THE MOON AND THE EARTH-MOON SYSTEM
The following topics are covered in this chapter:
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The Moon and Earth-Moon System: Introduction
Phases of the Moon and Eclipses
The Moon and Earth’s Tides
The Moon: Structure and Composition
Observing the Moon with a Telescope
Exploring the Moon
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Early Unmanned Lunar Missions
The Apollo Missions
The Clementine Mission
The Lunar Prospector Mission and MoonLink
Future Prospects for Lunar Exploration
THE MOON: INTRODUCTION
• Our Moon has a diameter about 1/4 that of Earth, and has
about 1/80 (1.2%) of Earth’s mass.
• The Moon orbits the Earth at a mean distance of about
384,000 kilometers (238,000 miles), once every 29.5 days
(referred to the Earth-Sun direction), nearly the period of
time we call a Month.
• The current theory of the origin of the Moon is that it was
created by a collision of another planet, nearly the size of
Mars, with Earth early in the history of the solar system.
• The Moon lacks both an atmosphere and water, and
hence cannot support life.
• The Moon is the only planetary object, outside of our
Earth, that has yet been visited by humans.
Ground-Based Telescopic Image of the Full Moon
Earth and Moon - Size Comparison
Sizes and Distances to Scale
THE EARTH-MOON SYSTEM
• The Moon keeps one side facing Earth as it revolves
around Earth, such that nearly half of the Moon is never
visible from Earth.
• The changing illumination of the Moon (as seen from
Earth) by the Sun is responsible for the Moon’s varying
phases.
• When the Earth, Moon, and Sun are all arrayed along a
line passing through all three objects, we observe
eclipses of the Moon or of the Sun.
• The Moon’s gravitational force on the Earth is largely
responsible for ocean tides.
• Tides in Earth’s oceans are due to the differential of
gravitational attraction on different parts of Earth, primarily
by the Moon, and secondarily by the Sun.
The Moon at Gibbous and Crescent Phases
ECLIPSES OF THE SUN AND MOON
• Eclipses of the Moon occur when the Moon passes through
Earth’s shadow as it orbits Earth.
• Lunar eclipses can occur only at Full Moon phase, but do NOT
occur every orbit of the Earth, since the Moon’s orbit is tilted
relative to the Earth’s orbit around the Sun.
• Eclipses of the Sun occur when the Moon passes in front of the
Sun in its orbit around the Earth.
• Solar eclipses can only occur at New Moon phase, but (like
lunar eclipses) do not occur every orbit of the Earth, because of
the tilt of the Moon’s orbit to that of Earth’s orbit around the Sun.
• Eclipses of the Sun, as seen from any position on Earth, are
much rarer than eclipses of the Moon, because the portion of
Earth from which a solar eclipse can be viewed is much smaller
than the portion of the Earth (nearly all of the night-side
hemisphere) from which a lunar eclipse can be viewed.
Stages of a Lunar Eclipse
Moon at Total Eclipse
(brightness enhanced by time
exposure)
Total Eclipse of the Sun by the Moon
THE MOON AND EARTH’S TIDES
• The part of the oceans on the side of the Earth facing the
Moon feels stronger gravity, and the part of the oceans on
the side of the Earth facing away from the Moon feels
weaker gravity, than does the center of the solid Earth.
• This causes rises in sea level on opposite sides of Earth.
• Tides result from horizontal motion of water, relative to
Earth’s center, not “lifting” of water by the Moon’s gravity.
• When the Sun and Moon are in line with the Earth, tides
are stronger than average, and when the Sun and Moon
are at right angles to each other, tides are weaker than
average.
• Tide heights are also affected by the local sea level and
coastal topography.
View of the directions and magnitudes of gravitational forces exerted by the
Moon on the solid Earth and oceans (G), resultant effect on the solid Earth (C),
and net effects on ocean waters (residual difference vectors).
Net effective translational force on Earth’s surface water. Note it is zero at 90 to
the Earth-Moon direction, as well as at the zenith (Z) and nadir (N) directions.
THE MOON: STRUCTURE AND COMPOSITION
• The manned Apollo missions, and unmanned space missions,
to the Moon have greatly increased our knowledge of the
Moon’s composition and history.
• The composition of the surface rocks on the Moon are similar to
those of igneous rocks on Earth.
• The highlands are composed primarily of anorthosite, an
igneous rock composed mainly of a version of the mineral
feldspar.
• The lowlands (“mare”, Latin for seas) are composed of basalt,
consisting largely of the minerals pyroxene and olivine.
• The basaltic lowlands, or mare, are darker in color than the
felsic highlands (a feature obvious even to the naked eye,
viewing the “man in the moon” from Earth).
• The Moon has only a very small iron core, and no magnetic
field. It has a very thick lithosphere, and an asthenosphere
between the lithosphere and the core.
• Lunar craters provide a record of the meteoroid bombardment
history of the early solar system, and seismometers emplaced
on the Moon record the current rate of bombardment.
