From Earth, the Moon's angular diameter is about 0.5°. Knowing that and the distance to the Moon, we can easily calculate its true size, as discussed in Chapter 1. 
(Sec. 1.5) The Moon's radius is about 1700 km, roughly one-fourth that of Earth. More precise measurements yield a lunar radius of 1738 km. We can determine Mercury's radius by similar reasoning. At its closest approach to Earth, at a distance of about 0.52 A.U., Mercury's angular diameter is measured to be 13" (arc seconds), implying a radius of about 2450 km, or 0.38 Earth radii. More accurate measurements by unmanned space probes yield a result of 2440 km.
As with all astronomical objects, we determine the masses of the Moon and Mercury by studying their gravitational pull on other objects in their vicinity, such as Earth and human-made spacecraft. (More Precisely 1-2 ) As mentioned in Chapter 6, even before the Space Age, the masses of both the Moon and Mercury were already quite well known from studies of their effects on Earth's orbit. (Sec. 6.2) The mass of the Moon is 7.3 
1022 kg, approximately 1/80 the mass of Earth. The mass of Mercury is 3.3 1023 kg—about 0.055 Earth masses.
The Moon's average density of 3300 kg/m3 contrasts with the average Earth value of about 5500 kg/m3, suggesting that the Moon contains fewer heavy elements (such as iron) than Earth. However, despite its many other similarities to the Moon, Mercury's mean density is 5400 kg/m3, only slightly less than that of Earth. Assuming that surface rocks on Mercury are of similar density to surface rocks on Earth and the Moon, we are led to the conclusion that the interior of Mercury must contain a lot of high-density material, most probably iron. In fact, since Mercury is considerably less massive than Earth, its interior is squeezed less by the weight of overlying material, so Mercury's iron core must actually contain a much larger fraction of the planet's mass than is the case for our own planet.
Because the Moon and Mercury are so much less massive than Earth, their gravitational fields are also weaker. The force of gravity on the lunar surface is only about one-sixth that on Earth; Mercury's surface gravity is a little stronger—about 0.4 Earth gravity. Thus, an astronaut weighing 180 lb on Earth would weigh a mere 30 lb on the Moon and 72 lb on Mercury. Those bulky spacesuits used by the Apollo astronauts on the Moon were not nearly as heavy as they appeared!
Astronomers have never observed any appreciable atmosphere on either the Moon or Mercury, either spectroscopically from Earth or during close approaches by spacecraft. As discussed in More Precisely 8-1, this is a direct consequence of these bodies' weak gravitational fields. Massive objects have a better chance of retaining their atmosphere, because the more massive an object, the larger are the speeds needed for atoms and molecules to escape. The Moon's escape speed is only 2.4 km/s, compared with 11.2 km/s for Earth; Mercury's escape speed is 4.2 km/s. Any primary atmospheres these worlds had initially, or secondary atmospheres that appeared later, are gone forever.
During its flybys of Mercury in 1974 and 1975, the U.S. space probe Mariner 10 found traces of what was at first thought to be an atmosphere on Mercury. (Sec. 6.6) However, this gas is now known to be temporarily trapped hydrogen and helium stolen from the solar wind. Mercury captures this gas and holds it for just a few weeks. Earth-based observations have also found an extremely tenuous envelope of sodium and potassium around the planet. Scientists believe that these atoms are torn out of the surface rocks following impacts with high-energy particles in the solar wind; they do not constitute a true atmosphere in any sense. Thus, neither the Moon nor Mercury has any protection against the harsh environment of interplanetary space. This fact is crucial in understanding their surface evolution and present-day appearance.
Lacking the moderating influence of an atmosphere, both the Moon and Mercury experience wide variations in surface temperature. Noontime temperatures on the Moon can reach 400 K, well above the boiling point of water. Because of its proximity to the Sun, Mercury's daytime temperature is even higher—radio observations of the planet's thermal emission indicate that it can reach 700 K. (Sec. 3.4) But at night or in the shade, temperatures on both worlds fall to about 100 K, well below water's freezing point. Mercury's 600 K temperature range is the largest of any planet or moon in the solar system.
The first observers to point their telescopes at the Moon, most notable among them Galileo Galilei, noted large dark areas, resembling (they thought) Earth's oceans. They also saw light-colored areas resembling the continents. Both types of regions are clear in Figure 8.4, a mosaic of the full Moon. The light and dark surface features are also evident to the naked eye, creating the face of the familiar "man in the moon."
Figure 8.4 (a) 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. (b) The Moon near third quarter. Notice that 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 topography. The light-colored crater just below the lower left corner of the breakout box is Copernicus. (c) 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 bottom right by the Apennine mountains. (d) An enlargement of a portion of (c). The crater at the bottom left is Eratosthenes; Archimedes is at top center.
