Deck 8: The Other Terrestrial Planets

Question 1

In 2011 a major event in the exploration of Mercury took place, namely

A) sending a radar pulse to bounce off Mercury's surface.
B) putting the MESSENGER spacecraft in orbit around the planet.
C) a soft, unmanned landing.
D) a manned landing.

Answer 1

putting the MESSENGER spacecraft in orbit around the planet.

Question 2

The BEST estimate for the time of production of most craters on both Mercury and the Moon is

A) relatively recently, about 4 billion years after planet formation.
B) in the first 800 million years after planet formation.
C) about halfway through their lives, or about 2.3 billion years after planet formation.
D) at a relatively constant rate throughout their lives, up to and including the present day.

Answer 2

in the first 800 million years after planet formation.

Question 3

Mercury can be seen easily from Earth only

A) during a lunar eclipse, when the sky is sufficiently dark near the Moon, because Mercury is always close to the Moon in the sky.
B) at midnight, when it is high in the sky.
C) just after sunset or just before sunrise.
D) in the winter, when the ecliptic plane is high in the sky at night.

Answer 3

just after sunset or just before sunrise.

Question 4

The albedo, or fraction of light reflected from the surface, of Mercury is

A) fairly high, about the same as that of Earth.
B) very low because of its dark rocky surface and absence of an atmosphere.
C) variable and relatively high because of variable cloud cover.
D) very high because of the light color of its surface.

Question 5

Mercury appears from spacecraft photographs to resemble the Moon in its surface features but with one important difference,

A) the presence of extensive plains between craters, in contrast to the surface of the Moon.
B) the presence of retrograde direction of spin compared to most other planets and moons.
C) evidence of active volcanoes on Mercury.
D) the presence of clouds and a measurable and significant atmosphere on Mercury.

Question 6

Mercury has a very

A) dim magnitude when viewed in the sky.
B) low albedo.
C) large magnetic field.
D) thick atmosphere.

Question 7

Mercury, unlike the Moon, has large crater-filled plains rather than maria. The reason is thought to be that

A) Mercury had no large low-lying regions that could be filled in by lava.
B) Mercury's interior lacks the raw material for lava.
C) because of its large iron core, Mercury has a thinner mantle and therefore has fewer craters produced by volcanoes.
D) Mercury is closer to the Sun and did not receive as many crater-forming impacts as the Moon.

Question 8

Why is it relatively difficult to observe details on the surface of Mercury from Earth?

A) The surface of Mercury is always completely covered in clouds.
B) The surface of Mercury appears very bright from reflected ultraviolet sunlight and emitted infrared radiation from its very hot surface.
C) Mercury is a small object that always appears close to the Sun in the sky.
D) Mercury's orbit always keeps it on the opposite side of the Sun from Earth.

Question 9

The Mariner 10 spacecraft was placed in an elliptical orbit around the Sun, with the period of its orbit twice that of the period of Mercury's orbit. What is the length of the semimajor axis of Mariner 10's orbit?

A) 0.48 au
B) 0.61 au
C) 0.39 au
D) 0.0125 au

Question 10

Why is Mercury difficult to observe from Earth?

A) Mercury remains close to the Sun in its orbit and is seen in a dark sky only at sunrise or sunset, close to the horizon.
B) Mercury orbits the Sun very rapidly and moves across the sky very quickly.
C) The orbit of Mercury is very elliptical and tilted at a large angle to the ecliptic, and any position on this orbit is visible only at midnight when Mercury is low in the sky.
D) Mercury is a very small object that orbits a long way from the Sun, thereby reflecting little light back to Earth.

Question 11

Which one of these features is common to Mercury and the Moon?

A) craters associated with volcanic activity
B) large flat plains dotted with relatively small craters
C) maria filled with solidified basalt and marked with few craters
D) scarps formed when the body shrank

Question 12

How has the age of Mercury's intercrater plains been estimated?

