Both the Moon and Mercury are airless, virtually unchanging worlds that experience extremes in temperature. Mercury has no permanent atmosphere, although it does have a thin envelope of gas temporarily trapped from the solar wind. The main surface features on the Moon are the dark maria and the lighter-colored highlands. The surfaces of both the Moon and Mercury are covered with craters of all sizes, caused by impacting meteoroids. Meteoritic impacts are the main source of erosion on the surfaces of both worlds. The lunar highlands are older than the maria and are much more heavily cratered. The rate at which craters are formed decreases rapidly with increasing crater size.
The high day-side temperatures and cold night-side temperatures on the Moon and Mercury result from the absence of significant heat conduction or atmospheric blanketing on the planet. Sunlight strikes the polar regions of both the Moon and Mercury at such an oblique angle that temperatures there are very low, with the result that both bodies may have amounts of water ice near the poles.
The tidal interaction between Earth and the Moon is responsible for the Moon's synchronous orbit, in which the same side of the Moon always faces our planet. The large lunar equatorial bulge probably indicates that the Moon once rotated more rapidly and orbited closer to Earth. Mercury's rotation rate is strongly influenced by the tidal effect of the Sun. Because of Mercury's eccentric orbit, the planet rotates not synchronously but exactly three times for every two orbits around the Sun. The condition in which a body's rotation rate is simply related to its orbit period around some other body is known as a spin-orbit resonance.
The Moon's surface consists of both rocky and dusty material. Highland rocks, which are less dense than rocks from the maria, are believed to represent the Moon's crust. Maria rocks are believed to have originated in the mantle of the Moon. Lunar dust, called regolith, is made mostly of pulverized lunar rock, mixed with a small amount of material from impacting meteorites. Evidence for past volcanic activity on the Moon is found in the form of solidified lava channels called rilles. Mercury's surface features bear a striking similarity to those of the Moon. The planet is heavily cratered, much like the lunar highlands. Among the differences between Mercury and the Moon are Mercury's lack of lunarlike maria, its extensive intercrater plains, and the great cracks, or scarps, in its crust. The plains were caused by extensive lava flows early in Mercury's history. The scarps were apparently formed when the planet's core cooled and shrank, causing the surface to crack. Mercury's evolutionary path was similar to that of the Moon for half a billion years after they both formed. Mercury's volcanic period probably ended before that of the Moon.
The absence of a lunar atmosphere and any present-day lunar volcanic activity are both consequences of the Moon's small size. Lunar gravity is too weak to retain any gases, and lunar volcanism was stifled by the Moon's cooling mantle shortly after extensive lava flows formed the maria more than 3 billion years ago. The crust on the near side of the Moon is substantially thicker than the crust on the far side. As a result, there are almost no maria on the lunar far side. Mercury has a large impact crater called the Caloris basin, whose diameter is comparable to the radius of the planet. The impact that formed it apparently sent violent shock waves around the entire planet, buckling the crust on the opposite side.
The Moon's average density is not much greater than that of its surface rocks, probably because the Moon cooled more rapidly than the larger Earth and solidified sooner, so there was insufficient time for significant differentiation to occur. The lunar crust is too thick and the mantle too cool for plate tectonics to occur. Mercury's average density is considerably greater—similar to that of Earth—implying that Mercury contains a large high-density core, probably composed primarily of iron. The Moon has no measurable magnetic field, a consequence of its slow rotation and lack of a molten metallic core. Mercury's weak magnetic field seems to have been "frozen in" long ago when the planet's iron core solidified.
The most likely explanation for the formation of the Moon is that the newly formed Earth was struck by a large (Mars-sized) object. Part of the impacting body remained behind as part of our planet. The rest ended up in orbit as the Moon.
1. Laser-ranging can determine the distance to the Moon to an accuracy of a few centimeters. HINT
2. Mercury and Earth have about the same average density. HINT
3. The Moon rotates on its axis about once per week. HINT
4. Mercury has a very small orbital eccentricity. HINT
5. Neither Mercury nor the Moon has any detectable atmosphere. HINT
6. Both the Moon and Mercury have nighttime low temperatures of 250 K, well below the freezing point of water. HINT
7. Telescopes on Earth could see the astronauts and the lunar landers on the Moon during the Apollo missions. HINT
8. Mercury has few lava-flow regions like the lunar maria. HINT
9. Mercury's solar day is actually longer than its solar year. HINT
10. Large craters are formed frequently today on the surface of the Moon. HINT
11. Some volcanic activity continues today on the surface of the Moon. HINT
12. Mercury's craters are more densely packed than are craters on the Moon. HINT
13. Scarps are found only on Mercury, not on the Moon. HINT
14. The Moon has an extensive iron core. HINT
15. Unlike the Moon, Mercury is differentiated. HINT
1. The most accurate method for determining the distance to the Moon is by _____. HINT
2. Mercury can be seen only just before _____ or just after _____. HINT
3. The radius of the Moon is about _____ Earth's radius; the radius of Mercury is about _____ that of Earth. (Give your answers as simple fractions, not decimals.) HINT
4. Because the Moon's average density is so much lower than Earth's average density, we can infer that the Moon must contain less _____. HINT
5. Mercury's iron core contains a _____ fraction of the planet's mass than does Earth's core. HINT
