5. The Moon: stepping stone to the planets


Lunar eclipse

Lunar eclipse

. During a lunar eclipse the initially full Moon passes through the Earth's shadow. A total lunar eclipse occurs when the entire Moon moves into the umbra. Because no portion of the Sun's surface can be seen from the umbra, it is the darkest part of the Earth's shadow. Only part of the Sun's surface is blocked out in the larger penumbra. A partial lunar eclipse occurs when the Moon's orbit takes it only partially through the umbra or only through the penumbra.


The blood-red Moon

The blood-red Moon

. If the Earth had no atmosphere, the Moon would disappear in darkness during a total lunar eclipse. As shown here, the Moon actually becomes dark red for an hour or so. This is because the Moon is illuminated by sunlight that is bent part way around the Earth and is reddened in passing through the Earth's atmosphere, just as the Sun is reddened at sunset. IF the Earth is heavily clouded, the sunlight is obstructed and the Moon is particularly dark during a lunar eclipse. (Courts of Eric Mandon, Observatoire Populaire de Rouen.)


Gossamer Corona

Gossamer Corona

. The Sunís corona as photographed during the total solar eclipse of 26 February 1998, observed from Oranjestad, Aruba. To extract this much coronal detail, several individual images, made with different exposure times, were combined and processed electronically in a computer. The resultant composite image shows the solar corona approximately as it appears to the human eye during totality. Note the fine rays and helmet streamers that extend far from the Sun and correspond to a wide range of brightness. (Courtesy of Fred Espenak.)


Celestial Paths of the Moon and Sun

Celestial Paths of the Moon and Sun

. The Moon's orbit is titled 5 degrees to the Sun's route across the sky, the ecliptic, allowing these paths to cross at two nodes. These are the only points at which eclipses can occur. During a lunar eclipse the Moon and Sun are located at opposing nodes, so that the Moon can move into the Earth's shadow cast by the Sun. A solar eclipse occurs when the Moon and Sun cross paths at the same node.


Solar Eclipse

Solar Eclipse

. During a solar eclipse, the Moon casts its shadow upon the Earth. No portion of the Sunís photosphere can be seen from the umbral region of the Moonís shadow (small gray spot); but the Sunís light is only partially blocked in the penumbral region (larger half circle). A total solar eclipse, observable only from the umbral region, traces a narrow path across the Earthís surface.


An enormous Moon

An enormous Moon

. In this awesome picture, a man, child seem enveloped by the huge Moon. Our brains trick us into thinking the Moon is much larger at the horizon, then it is when viewed overhead - see Fig. 5.7. (Courtesy of der Foto-Treff.)


Moon illusion

Moon illusion

. We make judgements about size because of our perceptions of distance. The two black disks in this figure are the same size, but we see the bottom one as smaller because we think it is closer. The top disk seems larger because it appears to be farther away. The Moon on the horizon is similarly thought to be huge because comparisons with objects on the ground make us think it is far away - see Fig. 5.6. When people look straight up at the Moon, in an otherwise empty sky, they no longer have land clues to compute the Moon's distance and it is perceived as being closer and smaller.


The full Moon

The full Moon

. The near side of the Moon that is always turned toward the Earth. This Earth-based view of the full Moon enhances the consrast between the dark maria and the bright-rayed craters, such as Tycho (near bottom center), named after Tycho Brahe. The dark circular Mare Imbrium (Sea of Rains) is prominent in the northwest (upper left), immediately above the bright rays of craters Copernicus and Kepler (middle left). The dark circular Mare Serenitatis (Sea of Serenity) lies to the east (right) of Imbrium. (Lick Observatory photograph.)


Lunar highlands

Lunar highlands

. The heavily cratered lunar highlands are shown in this picture of the southern hemisphere of the Moonís near side. Humboldt crater is near the center of the image and Smythís Sea is to the right. Impact craters of all sizes, including giant impact basins measuring hundreds of kilometers across, were formed during an intense bombardment of the Moon about 3.9 billion years ago. This image was obtained during the Apollo 15 mission in August 1971. (Courtesy of NASA.)


Lava flows in a maria

Lava flows in a maria

. Lunar volcanism is seen frozen into place on the Sea of Serenity in this Apollo 17 image taken in December 1972. Crater Condorcet (top) and Crater Bessel (bottom) are superposed on the lava, but the lunar maria contain relatively few craters when compared with the lunar highlands. The maria formed a secondary crust on the Moon, when lava filled the giant impact basins over a period of several hundred million years ending around 3.15 billion years ago. The fluid spread rapidly, creating thin extensive sheets rather than piling up to form volcanoes. (Courtesy of NASA.)


