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Astronomy 2020 – Space Astronomy & Exploration
Homework #8
Spring 2015
Due: Apr. 20, 2015
In questions 1-3 below, choose the best answer. Then explain your reasoning in a few complete
sentences, including why these statements are correct or incorrect. Each question is worth 2 pts.
1. Why does Neptune appear blue and Jupiter appear red?
a. Neptune is hotter, which produces bluer thermal emission.
b. Methane in Neptune’s atmosphere absorbs red light.
c. Neptune’s atmosphere scatters blue light, much as Earth’s atmosphere does.
A: b – Neptune’s, unlike Jupiter’s, atmosphere contains a significant amount of methane. The
lack of red light is perceived as blue.
2. Why is Jupiter’s moon Io more volcanically active than our own Moon?
a. Io is much larger.
b. Io is much smaller.
c. Io has a higher concentration of radioactive elements.
d. Io has strong tidal forces because it is close to Jupiter.
A: d – The tidal forces of Jupiter on Io create friction in Io’s interior, which heats it.
3. Based on everything we have learned about Venus and Mars, what is the most surprising
aspect of Earth's climate history?
a. the fact that the temperature of our planet has remained relatively steady
throughout our planet's history
b) the fact that Earth apparently got a lot of atmospheric gas from outgassing by
volcanoes
c) the fact that Earth had enough water to form oceans
d) the fact that Earth's climate can be affected by changes in its axis tilt
A: a – Whereas Venus suffered a runaway greenhouse effect, and Mars lost its magnetic field
and so its liquid water and atmosphere, the Earth has maintained a habitable climate.
4.
(4 pts). Suppose observers on Earth’s North Pole and South Pole use a transit of the Sun
by Venus to discover that the angular size of Earth as viewed by the Venus observations
would be 62.8 arcseconds. Earth’s radius is 6378 kilometers. Estimate the distance
between the Earth and Venus in kilometers and in AU (where 1 AU = astronomical unit =
distance between the Earth and Sun). A: Using the equation: Angular separation = 206,265 arcseconds ×
physical separation (km)
distance (km)
Solving for distance gives: distance (km) = 206,265 arcseconds
× physical separation (km) Angular separation (arcseconds)
If we use the angular separation of 62.5 arcseconds and the physical separation as the diameter of Earth (2 x 6378 km), then we get a distance of 4.2x107 km = 0.28 AU. 5. (4 pts). Of Mercury, Venus, the Moon, and Mars, which world has the greatest erosion?
Explain why. In your answer, include the potential sources of erosion and discuss their
effect on each planet.
A: Of these bodies, Mars has the greatest erosion. While Venus has a thick atmosphere, the
winds near the surface are very slow. This means that there is not much erosion. None of the
others have a substantial atmosphere, so their wind erosion is negligible, except on Mars,
which has global dust storms. These dust storms lead to a significant amount of erosion.
There is also evidence on Mars for water erosion in the past, and even some episodic water
erosion in current times. There was likely also water erosion on Venus before its runaway
greenhouse effect took place. Nowadays it is too hot on Venus for liquid water and so there
cannot be water erosion. Erosion on Mercury and the Moon is limited to the effects of the
solar wind and micrometeorites.
6. (3 pts). What do we mean by a Runaway Greenhouse Effect? Explain why the process
occurred on Venus but not on Earth.
A: Venus is closer to the Sun than Earth, and the sunlight is more intense. This more intense
sunlight warmed the surface of Venus, which evaporated water. Water vapor is a greenhouse
gas, and so the surface of Venus is warmed further. This warming increases the evaporation,
which again increases the temperature. And so on.
7.
(3 pts). Suppose you could choose any one moon (other than Earth’s Moon) to visit in the
solar system with a human mission. a. Which one would you pick, and why? b. What dangers would you face in your visit to this moon? c. What kinds of scientific instruments would you want to bring along for your studies? Example: I would choose Europa because of its possibility of harboring life, and its large
supply of water, which could be used as fuel and also to aid in survival. Dangers would
include the temperature (about 100 K = -170 C), lack of an atmosphere, less gravity than the
human body is accustomed to (about 13% of Earth’s surface gravity), and increased radiation
levels. I would want seismometers to determine if there is indeed a subsurface ocean or warm
ice mantel. Other useful scientific instruments would study the composition of the ice and
search for microbes.
8. (3 pts). Each ring particle in the densest part of Saturn’s rings collides with another about
every 5 hours. If a ring particle survived for the age of the solar system (4.5 billion
years), how many collisions would it undergo?
A: We simply divide the total time by the time per collision to find the number of collisions:
4.5 billion years 3.94×10!" hours
=
= 7.88×10!" = 7.88 trillion collisions
5 hours
5 hours
9. (3 pts). A number of planetary scientists strongly suspect that Europa has a subsurface
ocean, even though we cannot see through the surface ice. Explain why scientists think
the ocean exists. What is the evidence for such an ocean?
A: Water vapor plumes have been observed erupting on Europa, and that water vapor has to
come from somewhere. A subsurface ocean is good solution. There are also surface features
which look like icebergs broke apart and refroze. There are few craters, which would suggest
regular resurfacing by new ice.