1. Chapter 8 The Moon and Mercury
America’s manned exploration of the Moon was arguably the greatest engineering feat of the 20th century, perhaps one of the greatest of all time. Nine crewed missions were launched to the Moon, a dozen astronauts were landed, and all returned safely to Earth. Here, an Apollo 16 astronaut is prospecting near the rim of Plum Crater for rock samples that might help reveal the origin of the Moon. The “rover” that carried him several kilometers from his landing craft can be seen in the left background. Given the lack of wind and water on the Moon, the bootprints in the foreground are destined to survive for more than a million years. (NASA)

2. Units of Chapter 8 8.1 Orbital Properties 8.2 Physical Properties
8.3 Surface Features on the Moon and Mercury
8.4 Rotation Rates
Lunar Exploration
Why Air Sticks Around
8.5 Lunar Cratering and Surface Composition

3. Units of Chapter 8 (cont.)
8.6 The Surface of Mercury
8.7 Interiors
8.8 The Origin of the Moon
8.9 Evolutionary History of the Moon and Mercury

4. 8.1 Orbital Properties
Distance between Earth and Moon has been measured to within a few centimeters using lasers
Viewed from Earth, Mercury is never far from the Sun
Figure 8-1. Evening Sky (a) Mercury’s orbit has a semimajor axis of just 0.4 AU, so the planet can never be farther than 28° from the Sun, as seen from Earth. Mercury’s eccentric orbit means that this maximum separation is achieved only for the special configuration shown here, in which the Earth–Sun line is perpendicular to the long axis of Mercury’s orbit and Mercury is near aphelion (its greatest distance from the Sun). (b) Four planets, together with the Moon, are visible in this photograph taken shortly after sunset. To the right of the Moon (top left) is the brightest planet, Venus. A little farther to the right is Mars, with the star Regulus just below and to its left. At the lower right, at the edge of the Sun’s glare, are Jupiter and Mercury. (The Moon appears round rather than crescent shaped because the “dark” portion of its disk is indirectly illuminated by sunlight reflected from Earth. This “earthshine,” relatively faint to the naked eye, is exaggerated in the overexposed photographic image.) ( J. Sanford/Photo Researchers, Inc.)

5. Phases of Mercury can be seen best when Mercury is at its maximum elongation
Figure 8-2. Phases of Mercury Some views of Mercury at different points along its orbit. The best images of the planet are taken when it is at its maximum elongation (greatest apparent distance from the Sun) and show a “half Mercury” (cf. Figure 2.12a). (R. Beebe)

6. 8.2 Physical Properties Moon Mercury Earth 1738 km 2440 km 6380 km
Radius
1738 km
2440 km
6380 km
Mass
7.3 × 1022 kg
3.3 × 1023 kg
6.0 × 1024 kg
Density
3300 kg/m3
5400 kg/m3
5500 kg/m3
Escape Speed
2.4 km/s
4.2 km/s
11.2 km/s

7. 8.3 Surface Features on the Moon and Mercury
Moon has large dark flat areas, due to lava flow, called maria (early observers thought they were oceans)
Figure 8-3. Full Moon, Near Side A photographic mosaic of the full Moon, north pole at the top. Because the Moon emits no visible radiation of its own, we can see it only by the reflected light of the Sun. Some prominent maria are labeled. (UC/Lick Observatory)

8. Moon also has many craters (from meteorite impacts)
Figure 8-4. Moon, Close Up (a) The Moon near third quarter. Surface features are much more visible near the terminator, the line separating light from dark, where sunlight strikes at a sharp angle and shadows highlight the landscape. (b) Magnified view of a region near the terminator, as seen from Earth through a large telescope. The central dark area is Mare Imbrium, ringed at the bottom by the Apennine mountains. (c) Enlargement of a portion of (b). The smallest craters visible here have diameters of about 2 km, about twice the size of the Barringer crater on Earth shown in Figure (UC/Lick Observatory; Palomar)

9. Far side of Moon has some craters but no maria
Figure 8-6. Full Moon, Far Side The far side of the Moon, as photographed by the Apollo 16 manned mission. The large, dark region at center bottom outlines the South Pole–Aitken Basin, the largest and deepest impact basin known in the solar system. Only a few small maria exist on the far side. (NASA)

10. Mercury cannot be imaged well from Earth; best pictures are from Messenger
Figure 8-8. Mercury, Up Close Mercury is imaged here as a mosaic of photographs—a composite image constructed from many individual images—taken by the Messenger spacecraft in 2008 as it bypassed the planet. Notice the young, extensively rayed craters, here imaged with a resolution of about 5 km. (NASA)

11. Cratering on Mercury is similar to that on Moon
Figure 8-9. Mercury, Very Close Another photograph of Mercury by Messenger, this one taken at much higher resolution (about 300 m). The dark material around the crater at lower left exemplifies how many of the large craters on Mercury tend to have dark holes about them. The reason is not yet understood. (NASA)

12. 8.4 Rotation Rates
Moon is tidally locked to Earth—its rotation rate is the same as the time it takes to make one revolution, so the same side of the Moon always faces Earth
Figure The Moon’s Synchronous Rotation As the Moon orbits Earth, it keeps one face permanently pointed toward our planet. To the astronaut shown here, Earth is always directly overhead. In fact, the Moon is slightly elongated in shape owing to Earth’s tidal pull on it, with its long axis perpetually pointing toward Earth. (The elongation is highly exaggerated in this diagram.)

13. Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false.
Rather, Mercury’s day and year are in a 3:2 resonance; Mercury rotates three times while going around the Sun twice.
Figure Mercury’s Rotation Mercury’s orbital and rotational motions combine to produce a day that is 2 Mercury years long. The red arrow represents an observer standing on the surface of the planet. At day 0 (center right in Year 1 drawing), it is noon for our observer and the Sun is directly overhead. By the time Mercury has completed one full orbit around the Sun and moved from day 0 to day 88, it has rotated on its axis exactly 1.5 times, so that it is now midnight at the observer’s location. After another complete orbit, it is noon once again on day 176 (center right in Year 3 drawing). The eccentricity of Mercury’s orbit is not shown in this simplified diagram.

14. More Precisely 8-1: Why Air Sticks Around
Air molecules have high speeds due to thermal motion. If the average molecular speed is well below the escape velocity, few molecules will escape.
Escape becomes more probable:
For lighter molecules (higher speed for same kinetic energy)
At higher temperatures
For smaller planets (escape speed is less)

15. More Precisely 8-1: Why Air Sticks Around
Molecules in a gas have a range of speeds; the fastest (and those that are headed in the right direction) will escape

16. 8.5 Lunar Cratering and Surface Composition
Meteoroid strikes Moon, ejecting material; explosion ejects more material, leaving crater
Figure Meteoroid Impact Several stages in the formation of a crater by meteoritic impact. (a) A meteoroid strikes the surface, releasing a large amount of energy. (b, c) The resulting explosion ejects material from the impact site and sends shock waves through the underlying surface. (d) Eventually, a characteristic crater surrounded by a blanket of ejected material results.

17. Craters are typically about 10 times as wide as the meteoroid creating them, and twice as deep
Rock is pulverized to a much greater depth
Most lunar craters date to at least 3.9 billion years ago; much less bombardment since then

18. Craters come in all sizes, from the very large…
Figure Large Lunar Craters (a) A large lunar crater, called the Orientale Basin. The meteorite that produced this crater thrust up much surrounding matter, which can be seen as concentric rings of cliffs called the Cordillera Mountains. The outermost ring is nearly 1000 km in diameter. Notice the smaller, sharper, younger craters that have impacted this ancient basin in more recent times. (b) Two smaller craters called Reinhold and Eddington sit amid the secondary cratering resulting from the impact that created the 90-km-wide Copernicus crater (near the horizon) about a billion years ago. The ejecta blanket from crater Reinhold, 40 km across and in the foreground, can be seen clearly. The view was obtained by looking northeast from the lunar module during the Apollo 12 mission. (NASA)

19. …to the very small
Figure Microcraters Craters of all sizes litter the lunar landscape. Some shown here, embedded in glassy beads retrieved by Apollo astronauts, measure only 0.01 mm across. (The scale at the top is in millimeters.) The beads themselves were formed during the explosion following a meteoroid impact, when surface rock was melted, ejected, and rapidly cooled. (NASA)

20. Regolith: Thick layer of dust left by meteorite impacts
Moon is still being bombarded, especially by very small “micrometeoroids”; softens features
Figure Regolith The lunar soil, or regolith, is a layer of powdery dust covering the lunar surface to a depth of roughly 20 m. Note the bootprints in the foreground of the Apollo astronaut, seen here adjusting some instruments for testing the composition of soil near Mount Hadley. The astronaut’s weight has compacted the regolith to a depth of a few centimeters. Even so, these boot prints will probably survive for more than a million years. (NASA)

21. Meteorites also hit Earth; this crater is in Arizona
Figure Barringer Crater The Barringer Meteor Crater, near Winslow, Arizona, is 1.2 km in diameter and 0.2 km deep. (Note the access road at right for scale.) Geologists think that a large meteoroid whacked Earth and formed this crater about 25,000 years ago. The meteoroid was probably about 50 m across and likely weighed around 200,000 tons. The inset shows a closeup of one of the interior walls of the crater. (U.S. Geological Survey)

22. More than 3 billion years ago, the moon was volcanically active; the rille here was formed then
Figure Lunar Volcanism A volcanic rille, photographed from the Apollo 15 spacecraft orbiting the Moon, can be seen clearly here (bottom and center) winding its way through one of the maria. Called Hadley Rille, this system of valleys runs along the base of the Apennine Mountains (lower right) at the edge of the Mare Imbrium (to the left). Autolycus, the large crater closest to the center, spans 40 km. The shadow-sided, most prominent peak at lower right, Mount Hadley, rises almost 5 km high. (NASA)

23. 8.6 The Surface of Mercury
Mercury is less heavily cratered than the Moon
Some distinctive features:
Scarp (cliff), several hundred kilometers long and up to 3 km high
Figure Mercury’s Surface Scarps, or ridges, on Mercury’s surface, as photographed by Messenger. This cliff seems to have formed when the planet’s crust cooled and shrank early in its history, causing a crease in the surface. Running diagonally across the center of the frame, the scarp is several hundred kilometers long and up to 3 km high in places. (NASA)

