AST 105 HW #6 Solution
Week of September 28th, 2015
Note: All Problems are from The Cosmic Perspective (6ed)
Chapter 8
Review Problems
1. What are the four major features of our solar system that provide clues to how it formed? Describe each
one briefly.
 There are four properties to our solar system that provide us with clues to how it formed. The first is
the fact that the motions in our solar system are orderly. The planets orbit in the same plane, on
nearly circular orbits, and in the same direction around the Sun, and most of them spin in that same
sense. Most of the major moons of our solar system also follow these rules when orbiting around
their planets.
The second property of the solar system that provides clues about the formation is the existence of
two kinds of planets, jovian and terrestrial. The two types of planets are found in different parts of
the solar system (outer and inner, respectively) and are generally very different in nature. (Masses,
densities, and compositions are especially significant differences.)
The third property we use to understand the formation of our solar system is the existence of the
smaller bodies such as the comets and asteroids. Asteroids are rocky or metallic bodies that are
usually found between the orbits of Jupiter and Mars. Comets are icy bodies found farther out into
the solar system. Comets are found in two regions, the Oort cloud and the Kuiper belt.
Finally, there are exceptions to the rules. Venus spins backward, Uranus and Pluto spin on their
sides, Earth has an incredibly large moon for a terrestrial planet, and a number of smaller moons
(plus the large moon Triton) orbit their planets backward.
2. What is the nebular theory, and why is it widely accepted by scientists today?
 The nebular theory states that the solar system formed from a collapsing cloud of gas and dust billions
of years ago. It is widely accepted by scientists today because it explains the properties of the solar
system remarkably well. However, there is not observational evidence of other forming solar systems to
support this theory.
4. Describe each of the three key processes that led the solar nebula to take the form of a spinning disk.
What observational evidence supports this scenario?
 This cloud that formed the solar system began by collapsing under its own gravity. As it did so, to
conserve angular momentum it spun faster. When clumps of gas and dust collided, both took velocities
nearer the average. Since the average velocity tends to point along a flattened disk, the solar nebula
eventually took on a disk shape. Evidence for these processes can be found in the fact that the planets
orbit the Sun in nearly the same plane and in the same direction and that they spin in this same
direction.
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6. What was the frost line in the solar nebula? Explain how temperature difference let to the formation of
two distinct types of planets.
 The frost line is the point moving away from the Sun where it is cool enough for hydrogen compounds to
freeze. Since the solar nebula was hotter near the center of the disk, hydrogen compounds such as
water stayed gaseous in the inner solar system. Outside of the frost line, they froze. As a solid, it was
able to participate in building planets. Since there was so much more water (and, farther out, solid
ammonia and methane) in the nebula, the forming planets past the frost line were much larger than the
ones inside that limit. Furthermore, this large size of the "Jovian planet seed" allowed it to gather
hydrogen/helium gas and make truly He planets.
12. How do we think the Moon formed, and what evidence supports this hypothesis?
 We think that the Moon formed when a large impactor struck Earth with a glancing blow. Such a
collision would have blown a lot of material into orbit around Earth, temporarily forming a disk around
our planet. From this disk, the Moon formed. Evidence in favor of this theory includes the chemical
composition of the Moon: It is similar to Earth's mantle (which is where the material probably came
from) and it is deficient in easily vaporized compounds like water.
It is not really surprising that such an event could have affected Earth. There are other objects in the
solar system that seem to show the effects of similar collisions. Uranus is tipped over. Pluto is both
tipped over and has a large moon like we do. Mercury lacks most of the outer layers we would expect it
to have, which might have been blasted off in a collision. Astronomers also believe they have detected
the thermal radiation from the dust of a comparable impact around another star.
Quick Quiz
Choose the best answer to each of the following. Explain your reasoning with one or more complete
sentences.
25. How many of the planets orbit the Sun in the same direction that Earth does?
(a) a few (b) most (c) all
27. The solar nebular was 98%
(a) rock and metal.
(b) hydrogen compounds.
(c) hydrogen and helium.
30. Which lists the major steps of solar system formation in the correct order?
(a) collapse, accretion, condensation
(b) collapse, condensation, accretion
(c) accretion, condensation, collapse
32. Why didn't a terrestrial planet form at the location of the asteroid belt?
(a) There was never enough material in that part of the solar nebula.
(b) The solar wind cleared away nebular material there.
(c) Jupiter's gravity kept planetesmials from accreting.
34. About how old is the solar system?
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(a) 4.5 million years
(b) 4.5 billion years
(c) 4.5 trillion years
Quantitative Problems
Be sure to show all calculations clearly and state your final answers in complete sentences.
48. Radiometric Dating
You are analyzing rocks that contain small amounts of potassium-40 and argon-40. The half-life of
potassium-40 is 1.25 billion years.
a. You find a rock that contains equal amounts of potassium-40 and argon-40. How old is it? Explain
 Half of potassium-40 has decayed into argon-40. Since the half-life of potassium-40 is 1.25 billion years,
the rock has to be 1.25 billion years old.
b. You find a rock that contains three times as much argon-40 as potassium-40. How old is it? Explain
 The amount of potassium-40 is ¼ of the original amount. This implies that the rock went through halflife of potassium-40 twice since ¼ = ½ * ½ . So the rock is 2.5 billion years old.
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