A) from residual radioactivity within the plains compared to areas outside, as measured by the Mariner 10 spacecraft
B) by comparing the albedo of the plains to areas outside because material becomes lighter as it gets older, as it does on the Moon
C) from radioactive dating of rocks returned to Earth by the Mariner 10 spacecraft
D) by comparing the number of craters per unit area within the plains to areas outside

Question 13

Mercury's orbit has a semimajor axis of 0.387 au. How large would the Sun appear to be in the sky (in angular diameter) as seen by someone on Mercury, compared with its angular diameter from Earth?

A) same size because it is the same Sun
B) about 1.5 times larger
C) about 6.25 times larger
D) about 2.5 times larger

Question 14

Mercury is BEST seen from Earth with the unaided eye at what time of the day?

A) near midday, when it is seen through the least amount of Earth's atmosphere
B) close to dawn or dusk, when Mercury is at positions of greatest east or west elongation
C) at midnight, when Mercury is at opposition and highest in the sky
D) during the daytime, when Mercury is at superior conjunction

Question 15

Which of these statements about the similarities of Mercury and Earth's Moon is NOT true?

A) Both Mercury and the Moon have heavily cratered surfaces.
B) Both Mercury and the Moon have very dark surfaces, or low reflectivity, or albedos.
C) Both Mercury and the Moon have large, circular, and relatively flat basins, or maria, on parts of their surfaces.
D) Mercury and the Moon have no atmospheres.

Question 16

Approximately how many times in 1 Earth year does Mercury go completely around the Sun, as viewed from the distant stars?

A) 4 times
B) 3 times
C) Twice
D) Once

Question 17

Which planet MOST resembles the Moon in visible surface features and atmosphere?

A) Mars
B) Venus
C) Uranus
D) Mercury

Question 18

Craters on Mercury appear to have been produced by

A) both impacts from objects from space early in the planet's history and volcanic activity.
B) successive expansion and contraction of the planet's surface because of intense heating by the Sun and severe cooling during rotation because the craters appear to be in irregular lines across the surface.
C) volcanic eruptions early in the planet's history.
D) continuous impacts by objects from space throughout the planet's history, including very recently in geological time.

Question 19

How long is 1 solar day (e.g., noon to noon) on Mercury in Earth days?

A) 88 days
B) 1 day
C) 58.7 days
D) 176 days

Question 20

Mercury is always much closer to the Sun than Venus is, and yet it never appears brighter than Venus, even at maximum brightness. Explain.

A) We never see more than a small fraction of Mercury's illuminated surface because of its orbital path relative to that of Earth; hence it always appears dark.
B) Mercury has a thick atmosphere that impedes reflection from its surface and hence appears dark.
C) Mercury has a high albedo, or reflectivity, but it is very small and hence appears relatively dark.
D) Mercury is small, has a dark surface, and has no reflecting clouds.

Question 21

The huge impact of an object that produced the circular Caloris Basin and its ring of mountains on Mercury also produced

A) a very localized region of magnetic field, the only magnetic field possessed by Mercury.
B) melting of permafrost, causing massive floods that produced deep valleys on Mercury's surface.
C) a pattern of jumbled terrain diametrically opposite the basin, caused by the focused seismic waves from the original impact.
D) a very thick, dusty atmosphere that still obscures the planet's surface.

Question 22

Earth's average density (5520 kg/m³) is slightly higher than that of Mercury (5430 kg/m³) because

A) Earth has a larger iron core in proportion to its size than Mercury has.
B) Earth is more massive than Mercury and has therefore become more compressed, gravitationally.
C) Mercury's interior has expanded slightly because of radioactive heating, reducing its density since its formation.
D) Earth has a denser atmosphere than Mercury has.

Question 23

What caused the long, meandering scarps (cliffs) that can be seen on Mercury?

A) volcanic eruptions along crustal faults over hot spots in the mantle
B) crustal movement similar to continental drift on Earth, where plates have pressed against one another
C) shrinking and folding of the planet's surface as it cooled
D) large impacts near the end of the early period of heavy bombardment; the scarps are eroded crater walls

Question 24

Mercury's average density and the fact that it has a (weak) magnetic field lead to the conclusion that its central core is probably composed of

A) solid rocks of relatively low density.
B) solid and/or molten iron.
C) ices of water and methane.
D) molten rock.

