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Moons and Other
Solar System
Objects
The Moon
• July 20, 1969 – humans first landed on moon
• What was the first word transmitted from the
surface of the moon?
• “Houston …”
• Retroreflector – placed on moon and
designed to reflect a laser beam from earth
– Rates of Continental Drift on earth, change in
earth’s tilt, distance to moon, Gravitational
Constant (G)
• Average distance = 384,000 km (240,000 mi)
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The Moon
• Second brightest object in the sky
• “moon” – unknown origin of the word
• Many primitive and modern societies
base their religious ceremonies on the
cycles of the moon (e.g., new and full
moons).
• Our month is based on moon’s cycle.
• Human ovarian cycle is also
synchronized to the 29.5 day lunar
cycle.
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Composition and History of the Moon
• Since the first manned lunar landing on July
20, 1969, there have 5 other lunar landings –
Apollo 12, 14, 15, 16, & 17.
• 379 kg (over 800 lbs) of lunar material have
been brought back to earth.
• Using radiometric dating techniques
– Mountain rocks – 4.4 to 3.9 billion years old
– Plain rocks – 3.9 to 3.1 billion years old
• No rocks older than 4.4 or younger than 3.2
billion years old have been found.
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General Features of the Moon
• Largest moon of any of the terrestrial planets
(Mercury, Venus, Earth, and Mars)
• One complete revolution = 29.5 days
(approx.)
• The moon rotates on its axis at the same rate,
and therefore we only see one side of the
moon.
• Since the moon rotates every 29.5 days, the
sun would appear to rise/set every 29.5 days.
• The surface features of the moon formed
millions of years ago without erosion to erase
them.
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Characteristics of the Moon
• Nearly spherical, with a diameter of 3476 km
(2160 mi) – approx. ¼ the earth’s diameter
• Mass of the moon = 1/81 of the earth
• Average density of 3.3 g/cm3 (earth is 5.5)
• Surface gravity of the moon is only one-sixth
of Earth’s.
• Therefore one’s weight on the moon would
only be one-sixth of that on Earth.
• Average reflectance (albedo) = only 7% (only
7% of the light received from the sun is
reflected)
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Composition of the Moon
•
•
•
•
Core, mantle, crust
All thought to be solid
Crust, thicker on one side
Presently does not have a magnetic
field
• Interestingly though, the rocks brought
back show some magnetism, indicating
that the moon had a slight magnetic
field at the time of rock crystallization.
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Prominent Features of the Moon
• Other than the very obvious phases of
the moon, the moon’s most prominent
features include craters, basins, plains,
rays, rills, mountains, and faults.
• Crater – Greek for “bowl-shaped”
• Lunar craters are large to small and
believed to have been formed by
meteorite impacts.
• For individual craters, the volume of the
rim and the crater are about equal.
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Craters and Basins
• Craters are the best known features of
the moon’s surface.
• About 30,000 can be seen using an
earth-based telescope.
• Range in size – microscopic to 100’s km
• The shapes of the craters change with
diameter.
– 1 km – smooth bowl-shaped interiors
– >1 km – flat interiors
– >>1 km – flat floor with central peak
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Craters of the Moon
• Most of the craters were formed
between 4.4 and 3.9 billion year ago,
due to a time of intense meteorite
bombardment.
• After 3.1 billion years ago, the moon
cooled down enough that molten rock
could no longer get to the surface.
• The moon appears to have been
geologically quiet for the past 3.1 billion
years or so.
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Lunar Plains
• Also call maria (Latin for ‘seas’)
• Lunar plains - large, dark, flat areas on the
moon believed to be craters formed by
meteorite impact that then filled with volcanic
lava
• Dark in appearance because these flat plain
areas reflect even less light than the average
surface area of the moon
• Surface of the moon covered by a layer of
loose debris called regolith
• Rock samples examined - similar to volcanic
rock on earth
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Plains of the Moon
• Plains were produced by volcanic
eruptions and the resulting enormous
lava flows.
• Apparently the moon’s interior was hot
enough to cause these major eruptions
from 3.9-3.1 billion years ago
• Plains are more common on the near
side of the moon, probably due to the
thinner crust (where it is easier for the
molten material to reach the surface).
