Magnetic Field
The Earth has a modest magnetic field produced by electric currents in its iron and nickel core. The magnetic field forms a set of complete loops similar in shape to those around a bar magnet. The magnetic poles of the Earth are close to the rotational pole (about 11° apart); other planets have much larger angles between their magnetic and rotational poles.
The Earth’s magnetic field (magnetosphere) normally protects it from bombardment by energetic particles from space. Most of these particles originate from the Sun and are carried towards Earth by the solar wind. Near the Earth the solar wind has a speed of about 400 km/s. Charged particles in the solar wind get trapped in the magnetic field and circulate in a pair of doughnut-shaped rings called the Van Allen radiation belts. These belts were discovered in 1958 during the flight of America’s first successful Earth-orbiting satellite named after physicist James Van Allen, who insisted the satellite carry a Geiger counter to detect charged particles (see Fig. 1.8).
During times of solar flares and increased sunspot activity, the Van Allen Radiation belts become overloaded with charged particles. Some of the particles travel down into the Earth’s upper atmosphere where they interact with gases like nitrogen and oxygen to cause them to fluoresce (give off light). The result is a shimmering display called the northern lights (aurora borealis) or southern lights (aurora australis) depending on the hemisphere involved. Such events sometimes interfere with radio transmissions, communication satellites, and electrical power transmission.
The Moon
The Earth has only one natural satellite—the Moon. The Moon orbits Earth in an elliptical orbit at about 36,800 km/h. Its from Earth varies from 356,000 to 407,000 km. The Moon is about a quarter the size of Earth and has just more than 1 % of the mass of Earth. The density of the Moon is only 3.34 g/cm3 compared to Earth’s 5.52 g/cm3 (Fig. 6.5).
Fig. 6.5The Moon as imaged by the author through a 10 inch SCT telescope in April 2015 (Credit: J. Wilkinson).
The Moon is the second brightest object in the sky after the Sun. Unlike the Sun, which emits its own light and heat, the Moon only reflects sunlight. The Moon is also one of the most widely studied objects in the solar system. It has been studied with the naked eye, telescopes, and spacecraft. To date, the Moon is the only body in the Solar System to have been visited by humans; this occurred for the first time in 1969.
The Moon was probably formed at the same time as other planets in the solar system and gravitationally captured by the Earth. Others think it is a fragment torn out of Earth’s mantle. Yet another theory is that the Earth–Moon pair could be a double planet.
Early Views About the Moon
The presence of the Moon in our sky has captured human interest throughout history. The Moon is so large and close to Earth that some of its surface features are readily visible to the naked eye. For centuries people have thought that some of the features on the Moon looked like a human face looking down on them, and often talked about the ‘man in the Moon’. People also thought that the Moon had a fairly smooth surface, until Galileo’s telescope showed the surface was covered with many craters and mountains as well as plains (Table 6.3).Table 6.3Details of the Moon
Distance from Earth
384,400 km
Diameter
3476 km
Mass
7.35 × 1022 kg (0.012 Earth masses)
Density
3.34 g/cm3 or 3340 kg/m3
Orbital eccentricity
0.055
Period of revolution
27.3 Earth days (relative to stars)
29.5 Earth days (seen from Earth)
Rotation period
27.3 Earth days
Orbital velocity
36,800 km/h
Tilt of axis
6.7°
Day temperature
130 °C
Night temperature
−184 °C
Atmosphere
None
Strength of gravity
1.7 N/kg at surface
Probing the Moon
More spacecraft have been sent to the Moon than any other body in the solar system, simply because it is so close to Earth. Early probes to pass by the Moon included the USSR’s Luna probes and the USA’s Ranger and Surveyor probes.
The most significant missions to the Moon were those of the USA’s Apollo program. The first manned lunar fly-around involved 10 orbits of the Moon by Apollo 8, between 21 and 27 December 1968. Apollo 11 was the first manned lunar landing, on 20 July 1969. The landing site was Mare Tranquillitatis. Neil Armstrong was the first astronaut to walk on the Moon followed by Edwin Aldrin. The third crew member, Michael Collins, stayed in the Command module orbiting the Moon (Figs. 6.6 and 6.7).
Fig. 6.6View of the Moon’s surface, the Apollo 11 lander, and distant Earth (Credit: NASA).
Fig. 6.7Apollo 15 astronaut, lander and rover on the Moon’s surface (Credit NASA).
Apollo missions 12 and 14 through to 17 landed manned craft on the Moon’s surface. Of these missions, Apollo 15 was first to make use of a lunar rover vehicle. The rover allowed astronauts to travel several kilometres from the landing site. The last Apollo mission (Apollo 17) occurred in December 1972.
Exploration of the Moon continued with the Galileo and Clementine spacecraft. In October 1989, Galileo began a 6-year mission to Jupiter, but on its way passed the Moon twice. Data was returned to Earth on the composition of the lunar surface. Clementine went into orbit around the Moon in February 1994. Using laser-ranging techniques and high-resolution cameras, Clementine mapped the Moon’s topography in greater detail than had been done previously. In 1999, Prospector also mapped the lunar surface and detected ice buried beneath the ground in deep polar craters. More recently Japan, India and China have sent space probes to the Moon.