Craters Copernicus and Reinhold

Craters Copernicus and Reinhold

. Bright ejecta radiates outward from the crater Copernicus near the lunar horizon. It is one of the youngest lunar craters on the near side of the Moon, with an estimated age of 900 million years and a diameter of 93 thousand meters. The craters in the foreground are Reinhold A and B. (Courtesy of NASA.)


Lunar rays

Lunar rays

. White rays splash out across the Moon from crater Tycho at the upper right. Tycho is a large, young crater with a diameter of 85 thousand meters and an age of 107 million years. Only relatively recent craters retain their white rays, for those of older craters are darkened and worn away by continued meteorite impact. The dark, flat circular feature in the lower left is Mare Nectaris (Sea of Nectar). This clear image was produced using the unsharp masking technique that permits high contrast and fine resolution. (Anglo-Australian Telescope © 1976. Photo prepared by David F. Malin.)


Earthrise

Earthrise

. Expeditions to the Moon created a new image of the Earth as a blue and turquoise ball suspended all alone in the dark chill of outer space, light and round and shimmering like a bubble, flecked with delicate white clouds. This image was taken from the Apollo 8 spacecraft in 1968. (Courtesy of NASA.).


Landing sites

Landing sites

. The six Apollo (A) landing sites were located in safe places near the equator on the near side of the Moon. Within this constraint, the sites were designed to obtain samples from a wide variety of terrain. Apollo 11 and 12 respectively landed on Mare Tranquillitatis and Oceanus Procellarum. The spot chosen for Apollo 14 was the Fra Mauro Formation, which is covered with material ejected during the ancient impact that created the Imbrium Basin. By landing at a point just inside the Apennine Mountains, the Apollo 15 astronauts could sample highlands, maria and the Hadley Rille. The Apollo 16 mission sampled the highlands near crater Descartes, while Apollo 17 landed near Mare Serenitatis. The location of the three Soviet Luna (L) unmanned, sample return sites are also shown.


Boot prints on the Moon

Boot prints on the Moon

. On 20 July 1969, Neil Armstrong became the first human to walk on the Moon. His boot print, shown here, reveals a thin layer of Moon dust, about 0.01 meters thick. Because there is no atmosphere, water or weather on the Moon, the footprint will probably remain for 1 or 2 million years. By that time, the constant rain of micrometeorites will have erased it. Altogether, twelve astronauts have left boot prints on the Moon. (Courtesy of NASA.)


Moonwalk

Moonwalk

. Charles Duke strolls across the lunar surface during the Apollo 16 mission in April 1972. Small impacting particles have sandblasted the lunar surface, producing smoothed, undulating layers of fine dust and rounding the surfaces of lunar rocks. Larger meteorites have pounded and churned the surface, producing a layer of ground-up rocky debris. (Courtesy of NASA.)


Moon ride

Moon ride

. Eugene Cernan walks away from his lunar rover at the edge of the lunar highlands during the Apollo 17 mission in December 1972. Sculptured, rolling hills are present in the background, and the South Massif is at the far right. The mountains of the Moon have smooth, rounded contours, primarily because there has been no water or ice erosion to sculpt them into steep peaks and valleys. The astronauts used roving vehicles like this one to travel across the Moon's rugged terrain, gathering rocks from a wide variety of locations. The rovers were left on the Moon. Free from wind, rain and rust, they will remain intact for millions of years; one might even imaging a returning astronaut using one that was discarded hundreds of years before. (Courtesy of NASA.)


Moon rock

Moon rock

. Harrison Schmitt about to walk behind Split Rock during the Apollo 17 mission in December 1972. Eugene Cernan had already scooped up samples from the debris on the front side of the boulder. The huge rock rolled down about a billion years ago, splitting into five pieces during the fall. The total length of the boulder, when reassembled is about 20 meters. (Courtesy of NASA.)


Rare side view of the Moon

Rare side view of the Moon

. The Galileo spacecraft returned this image of the western limb and far side of the Moon on 8 December 1990, from a vantage point not possible from the Earth. It shows the bright far-side highland (left) and the near-side Oceanus Procellarum (top right). The dark spots near the center are Mare Orientale, about 900 thousand meters across; it is barely visible form Earth at the western limb of the lunar near side. The huge South Pole - Aitken impact basin, a large circular depression about 2.6 million meters in diameter, is the large dark region at the lower left. (Courtesy of JPL and NASA.)