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Craters on the Moon
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Rays
• Rays - streaks that extend outward from
some of the craters
• Thought to be pulverized rock that was
thrown out when the crater formed
• These rays are brighter (reflect more light)
than the crater – (powdered rock reflects
more light than regular-sized rock)
• Occasionally the ray systems are also
marked by secondary craters formed by flying
debris thrown from the primary crater.
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Rills
• Rills - long, narrow trenches or valleys
• Some are straight and some are curved.
• Thought to represent a separation (or
crack) caused by moonquakes
• Similar features are known to form
because of earthquakes.
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The Straight Wall
• A unique steep
slope on the
eastern side of
Mare Nubium
• The wall is 113
km long by 244
m in height.
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Mountain Ranges
• Mountain ranges on the moon can be
as high as 6100 m (>19,000 ft)
• All mountain ranges appear to be
formed in a circular pattern, bordering
the lunar plains.
• Therefore, they were probably not
formed by the same processes as
mountain ranges on earth.
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Faults
• Fault - a break or fracture in the surface
of the moon, along which movement
has occurred
• Results from the movement of the
moon’s crust along a break
• Since there is little/no erosion on the
surface of the moon, the difference in
elevation caused by these faults remain
for millions of years.
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Origin of the Moon??
• Must take into account these facts:
• Lunar rocks are similar to the earth’s mantle.
• Oxygen isotope ratios indicate that the earth
and the moon were formed at a similar
distance from the sun.
• There is no water in lunar rocks.
• There is a deficiency of volatile elements
(which were driven off by heat).
• Relative to earth, moon has less iron.
• 3.3 g/cm3 = moon; 5.5 g/cm3 = earth
• The oldest rocks on earth and moon are
similar.
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Origin of the Moon
• Most widely accepted theory is the great
impact theory.
• A planet-sized object (size of Mars)
struck the earth with a glancing blow 4.4
billion years ago, resulting in the
ejection of matter into orbit to form the
moon.
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Lunar Motions
• The lunar orbital plane does not coincide with
earth’s orbital plane.
– Approximately 5o with respect to earth’s orbital
plane.
• Due to this 5o tilt, it is possible for the moon
to be directly overhead at any latitude
between 28.5oN and 28.5oS.
• Both the rotation of the earth on its axis and
the moon’s revolution around the earth are
counterclockwise (from a N pole perspective).
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Relative
Motions of
the Moon
and Earth
Section 17.2
Two Different Lunar Months
• Sidereal Month – 27.33 days, lunar cycle with
respect to a star other than the sun
• Synodic Month – 29.5 days, lunar cycle with
respect to the sun
– From one’s perspective on earth the synodic
month is one complete month of lunar phases.
– In actuality the moon revolves more than 360o
during a synodic month.
• From earth it appears that every day the
moon rises in the east and sets in the west.
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Phases of the Moon
• The moon’s periodic change in appearance is
its most outstanding visual feature.
• One-half of the moon is always reflecting light
from the sun, but only once during that cycle
can an observer on earth see the entire
illuminated half, called a “full moon.”
• The starting point for the moon’s synodic
month is arbitrarily taken a the “new moon”
position.
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New Phase or New Moon
• Occurs when
earth, sun, and
moon are all in
the same
plane, with the
moon
positioned
between the
Sun and Earth
• At this
position, the
dark side of
the moon is
fully toward the
Earth (“dark of
the moon”).
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Lunar Phase Vocabulary
• Waxing phase - the illuminated portion
is getting larger
• Waning phase – the illuminated portion
is getting smaller
• Crescent moon – less than ½ of the
visible portion of the moon is illuminated
• Gibbous moon – more than ½ of the
visible portion of the moon is illuminated
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Phases of the Moon
As observed from any latitude north of 28.5oN
The “observer” is looking south; therefore east is on the left.