Question 25

A distinct area of unusually jumbled, hilly terrain has been found on Mercury. How is this terrain believed to have been formed?

A) by an intense shower of meteoroids near the end of the heavy bombardment era
B) by upwelling of magma while Mercury was still hot enough for its mantle to be tectonically active
C) by the impact of a large comet
D) by seismic waves generated by the Caloris impact

Question 26

Mercury has a higher proportion of iron in its makeup than does Earth. So how does Mercury's density compare to Earth's density?

A) Mercury must have the higher density.
B) Earth has the greater density because larger masses always have greater densities.
C) Mercury has the smaller density because the non-iron portion of its makeup is very light, porous materials that more than compensate for the greater proportion of iron.
D) Earth has the greater density because Earth's iron is more dense than Mercury's iron, thanks to the greater pressure experienced at the core of Earth compared to that at Mercury's core.

Question 27

Most of the information astronomers have about Mercury's surface comes from

A) Earth-based telescopic observations.
B) Mariner 10.
C) MESSENGER.
D) landings by robotic spacecraft.

Question 28

Mercury can be characterized as having

A) a Moonlike surface and an Earthlike interior.
B) both a surface and an interior like that of Earth's Moon.
C) an Earthlike surface and a Moonlike interior.
D) a surface and an interior nothing like either the Moon or Earth.

Question 29

The planet with the highest overall density is

A) Mercury.
B) Venus.
C) Earth.
D) Mars.

Question 30

The Caloris Basin, a huge circular region on Mercury surrounded by rings of mountains, appears to have been produced by

A) the impact of a massive object in the early phases of the planet's formation, soon after the initial cratering period.
B) successive expansion and contraction of the planet's surface by intense solar heating and severe cooling as the planet rotated, causing buckling in a similar way to the formation of the North American Rocky Mountains or the South American Andes.
C) wind erosion from huge atmospheric storms, similar to enormous hurricanes on Earth.
D) the eruption of a large and long-lived volcano that formed the mountains in a similar manner to the formation of the Hawaiian Islands on Earth.

Question 31

When were most of the existing lava plains formed on the surface of Mercury?

A) never-there is no evidence of lava flows on the surface of Mercury
B) when Mercury was first formed, about 4.5 billion years ago
C) near the end of the era of heavy bombardment, about 3.8 billion years ago
D) when the crust wrinkled to form the scarps

Question 32

The density of Mercury is about the same as that of Earth, but Mercury contains a larger proportion of iron. How is this possible?

A) The iron in Mercury's core is accompanied by a very low-density crust. The average density is thus comparable to the density of Earth.
B) The inner core of Mercury is hollow, thus reducing the overall density much below that of iron.
C) Mercury has only 1/18 Earth's mass. Thus, it must be composed of heavier materials (like iron) in order to match Earth's density.
D) Earth contains, proportionally, a smaller volume of iron, but because of Earth's greater mass, Earth's iron has been compressed to a higher density than the iron on Mercury.

Question 33

The Caloris Basin is an immense impact crater on Mercury, which is unusual in that the opposite side of the planet shows jumbled terrain that resulted from shock waves created by the impact. This combination has also been found

A) nowhere else in the solar system.
B) on the Moon.
C) on Earth.
D) on Saturn.

Question 34

Mercury's iron core takes up approximately what fraction of the mass of the planet?

A) about 32 percent, very similar to that of Earth
B) greater than 95 percent
C) about 60 percent
D) about 10 percent

Question 35

What is the major difference between Mariner 10 and MESSENGER?