6. Mercury's daytime temperature is higher than the Moon's because it is _____. HINT
7. The _____ on the Moon are dark, flat, roughly circular regions hundreds of kilometers in diameter. HINT
8. Craters on the Moon and Mercury are primarily due to _____. HINT
9. Mercury's rotation rate was first measured using _____. HINT
10. Although Mercury's daytime temperatures are always very hot, it may still be possible for it to have sheets of water ice at its _____. HINT
11. The crater produced by the impact of a meteoroid on the Moon has a typical diameter about _____ times the diameter of the meteoroid. HINT
12. The lunar crust is significantly _____ than Earth's crust. HINT
13. The lunar maria's dark, dense rock originally was part of the lunar _____. HINT
14. Mercury's _____ is about 1/100 that of Earth and was originally thought not to exist at all. HINT
15. The most likely scenario for the formation of the Moon is a collision between Earth and a _____. HINT
1. Why is Mercury seldom seen with the naked eye? HINT
2. Employ the concept of escape speed to explain why the Moon and Mercury have no significant atmospheres. HINT
3. What does it mean to say that Mercury has a 3:2 spin—orbit resonance? Why didn't Mercury settle into a 1:1 spin—orbit resonance with the Sun as the Moon did with Earth? HINT
4. Why is the surface of Mercury often compared with that of the Moon? List two similarities and two differences between the surfaces of Mercury and the Moon. HINT
5. What is a scarp? How are scarps thought to have formed? Why do scientists believe that the scarps formed after most meteoritic bombardment ended? HINT
6. What is the primary source of erosion on the Moon? Why is the average rate of lunar erosion so much less than on Earth? HINT
7. Name two pieces of evidence indicating that the lunar highlands are older than the maria. HINT
8. In contrast with Earth, the Moon and Mercury undergo extremes in temperature. Why? HINT
9. How is Mercury's evolutionary history like that of the Moon? How is it different? HINT
10. Describe the theory of the Moon's origin favored by many astronomers. HINT
11. Because the Moon always keeps one face toward Earth, an observer on the moon's near side would see Earth appear almost stationary in the lunar sky. Still, Earth would change its appearance as the Moon orbited Earth. How would Earth's appearance change? HINT
12. The best place to aim a telescope or binoculars on the Moon is along the terminator line, the line between the Moon's light and dark hemispheres. Why? If you were standing on the lunar terminator, where would the Sun be in your sky? What time of day would it be if you were standing on Earth's terminator line? HINT
13. Where on the Moon would be the best place from which to make astronomical observations? What would be this location's advantage over locations on Earth? HINT
14. Explain why Mercury is never seen overhead at midnight in Earth's sky. HINT
15. How is the varying thickness of the lunar crust related to the presence or absence of maria on the Moon? HINT
1. How long does a radar signal take to travel from Earth to Mercury and back when Mercury is at its closest point to Earth?
2. The Moon's mass is 1/80 that of Earth, and the lunar radius is 1/4 Earth's radius. Based on these figures, calculate the total weight on the Moon of a 100-kg astronaut with a 50-kg spacesuit and backpack, relative to his weight on Earth. HINT
3. Based on the data presented in the Moon's data box, verify the values given for the Moon's perigee (minimum distance from Earth) and apogee (maximum distance from Earth), and estimate the Moon's minimum and maximum angular diameter, as seen from Earth. Compare these values with the angular diameter of the Sun (or actual diameter 1.4 million km), as seen from a distance of 1 A.U.
4. The Hubble Space Telescope has a resolution of about 0.05"". What is the size of the smallest feature it can distinguish on the surface of the Moon? On Mercury? Give your answers in meters.
5. What was the orbit period of the Apollo 11 command module, orbiting 10 km above the lunar surface?
6. What is the angular diameter of the Sun, as seen from Mercury, at perihelion?
7. Compare the gravitational tidal effect of the Sun on Mercury with the tidal effect of Earth on the Moon (see Section 7.6).
8. Mercury's average orbital speed around the Sun is 48 km/s. Use Kepler's second law (see Section 2.4) to calculate Mercury's speed (a) at perihelion and (b) at aphelion. Convert these speeds to angular speeds (in degrees per day), and compare them with Mercury's 6.1°/day rotation rate.
9. Using the rate given in the text for the formation of 10-km craters on the Moon, estimate how long would be needed for the entire Moon to be covered with new craters of this size. How much higher must the cratering rate have been in the past to cover the entire lunar surface with such craters in the 4.6 billion years since the Moon formed? HINT
10. Assume that a planet will have lost its initial atmosphere by the present time if the average molecular speed exceeds one-sixth of the escape speed (see More Precisely 8-1). What would Mercury's mass have to be in order for it still to have a nitrogen (molecular weight 28) atmosphere? HINT
1. Observe the Moon during an entire cycle of phases. When does the Moon rise, set, and appear highest in the sky at each major phase? What is the interval of time between each phase?
2. If you have binoculars, turn them on the Moon when it appears at twilight and when it appears high in the sky. Draw pictures of what you see. What differences do you notice in your two drawings? What color is the Moon seen near the horizon? What color is the Moon seen high in the sky? Why is there a difference?
3. Watch the Moon over a period of hours on a night when you can see one or more bright stars near it. Estimate how many moon diameters it moves per hour. Knowing the Moon is about 0.5° in diameter, how many degrees per hour does it move? What is your estimate of its orbital period?
4. Try to spot Mercury in morning or evening twilight. (Hint: As seen from the Northern Hemisphere, the best evening apparitions of the planet take place in the spring, and the best morning apparitions take place in the fall.)