ROCK AND MINERAL TYPES
• Examples of Igneous Rocks and their constituent minerals
include:
• Basalt, Gabbro, Rhyolite, Granite
– Feldspar, Quartz, Amphibole, Pyroxine, Olivine
• Basalt and Gabbro differ not in composition, but in crystallinity;
likewise Rhyolite and Granite.
• Basalt and Gabbro differ in relative composition, of the five
major constituents above, from Rhyolite and Granite.
• Basalt and Rhyolite have small crystals because they are
erupted by volcanoes and cool rapidly from the molten state.
• Gabbro and Granite have large crystals because they cool
slowly from the molten state, underground.
COMMON MINERALS IN THE EARTH
• QUARTZ (Silica)
SiO2
• CORUNDUM (Alumina) Al2O3
• HEMATITE
Fe2O3
• CALCITE
CaCO3
• DOLOMITE
CaMg(CO3)2
• GYPSUM
CaSO4
• FLUORITE
CaF2
• PYRITE
FeS2
• OLIVINE
(Mg,Fe)2SiO4
• PYROXENE
(Mg,Fe)SiO3
• AMPHIBOLE
(Ca2Mg5)Si8O22(OH)2
• FELDSPARS
Albite
Orthoclase
Anorthite
• KAOLINITE
NaAlSi3O8
KAlSi3O8
CaAl2Si2O8
Al2Si2O5(OH)4
Silicate minerals, including olivine, pyroxene, and feldspar, are also found
in Moon rocks. Not found, or found in much lower relative abundances
(compared to Earth) in Moon rocks (to date), are the hydrated (OH)
minerals, carbonates, sulfides, sulfates, and halides.
THE MOON: STRUCTURE AND COMPOSITION
• The currently leading theory of the origin of the Moon is that a
large protoplanet, nearly as large as Mars, collided with the
proto-Earth more than 4 billion years ago, ejecting materials
which re-collected to form the Moon.
• The Apollo astronauts returned anorthosite rock samples from
the lunar highlands with ages in the range 4.0-4.6 billion years.
• The mare basalts are younger, in the 3.1 to 3.8 billion year age
range, indicating that volcanic activity or lava flows were
occurring during the first billion years of the Moon’s existence.
• Most of the larger craters visible on the Moon were created in
the first billion years of the Moon’s existence, but some craters
are known to be much younger.
• The intense bombardment of the Moon by asteroids and
meteorites has largely mixed the various rock types as found
near the surface of the Moon, creating a layer of loose soil and
rocks known as the regolith.
OBSERVING THE MOON
• The Moon is the easiest extraterrestrial object to observe
from the Earth’s surface.
• A relatively small and inexpensive telescope is adequate
to observe most of the major features of the Moon’s
surface, including craters, mountain ranges, and other
features as small as 2 or 3 miles in size.
• Because the Moon is usually very bright, it is easy to take
photographs or video recordings for later viewing.
• Observations should be made over a wide range of lunar
phases (times of the month) to obtain the best
observations of all accessible surface features.
• During crescent phases of the Moon, the night side of the
Moon is faintly visible by reflected Earth shine.
HST VIEW OF THE MOON
GALILEO VIEW OF THE MOON
False colors indicate presence of various minerals,
based on both visible and infrared brightnesses
Galileo False Color Imagery of Lunar Surface Minerals
EXPLORING THE MOON
• The exploration of the Moon, using unmanned spacecraft,
began in the early 1960s.
• U.S.-launched missions included the Rangers (crashlanders which returned close-up images), the Lunar
Orbiters, and the Surveyor landers.
• The manned Apollo missions to the Moon began in 1968,
with the Apollo 8 orbiter.
• The first manned lunar landing and sample returns were
accomplished with the Apollo 11 mission in 1969.
• There were a total of 6 manned landings on the Moon,
ending with the Apollo 17 mission in 1972.
• Analysis of the rock and soil samples brought back from
the Moon in the Apollo missions verified that some rocks
were as much as 4 billion years old, and that active
volcanism and lava flows on the Moon ended more than
3.5 billion years ago.
View of Moon from Apollo Mission
Direction of
Earth
Earthrise Seen from Apollo in Moon Orbit
POST - APOLLO LUNAR MISSIONS
• The first lunar mission launched by the U.S. since the end of
the Apollo program was the unmanned Clementine mission,
launched in 1995.
• The Clementine mission was sponsored by the U. S.
Department of Defense, as a means for testing new
technologies for remote sensing in the space environment.
• The Clementine spacecraft went into polar orbit around the
Moon and obtained the first nearly complete mapping of the
lunar surface.
• The first NASA lunar mission since Apollo was the Lunar
Prospector mission, launched in January, 1998.
• The Lunar Prospector included several non-imaging remote
sensing instruments, used to determine the composition and
other properties of lunar materials.
THE CLEMENTINE MISSION
• The Clementine mission (also known as the Deep Space
Program Science Experiment, DSPSE), was launched to
the Moon in January, 1994.
• Clementine was sponsored by the Strategic Defense
Initiative Organization (SDIO), and the spacecraft was
designed and built at the Naval Research Laboratory.