A) Mariner 10 flew past Mercury several times. MESSENGER went into orbit around the planet.
B) MESSENGER made a soft landing on Mercury, but Mariner 10 only flew by.
C) The two spacecraft were essentially identical, except that Mariner 10 landed on the daylight side of the planet and MESSENGER landed on the night side.
D) Mariner 10 was a failed mission. It crashed into Mercury. MESSENGER conducted a successful flyby.

Question 36

How do astronomers know that the Caloris Basin on Mercury was formed late in the period of intense bombardment?

A) Radioactive dating of rock samples has fixed the date of formation.
B) The basin contains very few craters.
C) The surface of the basin is much darker than the rest of the planet, suggesting that the surface had not yet been churned over by meteoric impacts.
D) Mountain ranges in the basin are very high because they have not had time to suffer significant erosion.

Question 37

The history of Mercury can be summarized as

A) mostly volcanic activity and crustal deformation; the only craters to survive were formed within the last billion years.
B) thin crust forming at first, followed by extensive bombardment, the craters from which were then covered by lava flows that produced extensive lava plains between the remaining craters.
C) early cooling to form a thick crust, with no evidence of volcanism or lava flows at any time.
D) rapid cooling to form a thick crust during the early bombardment, with a few isolated lava flows and very dense cratering everywhere else.

Question 38

The impact that formed the Caloris Basin on Mercury also created intense seismic waves that disrupted the surface of Mercury on the opposite side of the planet. Which other solar system feature is known to have involved a major impact with a similar result?

A) Mare Orientalis on the Moon
B) Hellas Basin on Mars
C) Great Rift Valley on Earth
D) Cleopatra Caldera on Venus

Question 39

What is the Caloris Basin?

A) lunar mare, on the far side of the Moon
B) volcanic caldera on Mount Maxwell, on Venus
C) large, lowland area on Mars
D) multiringed impact basin on Mercury

Question 40

Where in the solar system would one find the Caloris Impact Basin?

A) on Jupiter's outer Galilean satellite, Callisto
B) on Mars
C) on Mercury
D) on the Moon

Question 41

Mercury rotates once in 59 days with respect to the distant stars and has a sidereal period of 88 days. If a star is seen to be overhead to an observer on Mercury at midnight on a particular day, how many times will Mercury orbit the Sun before this star is again above the observer's head in a nighttime sky?

A) 3 times
B) twice
C) It will never again reach this specific position.
D) once

Question 42

If one is on Mercury and the time is noon (Sun directly overhead), what time of day will it be 1 Mercurian year later (after Mercury has orbited the Sun once)?

A) just after sunset
B) any time because Mercury rotates independently of its revolution
C) noon
D) midnight

Question 43

How many times does Mercury rotate with respect to the Sun in one sidereal orbital period?

A) many times because Mercury rotates rapidly
B) once
C) 1 $1 / 2$ times
D) twice

Question 44

Both Mercury and Earth have global magnetic fields. Compared to Earth, which feature is Mercury known to lack that is thought necessary for generating a STRONG field?

A) Mercury does not lack any such feature.
B) large iron core
C) rapid planetary rotation
D) molten iron core

Question 45

How many times does Mercury rotate around its axis with respect to the distant stars in one orbit around the Sun?

A) 1 $1 / 2$ times
B) 3 times
C) 2/3 times
D) twice

Question 46

Mercury is unique among the inner planets in having

A) one hemisphere with many craters and few volcanoes and the other hemisphere with few craters and many volcanoes.
B) extensive volcanic activity and crustal deformation over most of its history but no evidence of plate tectonics.
C) no evidence of an iron core.
D) an iron core that occupies almost half its volume.

Question 47

"This planet has a large iron core, a heavily cratered surface, and no atmosphere." Which planet in Earth' solar system fits this description?

A) Mars
B) Neptune
C) Mercury
D) Earth

Question 48

What is the observed feature that suggests that Mercury's iron core is at least partly molten?

A) presence of a global magnetic field
B) background sloshing sound in radio transmissions sent back by the Mariner 10 spacecraft
C) irregularities in Mercury's rotation rate
D) extensive lava flows near the regions of greatest tidal stress, such as the Caloris Basin

Question 49

Many planetary scientists believe Mercury suffered a major collision early in the history of the solar system. How could such a collision explain some of Mercury's peculiar properties?