Today we know that the dark areas are not oceans but extensive flat areas that resulted from lava flows during a much earlier period of the Moon's evolution. Nevertheless, they are still called maria, a Latin word meaning "seas" (singular: mare). There are 14 maria, all roughly circular. The largest of them (Mare Imbrium) is about 1100 km in diameter. The lighter areas, originally dubbed terrae, from the Latin word for "land," are now known to be elevated several kilometers above the maria. Accordingly, they are usually called the lunar highlands.
The smallest lunar features we can distinguish with the naked eye are about 200 km across. Telescopic observations further resolve the surface into numerous bowl-shaped depressions, or craters. Most craters apparently formed eons ago primarily as the result of meteoritic impact. Craters are particularly clear in Figures 8.4(b) and (c) near the terminator (the line that separates day from night on the surface), where the Sun is low in the sky and casts long shadows that enable us to distinguish quite small surface details.
Due to the blurring effects of our atmosphere, the smallest lunar objects that telescopes on Earth's surface can resolve are about 1 km across (see Figure 8.4d). Much more detailed photographs have been taken by orbiting spacecraft and, of course, by visiting astronauts. Figure 8.5 is a view of some lunar craters taken from an orbiting spacecraft, showing features as small as 500 m across. Craters are found everywhere on the Moon's surface, although they are much more prevalent in the highlands. They come in all sizes—the largest are hundreds of kilometers in diameter; the smallest are microscopic.
Figure 8.5 The Moon, as seen from the Apollo 8 orbiter during the first human circumnavigation of the Moon in 1968. Craters ranging in size from 50 km to 500 m (also the width of the long fault lines) can be seen.
All the Moon's significant surface features have names. The 14 maria bear fanciful Latin names—Mare Imbrium ("Sea of Showers"), Mare Nubium ("Sea of Clouds"), Mare Nectaris ("Sea of Nectar"), and so on. Most mountain ranges in the highlands bear the names of terrestrial mountain ranges—the Alps, the Carpathians, the Apennines, the Pyrenees, and so on. Most of the craters are named after great scientists or philosophers, such as Plato, Aristotle, Eratosthenes, and Copernicus.
Because the Moon rotates once on its axis in exactly the same time as it takes to complete one orbit around Earth, the Moon has a "near" side, which is always visible from Earth, and a "far" side, which never is (see Section 8.3). To the surprise of most astronomers, when the far side of the Moon was mapped, first by Soviet and later by U.S. spacecraft (see Interlude 8-1), no major maria were found there. The lunar far side (Figure 8.6) is composed almost entirely of highlands. This fact has great bearing on our theory of how the Moon's surface terrain came into being, for it implies that the processes involved could not have been entirely internal in nature. Earth's presence must somehow have played a role.
Figure 8.6 The western hemisphere of the Moon, as seen by the Galileo probe en route to Jupiter. The large, dark region is part of the face visible from Earth. This image shows only one or two small maria on the far side.
Mercury is difficult to observe from Earth because of its closeness to the Sun. Even with a fairly large telescope, we see it only as a slightly pinkish disk. Figure 8.7 is one of the few photographs of Mercury taken from Earth that shows any evidence of surface markings. Astronomers could only speculate about the faint, dark markings in the days before Mariner 10's arrival. We now know that these markings are much like those seen when gazing casually at Earth's Moon. The largest ground-based telescopes can resolve surface features on Mercury about as well as we can perceive features on the Moon with our unaided eyes.
Figure 8.7 Photograph of Mercury taken from Earth with one of the largest ground-based optical telescopes. Only a few surface features are discernible.
In 1974, Mariner 10 approached within 10,000 km of the surface of Mercury, sending back high-resolution images of the planet. (Sec. 6.6) These photographs, which showed surface features as small as 150 m across, revolutionized our knowledge of the planet. Figures 8.8 (a) and (b) show views of Mercury taken by Mariner 10 from a distance of about 200,000 km. Together, these two mosaics cover the known surface of Mercury. No similar photographs exist of the hemisphere that happened to be in shadow during the encounters. Figure 8.9 shows a higher-resolution photograph of the planet from a distance of 20,000 km. The similarities to the Moon are very striking. We see no sign of clouds, rivers, dust storms, or other aspects of weather. Much of the cratered surface bears a strong resemblance to the Moon's highlands. Mercury, however, shows few extensive lava flow regions akin to the lunar maria.
Figure 8.8 (a) Mercury is imaged here as a mosaic of photographs taken by the Mariner 10 spacecraft in the mid-1970s during its approach to the planet. At the time, the spacecraft was some 200,000 km away. (b) Mariner 10's view of Mercury as it sped away from the planet after each encounter. Again, the spacecraft was about 200,000 km away when the photographs making up this mosaic were taken.
Figure 8.9 Another photograph of Mercury by Mariner 10, this time from about 20,000 km above the planet's surface. The double-ringed crater at the upper left, named C. Bach, is about 100 km across; it exemplifies many of the large craters on Mercury, which tend to have double, rather than single, rings. The reason is not yet understood.
