The Moon
• The Moon has 1/80 the mass of the Earth and 1/6 its
gravity
The Moon has no atmosphere because of its low gravity
•
• The density of the moon
•
•
is 3.3 g/cm3 compared
with Earth’s 5.5 g/cm3
The Moon rotates on its
axis every 27.3 days
which is exactly equal
to the time it takes to
orbit Earth
Views of the moon from
Redshift
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
1
Exploration of the Moon
• The Russian Luna 9 was the first spacecraft to land on the Moon in 1966
• On July 20, 1969, Neil Armstrong stepped out on the surface of the
•
•
Moon
9 Apollo missions were flown (list given in Table 8.2)
 3 did not land and 6 landed on the surface
The last mission was flown in December, 1972
 Here is Jack Schmidt during Apollo 17
• Here is the lunar
module from Apollo 17
leaving the surface of
the moon
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
2
Top 10 Discoveries from Apollo
• The Moon is not a primordial object; it is an evolved
terrestrial planet with internal zoning similar to that of
Earth.
• The Moon is ancient and still preserves an early history (the
first billion years) that must be common to all terrestrial
planets.
• The youngest Moon rocks are virtually as old as the oldest
Earth rocks. The earliest processes and events that probably
affected both planetary bodies can now only be found on
the Moon.
• The Moon and Earth are genetically related and formed
from different proportions of a common reservoir of
materials.
• The Moon is lifeless; it contains no living organisms,
fossils, or native organic compounds.
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
3
Top 10 Discoveries from Apollo, cont.
• All Moon rocks originated through high-temperature processes
•
•
•
•
with little or no involvement with water. They are roughly divisible
into three types: basalts, anorthosites, and breccias.
Early in its history, the Moon was melted to great depths to form a
"magma ocean." The lunar highlands contain the remnants of early,
low density rocks that floated to the surface of the magma ocean.
The lunar magma ocean was followed by a series of huge asteroid
impacts that created basins which were later filled by lava flows.
The Moon is slightly asymmetrical in bulk form, possibly as a
consequence of its evolution under Earth's gravitational influence.
Its crust is thicker on the far side, while most volcanic basins -- and
unusual mass concentrations -- occur on the near side.
The surface of the Moon is covered by a rubble pile of rock
fragments and dust, called the lunar regolith, that contains a unique
radiation history of the Sun which is of importance to
understanding climate changes on Earth.
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
4
Composition of Moon
•
•
•
•
•
•
The Moon is made of lighter silicate rock
The Moon has no iron core like Earth
The Moon has no current seismic activity
The moon has no magnetic field
The Moon has no volatile materials
It is as if the Moon were composed of the same basic
materials as the Earth’s crust with the volatile material
and the core metals missing
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
5
General Appearance of the Moon
• To the naked eye, the Moon has seas and
continents


Craters are not visible
Features are named for seas

Maria are younger



Dark, round plains much less cratered
Sea of Tranquility, Sea of Clouds, etc.
Continents or plains are not named

Highlands are older

Low density rock formed early, severe cratering
• Since the invention of the telescope, craters have
been observed and named
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
6
Impact Craters
• The moon provides an important benchmark for
•
understanding the history of the solar system
The Moon preserves the history of the solar system
through its uneroded record of impact craters
• The observed craters on the moon
•
were once thought to be volcanic in
origin
Now the craters are understood to be
all from the impact of meteors
Terrestrial Volcano
Lunar Impact Crater
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
7
The Cratering Process
• Any object striking moon
does so at least escape
velocity (2.4 km/h or 5400
mi/h)
• This impact creates an
explosion the produces a
round crater
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
8
Using Crater Counts
• We can use the number of impact craters on the Moon’s
•
surface to estimate its age
By looking at the current population of asteroids and
comets one can deduce that



1 km crater produced every 200,000 years
10 km crater produced every million years
1 or 2 100 km craters produced every billion years
• This calculation gives 3.3 to 3.8 billions years for the age
of the Moon

Compares well with radioactive dating
• There is evidence that the cratering rate was much higher
just after the formation of the solar system 4.5 billion
years ago

Highlands have 10 times more craters than maria
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
9
Deducing Information from Craters
• If a crater forms on top of
another crater, that crater
is newer
• Looking at the edges of a
crater one can say


Old craters have smoother
edges from meteor
erosion
Young craters have sharp
edges
ISP 205 - Astronomy Gary D. Westfall
Ptolemaeus Crater taken
by Apollo 16
Lecture 10
10
Theories for the Origin of the Moon
• Theories for the origin of the Moon fall into
three categories

The fission theory


The sister theory


The Moon was once part of the Earth but separated
early in its history
The Moon formed at the same time as the Earth but
independently
The capture theory

The Moon was formed elsewhere in the solar
system and later captured by Earth
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
11
Evidence Against These Theories
• The fission theory


Modern calculations show that fission cannot occur
Recently discovered chemical differences rule out this
theory
• The sister theory

Moon should have much larger iron core like Earth
• The capture theory


A body can only be captured into orbit if there is a
mechanism to slow the body - there is none
Orbit would be very elliptical - it is circular
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
12
The Giant Impact Theory
• Moon was created by collision of Earth with an
object about 1/10 the size of Earth
• The Mars-like object might have stuck the Earth
and ejected a large chuck of the crust into orbit
around the Earth that formed the Moon

Explains lack of volatiles and chemical composition
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
13
The Planet Mercury
• Mercury is the closest planet to the Sun
• Mercury orbits the Sun in 88 days
• Mercury has an
eccentric orbit (e =
0.206) with a
semimajor axis of 58
million km (0.39 AU)
• The plane of
Mercury’s orbit is
tilted 7 degrees from
the ecliptic
• Interactive Solar System
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
14
Mercury’s Strange Rotation
• Mercury was once thought to rotate once per orbit
• Radar observations showed that Mercury does not
keep one face toward the Sun


Takes 59 days to rotate on it axis
Takes 88 days to orbit Sun
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
15
Viewing Mercury in the Sky
• Position of Mercury in the sky at 6 pm, Oct. 3 looking SW
• Mercury Sky View.RS3
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
16
Composition and Structure
• Mercury’s mass is 1/18 that of the Earth

2nd smallest
• Mercury’s diameter is 4880 km

2nd smallest
• Mercury’s density is 5.4 g/cm3

2nd densest
• Mercury is thought to consist of an iron-nickel
core surrounded by a silicate crust


Core is roughly the size of the Moon
Probably molten because Mercury has a magnetic field
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
17
The Surface of Mercury
• Mercury strongly resembles the Moon in
appearance
• Mercury is covered with thousands of craters
and larger basins up to 1300 km in diameter
• Some of the brighter craters are rayed
• Mercury’s basins resemble the
•
•
ISP 205 - Astronomy Gary D. Westfall
Moon’s maria
There is no evidence of plate
tectonics on Mercury
Long scarps are visible cutting across
craters
Lecture 10
18
Fly-by of Mercury
• Here is a series of photos of Mercury shot by
Mariner 10
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
19
Origin of Mercury
• The problem with understanding the origin of
Mercury is the opposite of understanding the
origin of the Moon
• Mercury has too little silicates and too much iron
• Mercury may have suffered a catastrophe early in
its history that tore away a fraction of its mantle
• Both the Moon and Mercury have strange
compositions that tell astronomers that the early
solar system was chaotic
ISP 205 - Astronomy Gary D. Westfall
Lecture 10
20