Homework KEY
AT 9th Ed.
Chapter 6 Overview of the Solar System
Assigned: 1-15 (odd), Multiple Choice 1-9 odd

Review and Discussion

1. What is comparative planetology?  Why is it useful?
   What is its ultimate goal?
   Comparative planetology involves looking at all of
the similarities and differences between the planets.
Since the Earth is most important to us, we usually
start by comparing to the Earth.  This helps us
refute any incorrect generalizations about planets
that were based on Earth alone.  For example, we
might generalize that plate tectonics exists on all
terrestrial planets, but this is not true.
It is useful for gaining a better understanding of
the conditions under which planets form and evolve.
It is especially useful in noticing things that are
absent in one object but not another. The goal is
a comprehensive theory of the origin and evolution
of our planetary system.

2. Name and describe all of the different types of
objects found in the solar system. Give one distinguishing
characteristic of each. Mention interplanetary space.
   Our solar system contains 8 planets, 1 Sun (star),
asteroids, meteoroids, comets, over 170 moons, and
interplanetary dust.  There are two main types of planets:
terrestrial and Jovian, with the terrestrials found close
to the Sun and the Jovians found between 5 and 30 AU.
The space between the planets is almost a vacuum except
for dust, gas and charged particles (mostly coming from
the Sun).

3. Why is it necessary to know the distance to a planet in order
to determine its mass?
    A planet's mass is determined using one or more of its
moons and Kepler's Third Law.  This law requires
that we know the size of the orbit (in m or km), and this
can only be determined if we know the distance to the 
planets.  From Earth, we can measure the angular distance
between the planet and its Moon, call it A, and we can
measure the actual orbital radius, R, if we know the
distance to the planet, D using:  A*D = R.  (This assumes
that A is in radians, and R and D are in the same
length unit.)

4. Name 3 important differences between the terrestrial and
Jovian planets.
First, the terrestrial planets are all in the inner 1.5 AU of the 
solar system, while the jovian planets are scattered from 5 to 30 AU. 
Second, the jovian planets are all substantially larger, in both mass 
and radius. Finally, the terrestrial planets are mostly made of rock 
and metal, with a much higher density than the jovian planets, which 
are made mostly of light gases and liquid.

5. Why are asteroids and meteoroids important to 
planetary scientists?
    These objects have not been transformed as much
as the planets since the solar system formed.  They
are remnants of "planetesimals", the fragments that
went into forming planets and their moons.

[ 5. (7th Ed.) Disorderly properties of the Solar system include the
way that Mercury has a more tilted orbit than the others.
The spin axes of Venus and Uranus are tipped more than 90
degrees so we consider their rotation to be "retrograde".
Some moons, like Nereid, have retrograde orbits.  Comets
come in from the Oort cloud at all angles.  And collisions
between meteoroids and other bodies in the solar system
occur fairly randomly. ]

6. Comets generally vaporize upon striking Earth's
atmosphere.  How, then, do we know their composition?
  We know of the composition of comets in a few ways.
First, we can do spectroscopy of the comet and see which
gases are in its tail.  Second, we have sent spacecraft
to collect samples from comets (e.g., "Stardust" collected
some of the coma of a comet using aerogels).
Comets also release dusty particles which reflect sunlight in 
different ways, and reveal information about their size and composition.

7. What is the key ingredient in the modern condensation theory
of the solar system's origin that was missing or unknown in
the nebular theory?
   They key ingrediant that makes condensation theory more successful
than nebular theory at explaining planet formation is the presence
of DUST in the gases.  These serve as nuclei for condensation and
so they can grow and clump together through weak electrostatic
forces (stickiness).

8. Give three examples of how the condensation theory explains
the observed features of the present-day solar system.
    The condensation theory predicts higher density planets close
to the Sun because only metals and rocks could condense there.
It also predicts a change in composition and density of
planetesimal-like objects (asteroids, KBOs, comets) with distance
such that the higher density, metal-rich objects are close
to the Sun and objects rich in frozen volatiles are far from
the Sun.  The Moons around Jovians have crusts made largely of
water ice indicate of the composition of the outer solar nebula.

9. Describe how the terrestrial planets formed.
Mercury, Venus, earth and Mars formed out of the inner, hottest
part of the solar nebula.  The grains of dust were first 
thought to have stuck together due to electrostatic forces.
They grew in gentle encounters until they formed planetesimals
that could grow by gravitational attraction of smaller
pieces.  The largest planetesimals could survive further collisions
and eventually became "protoplanets".  These evolved through 
processes like differentiation into the planets.

10. Why are the Jovians so much more massive than the terrestrials?
The gases were able to condense into ices a join with 
rock and metals to form much larger planet cores in the outer
solar system.  These cores could then gravitationally attract 
even the lightest gases (H and He) to further build the
planet in ways that terrestrials could not.
Also, the Jovians had a much larger reservoir of material
in the vicinity of their orbits to attract.
(Picture the bigger gaps between the orbits of Jupiter, Saturn,
etc., compared to between Mercury, Venus, Earth, etc.)

