KEY
Planetary Sciences 206
Midterm Examination #2
9:30-10:45 a.m., Thursday, February 27, 2014
INSTRUCTIONS: There are 35 multiple-choice questions, which are worth 2 points each. The last two
questions (#36 and #37) are essay questions, and are worth 15 points each. Mark your multiple-choice
answers on this test and on the bubble sheet. This is a CLOSED BOOK, CLOSED NOTES examination.
1. In general, the closer satellites of planets tend to rotate so as to keep the same side pointed toward the
planet. This is caused by
m A. friction from the solar wind acting on the planet and satellite
m B. constant bombardment of the satellite by meteoroids over 4.5 Gyr
m C. volcanism on the satellite due to radioactive heating processes
m D. differential bombardment from meteoroids
l E. friction from tides raised on the satellite by the planet
2. Which statement about the Moon is correct?
l A. its minerals were mostly formed at high temperatures
m B. its chemical composition is very similar to the Sun’s
m C. it is known to have a substantial iron core like the Earth
m D. it was captured by the Earth 1 million years ago
m E. according to the latest theories, the Moon formed near the orbit of Mercury
3. The reason that a 1000-kg object will have a noticeable effect when it hits the Earth from orbit is m A. chemicals in its interior react strongly when exposed to the Earth's atmosphere m B. many meteoroids are made of antimatter, which annihilates ordinary matter on contact l C. its energy release will exceed 10 truck-bombs of explosive power m D. it is accelerated strongly by the solar wind just before it reaches the Earth m E. it has been heated to 900 K in space by solar ultraviolet photons
4. Most of the heavy cratering on the Moon took place
l A. more than 3 gigayears ago
m B. about 2 gigayears ago
m C. about 1 gigayear ago
m D. uniformly throughout the age of the solar system
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m E. during the last 10 years
5. Which of the following is a siderophile element?
m A. hydrogen
m B. argon
l C. iridium
m D. helium
m E. oxygen
6. Radar is used to study Venus' surface because m A. it can measure the surface wind speed more precisely than other methods m B. it is sensitive to basins that are filled with water l C. Venus' clouds block observations from space at visual wavelengths m D. radar equipment is cheaper to operate than visual imaging cameras m E. no visible images at Venus' surface have ever been made
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7. The isotope Al
left? m A. none
m B. 1/2 m C. 1/4
l D. 1/8 m E. 1/16
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has a half-life of 10 years. After 3 X 10 years, what fraction of the original number is
8. The period of rotation of the Moon about its axis m A. is exactly 1/2 of the Moon's orbital period around Earth m B. is exactly twice the Moon's orbital period around Earth m C. is exactly 3/2 of the Moon's orbital period around Earth m D. is gradually getting shorter by about one day per year l E. is exactly equal to the Moon's orbital period around Earth
9. A lunar sample contains 40000 nuclei of a certain radioactive isotope. The half-life of the isotope is
10000 years. After 10000 years, how many nuclei of the isotope are left in the sample?
m A. none
l B. 20000
m C. 10000
m D. 5000
m E. 4000
10. The following is evidence that the Earth has a liquid outer core: m A. the Earth has the thinnest lithosphere of any of the terrestrial planets m B. the Earth once had a magnetic field but does no longer m C. volcanoes on the Earth's surface erupt liquid iron from the core m D. P-waves can only be seen within 100 km of an earthquake source l E. S-waves don't go through the center of the Earth
11. The radioactive decay of unstable isotopes m A. is responsible for the existence of the solar wind l B. can be used to date the time since formation of minerals m C. is responsible for the existence of the Van Allen belts m D. happens only in laboratories, never in natural settings m E. has been prohibited by the Environmental Protection Agency
12. A major difference between Mercury and the Moon is
m A. the Moon's density is much higher, indicating more iron in the Moon m B. much of the Moon's surface is saturation-cratered, but Mercury's is not l C. Mercury's density is much higher, indicating an iron core in Mercury m D. Mercury's surface is warmed substantially by the greenhouse effect m E. the Moon has a spin-orbit resonance, but Mercury does not
13. The reason that Mariner 10 was able to encounter Mercury multiple times was m A. Mariner 10 kept getting the same gravitational assist from Venus l B. Mariner 10 was placed in an orbit with a period equal to exactly twice Mercury’s orbital period m C. NASA engineers were able to change Mercury’s orbital period so that it matched Mariner 10’s m D. Mercury rotates retrograde m E. Mariner 10 executed a soft landing on Mercury
