 kilometers in diameter
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Meteor Crater (also known as Barringer Crater), Arizona, with a diameter of
approximately 1.2 kilometers, was the first terrestrial impact crater to be
recognized as such. Its impact origin was first suspected late in the nineteenth
century, when abundant iron meteorite fragments were discovered in the
immediate vicinity of the crater. This finding led the mining engineer Daniel
Moreau Barringer to embark, between about 1905 and 1928, on a drilling project
to find a suspected large iron meteorite body underneath the crater floor. At this
time, however, researchers did not yet have a clear understanding of the immense
energy that is liberated when an extraterrestrial body hits the surface of the Earth
with cosmic velocity. It was only in the 1920s that the first quantitative studies
revealed the explosive nature of meteorite impact. Under impact conditions,
tremendous amounts of energy are released instantaneously, completely
destroying the cosmic projectile and generating a crater that is many times larger
than the original meteoroid. In the case of Meteor Crater, an iron meteorite body
only about 30–50 meters in diameter was sufficient to create a crater 1.2
kilometers in diameter.
http://www.lpi.usra.edu/publications/slidesets/craters/
Lecture 3
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Crater formation and structure
Comets and asteroids strike the Moon at a wide range of impact speeds, with 20 km/sec being
typical. Such a high-speed impact will produce a crater that is 10 to 20 times larger in diameter
than the impacting object. The detailed form of the crater depends on its size:
Figure shows idealised cross-sections of
the structure of small, simple craters (top)
and of larger, more complex craters
(bottom).
Simple craters have bowl-shaped
depressions and are the typical crater form
for structures on the Moon with rim
diameters (D in the figure) of less than
about 15 kilometers.
Moon craters with diameters >15 km have
more complex forms, including shallow,
relatively flat floors, central uplifts, and
slump blocks and terraces on the inner wall
of the crater rim.
Near the surface is a layer of breccia (a
type of rock composed of coarse, angular
fragments of broken-up, older rocks).
http://www.lpi.usra.edu/expmoon/science/craterstructure.html
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In craters on the Moon with diameters between about 20 and 175 kilometers, the
central uplift is typically a single peak or small group of peaks. Craters on the
Moon with diameters larger than about 175 kilometers can have complex, ringshaped uplifts. When impact structures exceed 300 kilometers in diameter, they
are termed impact basins rather than craters. More than 40 such basins are known
on the Moon, and they have an important control on the regional geology of the
Moon.
Much of the material ejected from the crater is deposited in the area surrounding
the crater. Close to the crater, the ejecta typically forms a thick, continuous layer.
At larger distances, the ejecta may occur as discontinuous clumps of material.
Some material that is ejected is large enough to create a new crater when it comes
back down. These new craters are termed secondary craters and frequently occur
as lines of craters that point back to the original crater.
Material below the surface of the crater is significantly disrupted by the shock of
the impact event. Near the surface is a layer of breccia (a type of rock composed
of coarse, angular fragments of broken-up, older rocks). Rocks at deeper depths
remain in place (and are termed bedrock) but are highly fractured by the impact.
The amount of fracturing decreases as the depth below the surface increases. The
energy of the impact typically causes some material to melt. In small craters, this
impact melt occurs as small blobs of material within the breccia layer. In larger
craters, the impact melt may occur as sheets of material.
Lecture 3
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Lunar Craters (1)
Bessel Crater, 16 km
diameter, 2 km deep
Moltke Crater, 7 km diameter
King Crater, 77 km diameter,
more than 5 km deep
http://www.lpi.usra.edu/expmoon/scie
nce/craterstructure.html
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Paul Sellin
Euler Crater, 28 km diameter, about 2.5 km deep
Moltke Crater, 7 kilometers in diameter, is an excellent example of a simple
crater with a bowl-shaped interior and smooth walls. Such craters typically have
depths that are about 20 percent of their diameters. The hummocky material
surrounding the crater is Moltke's ejecta deposit. (Apollo 10 photograph AS1029-4324.)
Bessel Crater, 16 kilometers in diameter and 2 kilometers deep, is an example of
a transitional crater between simple and complex craters. Slumping of material
from the inner part of the crater rim destroyed the bowl-shaped structure seen in
smaller craters and produced a flatter, shallower floor. However, wall terraces
and a central peak have not developed. (Part of Apollo 15 Panoramic photograph
AS15-9328.)
Euler Crater, 28 kilometers in diameter and about 2.5 kilometers deep, is a good
example of complex crater morphology. It has a flattened floor, a small central
peak, and material that has slumped off the inner crater rim. The blanket of ejecta
surrounding the crater is quite clear. (Part of Apollo 17 Metric photograph AS172923.)
King Crater, on the Moon's farside, is 77 kilometers in diameter and more than 5
kilometers deep. The terraces and slump blocks on the inside of the crater rim and
the relatively flat floor are both typical of large lunar craters. However, the
central peak is much larger at King Crater than at other lunar craters of similar
size, such as Copernicus or Tycho. (Apollo 16 Metric photograph AS16-1580.)
http://www.lpi.usra.edu/expmoon/science/craterstructure.html
Lecture 3
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Lunar craters (2)
Schrodinger Impact Basin, 320 km in
diameter (inner ring 150km diameter)
Copernicus Crater, 93 km diameter
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Paul Sellin
Copernicus Crater, 93 kilometers in diameter, is one of the youngest and freshest
impact craters on the nearside of the Moon. Like King Crater, Copernicus is a
well-developed complex crater, with a prominent central peak and a relatively
flat floor. This oblique photograph clearly shows the terracing and slump blocks
on the inside of the crater rim and the rough ejecta deposit outside the crater.
