Download Due: January 7, 2014 Name

Homework Chapters 7-8
Due: January 7, 2014
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 1/3
Name: __________________________________
1.
The correct sequence of planets in our solar system from the Sun outward is
a. Mercury, Venus, Earth, Mars, Saturn, Uranus, Jupiter, Neptune, Pluto.
b. Mercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus, Pluto, Neptune.
c. Mercury, Venus, Mars, Earth, Jupiter, Saturn, Uranus, Pluto, Neptune.
d. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.
2.
Which of the following statements about the solar system is false?
a. All planets revolve around the Sun in the same direction.
b. All planets rotate on their axis in the same direction as they orbit about the Sun.
c. All planets orbit in roughly the same plane.
d. Most planetary orbits are not highly elliptical.
3.
The average density of which of the following planetary groups is close to that of water (1000 kg/m3)?
a. Mercury and Venus, because they are close to the Sun.
b. Terrestrial planets, because they are of relatively low mass and have been compressed
very little by gravitational forces.
c. Asteroids, because they are very small objects.
d. Jovian planets, because of their H and He composition.
4.
What was the form of the material from which the solar system formed?
a. A nebula made mostly of hydrogen and helium gas, but enriched in heavier elements from
supernova explosions.
b. A nebula made mostly of heavy elements, but enriched in hydrogen and helium from the
supernova explosions.
c. A nebula made entirely of hydrogen and helium gas.
d. Debris from the explosion of a massive star.
5.
What is meant by a substance's condensation temperature? What role did condensation temperatures play in the
formation of the planets?
The condensation temperature of a substance is the temperature at which the substance solidifies
from gas. In the inner part of the solar system where the temperature was high, only substances
with high-condensation temperatures could become planets. In the outer part of the solar system,
substances with low-condensation temperatures could also become part of planets. Thus the inner
planets are rocky while the outer planets contain more liquids and gases.
6.
Why did the terrestrial planets form close to the Sun while the Jovian planets formed far from the Sun?
Terrestrial planets are made of substances with high condensation temperature, substances that will
not vaporize at the high temperatures that exist close to the Sun. Jovian planets are made of
substances with low condensation temperatures, such as hydrogen and helium, and they can only
form where temperatures are very cold, far from the Sun.
7.
According to modern theories, the most significant difference between the formation of the terrestrial and Jovian planets
is that
a. both formed by accretion of planetesimals, but the Jovian planets became massive enough to attract
gas onto them directly from the solar nebula.
b. both formed by accretion of rocky and icy planetesimals, but the terrestrial planets were close enough
to the Sun that almost all of ices escaped back to space after the planets formed.
c. the terrestrial planets formed close to the Sun where there was lots of rock but no ice, whereas the
Jovian planets formed far from the Sun where there was lots of hydrogen and ice but no rocky material.
d. the terrestrial planets formed by accretion of planetesimals, whereas the Jovian planets formed from
streamers of hot gas which shot out of the protostar.
Homework Chapters 7-8
8.
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 2/3
The protoplanetary disk, or proplyd, in the figure below is seen edge-on. The diameter of the disk is about 700
AU. (a) Make measurements on this image to determine the thickness of the disk in AU. Explain why the disk will
continue to flatten as time goes by.
The ratio of the proplyd’s width to its thickness is about 4 to1.
thickness 
1
 700 AU   175 AU
4
The disk will continue to flatten as gravity compresses the material along the axis of rotation, but
the centrifugal force prevents the material from compressing perpendicular to the rotation axis.
9.
Run the “Solar System Builder” from the chapter 7 section of the textbook companion site. Click “Reset” and then
check the Pause checkbox. Move the slider (if necessary) to choose an Earth planet. Click once when the cursor
position (see lower right) is at Y = 0, X = 1.0 AU. Now move the planet slider to Jupiter. Click once at Y = 0,
X = 5.0 AU. Now add 5 more Jupiters by clicking at different places along the line between Jupiter and the Earth, then
move the planet slider to Earth and add 5 more Earths in that same region. Now uncheck the Pause checkbox and watch
the system for 30 years. Describe the state of the system in comparison to how it began. Try the same simulation two
more times, but vary the distribution of planets along the initial line a little bit. What common themes do you see in the
outcomes of the three simulations?
After 30 years some planets have collided and some have been flung off into space, so that only a
few planets remain and the resulting orbits are noncircular and chaotic. The closer the planets are
initially placed to one another, the stronger the interactions between them.
10. One theory about the origin of the Moon says that the Moon was formed from debris thrown out when a Mars-sized
object collided with Earth. One fact that strongly supports this theory is that
a. the Moon has several smooth plains formed by ancient lava flows.
b. the Moon always turns the same side toward Earth.
c. impact breccias (rock fragments cemented together by an impact) are common on the Moon.
d. Moon rocks are very similar to those of Earth but are depleted in elements that melt at relatively
low temperatures.
11. A planet [Mars] has two small satellites, [Phobos and Deimos]. Phobos circles the planet once every 0.31891 day at an
average altitude of 5980 km above the planet's surface. The diameter of the planet is 6794 km. Using this information,
calculate the mass and average density of the planet.
First of all, you must calculate Phobos’ orbital radius:
a  5980 km  12  6794 km  9377 km  9.377  106 m
From the Extending Our Reach on page 82 we see that the appropriate form of Kepler’s third law is
M 
4 2 d 3
, where M is the mass of Mars. The orbital period of Phobos is
G P2
P = 0.31891 days = 2.755  104 s. Applying the formula yields:
 4π 2
M 
 G
6
 d3 
  9.377  10 m 
4π 2
 2 
11
2
2 
2
P
 6.67  10 N·m /kg   2.755  104 s  .
M  6.43  1023 kg
Now calculate the density:

M
M
6.43 1023 kg
4 3
 3920 kg/m3
3
3
4
1
V
3r
3   2 6794  10 m 
3
Homework Chapters 7-8
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 3/3
12. Describe the differences between the maria and the lunar highlands. Which kind of terrain is more heavily cratered?
Which kind of terrain was formed later in the Moon's history? How do we know?
The maria are lava-flooded areas that are dark gray in color and are lower in elevation than the
average. The highlands are light gray in color, rough in appearance, and higher than average in
elevation. The highlands are much more heavily cratered. The maria were formed later in the
Moon’s history. We know this because the crater density on the maria is lower than the the crater
density on the highlands.
13. Rocks found on the Moon are between 3.1 and 4.6 billion years old. By contrast, the majority of the Earth's surface is
made of oceanic crust that is less than 200 million years old, and the very oldest Earth rocks are about 3 billion years
old. If the Earth and Moon are essentially the same age, why is there such a disparity in the ages of rocks on the two
worlds?
On the Earth, rocks that formed before about 3 billion years ago have been subducted into the
mantle and melted. Old crust is continually pushed down into the mantle and replaced along the
mid-oceanic ridges by new crust. These processes do not occur on the Moon, so ancient rocks are
still found there on the surface.
14. In a whimsical moment during the Apollo 14 mission, astronaut Alan Shepard hit two golf balls over the lunar surface.
Give two reasons why they traveled much farther than golf balls do on Earth.
Golf balls hit on the Moon travel farther than on Earth because there is no air resistance on the Moon to
slow the balls down, and the Moon's weaker gravity takes longer to pull the balls back down to the ground.
15. How much would an 80-kg person weigh on the Moon? How much does that person weigh on the Earth? Use
Appendix Tables 3 and 5 together with the Universal Law of Gravity.
The Universal Law of Gravity (p. 79) can be used to find these answers.
Tables 3 and 5:
On the Moon,
Weight  F  G
Mm
d
2

 6.67  1011 N  m 2 /kg 2
or about 29.2 pounds. On Earth,
F G
Mm
d
2

 6.67 10
11
N  m /kg
2
2


 7.349 10
5.974 10
22

kg 80 kg 
1738 km 100 m/km 2
24

kg 80 kg 
 6, 378 km 100 m/km 2
 130 N
 784 N  176 lb
Using Appendix