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14. Which of the following statements about the Moon is not true? m A. the Moon's surface was once at high temperatures m B. the Moon's composition shows that it came from hot Earth mantle material l C. the Moon has an iron core proportionally the same size as the Earth's m D. the Earth-facing side is one of the heavy sides of the Moon m E. the lunar farside has almost no maria
15. The main difference between the Moon's nearside and farside is
m A. the farside is much more recent geologically than the nearside
m B. the farside never receives any sunlight
m C. the nearside has many more impact craters than the farside
l D. the farside is almost all heavily-cratered highlands
m E. the farside is lower and smoother than the nearside
16. Which of the following statements is true? m A. there is no evidence for volcanoes on Venus m B. it is correct to refer to our local desert surface as regolith
m C. there is evidence that water once flowed on the Moon m D. the Moon has no complex craters l E. the terrestrial planets that have magnetospheres also have iron cores
17. The reason that we did not see the Moon's farside until 1959 is because m A. NASA's Vanguard rocket program didn't start working until then l B. the Moon rotates about its axis and orbits Earth with the same period m C. Earth's rotation period and the Moon's orbital period are exactly the same
m D. the Moon's farside can only be imaged with telescopes that didn't exist before 1959 m E. the Sun never illuminates that side of the Moon, so advanced radar techniques were needed
18. Siderophiles in the Moon are m A. overabundant because refractory elements are overabundant m B. mainly located in mare basins l C. depleted because iron is depleted m D. mainly found on the lunar farside m E. mainly found at the lunar poles
19. One of the sources of information about the size of an iron core on the Moon is l A. measurement of deformation of solar magnetic field lines at the Moon m B. measurement of heat flow from the lunar farside m C. the rate of change of the Earth-Moon distance m D. the size of mountain ranges on the Moon m E. the size of craters on the lunar farside
20. Refractory elements in the Moon are l A. overabundant because the early Moon was subjected to very high temperatures
m B. overabundant because the early moon was hit by planetesimals which were rich in refractories m C. extremely rare because the Moon formed inside the present-day orbit of Mercury m D. mainly found on the lunar farside m E. mainly found at the lunar poles
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21. One method for dating samples from the Moon makes use of
m A. measurement of the densities of the samples at a temperature of 300 K
m B. the appearance of the ablation crusts formed when they entered the atmosphere
m C. the absence or presence of CAI's
l D. decay of uranium isotopes into lead
m E. measurement of the relative amounts of water in the samples
22. The Apollo 11 mission to the Moon involved
m A. a hyperbolic trajectory from Florida to Tranquility Base
m B. rendezvous with a return vehicle already cached on the lunar surface
l C. a Hohmann transfer to the Earth-Moon L1 point
m D. landing on the lunar farside, since that required less energy
m E. a continuous rocket burn from Earth's Roche limit to Moon's Roche limit
23. The reason that we don't see saturated cratering on the Earth's surface is m A. the Earth's atmosphere keeps crater-forming objects from hitting the surface l B. the Earth's surface is active and has erased any ancient cratered regions m C. the Moon has absorbed all of the possible impacting objects m D. the Earth's surface is much older than the Moon's surface, on average m E. the Earth's magnetic field keeps crater-forming objects from hitting the surface
24. The polar caps of Mars m A. are made up of frozen methane m B. turned out to be optical illusions after Mars was imaged by spacecraft m C. unlike Earth's, do not change their size with the Martian seasons l D. are made up of water ice beneath dry ice m E. appear to melt each spring and drain into flow channels
25. "regolith" is
l A. the outermost soil of the Moon produced by impact "gardening"
m B. the rigid plates on Earth that participate in continental drift
m C. the region within a planet where the magnetic field is produced
m D. the plastic layer within the Earth's mantle which slowly convects
m E. seen only on the Moon's farside
26. Mercury rotates about its axis in such a way that
m A. the same side of Mercury always points to the Sun at periapse
l B. its "hot poles'', points 180o apart, point alternately at the Sun at periapse
m C. the same side of Mercury always points to the Earth when Mercury is closest to the Earth
m D. the length of Mercury's day is about 30 minutes longer than the length of a terrestrial day
m E. the planet rotates prograde one orbit, then rotates retrograde the next orbit, etc.