(Apollo 17 photograph AS17-151-23260.)
Schrodinger is 320 kilometers in diameter, large enough to be considered an
impact basin rather than a crater. In addition to the main, outer rim, Schrodinger
also has an inner ring that is 150 kilometers in diameter and about 75 percent
complete. Schrodinger is one of the youngest, freshest impact basins on the
Moon. (Mosaic of Clementine images. Image processing by Ben Bussey, LPI.)
http://www.lpi.usra.edu/expmoon/science/craterstructure.html
Lecture 3
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The crater Clavius
Clavius is one of the largest craters
on the Moon, with a diameter of 232
km and a depth of 4.9 km measured
from the crater’s floor to the top of the
surrounding rim
This photograph was taken from
Earth at the 5m Mount Palomar
observatory
The crater was probably produced by
a fast moving meteorite only a few
kilometres in radius
Page 35photographed by Ranger 9 shortly before impact
Paul Sellin
The Alphonsus crater
Lecture 3
Page 35
The Moon’s Maria and Terrae
The ‘man on the moon’ pattern observed on the Moon’s surface is due to the large dark
coloured maria. Early astronomers thought these regions were seas of water, and hence
named them ‘Sea of tranquillity’, ‘Sea of clouds’ etc
Maria are due to large flows of solidified larva, and tend to be circular in shape
These very large depressions in the lunar surface were caused by very early impacts with
large (tens of kilometres) meteoroids which formed basins that flooded with larva due to
cracking of the lunar crust
Such massive impacts produced mountain
ranges around the edges of the basins, for
example around the edge of the Mare
Imbrium (Sea of showers)
The older light-coloured terrae are often
called the lunar highlands, and are heavily
cratered
The maria cover 15% of the lunar surface and
the terrae cover the remaining 85%
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Lecture 3
Paul Sellin
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Mare and Terrae (2)
The Earth-facing side of the Moon displays light-coloured,
heavily cratered highlands and dark-coloured, smoothsurfaced maria. The Moon’s far side has almost no maria
Photograph from lunar orbit shows numerous tiny craters
and cracks in the surface of a typical mare. The Apollo 11
landing site is near the centre of the top edge of the image
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Lecture 3
Paul Sellin
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The Lunar Regolith
The lunar regolith, or "soil," consists of tiny rock fragments that have been broken up by the
bombardment of meteorites on the lunar surface over the eons. Most regolith particles are the
size of fine-grained silt or sand. However, there are also larger fragments, including pebble-sized
objects and even some boulders. At the surface, the regolith is somewhat porous, but it becomes
more densely packed as the depth below the surface increases.
One important objective of the Apollo 11 mission was to observe the properties of the regolith
and assess how these properties affected the crew's ability to move about on the lunar surface.
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Paul Sellin
"The surface is fine and powdery. I can kick it up loosely with my toe. It does adhere
in fine layers like powdered charcoal to the sole and sides of my boots. I only go in a
small fraction of an inch, maybe an eighth of an inch, but I can see the footprints of my
boots and the treads in the fine sandy particles." - Neil Armstrong (Apollo 11
photograph AS11-40-5977.)
Left figure: The footpads on the lunar module's legs penetrated 2-8 centimeters into the
regolith. This is about the amount of penetration that was predicted prior to the
mission. The rod-shaped object protruding from beneath the footpad is a contact probe.
These probes originally extended about 1.5 meters vertically below the footpads and
were used to indicate when the lunar module was approaching the Moon's surface. This
allowed the descent rocket engine to be turned off just prior to landing. (Apollo 11
photograph AS11-40-5925.)
Right figure: The exhaust gas from the lunar module's descent engine caused some
scouring of dust on the surface during landing. On Apollo 11, a significant dust cloud
was visible when the lunar module was still 30 meters above the surface. The scouring
of the surface is visible in this photograph of the region below the descent engine,
although the scouring did not form any sort of hole in the surface. (Apollo 11
photograph AS11-40-5921.)
http://www.lpi.usra.edu/expmoon/Apollo11/A11_LandingSite_viewsfrom.html
Lecture 3
Page 38
Conclusions
 Appearance: the Earth-side facing surface contains dark coloured Maria and light
coloured heavily cratered highlands. The far side contains almost not Maria. There is no
evidence for plate tectonics or volcanism
 Internal structure: the Moon contains a thick crust, a large mantle, and a small iron
core. The crust is thicker on the far side, evidenced by the lack of flooded maria. The Moon’s
crust and lithosphere are both much thicker than Earth. There is no global magnetic field
 Geological history: the anorthositic crust exposed in the highlands was formed 4.0 - 4.3
billion years ago. The mare basalts solidified 3.1 - 3.8 billion years ago. There has been little
major change to the surface in the last 3 billion years. Lunar rocks are formed of minerals
similar to Earth, although depleted in the volatile elements. Lunar rock contains no water.
 Cratering: crater damage and large impacts from meteoroids are the only significant
source of weathering of the Moon’s surface. The period of intense bombardment was 4.0 –
4.6 billion years ago. The regolith on the Moon’s surface is a fine rock powder formed by
meteoritic action
 Formation of the Moon: formed by collision between Earth and a Mars-sized planet.
Initially molten, the Moon solidified to form an anorthositic crust, with the lower density lava
floating to the surface. Tidal interactions with the Earth have deformed the Moon, and lock it
into a synchronous orbit
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Lecture 3
Paul Sellin
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