Download Due: January 7, 2014 Name

Homework Chapter 9
Due: January 7, 2014
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 1/3
Name: ______________________________________
1.
Earth's atmosphere differs from those of near-neighbor planets, Venus and Mars, in one important respect in that
a. it has a significant fraction of oxygen in it.
b. it has a much larger fraction of CO2 than either of the other two atmospheres.
c. the atmospheric temperature at the surface is much higher than those of the other two planets.
d. it has a higher pressure than these other planetary atmospheres.
2.
How does the greenhouse effect influence the temperature of the atmosphere? How does this effect differ from what
happens in an actual greenhouse made out of glass? (see pp. 167-168)
The greenhouse effect causes heat to be retained in the lower part of the atmosphere by absorbing
infrared radiation that is trying to escape from the Earth’s surface. In an actual greenhouse the
heat is retained by preventing its removal by convection.
3.
Mercury is much closer than Venus to the Sun, and yet it never appears brighter than Venus, even when both are at
maximum brightness, because
a. Mercury is larger than Venus, but it has a thick atmosphere which impedes reflection from its surface,
making it appear dark.
b. we never see more of Mercury than a thin crescent because of its orbital path relative to that of Earth.
c. Mercury is small, has a dark surface, and has no reflecting clouds.
d. Mercury has a highly reflective surface but is very small and hence appears relatively dark.
4.
The surface of Mercury, unlike the highland regions of the Moon, has extensive plains between the craters. The reason
for this is thought to be that
a. Mercury has a thinner mantle because of its large iron core and therefore experienced less
volcanism.
b. Mercury is closer to the Sun and did not receive as many crater-forming impacts as the Moon.
c. the older craters were worn down and obliterated by the solar wind.
d. Mercury is larger than the Moon, and it cooled more slowly, leaving lava to flow more
recently across its surface than it has over the lunar highlands.
5.
The density of Mercury is about the same as that of Earth, but Mercury contains a larger proportion of iron. How is this
possible?
a. The inner core of Mercury is hollow, thus reducing the overall density much below that of iron.
b. The iron in Mercury's core is accompanied by a very low-density crust. The average density is
thus comparable to the density of Earth.
c. The Earth contains, proportionally, a smaller volume of iron, but because of Earth's
greater mass this iron has been compressed to a larger density than the iron on Mercury.
d. Mercury has only one eighteenth the Earth's mass. Thus it must be composed of heavier
materials (like iron) in order to match Earth's density.
6.
Why does the surface temperature on Mercury’s equator show such an extreme variation between day and night?
The extreme temperature variations result from Mercury’s closeness to the Sun, its slow rotation, and
its lack of atmosphere. The slow rotation allows the surface to heat up strongly during the Mercurian
day and cool down greatly at night. The absence of an atmosphere means that nothing moderates the
inflow of sunlight during the day or retains the heat during the night.
Homework Chapter 9
7.
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 2/3
The Mariner 10 spacecraft, out of fuel since March 1975 and damaged by solar radiation, is still orbiting the Sun.
Suppose the orbital period of Mariner 10 is 2.00 times that of Mercury (87.97 days). Use this fact to calculate the length
of the semimajor axis (in AU) of the spacecraft's orbit. (When converting days to years, use 365.25 days/year.)
From Kepler’s third law we know that P2 = a3.
1.00 y
For Mercury, PMercury  87.97 d 
 0.2409 y
365.25 d
For Mariner 10, PMariner  2.00 PMercury   2.00  0.2409 y   0.4818 y
2
Then a  3 PMariner
 3  0.4818 y   0.615 AU
2
8.
The severe atmospheric conditions that quickly destroyed spacecraft, which were soft-landed on the surface of Venus,
were
a. intense sunlight, including UV, very high pressures, and very low temperatures.
b. very low temperatures, a near vacuum, and corrosive alkaline clouds and mist.
c. high temperatures, low atmospheric pressure, and intense UV radiation from the Sun.
d. high temperatures, high pressures, and corrosive acid clouds and mist.
9.