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Waxing Phases of the Moon
• Waxing crescent phase – appears as a
crescent moon, less than 90o east of
sun (7.375 days)
• First-quarter phase – when the moon is
exactly 90o east of sun
• Waxing gibbous phase – appears larger
than ½ illuminated, but < a full moon
(7.375 days)
• Full Moon – 180o
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Waning Phases of the Moon
• Waning gibbous phase – appears < a
full moon, but larger than ½ moon
(7.375 days)
• Last-quarter phase – when the moon is
exactly 270o east of sun
• Waning crescent phase – appears
smaller than ½ illuminated (7.375 days)
• New Moon – 360o
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Phases of the Moon
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Altitude of the Full Moon
• A full moon is always on the opposite
side of the earth from the sun.
• Thus when the sun is at its lowest
position (winter solstice), the moon will
be at its highest.
• Due to the 5o difference in the earth’s
and moon’s orbital planes, the moon will
be situated directly over the 28.5o N
latitude line on the winter solstice.
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Eclipses
• The sun provides the light for our solar
system
• Planets and moons within the solar system
cast shadows that extend away from the sun
– The size and shape of the shadow cast depends
on the object’s size, shape, and distance from the
sun
– The Earth and the Moon cast conical shadows, as
viewed from space
• Eclipse – the darkening of the light of one
celestial body by another
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Eclipses
• The shadows cast by the Earth and the
Moon cast two different degrees of
darkness
• Umbra – the darkest and smallest
region
– A total eclipse occurs within the umbra
region
• Penumbra – the semidark region
– A partial eclipse occurs within the
penumbra region
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Solar Eclipse
• Solar eclipse – occurs when the moon
blocks some or all of the sun’s rays from
an observer on Earth
• A solar eclipse occurs when the moon is
at or near new phase and is in or near
the ecliptic plane
• When the moon lies between the sun
and Earth in nearly a straight line, the
moon’s shadow will fall on the Earth
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Positions of the Sun, Moon, and Earth
During a Total Solar Eclipse
The umbra and penumbra are,
respectively, the dark and semidark
shadows cast on the Earth by the
Moon
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Types of Solar Eclipses
• The length of the moon’s shadow varies with
the moon’s distance from the sun
– In some cases the moon’s umbra does not reach
all the way to Earth
• If the umbra does not reach Earth, an
observer on Earth would see the moon’s disk
projected against the sun and a bright ring or
annulus outside the dark moon – this is called
an annular eclipse
• The maximum diameter of the umbra on
Earth is about 270 km
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Solar Eclipses
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Lunar Eclipse
• Lunar eclipse - occurs when the Earth blocks
some or all of the sun’s rays to the moon
• A lunar eclipse occurs when the moon is at or
near full phase and is in or near the ecliptic
plane
• When the Earth lies between the sun and
moon in nearly a straight line, the Earth’s
shadow will conceal the face of the moon
– A total lunar eclipse can last for more than 1.5
hours, and partial eclipses may last for over 3
hours
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A Lunar Eclipse
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Spring Tide
• Spring tide occurs when the sun, moon, and
Earth are all positioned in nearly a straight
line
• In this situation, the gravitational forces of the
sun and moon combine to produce higher
high tides and lower low tides
• The variations between high and low tides
are the greatest during a spring tide
• Spring tides occur twice during each lunar
month; at new and full moons
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Neap Tide
• Neap tide occurs when the sun and the moon
are at angles of 90o with respect to the Earth
• In this situation, the gravitational forces of the
sun and moon tend to cancel and produce
lower high tides and higher low tides
• The variations between high and low tides
are at a minimum during the neap tide
• Neap tides occur twice during each lunar
month; at first and last quarter
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Spring and
Neap Tides
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Moons on the Terrestrial Planet
•
•
•
•
Mercury: No moons
Venus: No moons
Earth: 1 moon
Mars: 2 known moons
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Moons of the Jovian Planets and Pluto
• Jupiter: 62 known moons
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The
Galilean
Moons of
Jupiter
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The Moons of Jupiter
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Saturn: 35 known moons—Titan, the largest
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The Moons of Uranus
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Other Solar System Objects:
•
•
•
•
Asteroids
Meteoroids
Comets
Interplanetary Dust
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Asteroids
or Minor
Planets
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Meteroids
• Interplanetary
metallic and
stony objects
that range in
size from a
fraction of a
millometer to
a few
hundred
meters
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