• The objectives of Clementine were to test new remote
sensing techniques for SDIO, and to obtain new scientific
information about the Moon and an asteroid using these
technologies.
• Clementine was placed in a polar orbit around the Moon,
which gave it the capability to map the entire surface of
the Moon over a period of a month or more.
THE CLEMENTINE MISSION
• The Clementine instrumentation included electronic
imaging cameras with coverage of several ultraviolet,
visible, and infrared spectral ranges.
• These images were used to obtain nearly complete maps
of the Moon and to provide information about the
composition of surface materials.
• Measurements of radio wave radiation from Clementine,
scattered from the lunar surface back to Earth, gave
tentative indications of the presence of water ice in
permanently shadowed regions near the Moon’s south
pole.
Clementine Mosaic of Lunar South Polar Region
Clementine Mosaic and High-Resolution Strip View of
Moon’s North Polar Region
THE LUNAR PROSPECTOR MISSION
• The Lunar Prospector mission was NASA’s first mission to
the Moon since the end of the Apollo program.
• The Lunar Prospector was the first of NASA’s new
Discovery series of low-cost planetary and deep space
missions.
• Lunar Prospector was launched January 6, 1998 and went
into a low-altitude (100 km) polar orbit around the Moon,
with a nominal one-year mission lifetime.
• Lunar Prospector incorporated many new and improved
remote sensing technologies to improve our
understanding of the Moon and its evolutionary history.
THE LUNAR PROSPECTOR MISSION
SCIENCE OBJECTIVES
The following questions are the basis of some of the
science objectives and measurement requirements
necessary for full scientific exploration of the Moon.
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How did the Earth-Moon System Form?
How did the Moon Evolve?
What is the Impact History of the Moon’s Crust?
What Constitutes the Lunar Atmosphere?
What can the Moon tell us about the History of the Sun
and Other Planets in the Solar System?
THE LUNAR PROSPECTOR MISSION
SCIENTIFIC INSTRUMENTATION
• Neutron Spectrometer
– Neutrons produced by cosmic rays provided evidence of water ice
in lunar material near the poles
• Alpha Particle Spectrometer
– Detected radioactive materials such as Radon; tracer of other gas
releases
• Magnetometer
– Measured and Mapped Moon’s Magnetic Field
• Electron Reflectometer
– Alternate Means for Magnetic Field Measurement
• Gamma Ray Spectrometer
– Measured and mapped abundances of solid radioactive materials,
and others activated by cosmic rays
• Doppler Gravity Experiment
– Used spacecraft motions to map Moon’s gravity field
Lunar Prospector Spacecraft and Launch
Lunar Prospector (Artist’s Interpretation)
THE LUNAR PROSPECTOR MISSION
• The results of the Lunar Prospector mission presently include:
– Confirmation and extension of measurements of water ice
mixed with soil near the Moon’s north and south poles
– More detailed measurements of the Moon’s magnetic field
and associations with surface features, such as mare and
large craters
– More detailed mapping of surface material compositions.
• The Lunar Prospector mission also had associated with it an
Education and Public Outreach component, called MoonLink,
which allowed pre-college students to participate in the mission
and to analyze results.
• MoonLink participants included students and teachers at Ballou
and Anacostia high schools, in Washington, DC.
Principle of Neutron Spectrometer Detection of Hydrogen
The collision of a fast neutron with the nucleus of a hydrogen atom (proton)
results in transfer of momentum to the proton and slows down the neutron.
Lunar Prospector Neutron Spectrometer
Lunar Prospector Neutron Spectrometer
Lunar Prospector Neutron Spectrometer
Lunar Hydrogen Distribution from Lunar Prospector
Neutron Spectrometer Observations
MoonLink Participants at Ballou H.S., DC, February 1998
FUTURE MISSIONS TO THE MOON
• Currently, there are a number of plans and proposals for future
missions for exploration and study of the Moon.
• The first of these missions, currently under development, is the
Lunar Reconnaissance Orbiter (LRO), planned for launch in late
Fall, 2008.
• The LRO is planned to orbit the Moon for at least a 1-year
mission.
• Among the highest scientific measurement priorities of LRO are:
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Characterization of deep-space radiation environment in lunar orbit
Geodetic global topography
High spatial resolution hydrogen mapping
Temperature mapping in polar shadowed regions
Imaging of the lunar surface in polar shadowed regions
Identification of near-surface water ice in polar cold traps
Assessment of features for future landing sites
Characterization of the polar region lighting environment
THE LUNAR RECONNAISSANCE ORBITER
Artist’s Concepts of the Lunar Reconnaissance
Orbiter, approaching and orbiting the Moon
LUNAR CREW EXPLORATION
• NASA has proposed to return astronauts to the Moon by the end of
this decade, using a newly developing Crew Exploration Vehicle.
• This vehicle would have two parts, a lunar lander and a transport
vehicle, both similar to but more advanced than the Apollo lunar
vehicles, which would have the International Space Station as the
departure and return site.