Compositional variations

Compositional variations

. This enhanced color mosaic shows volcanic flows with relatively high titanium content (blue), volcanic flows that are low in titanium but rich in iron and magnesium (green, yellow and light orange), and heavily cratered highlands that are typically poor in titanium, iron and magnesium (pink and red). In this view, taken by Galileo as it flew over the northern region on 7 December 1992, bright pink highlands surround the lava-filled Crisium impact (bottom) and the dark blue Mare Tranquillitatis (left) is richer in titanium than the green and orange maria above it. The youngest craters have prominent blue rays extending from them. (Courtesy of JPL and NASA.)


Bottom of the Moon

Bottom of the Moon

. The laser altimeter on Clementine provided a detailed topographic map of the South Pole of the Moon for the first time, revealing the full extent of the South Pole - Aitken basin (center). Colors indicate relative elevation, with orage being high and blue being low. The basin is about 2.6 million meters and over 12 thousand meters deep. It is the biggest, deepest impact feature known in the solar system, and dominates the relief of the far side of the Moon. Water ice might be preserved in the permanently shadowed regions at the Moon's south pole. (Courtesy of Paul D. Spudis, Lunar and Planetary Institute.)


Lunar interior

Lunar interior

. A schematic cross section of the Moon shows its internal structure. The lunar crust is thinner on the near side that faces the Earth, and thicker on the far side. Fractures in the thin crust have allowed magma to reach the surface on the near side, where the lava-filled maria are concentrated. The Moon has an iron-rich core with a radius of about 20 percent of the Moon's average radius. The Moon's center of mass (CM) is offset by 2 thousand meters from its center of figure, CF, so an equipotential surface, which experiences an equal gravitation force at all points, lies closer to the lunar surface on the hemisphere facing Earth. Therefore magmas originating at equipotential depths will have greater difficulty reaching the surface on the far side.


Composition of Aristarchus plateau

Composition of Aristarchus plateau

. This colored Clementine mosaic illustrates the composition and mineralogy in the Aristarchus plateau, an elevated block of crust surrounded by the vast mare lava plains of Oceanus Procellarum. The fresh, excavated highland materials appear blue, mare lava flows are reddish purple, fresh basalt in the crater and and rilles is yellow, and ash and volcanic glass is dark red. The two dark blue spots in the center of Aristarchus are consistent with a composition of almost pure anorthosite, a primitive rock type that floated to the top of the ancient magma ocean. The sinuous rille is SchrŲter's Valley, or Vallis Schroteri, a large lava cannel stretching for about 160 thousand meters across the plateau. (Courtesy of JPL, NASA and the US Geological Survey.)


Lunar topography

Lunar topography

. The laser altimeter on Clementine provided the first comprehensive topographic map of the Moon. The near side (left) is relatively smooth and low (blue and purple), primarily because of the prominent impact basins, including Imbrium, Crisium and Nectaris, that are at least partly filled with mare basalt. In contrast, the far side (right) shows high relief (red) and extreme topographic variation comparable to that of the Earth. The Moon's wide range of relief is attributed to ancient impact basins that have been preserved for about 3.9 billion years, while the Earth's wide range stems from ongoing mountain building by colliding tectonic plates. The large circular feature on the southern far side (right bottom) is the South Pole - Aitken basin, 2.6 million meters in diameter and 12 thousand meters deep. (Courtesy of Paul D. Spudis, Lunar and Planetary Institute.)


Dark, cold lunar poles

Dark, cold lunar poles

. The near-vertical orientation of the Moon's north-south rotation axis to the ecliptic plane creates permanent night and a permanent deep freeze at the floor of craters located at the lunar poles. These regions might be reservoirs of water ice, delivered there by comets. The angle between the Earth's equator and the ecliptic, or the plane of the Earth's orbit around the Sun, is 23.5 degrees, and this tilt produces the seasons. The Moon provides a steadying influence for the Earth's tilt, keeping it from varying widely and producing dramatic climate variations. Also note that the plane of the lunar orbit falls neither in the Earth's orbital plane nor in the ecliptic.