A) Mercury's original core, along with a relatively thin mantle, formed the present Mercury. The remaining material was deflected out into the inner solar system.
B) The impactor, a large iron meteorite, was absorbed by Mercury, enhancing its iron core.
C) Mercury's original core, plus a very thin mantle, formed the present Mercury. The remaining material formed Vulcan, the small planet inside Mercury's orbit.
D) The result was dense Mercury and non-dense Venus.

Question 50

The internal structure of Mercury is a

A) dense iron core taking up almost half the volume of the planet and a rocky mantle surrounding the core.
B) rocky core with a liquid (or perhaps frozen) water mantle and icy surface.
C) thick rocky mantle taking up most of the volume of the planet, overlaying a small but dense iron core.
D) rocky core surrounded by liquid metallic hydrogen and a hydrogen-helium atmosphere.

Question 51

How does Mercury rotate, relative to the Sun?

A) Mercury rotates relatively quickly, so that a solar day (from sunrise to the next sunrise) on Mercury is just a bit shorter than on Earth.
B) Mercury rotates in a retrograde direction; consequently, the Sun rises in the west each day and sets in the east.
C) Mercury turns the same side (Caloris Basin) toward the Sun at all times, much as the Moon turns the same side toward Earth.
D) Mercury alternately turns one side (Caloris Basin) toward the Sun at one perihelion and the opposite side toward the Sun at the next perihelion.

Question 52

Which physical feature of the planet Mercury interacts with solar wind particles near the planet?

A) stream of ionized atoms, heated by the intense sunlight, that are emitted continuously by the planet
B) dense, ionized atmosphere surrounding the planet
C) magnetic field that, like Earth's, produces a magnetosphere surrounding the planet
D) Mercury's gravitational field

Question 53

Why has Mercury become locked into a 3-to-2 spin orbit coupling instead of a 1-to-1 coupling like the Moon around Earth?

A) Mercury feels a stronger gravitational pull from the more massive Sun than the Moon does from Earth, so its rotation became locked on the Sun before it had slowed enough for a 1-to-1 lock.
B) Mercury's orbit is very eccentric, so its orbital speed varies while its rotation rate remains constant, preventing a 1-to-1 lock.
C) Mercury's rotation rate is still slowing down, and the apparent 3-to-2 lock is actually just coincidence.
D) Mercury's orbit is highly inclined (tipped up at an angle to the orbits of the other planets), so the Sun sometimes pulls more on Mercury's north pole and sometimes more on its south pole.

Question 54

Suppose Mercury had oceans. What would be the length of time between successive high tides?

A) Mercury does not have a moon, so it does not have tides.
B) 58.7 days
C) 88 days
D) 176 days

Question 55

When the solar wind (mostly protons and electrons, emitted continuously at relatively high speeds by the Sun) approaches Mercury, it

A) is deflected around the planet by the magnetic field.
B) is absorbed and stopped by the atmosphere of the planet.
C) hits the surface directly, causing heat and a continuous fluorescent glow.
D) is repelled by the high electrostatic charge on the planet.

Question 56

What is the relationship between Mercury's rotation about its axis (measured relative to the distant stars) and its revolution around the Sun

A) Mercury rotates once for every 3 revolutions around the Sun.
B) Mercury rotates exactly once during each revolution around the Sun.
C) Mercury rotates approximately 88 times during each revolution around the Sun.
D) Mercury rotates 3 times for every 2 revolutions around the Sun.

Question 57

The interior of Mercury contains

A) significant amounts of water as permafrost because occasional melting has produced deep water-formed valleys that crisscross the planet's surface.
B) mostly ices and very low-density material because the planet's average density is close to that of water.
C) an iron core that occupies a large fraction of the volume of the overall planet and produces a magnetic field.
D) very little iron, in contrast to Earth, and so the planet has no magnetic field.