The geology and geography of the surface of Venus is best described as
a. mostly volcanic plains, with two continent-sized uplands and a number of large volcanoes.
b. volcanoes and volcanic uplifts in the northern hemisphere and cratered plains in the southern hemisphere.
c. colliding surface plates with long mountain chains, rift valleys, and deep subduction trenches.
d. heavily cratered, with no major volcanoes or lava flows.
10. What is so remarkable about the surface of Venus?
a. There is almost no evidence of volcanic activity anywhere, suggesting a very thick planetary crust.
b. Impact craters are sparse and uniformly distributed, suggesting that the entire surface is young and
about the same age.
c. Impact craters are as densely packed as on Mercury, suggesting the thick atmosphere that currently protects
the planet from most impacts is fairly young.
d. It is the only other planetary surface that is currently being shaped by active plate tectonics.
11. Planetary geologists tend to agree that the amount of water in the rocks of Venus and Earth plays a key role in
explaining why Venus and Earth have such different surface features and subsurface activity. What effect does the
abundant water in the Earth’s rocks have upon the properties of the Earth’s crust when compared to the crust of Venus?
Rocks with trapped water melt at a lower temperature and are “runnier,” allowing vigorous
convection in the mantle and a thinner crust on Earth than on Venus. The thinner crust allows
more heat to escape and allows for plate tectonics, which produce many of the surface features we
see on Earth but not on Venus.
12. Earth and Mars are similar planets in many respects. Which of the following pairs of physical characteristics are most
alike for these planets?
a. length of solar day and inclination of spin axis to the ecliptic
b. length of solar day and diameter
c. overall mass and diameter
d. planet diameter and inclination of spin axis to the ecliptic plane
Homework Chapter 9
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 3/3
13. A major feature of the atmosphere of Mars is
a. a chemical mixture very similar to that of Earth.
b. very high temperatures and pressures.
c. occasional strong winds and dust storms.
d. very dense clouds shrouding most of the planet.
14. The rotation of Mars and the orbital motion of its inner moon, Phobos, are in the same direction and Phobos orbits near
the equatorial plane of Mars in just over 7.5 hours. As a result, what is the motion of Phobos across the Martian sky as
seen from the surface of the planet?
a. It rises in the east, moves rapidly to set in the west, and appears several times per Martian day.
b. It rises in the west, moves rapidly to set in the east, and appears several times per Martian day.
c. It stays almost stationary in the sky in almost synchronous orbit, since its period is close to Mars's rotation
period.
d. It rises in the west, moves rapidly to set in the east, and appears only once per Martian day.
15. Mars Global Surveyor (MGS) is in a nearly circular orbit with an orbital period of 117 minutes. (a) Using the data in
Appendix Table 3, find the radius of the orbit. (b) What is the average altitude of MGS above the Martian surface? (c)
The orbit of MGS passes over the north and south poles of Mars. Explain how this makes it possible for the spacecraft
to observe the entire surface of the planet.
(a) The radius of the orbit is given by Newton’s form of Kepler’s third law:
GMP3
4π 2 3
.
a , or a  3
4π 2
GM
60 s
With P  117 min 
 7020 s, M  6.42 1023 kg, then
1 min
P2 
a
3
 6.67 10
–11
N  m 2 /kg 2  6.42  1023 kg  7020  103 m 
2
4π 2
 3.77  106 m  3770 km
(b) The radius of Mars is r  12  6794 km  3397 km . The altitude of MGS is
3770 km – 3397 km = 373 km above the surface.
(c) The polar orbit makes it possible for the spacecraft to observe the entire surface of Mars
because Mars rotates whereas the orbital plane of the spacecraft remains fixed. (Note:
Earth-orbiting ‘spy’ satellites are often in polar orbits.)
16. Carbon dioxide comprises about 95% of the present-day atmospheres of both Mars and Venus. Why, then, is there a
strong greenhouse effect on Venus but only a weak greenhouse effect on Mars?
The very thin atmosphere on Mars produces only a slight greenhouse effect compared to Venus.