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Part 1: Each Question in this Section is
worth 5 points: (40 points total)
1. A new object is discovered in the Solar
System. Its orbital period is Approximately
how far way from the sun is this object?
Mixed Performance
p a
2
3
20  400  a
2
a  (400)
3
1/ 3
2.What is the best method of probing the
nature of the surface features of Venus?
What are the limitations of this method?
Answered pretty well
 Radar Topographic Measurements
from Orbiting Satellite
 Limitations are a) can’t actually
sample surface and b) resolution
limited in the size of detected
topographic features
3. A newly discovered planetary system
has a planet with a mass of 2 earth masses.
What must the radius of this planet be for it
to have the same density as that of the
Earth? Mixed Performance
density 
m
 1  (earth)
r
3
m  2m
r 2
3
r2
1/ 3
Most of you did not do any reality check
and just said the new radius had to be
twice the old radius. In that case, the new
density would be 2/8 = ¼ of an Earth
density.
4. A newly discovered planetary system has
a planet with a mass of 20 earth masses and
it is located in a similar part of that solar
system as Uranus and Neptune in our own
solar system. What would the approximate
radius of this new planet be (in units or
Earth Radii?) Mixed Performance
Remember the density in this part of the
solar system is about 1.5 g/cc or about 4
times less than that of the Earth.
m
density   .25
r
m  20m
3
therefore
20
r 
 80
.25
r  80
3
1/ 3
5. Briefly explain why the Earth has a dense
iron-nickel core and a lighter silicon-oxygen
crust. Answered pretty well
Gravitational separation of dense and less
dense elements in a molten fluid; ironnickel sinks to core; silicon-oxygen rises
to the top and cools forming the crust.
6.Below are two images of a planetary
surface. Which one is younger and
explain how you know this. Answered
pretty well Right one; Less craters
7. What is the principle heat source in the
interior of the Earth that drives plate
tectonics?
Mixed Performance
The decay of Uranium 238 which has a
half-life of 4.5 billion years
8. On what planet is the Caloris Basin and
why is this feature of significance in
understanding planetary surface features.
Answered pretty well
Mercury (not Venus or Saturn)  shows
that planetary surfaces can withstand
very large impacts and that such impacts
can shock the opposite side of the planet
causing geological features to emerge
Part II: Medium Answer Questions
worth 10 points each (60 points total)
9. Explain the balance condition, according
to Newton’s laws, for an orbit to be stable.
Answered pretty well
Balance between inertial and
gravitational forces
10. Explain what is meant by a planetary
surface that is “saturated” with impact
craters. Answered very well
Very high crater density; new craters just
are put on top of older craters;
overlapping craters.
11. Explain why the observed retrograde
motion of Mars is a problem in the
Aristotelian Cosmology and what Ptolemy
had to construct to resolve this problem.
Answered pretty well but two key words (in
bold below) were usually left out
 All the planets have to move in the same
direction since they are stuck to spheres
(crystalline) that revolve around the
Earth. Therefore no planet can move
backwards
 Wait, Mars moves backwards (i.e.
exhibits retrograde motion), how can
that be?
 Ptolemy builds a system with epicycles
that allows Mars to periodically move
backwards in its orbit about the earth.
12. How has the recent measurement that
the Martian Polar Caps are mostly water ice
altered our view of the early geologic
history of the Martian surface? Answered
pretty well
From the last Homework Assignment:
Mars appears to have significantly less CO2
incorporated into the ice caps as previously
thought. This means that the total amount of
CO2 available to Mars is less than once
thought making it unlikely that Mars ever
had a sufficient greenhouse effect for it to
once have been warm and wet.
13. Explain what is anomalous about the
bulk properties of the moon and why this
suggests it must have a special origin.
Answered pretty well
 If the moon formed by accretion then its
density should be identical to the Earth’s
 Observed density is significantly lower,
at 3.5 g/cc  similar to the density of
Mars or the Earth’s Mantle
 Implies Moon formed in the vicinity of
Mars or was somehow ripped out from
the Earth’s mantle.
 Also the mass ratio of the moon to the
earth is very high.
14. Describe Tycho’s experiment to prove
that the Earth orbited the Sun. Why was he
unable to prove this assertion from his data?
Mixed Performance
Stellar Parallax:
Observe stars 6 months apart
and see if there is any
movement of that particular
star with respect to the
background stars.
Tycho had no telescope and with the
naked eye, the real parallax angle is less
than the position accuracy of the
measurements. This is because even the
nearest stars are unimaginably far away.
Part III: Longer answer Questions:
Each question worth 15 points (60 points
total)
15. Summarize the major geological events
that have occurred on the lunar surface and
the evidence acquired to support that idea.
Answered unnecessarily poorly
The age dating of the lunar rocks have
allowed us to identify four distinct
periods in its geological history:



The moon solidified and cooled 4.4
billion years ago
Between 4.4 and 4.2 billion years and
intense period of bombardment
occured from the material that was
left over from the acretion process.
Most of this material was chunks of
rock less than 10 km in size. Similar
debris reigned down on the surfaces of
Mercury, Venus, Earth and Mars.
Since the moon is not geologically
active, this record of bombardments is
largerly maintained on its surface.
By now the moon is tidally locked to
the earth. At 3.9 billion years there
was another period of bombardment
which involved a few large (> 100 km)
pieces of debris. The earth acted as a
gravitational focussing mechanism
which caused most of these impacts to
occur on the side of the moon always
facing the earth (there are no lunar
maria on the far side of the moon).

These large impacts produced the
lunar maria. This was a severe shock
to the crust of the moon and over
time, molten basalt would flow out of
the deep fissures and cracks in the
crust that were caused by the initial
impact. This took 200 million years to
occur and is the last geological process
which occurred on the moon.
16. Explain how the observed density
gradient in the solar system was produced
by its formation process. Answered Pretty
well
 The sun has to be radiating energy prior
to planet formation
 Only material that can exist as a solid at
temperatures of 1000 K or so are rocks
 90% of the initial protoplanetary
material is ice and 10% is rock
 With the sun as a heating source – only
small rocky planets can form in the inner
solar system but large balls of mostly ice
can form in the outer solar system
17. Explain why there are no earth mass
sized rocky planets located in the outer part
of the solar system (i.e. beyond Jupiter).
Mixed Performance
 Primary reason; there are earth mass
sized objects in the outer solar system,
they are just covered over by lots of
earth masses of ice because the ratio
of icy material to rocky material is
very high.
 Secondary reasons: formation
potential earth size objects near
Jupiter or Saturn would have been
disrupted by those large bodies.
18. Explain how the density of a planet is
determined and how this measured density
provides information on the bulk
composition of that planet. Mixed
Performance as most all left out the first
part of the question!
 Need to find mass and radius first
 Radius comes from imaging and
knowing the distance
 Mass requires a satellite in orbit about
the planet
 Once density is then determined one
can tell if the object is primarily made
of gases, rocks, or liquids/ices and the
relative proportional mixture of those
constituents.