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. Highland rocks are less dense than rocks from the maria, and are believed to represent the Moons crust. Maria rocks are thought to have originated in the lunar mantle. 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 significant amounts of water ice near the poles.
The tidal interaction between Earth and the Moon is responsible for the Moons 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. Mercurys rotation rate is strongly influenced by the tidal effect of the Sun. Because of Mercurys eccentric orbit, the planet rotates not synchronously but exactly three times for every two orbits around the Sun. The condition in which a bodys rotation rate is simply related to its orbit period around some other body is known as a spinorbit resonance.
The Moons surface consists of both rocky and dusty material. 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. Mercurys 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 Mercurys 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 Mercurys history. The scarps were apparently formed when the planets core cooled and shrank, causing the surface to crack. Mercurys evolutionary path was similar to that of the Moon for half a billion years after they both formed. Mercurys 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 Moons small size. Lunar gravity is too weak to retain any gases, and lunar volcanism was stifled by the Moons cooling mantle shortly after extensive lava flows formed the maria more than 3 billion years ago. The crust on the far side of the Moon is substantially thicker than the crust on the near 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 Moons 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 less time for differentiation to occur, although the Moon probably has a small iron-rich core. The lunar crust is too thick and the mantle too cool for plate tectonics to occur. Mercurys average density is considerably greatersimilar to that of Earthimplying that Mercury contains a large high-density core, probably composed primarily of iron. The Moon has no measurable large-scale magnetic field, a consequence of its slow rotation and lack of a molten metallic core. Mercurys weak magnetic field seems to have been frozen in long ago when the planets 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.
|PROBLEMS||Algorithmic versions of these questions are available in the Practice Problems Module of the Companion Website.|
The number of squares preceding each problem indicates the approximate level of difficulty.
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? HINT
2. The Moons mass is 1/80 that of Earth, and the lunar radius is 1/4 Earths 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. What would be the same astronauts weight on Mercury? HINT
4. Based on the data presented in the Moon Data box, verify the values given for the Moons perigee (minimum distance from Earth) and apogee (maximum distance from Earth), and estimate the Moons minimum and maximum angular diameter, as seen from Earth. Compare these values with the angular diameter of the Sun (of actual diameter 1.4 million km), as seen from a distance of 1 A.U. HINT
5. What is the angular diameter of the Sun, as seen from Mercury, at perihelion? At aphelion? HINT
6. 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 (distance = 380,000 km)? On Mercury, at closest approach to Earth? HINT
7. What was the orbital period of the Apollo 11 command module, orbiting 10 km above the lunar surface? HINT
9. Mercurys average orbital speed around the Sun is 47.9 km/s. Use Keplers second law to calculate Mercurys speed (a) at perihelion and (b) at aphelion (Sec. 2.5). Convert these speeds to angular speeds (in degrees per day), and compare them with Mercurys 6.1°/day rotation rate. HINT
10. Calculate the lengths of a sidereal and a solar day on Mercury if the planet were in a 4:3 spinorbit resonance instead of the 3:2 resonance actually observed. HINT
11. Assume that a planet will have lost its initial atmosphere by the present time if the average molecular speed exceeds 1/6 of the escape speed (see More Precisely 8-1). What would Mercurys mass have to be in order for it to still have a nitrogen (molecular weight 28) atmosphere? HINT
12. With the same assumptions as the previous question, estimate the minimum molecular mass that might still be found in Mercurys atmosphere. HINT
13. 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
14. Repeat the previous question, for meter-sized craters. HINT
15. Using the data given in the text, calculate how long erosion would take to obliterate (a) the bootprint in Figure 8.17, (b) the Barringer Meteor Crater in Figure 8.19, (c) lunar crater Reinhold in Figure 8.15(b). HINT
1. New Views of Mercury. The craters on the Moon are named after great scientists and philosophers. As a group, propose new names for the 10 largest craters found on Mercury when its "other" side is imaged by the Mercury MESSENGER mission in 2007 and explain your reasoning.
2. Outpost on the Moon. The anticipated cost of transporting a gallon of water from Earth to the Moon is $15,000. Estimate the cost of taking a single-day's supply of water for your group to the Moon by determining how much water each of the group members use in a single day.
3. Lunar Mineral Rights. As a group, decide who owns the rights to mine mineral resources from the Moon and explain your reasoning.
RESEARCHING ON THE WEB
To complete the following exercises, go to the online Destinations Module for Chapter 8 on the Companion Website for Astronomy Today 4/e.
1. Access the "Top 10 Scientific Discoveries from Apollo" page and describe the three most interesting scientific discoveries from the Apollo missions to the Moon.
2. Access the "Virtual Reality Moon Phase Pictures" page and determine the phase of the Moon on the day you were born.
3. Access the "Mercury Fact Sheet" page from NASA and determine the maximum and minimum distances Mercury is from Earth.
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, relative to the stars. 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.)
|SKYCHART III PROJECTS||The SkyChart III Student Version planetarium program on which these exercises are based is included as a separately executable program on the CD in the back of this text.|
1. With SkyChart III configured for normal viewing, center on the Moon at midnight. Set Draw/Horizon Mask deselect. Use a field of view of 3° 45 minutes and time step of one day. Use F6 to step through a month and observe the changes in the phases of the Moon. Repeat until you have a firm grasp on the relationship between phase and orientation with respect to the Sun.
Set DRAW/Mouse Coordinates and determine the position of the Moon. Advance time one day and again determine the position of the Moon. Note that it should advance 360°/27.2 days = 13.2° per day, or approximately 0.5° per hour against the background stars, while the stars advance approximately one degree per day. On some clear evening with a Moon, make careful observation of the position of the Moon and then again four hours later. You should be able to see the progress the Moon has made against the star background just as you do with SkyChart III.
2. Start viewing the Moon just before the full lunar eclipse of May 15, 2003. If you advance the time slowly, you will notice a dimming of the Moon. What are the magnitude and the illumination of the Moon at that time? Change your point of view to be directly above the ecliptic plane at 60 A.U. from the Sun. Verify that at the time of the lunar eclipse you can draw a line from the Moon, through the center of Earth to the Sun.
3. When one observes the full Moon rising, it appears to be huge. Later in the evening, the same full Moon will appear to be smaller. This is an illusion. From your location, plot the size of the Moon for one month. Use a time interval of one hour (Select ANIMATION/Set Time Step and enter: "0 01:00:00"). Observe the size at the Moon's rise and at midnight. At midnight, the Moon will be larger because it is closer to where you are standing (unless you are on one of the poles). Can you explain why?
4. Configure SkyChart III with a 90° field of view and center on Mercury. Set ANIMATE/Trail For/Mercury with Record apparent positions against fixed stars. Animate with one-day steps and observe the motion of Mercury against the background stars. Note how the orbit passes above and below the plane of the ecliptic. Allow the animation to proceed for at least a year to observe the varied patterns of the motion. Explain the motion by adding the effects of observing from a moving platform, Earth, to the regular elliptical orbit of Mercury.
5. Determine the day of the next new Moon, first quarter Moon, full Moon, and last quarter Moon. You can export the relevant data using: File/Export/Ephemeris as Text and then analyze it using a spreadsheet.
In addition to the Practice Problems and Destinations modules, the Companion Website at http://www.prenhall.com/chaisson provides for each chapter an additional true-false, multiple choice, and labeling quiz as well as additional annotated images, animations, and links to related Websites.