Download (6-4 thru 6-6 PPT Questions) Why do you think craters on the Moon

6-1, 6-4 thru 6-6
The EarthMoon
System
© 2007 Jones and Bartlett Publishers
Courtesy of NASA/JPL/Northwestern University
Chapter 6
6-1 Measuring the Moon’s Distance and Size
The Distance to the Moon
1. Using parallax, Ptolemy determined that the
distance from the Earth to the Moon is 27.3 Earth
diameters— close to the correct average distance of
30.13.
2. Since the Earth’s diameter is about 12,800 km, thirty
Earth diameters puts the Moon at about 380,000 km
from Earth.
The Size of the Moon
1. Angular size of the Moon is close to 0.5°.
2. The Moon’s apparent diameter depends
on its distance from the observer and on
its angular size.
Figure 6.04: The Moon’s angular diameter is
about 1/2 degree.
The Small-Angle Formula
1. The diameter (width) of an
object is directly
proportional to its angular
size and its distance from
the observer.
2. Small angle formula is
accurate for angles less
than 5°.
3. Small angle formula yields
a value of 3,480 km (2,160
mi) for Moon’s diameter.
Figure 6.05: The angular size and
diameter of an object are related to its
distance from the observer.
Summary: Two Measuring Techniques
1. The triangulation (parallax) method relies on the
relationship among size of the baseline, angle of
parallax, and distance to the object.
2. Another important relationship exists among
angular size, actual diameter, and distance.
The Moon’s Changing Size
1. Larger apparent diameter of the Moon occurs at
perigee—the point in the orbit of an Earth
satellite where it is closest to Earth—which is at
a distance of 363,300 km.
2. Smaller apparent size of the Moon occurs at
apogee—the point in the orbit of an Earth
satellite where it is farthest from Earth—which is
at a distance of 405,500 km.
Figure 6.08: Moon near to and far from Earth
Courtesy of Galileo Project, NASA
Question 1 (6-4 thru 6-6 PPT Questions)
Because Ptolemy lived long before instant distant
communication was possible, he was not able to
coordinate his observations of the Moon with
someone far around the Earth. Propose a method
by which he might have been able to observe
parallax of the Moon.
6-4 The Moon’s Surface
1. The surface of the Moon can be divided into maria
and mountainous, cratered regions.
2. Mare (plural maria) are any of the lowlands of the
Moon or Mars that resemble a sea when viewed
from Earth.
3. Most craters on the Moon are the result of impacts
by meteorites—an interplanetary chunk of matter
that has struck a planet or moon.
4. Earth has few impact craters because its
atmosphere keeps all but the largest meteorites
from reaching the surface.
Over time, erosion and tectonic plate movement has
erased all but a relative few of the largest craters.
On the airless Moon, Mercury, satellites of other
planets, and even asteroids, craters remain intact
and visible for billions of years.
Figure 6.26b: A close up of Tycho shows that it has a
prominent central peak
Photo courtesy of UCO/Lick Observatory
Photo courtesy of UCO/Lick Observatory
5. Lunar ray is a bright streak on
the Moon caused by material
ejected from a crater.
6. The Moon’s maria are the result
of volcanic action leading to
massive lava flows.
7. The Moon’s crust ranges in
depth from 60–100 km and is
thinner on the side facing the
Earth.
8. Mountains on the Moon are the
result of extensive cratering
over eons.
Figure 6.26a: Lightcolored rays can be seen
radiating from the Tycho
crater on the Moon.
8. Mountains on the Moon are the result of extensive
cratering over eons.
9. The Moon’s density is 3.35 g/cm3. Its core, if
composed of iron, must be small.
10. The Moon’s weak magnetic field—10−4 times that of
Earth’s magnetic field—suggests the presence of a
small iron core, though this has not been
confirmed.
11. Sensors on the Moon have detected very weak
natural moonquakes.
Question 2 (6-4 thru 6-6 PPT Questions)
Why do you think craters on the Moon could only be
from meteorite impacts and not from extensive
volcanic activity?
6-5 Theories of the Origin of the Moon
1. Evidence indicates that the Moon formed about 4.6
billion years ago.
2. According to the double planet theory, first
suggested in the early 1800s, the Earth and Moon
formed at the same time from the same rotating disk
of material.
The different densities of the Earth and Moon seem to rule out
this scenario.
3. According to the fission hypothesis, the large basin
of the Pacific Ocean is the place from which the
Moon was ejected.
This theory cannot explain the Moon’s current orbit nor offer
an adequate rationale for what force could have caused
the Moon to be torn from the Earth.
4. According to the capture theory, proposed early in
the 20th century, the Moon was originally solar
system debris that was captured by the Earth’s
gravitational field.
Dynamically, a third object is required for capture, and the
chance of this happening with the Moon and Earth is
remote at best.
5. The Moon’s chemical composition is similar to that
of the Earth’s crust, but the Moon has smaller
proportions of volatile—easily vaporized—
substances than the Earth.
The Large Impact Theory
1. According to the large impact theory, proposed in
the 1970s, the Moon formed as the result of a
glancing impact between a large Mars-sized object
and the Earth.
This theory can explain the relative compositions of the Earth
and Moon, the Moon’s orbit, and the Earth’s rotation rate.
This theory has also been successfully modeled on a
supercomputer.
Figure 6.30
6-6 The History of the Moon
1. Radioactive dating techniques on the 840 pounds of
Moon rocks brought back to Earth by the Apollo
astronauts have been indispensable in forming a
model of the Moon’s history.
2. The Moon formed about 4.6 billion years ago.
3. Most craters formed between 4.2 and 3.9 billion
years ago.
Giant impacts near the end of the cratering period formed the
maria.
6-6 The History of the Moon
4. After cratering ended, the Moon’s interior became
hot from radioactive decay and molten lava flowed,
ending about 3.1 billion years ago.
The Moon has probably remained relatively unchanged since
then.
5. Micrometeorites (tiny meteorites) still hit the Moon,
but no new large crater has ever been observed.
Figure 6.B04: Astronaut collecting lunar rocks
Courtesy of NASA/JPL-Caltech
Figure 6.32a: Lunar eclipse on May 5, 2004
© Kyodo/Landov
Question 3 (6-4 thru 6-6 PPT Questions)
Name and describe three different theories of the
formation of the Moon. Which of these theories is
considered most likely correct? Describe the
evidence that leads us to that conclusion.