The Moon 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 boot prints in the foreground are destined to survive for more than a million years. (NASA)
Moon Formation 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
The Moon’s Crust The side of the moon that faces Earth is called the near side. The other side of the moon that faces away from Earth is called the far side. The pull of Earth’s gravity during the moon’s formation caused the crust on the far side of the moon to become thicker than the crust on the near side. The crust on the near side is about 60 km thick. The crust on the far side is up to 100 km thick.
satellite a natural or artificial body that revolves around a celestial body that is greater in mass • moon a celestial body that revolves around a body that is greater in mass; a natural satellite • Six of the planets in our solar system have natural satellites, or moons. Our moon is Earth’s satellite. • The Apollo space program sent six spacecraft to study the moon. Scientists were able to gather data about the moon’s weak gravity and its effect on astronauts, as well as data about the moon’s surface.
Moon’s Rotation 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 8-10. 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.)
Surface Features 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)
Surface Features (cont.) • Any feature of the moon is referred to as lunar. • Light and dark patches on the moon can be seen with the unaided eye. • The lighter areas are rough highlands composed of rocks called anorthosites. The darker areas are smooth, reflect less light, and are called maria. • mare a large, dark area of basalt on the moon • Maria are plains of dark, solidified lava which formed more than 3 billion years ago when lava slowly filled basins that were created by impacts of massive asteroids.
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)
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 8.18. (UC/Lick Observatory; Palomar)
Lunar Craters Meteoroid strikes Moon, ejecting material; explosion ejects more material, leaving crater Figure 8-13. 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.
Lunar Craters (cont.) • 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
Lunar Surface (cont.) • Craters, Rilles, and Ridges • crater a bowl-shaped depression that forms on the surface of an object when a falling body strikes the object’s surface or when an explosion occurs • The surface of the moon is covered with craters, rilles, and ridges. Most of the craters formed when debris struck the moon about 4 billion years ago. • Rilles are long, deep channels that run through the maria. Rilles are thought to be leftover lava channels from the formation of the maria. • The moon’s surface also has several ridges, which are long, narrow elevations of rock that rise out of the surface and criss-cross the maria.
Craters come in all sizes, from the very large… Figure 8-14. 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)
…to the very small Figure 8-15. 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)
Lunar Surface (cont.) Regolith: Thick layer of dust left by meteorite impacts Figure 8-16. 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 boot prints 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)
Lunar Surface (cont.) Regolith More meteorites have reached the surface of the moon than have reached Earth’s surface because the moon has no atmosphere for protection. Over billions of years, these meteorites crushed much of the rock on the lunar surface into a layer of dust and small fragments called regolith. The depth of regolith layer varies from 1 m to 6 m.
More than 3 billion years ago, the moon was volcanically active; the rille here was formed then Figure 8-20. 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)
Lunar Rocks • Lunar Rocks • Lunar rocks are igneous, and most rocks near the surface are composed mainly of oxygen and silicon. • Rocks from the lunar highlands are light-colored, coarse-grained anorthosites rich in calcium and aluminum. • Rocks from the maria are fine-grained basalts and contain titanium, magnesium, and iron. • Breccia is found in both maria and the highlands. Lunar breccia formed when meteorites struck the moon.
Moon’s Interior 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 8-24. 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.
Interior of the Moon • The interior of the moon is less dense than the interior of Earth. • Most of the information about the interior of the moon comes from seismographs that were placed on the moon by the Apollo astronauts. • More than 10,000 moonquakes have been detected. From these moonquakes, scientists learned that the moon’s interior is layered.
Mercury is much denser than the Moon and has a weak magnetic field—not well understood!
Eclipses • eclipse an event in which the shadow of one celestial body falls on another • Bodies orbiting the sun, including Earth and its moon, cast long shadows into space. An eclipse occurs when one body passes through the shadow of another. • Shadows cast by Earth and the moon have two parts: the inner, cone-shaped part of the shadow called the umbra and the outer part of the shadow called the penumbra.
Solar Eclipses Click on the Eclipse • Solar Eclipses • solar eclipse the passing of the moon between Earth and the sun; during a solar eclipse, the shadow of the moon falls on Earth. • During a total solar eclipse, the sun’s light is completely blocked by the moon. The umbra falls on the area of Earth that lies directly in line with the moon and the sun. • Outside the umbra, but within the penumbra, people see a partial solar eclipse. The penumbra falls on the area that immediately surrounds the umbra.
Lunar Eclipses To the Moon! Click it • Lunar Eclipses • lunar eclipse the passing of the moon through Earth’s shadow at full moon • A lunar eclipse occurs when Earth is positioned between the moon and the sun and when Earth’s shadow crosses the lighted half of the moon. • When only part of the moon passes into Earth’s umbra, a partial lunar eclipse occurs. When the entire moon passes through Earth’s penumbra, a penumbral eclipse occurs. • Even during a total lunar eclipse, sunlight is bent around Earth through our atmosphere. Mainly red light reaches the moon, so the totally eclipsed moon appears to have a reddish color.
Frequency of Solar and Lunar Eclipses • As many as seven eclipses may occur during a calendar year. Four may be lunar, and three may be solar or vise versa. • Total eclipses of the sun and the moon occur infrequently, because the orbit of the moon is not in the same plane as the orbit of Earth around the sun. • Lunar eclipses are visible everywhere on the dark side of Earth. • A total solar eclipse, can be seen only by observers in the path of the moon’s shadow as it moves across Earth’s lighted surface. A partial solar eclipse can be seen for thousands of kilometers on either side of the path of the umbra.
New Moon • Time from New Moon to New Moon • The moon revolves around Earth in 27.3 days, however, the period from one new moon to the next one is 29.5 days. • This difference of 2.2 days is due to the orbiting of the Earth-moon system around the sun. • In the 27.3 days in which the moon orbits Earth, the two bodies move slightly farther along their orbit around the sun. So, the moon must go a little farther to be directly between Earth and the sun. About 2.2 days are needed for the moon to travel this extra distance.
Wax on Wan off • Waxing Phases of the Moon • When the size of the lighted part of the moon is increasing, the moon is said to be waxing. • When a sliver of the moon’s near side is illuminated, the moon enters its waxing-crescent phase. • When the waxing moon becomes a semicircle, the moon enters the first-quarter phase. • When the lighted part of the moon’s near side is larger than a semicircle and still increasing in size, the moon is in the waxing-gibbous phase. • The moon continues to wax until it appears as a full circle. At full moon, the entire near side of the moon is illuminated.
Waxing vs. Waning • Waning Phases of the Moon • When the lighted part of the near side of the moon appears to decrease in size, the moon is waning. • When the moon is waning, but is still larger than a semicircle, the moon is in the waning-gibbous phase. • When the moon is waning, and it is a semicircle, the moon enters the last-quarter phase. • When only a sliver of the near side is visible, the moon enter the waning-crescent phase. • After this phase, the moon becomes a new moon, in which no lighted area of the moon is visible from Earth.
Tides (cont.) Click, watch, and listen to a tall tale of ye tides mateies, arghhhh. • Bulges in Earth’s oceans, called tidal bulges, form because the moon’s gravitational pull on Earth decreases with distance from the moon. • As a result, the ocean on Earth’s near side is pulled toward the moon with the greatest force. • The solid Earth experiences a lesser force. The ocean on the far side is subject to less force than the solid Earth is. • These differences cause Earth’s tidal bulges. Because Earth rotates, tides occur in a regularly at any given point on the surface each day.