11. How did the temperature structure of the solar nebula
determine planetary composition?
As a given material increases in temperature it can more readily
sublimate (transform from solid to gas) or melt (transform from solid
to liquid).  (Under the low pressure of space, some materials will 
only be a solid or a gas, skipping over the liquid phase.)
If the temperature of the nebula decreases 
with distance from the Sun, there will be certain distances 
corresponding to these melting and sublimation points.  
Closer to the Sun, only the highest melting point materials will 
be able exist as dust grains and stick together and grow into 
planetesimals.  Farther from the Sun, both low and high melting 
point materials will adhere to dust grains.  If it's cold enough, 
the grains can contain frozen gases / volatiles like CO2 and H2O.  
So planetesimals forming at large distances from the Sun 
will contain more gases and volatiles than those at small
distances.  The planets, in turn, form out of the planetesimals,
so outer planets will contain more low-sublimation point 
materials than inner planets.

12. Describe two possible ways in which the jovian planets
formed.  What role did the Sun play in the process?
    One proposal, the core-accretion theory, is that the Jovian planets 
formed into protoplanets by the process of accretion, as the inner 
planets did.  However, because of the abundance of lighter materials 
in the outer solar system, these protoplanets were able to wrap 
themselves up in thick atmospheres.  A big problem with this idea is 
that there might not have been enough time to gather up those 
atmospheres, before the young sun's "T Tauri wind" drove material 
out of the solar system.  And so a second idea has sprung up, the 
gravitational instability theory, which proposes that large sections 
of the original solar nebula began to contract under the influence 
of gravity to form the Jovian planets and their ring and moon systems.  
In this scenario, the Jovian planets formed much like the Sun and other 
stars.  The first scenario leads to much larger rocky-ice cores in the 
giant planets, so further investigation of the interiors of these 
planets may help us decide between the two proposals.

13. What happened to the outer planets as the solar system
was cleared of icy planetesimals?
    Jupiter migrated inward as it looses some of its kinetic 
energy to encounters with planetesimals.
Saturn, Uranus and especially Neptune probably migrated 
outward due to influence of Jupiter, and from sending
planetesimals inward.

[ 13. Scientists use gravitational assists to accelerate their spacecraft
so that they can travel to the outer planets, overcoming the gravity
of the Sun.  They do this by planning a trajectory that brings the
spacecraft towards a planet in such a way that it approaches from
behind the planet (so the craft is already going in the same direction 
as the planet).  Then the planet's gravity will mostly 
speed up the spacecraft rather than slow it down. ]

14.  How did the Kuiper belt and the Oort cloud form?
   In the early days of the solar system, there were an immense number 
of icy planetesimals mingled with the Jovian planets.  Over time, 
these icy chunks would be attracted to the larger planets.  Many 
would simply collide with the planets or be captured as moons, but 
some would be traveling so fast, and at just the right angle, to 
fall toward the planet and "miss."  Much like the spacecraft that 
we often "slingshot" around the planets, these icy planetesimals 
would go flying off into space.  Some - especially those interacting 
with Uranus and Neptune - would go on to populate the Kuiper Belt 
along with very distant planetesimals that had no interactions.  
Others - especially those that pass by mighty Jupiter - could be 
flung out to the Oort Cloud.

15. Describe a possible history of a single comet now
visible from Earth, starting with its birth in the
solar nebula somewhere near the orbit of Jupiter.
  Perhaps some comets formed as close as 5 AU from
the Sun (this is beyond the frost line after all).
(Others may have formed at much greater distances.)
A comet that formed close to Jupiter would have its orbit
altered by Jupiter.  It would likely be given a high
speed which would give it a large, eccentric, and highly
inclined orbit, possibly spending most of its time in
the Oort cloud but occasionally returning to the inner
solar system.  Earthlings could observe this as a 
"long-period comet" coming in towards the Sun on a 
nearly parabolic orbit.  There the light and heating of the 
Sun would create a temporary tail behind the comet and make 
it more visible to those on Earth.

[ 15. Galileo and Cassini took such circuitous routes compared to
Pioneer and Voyager primarily so that they could use gravitational
assists from the inner planets to accelerate without using extra
fuel/power. ]

[ 19. The Jovian planets are more massive than the terrestrials
because, after the formation of their solid cores, they had
a much larger reservoir of gas in their orbits to attract.
(Picture the bigger gaps between the orbits of Jupiter, Saturn,
Uranus, etc. compared to the gaps between Mercury, Venus, Earth, etc.)
The more massive they got, the more effect they were at capturing
other material.  Also, the winds and radiation pressure from the
forming Sun would be more effective at pushing away the gas from
the vicinity of the terrestrials than the Jovians, and the gas
that is pushed outside of Mars' orbit may end up joining with the
gas planets. ]

Conceptual Self-Test: Multiple Choice

Long key:
1. A planet's mass can most easily be determined by measuring the
   planets _____
   A) moon's orbits
2. If we were to construct an accurate scale model of the solar system
   on a football field with the Sun at one end and Neptune at the other,
   the planet closest to the center of the field would be _____
   D) (Uranus barely beats choice C, Saturn)
3. The inner planets tend to have  _____
   A) fewer moons
4. A solar system object of rocky composition and comparable in size
   to a small city is most likely 
   C) an asteroid
5. The asteroids are mostly _____
   A) found between Mars and Jupiter
6. In the leading theory of solar system formation, the planets ____
   B) formed from the same gas cloud as the Sun
7. The inner planets formed ________
   C) by collisions and mergers of planetesimals
8. The solar system is differentiated because _____
   D) only rocky and metallic particles could form close to the Sun
9. According to figure 6.18, the jovian planets formed _____
   C) within a few million years of the Sun's formation
10. According to Figure 6.14, the temperature in the solar nebula
    at the location now at the center of the asteroid belt was ____
   C) 400 K