Question 58

If one was watching the sunrise on Mercury when Mercury was near perihelion, what would they see?

A) The Sun would rise slowly, gradually speed up over a period of a few days, then slow down again.
B) The Sun would rise, cross the sky, and set again, all in a few days, then spend the rest of Mercury's "year" below the horizon.
C) The Sun would rise, stop, turn back toward the east, stop, and then continue westward.
D) The Sun would rise, slowly come to a standstill, then start rising again.

Question 59

Compared with that of Earth, Mercury's magnetic field is

A) weak but clearly present.
B) below the limit of detection and therefore still needs confirmation.
C) similar in strength.
D) much more powerful.

Question 60

In one orbit around the Sun, Mercury rotates around its axis how many times?

A) twice
B) 3 times
C) 1 $1 / 2$ times
D) once, in a synchronous orbit

Question 61

How do planetary scientists explain the small amounts of hydrogen and helium in Mercury's atmosphere?

A) Mercury's large iron core has allowed it to retain these light gases from its primordial atmosphere.
B) They have escaped from the Sun and drifted to Mercury. Mercury does not retain them, but they are continually replenished from the Sun.
C) They are outgassed from Mercury's active volcanoes.
D) The hydrogen at least is formed when the Sun's intense ultraviolet radiation breaks apart the hydrocarbons on Mercury's surface.

Question 62

Because Mercury has no appreciable atmosphere, its surface shows extreme temperature changes between night and day, with a range of about

A) 600 K.
B) 900 K.
C) 20 K.
D) 60 K.

Question 63

How is water ice able to remain on the surface of Mercury, despite the planet's close proximity to the Sun?

A) The water ice is continuously replenished by condensation of water vapor from volcanoes.
B) The water ice is continuously replenished by fresh impacts from comets.
C) The water ice exists as permafrost below the thermally insulating surface and is exposed only by occasional impacts.
D) The water ice is permanently shielded from the Sun by crater walls at the north and south poles.

Question 64

There are traces of oxygen in Mercury's atmosphere. The source of this is believed to be

A) the sublimation of polar ice.
B) the remnants of the primordial solar nebula.
C) outgassing from within the planet.
D) a continual rain of ice-coated cometary fragments into the atmosphere of Mercury.

Question 65

"This planet has a solid, cratered surface, which is alternately very hot and very cold, and it has no permanent atmosphere." Which of the planets fits this description?

A) Venus
B) Mercury
C) Jupiter
D) Mars

Question 66

Suppose Mercury had 5-to-3 spin-orbit coupling and that its sidereal period remained 88 days, as now. An observer sees the Sun directly overhead in the Mercurian sky and then observes again from the same location 88 days later. Where will the Sun be in the sky? (Mercury, like Earth, rotates toward the east.)

A) below the eastern horizon
B) below the western horizon
C) directly overhead again
D) directly on the opposite side of the planet

Question 67

Suppose Mercury had 5-to-3 spin-orbit coupling and that its sidereal period remained 88 days, as now. What would be the time from noon to noon on Mercury, in days?

A) 88
B) 147
C) 264
D) 440

Question 68

Tomorrow's weather forecast for the Caloris Basin on Mercury's equator is

A) overcast, light winds, occasional acid rain.
B) hot and humid by mid-afternoon; cooling to near freezing overnight.
C) sunny with scattered clouds, possible afternoon dust storms.
D) lead and zinc melting by noon; temperatures well below freezing overnight.

Question 69

Which unexpected chemical compound has recently been discovered on Mercury?

A) nitrogen dioxide in Mercury's upper atmosphere
B) cyanide, poisonous to humans, in the troposphere
C) ammonia, escaping from fissures in the Caloris Basin
D) water ice at the north and south poles

Question 70

Where has water ice been found on Mercury?