Varying crater rate

Varying crater rate

. The rate of forming craters on the lunar surface is plotted against time. The arrows point to the crater rate and rock ages at various Apollo landing sites. The crater rate was very high during an intense bombardment that occurred 3.9 billion years ago. The rate dropped rapidly during the subsequent billion years, giving way to the lower steady rate of crater production that has persisted for the last 3 billion years. With such a curve, we can obtain approximate surface ages just by counting the number of craters in different parts of the Moon. Recent data have provided evidence of an increase in the cratering rate over the last 400 million years.


Mare formation

Mare formation

. Disintegrating and vaporizing as it strikes, a meteorite blasts a huge impact basin out of the lunar surface (left), while the associated shock waves create fractures in the rock beneath the basin. The blast hurls up mountain ranges around the basin (middle), and the underlying rock adjusts to the loss of mass above it by rebounding upward. The uplifted mantle causes additional fractures in the rock, while a pool of shock-melted rock solidifies in the basin. All the major impact basins on the Moon were created in this way between 3.9 and 4.3 billion years ago. Later, interior heat from radioactivity caused partial melting inside the Moon, and magma rose along the fractures, filling the basin with layer by layer with lava to form a dark mare (right). The lunar maria were fill by this volcanic outpouring between 3.1 and 3.9 billion years. ago.


Cause of the tides

Cause of the tides

. The Moon's gravitational attraction causes two tidal bulges in the Earth's ocean water, one on the closest side to the Moon and one on the farthest side. The Earth's rotation twists the closest bulge ahead of the Earth-Moon line (dashed line), and this produces a lag in time between the time the Moon is directly overhead and the highest tide. The Moon pulls on the nearest tidal bulge, slowing the Earth's rotation down. At the same time, the tidal bulge nearest the Moon produces a force that tend to pull the Moon ahead in its orbit, causing the Moon to spiral slowly outward.


Spring and neap tides

Spring and neap tides

. The height of the tides and the phase of the Moon depend on the relative positions of the Earth, Moon and Sun. When the tide-raising forces of the Sun and Moon are in the same direction, they reinforce each other, making the highest high tides and the lowest low tides. These spring tides (left) occur at new or full Moon. The range of tides is least when the Moon is at first or third quarter, and the tide raising forces of the Sun and Moon are at right angles to each other. The tidal forces are then in opposition, producing the lowest high tides and the highest low tides, or the neap tides (right). The height of the tides have been greatly exaggerated in comparison to the size of the Earth.


Steadying influence

Steadying influence

. The Moon holds the Earth upright in space, stabilizing its orientation and keeping the planet from tilting over. Without the Moon's influence, chaotic forces could tip the Earth's rotation axis down so far that its poles are pointing at or away from the Sun, producing wild swings in the Earth's climate. This image of the Moon and Earth was taken from a distance of 6.2 billion meters, by the Galileo spacecraft on 16 December 1992, soon after swinging around the Earth on its way to Jupiter. (Courtesy of JPL and NASA.)


Classical origin theories

Classical origin theories

. According to the fission theory (left), the rotational speed of the young Earth was great enough for its equatorial bulge to separate from the Earth and become the Moon. In the capture theory (middle), a vagabond moon-sized object once passed close enough to be captured by the Earth's gravitational embrace. We have pictured disruptive capture, with subsequent accretion, but the Moon might have been captured intact. The accretion theory (right) assets that the Moon formed from a disk near the Earth.


Giant impact theory

Giant impact theory

. According to the giant impact hypothesis, a massive projectile (A), about the size of Mars, struck the young, still-forming Earth (B) in a catastrophic, glancing blow nearly 4.6 billion years ago, resulting in a tremendous explosion and the jetting outward of both projectile and proto-Earth mass. Some fraction of this mass remained in Earth orbit (C), while the rest escaped Earth or impacted again on Earth's surface. A proto-Moon began to form from the orbiting material (D), accreting neighboring matter, and finally became the Moon (E). It may be mostly derived from the crust and mantle of the Earth and/or the impacting object, accounting for the Moon's relatively low mean mass density and lack of iron when compared with the Earth. The Moon accumulated so rapidly that the outer crust was molten, helping to account for the relative lack of water and other volatile elements. Then, as the crust cooled, the newborn Moon swept up the remaining objects nearby, blasting out impact basins and pockmarking the surface with numerous craters. (Courtesy of Alan P. Boss, Carnegie Institution of Washington.)


Summary diagram

Summary diagram

. Summary diagram