24. Caloris Basin, very large impact feature on opposite side of planet
Figure Mercury’s Basin Mercury’s most prominent geological feature—the Caloris Basin—measures about 1400 km across and is ringed by concentric mountain ranges that reach more than 3 km high in places. This huge circular basin, shown here in orange in this false-colored visible image from Messenger, is similar in size to the Moon’s Mare Imbrium and spans more than half of Mercury’s radius. (NASA)

25. “Weird terrain” is thought to result from focusing of seismic waves
Figure Weird Terrain The refocusing of seismic waves after the Caloris Basin impact may have created the weird terrain on the opposite side of the planet.

26. 8.7 Interiors
Moon’s density is relatively low, and it has no magnetic field—cannot have sizable iron/nickel core
Crust is much thicker than Earth’s
Figure Lunar Interior Cutaway diagram of the Moon. Unlike Earth’s rocky lithosphere, the Moon’s is very thick—nearly 1000 km. Below the lithosphere is the inner mantle, or lunar asthenosphere, a semisolid layer similar to the upper regions of Earth’s mantle. At the center lies the core, which may be partly molten.

27. Mercury is much denser than the Moon and has a weak magnetic field—not well understood!
Figure Terrestrial Interiors The internal structures of Earth, the Moon, and Mercury, drawn to the same scale. Note how large a fraction of Mercury’s interior is the planet’s core.

28. 8.8 The Origin of the Moon
Current theory of Moon’s origin: Glancing impact of Mars-sized body on the still-liquid Earth caused enough material, mostly from the mantle, to be ejected to form the Moon
Computer model
Figure Moon Formation This sequence shows a simulated collision between Earth and an object the size of Mars. The sequence proceeds from top to bottom and zooms out dramatically. The arrow in the final frame shows the newly formed Moon. Red and blue colors represent rocky and metallic regions, respectively, and the direction of motion of the blue material in frames 2, 3, 4, and 5 is toward Earth. Note how most of the impactor’s metallic core becomes part of Earth, leaving the Moon composed mainly of rocky material. (W. Benz)

29. 8.9 Evolutionary History of the Moon and Mercury
Time before present
Event
4.6 billion yr
Formation of Moon; heavy bombardment liquefies surface
3.9 billion yr
Bombardment much less intense; lunar volcanism fills maria
3.2 billion yr
Volcanic activity ceases

30. 8.9 Evolutionary History of the Moon and Mercury
Mercury much less well understood
Formed about 4.6 billion years ago
Melted due to bombardment, cooled slowly
Shrank, crumpling crust

31. The oldest moon rocks are older than the most ancient rocks found on the Earth because
the moon was formed long before the Earth.
the radioactive elements used for dating are rare on Earth.
the oldest Earth rocks were destroyed (recycled) by erosion and continental draft.
the moon formed from older material than the Earth did. 

32. Examination of the whole surface of the moon shows us that
craters exist only on one side of the moon.
the northern hemisphere is distinctly different from the southern hemisphere.
the moon can be considered as having two distinctly different sides, that seen from Earth, and that hidden from Earth.
there are no differences in surface features around the whole moon. 

33. We know the lunar highlands are older than the maria because
radioactive dating of lunar samples showed the maria rocks to be younger.
the maria have fewer craters than the highlands.
highland regions on the Earth are older than the ocean basins.
all of the above.
only a and b above. 

34. Erosion on the moon is brought about by
the seas.
micrometeorites.
the solar wind.
astronauts. 

35. The Earth and the moon are both about the same distance from the sun, yet the Earth (on the average) is much warmer than the moon. Why?
The moon is smaller than the Earth.
The moon's night is longer than the Earth's.
The moon has almost no atmosphere compared to the Earth.
The surface of the moon is, on the average, darker than the surface of the Earth. 

36. The property of Mercury that makes its temperature variations greater than those of any other planet is primarily
its lack of atmosphere.
its closeness to the sun.
its small size.the carbon dioxide in its atmosphere.
the statement is false; Mercury is so close to the sun that it is always hot. 

37. Mercury differs from the moon in that:
it has a substantial magnetic field.
it has a heavily cratered surface.
it rotates at the same rate as it revolves.
it has a dense atmosphere. 

38. Mercury is difficult to see from Earth primarily because
it is such a small planet.
it is very faint.
it always appears near the sun.
the orbit is highly elliptical. 

39. Mercury is not expected to have a magnetic field because of its
slow rotation.
high surface temperature.
high density.
spin-orbit coupling.

40. Summary of Chapter 8 Main surface features on Moon: maria, highlands
Both heavily cratered
Both have no atmosphere, and large day–night temperature excursions
Tidal interactions responsible for synchronicity of Moon’s orbit, and resonance of Mercury’s

41. Summary of Chapter 8 (cont.)
Moon’s surface has both rocky and dusty material
Evidence for volcanic activity
Mercury has no maria but does have extensive intercrater plains and scarps