A) nowhere
B) suspended as microscopic crystals in the stratosphere
C) in permanently shadowed crater floors at the north and south poles
D) in deep fissures on the night side

Question 71

Mercury's atmosphere consists mostly of

A) sulfur dioxide and hydrogen sulfide from volcanoes.
B) carbon dioxide.
C) nitrogen and oxygen, with traces of carbon dioxide and argon.
D) hydrogen, helium, potassium, sodium, and oxygen.

Question 72

Recent radar observations of Mercury have suggested that Mercury may

A) have one or more tectonic rift valleys.
B) have ice at its north and south poles.
C) have geologically recent lava flows on the side that was not photographed by Mariner 10.
D) not be precisely locked into its hypothesized 3-to-2 spin-orbit coupling.

Question 73

The highest daytime temperature reached on the surface of Mercury is

A) 961°C, high enough to melt silver.
B) 430°C, high enough to melt lead and tin.
C) 0°C, high enough to melt water ice.
D) 100°C, high enough to boil water.

Question 74

Temperatures on the surface of Mercury are seen to fluctuate between very cold 100 K (-173°C) and extremely hot 700 K (427°C). What does this measurement indicate about conditions on Mercury?

A) Mercury has a very elliptical orbit, and varying distance from the Sun produces these large temperature fluctuations because intensity varies as the inverse square of the distance from the Sun.
B) The planet has an atmosphere in which the greenhouse effect captures solar radiation to heat the sunlit hemisphere of the planet.
C) The planet is close to the Sun, has no atmosphere to maintain heat from the Sun, and is rotating slowly.
D) Erupting volcanoes occasionally heat the planet's surface to extreme temperatures.

Question 75

The source of the water found in the form of ice near Mercury's north and south poles is

A) comet impacts.
B) unknown at the present time.
C) rainfall early in Mercury's history, when the planet had a dense atmosphere.
D) a former global ocean, now almost entirely evaporated and lost to space.

Question 76

Mercury's atmosphere is

A) almost nonexistent.
B) relatively dense, composed mostly of nitrogen (80 percent) and oxygen (20 percent).
C) very thin, made up of sulfur dioxide and hydrogen sulfide from volcanoes.
D) relatively thin, composed of carbon dioxide with small quantities of nitrogen and argon.

Question 77

What do most astronomers believe is the source of the minute amount of oxygen in Mercury's atmosphere?

A) sublimation of water molecules from ice at the poles
B) ions captured from the solar wind
C) decay of sulfur dioxide ejected from volcanic eruptions
D) bacterial activity in a permafrost layer below the surface

Question 78

When Mariner 10 passed by Mercury, the Caloris Basin was crossed by the terminator line. Which statement correctly evaluates this situation?

A) This is not surprising because the Caloris Basin is so large that the terminator line always crosses part of it.
B) This is not surprising. Because of Mercury's synchronous rotation orbit, the terminator line does not move across the face of the planet, and it lies perpetually on the Caloris Basin.
C) Because of Mercury's 3-to-2 spin-orbit coupling the terminator line moves slowly across the face of Mercury. When Mariner 10 flew by it happened to lie across the Caloris Basin.
D) There is a direct correlation between the Caloris Basin and Mercury's unusual rotation. This unusual rotation occurs because of the bulge on Mercury-and this was caused by the impact that produced the Caloris Basin. So, it is not surprising that the terminator line was found across the Caloris Basin.

Question 79

The highest temperature on Mercury is 700 K, which is 50 K LESS than the highest temperature on Venus. How, then, can Mercury experience the greatest temperature range in the solar system? Which of these does NOT contribute to this great range of temperature?

A) Mercury has no significant atmosphere to retain heat on its night side.
B) Mercury has no greenhouse gases to keep the planet warm at night.
C) The ice near the poles contributes to the very cold night temperature.
D) Mercury has a slow rotation rate, which allows it to cool off easily on the night side.

Question 80

How was water ice discovered near Mercury's poles?

A) by studying reflections of radio waves sent to Mercury from Earth
B) visually by cameras aboard Mariner 10
C) by a surface rover sent out by MESSENGER
D) visually from the Hubble Space Telescope