27. Mercury's orbital eccentricity e is 0.20, and its orbital semimajor axis a is 0.4 AU. Therefore
m A. Mercury's distance to the Sun is always exactly 0.6 AU
m B. Mercury's distance to the Sun continually increased with time during the last 0.4 Gyr
m C. Mercury is on a hyperbolic orbit, unlike the other terrestrial planets
l D. Mercury's distance to the Sun can vary between 1.2 X 0.4 AU and 0.8 X 0.4 AU
m E. Mercury's distance to the Sun can vary between 1.4 X 0.4 AU and 0.6 X 0.4 AU
28. Which of the following statements about Venus is not true?
m A. Venus has mountain ranges
m B. Venus has volcanoes
l C. Venus has liquid water oceans
m D. Venus has "continents''
m E. Venus' surface is always shrouded by clouds
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29. The Moon's maria are
l A. relatively smooth basins filled with lavas that solidified about 3.8 to 3.1 Gyrs ago
m B. legendary creatures from Greek mythology used to name craters
m C. craters which can be seen in astronaut photos but are too small to see from Earth
m D. mountain ranges on the Moon which are created at the subduction zones of lunar plates
m E. craters which have central peaks produced by rebound from shock waves
30. The temperature of the Earth's core is
m A. too cold to transmit earthquake energy
m B. about the same as the temperature at the Earth’s surface
m C. cold enough to freeze water
l D. similar to the temperature at the Sun’s surface
m E. completely unknown as we have no way to measure it
31. Average temperatures reach 100 degrees C (boiling point of water) at about this depth below the
Earth's surface:
m A. a few centimeters
m B. a few meters
l C. a few kilometers
m D. a few hundred kilometers
m E. a few thousand kilometers
32. Which of the following statements about Venus is not true?
l A. Venus rotates prograde
m B. Venus is the second closest planet to the Sun
m C. Venus' orbit is more circular than Mercury's
m D. A Venus year is shorter than an Earth year
m E. Venus has no magnetosphere analogous to Earth's
33. Which of the following statements about Mars is not true?
m A. it has much larger volcanoes than any on Earth
m B. its surface is more heavily cratered than Earth's, on average
l C. there is no evidence that its surface has ever had water present
m D. on average, surface temperatures are colder than on any other terrestrial planet
m E. the surface has a reddish color on average, probably due to iron oxides
34. Which of the following statements about Venus is not true? m A. Venus has strange circular tectonic features called coronae m B. Venus has no global magnetic field m C. most of the surface of Venus has only been imaged by radar m D. most features on Venus are named after women l E. Venus is the largest terrestrial planet
35. Which statement about Mercury is not correct?
m A. large impact structures are seen on its surface
m B. there is evidence for a water ice deposit near its north pole
m C. it appears to have a substantial iron core
l D. it rotates so as to keep the same face pointed toward the Sun at all times
m E. there is evidence that some surface features were formed by crustal shrinkage
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ESSAY QUESTIONS
Write your answers in the spaces provided; use the back if you need more room.
36.
Give two lines of evidence to show that the Moon’s core is proportionally much smaller than the Earth’s.
(15 pts.)
(1) The mean density of the Moon is 3.34 g/cm3, which is very close to that of
uncompressed magnesium-silicate rocks.
(2) It is possible to check how much the Moon's density varies with depth, using a
method related to the method used for the Earth, which makes use of the Earth's
oblateness. This method tells us that there is very little density variation inside the
Moon.
(3) Solar wind carries solar magnetic field lines past the Moon. The field lines would be
caught by a conducting iron core in the Moon, since conducting iron acts like a
plasma. We see little evidence for this.
(4) Seismic evidence from moonquakes.
[7 pts. each for any 2 of the above, plus 1 point]
37. (a) Explain what is meant by saturation cratering. (8 pts.) Saturated craters occur when new craters are on top of older craters. Diagram showing
the difference between saturated cratering and unsaturated cratering:
(b) Are there any regions on the Earth that are saturation-cratered? Why or why not? (7 pts.) No. These types of terrains are associated with very old surfaces, dating back to more
than 3 Gyrs ago, when planets were still being formed during the accumulation phase.
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The Earth’s active surface has long since erased any trace of such terrains.
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