These recent missions have been successful at 3D mapping of the Moon, spectral analysis of the surface and interior, measurement of the gravitational field, surveying lunar resources and identifying future landing sites (see Table 6.4).Table 6.4Recent space probes to the Moon
Spacecraft
Country of origin
Date
Mission focus
SMART-1
Europe
2004
Lunar geology
Selene (Kaguya)
India
2007
Minerals, geographic
Chang’e 1
China
2007
Mapping, geology
Chandrayaan 1
India
2008
Spectral analysis
Moon Impact Probe
India
2008
Close range imaging
Lunar Recon. Orbiter
USA
2009
Resources, mapping
Chang’e 2
China
2010
Imaging, analysis
ARTEMIS P1
USA
2011
Monitor solar wind
ARTEMIS P2
USA
2011
Monitor solar wind
GRAIL A
USA
2011
Measure gravity
GRAIL B
USA
2012
Measure gravity
LADEE
USA
2013
Lunar exosphere
Chang’e 3
China
2013
Soft lander, rover
In December 2013, China successfully carried out the world’s first soft landing of a space probe on the Moon in nearly four decades. The touchdown of the unmanned Chang’e 3 lander was the latest mission in the country’s ambitious space programme, which is intended to put a Chinese astronaut on the Moon early next decade. The lander carried a six-wheeled rover called Yutu (Jade Rabbit), to its landing place on a flat plain known as the Sinus Iridum, or Bay of Rainbows, after hov
ering over the surface for several minutes before selecting the best available landing spot. This region was selected for the craft’s landing because lunar probes had not previously studied it. The spaceship’s rover was remotely controlled by Chinese control centres with support from a network of tracking and transmission stations around the world operated by the European Space Agency. After it touched down on the Moon, Chang’e 3’s solar panels, which are used to generate power from sunlight, unfolded and the spacecraft began transmitting pictures back to Earth.
India expects to launch another lunar mission by 2016, which would place a motorised rover on the Moon. China plans to conduct a sample return mission with its Chang’e 5 spacecraft in 2017.
The Japanese Aerospace Exploration Agency (JAXA) plans a manned lunar landing around 2020 that would lead to a manned lunar base by 2030.
Position and Orbit
The Moon spins like a top on its axis as it travels around the Earth, and, at the same time the Earth is orbiting the Sun. The Moon’s orbit is slightly elliptical and its mean distance from the Earth is 384,400 km. At closest approach (perigee) it is 356,000 km from Earth and its furthest distance (apogee) is 407,000 km.
The Moon takes 27.3 days to go once around the Earth and it also takes this time to rotate once on its own axis. Because of this, we always see the same side of the Moon from Earth. As the Moon orbits the Earth, the angle between the Earth, Moon and Sun changes, and we see this as the cycle of Moon’s phases (see Fig. 6.8). The time between successive new Moons is actually 29.5 days, slightly different from the Moon’s orbital period (measured against the stars) because the Earth moves a significant distance in its orbit around the Sun in that time.
Fig. 6.8Phases of the Moon.
The Moon appears to wobble on its axis due to its elliptical orbit. As a result we can see a few degrees of the far side of the Moon surface from time to time. Most of the far side was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. The far side of the Moon gets sunlight half the time. Whenever we see less than a full Moon, some sunlight is falling on the far side. Throughout each cycle of lunar phases all parts of the Moon get equal amounts of sunlight.
Gravity keeps the Moon in orbit around the Earth and produce the tides. Tidal forces deform the oceans, causing them to rise at some places and to settle elsewhere. There are two areas of high tide on the Earth at any given time, one on the side closest to the Moon, the other on the opposite side of the Earth. Low tides occur where the Moon is on the horizon.
The Sun also distorts the shape of the oceans, but only half as much as the Moon, because the Sun is nearly 400 times farther away.
Instruments placed on the Moon by the Apollo 12 astronauts have enabled us to measure precisely the distance to the Moon. From such measurements over time, astronomers have found that the Moon is spiralling away from the Earth as a rate of about 4 cm per year. The cause of this motion is thought to be due to tidal interactions between the Moon and Earth. Tidal forces are also causing the Earth’s rotation to slow by about 1.5 ms per century.
Density and Composition
The Moon has a smaller mass, diameter and average density than Earth. Because of this, the strength of gravity on the Moon’s surface is one sixth that of Earth. A 75 kg person on Earth weighs 735 N, but on the Moon the same person would only weigh 122 N. The Moon’s escape velocity is only 2.4 km/s.
Scientists have found out about what the interior of the Moon is like from seismic (moonquake) evidence made at the Apollo landing sites. The Moon has a crust, mantle and core. Although these regions are similar to those of the Earth, the proportions are quite different. The Moon’s crust averages 68 km thick and varies from a few kilometres under Mare Crisium on the visible side, to 107 km near the crater Korolev on the far side. The GRAIL mission showed the lunar crust is thicker on the far side (60 km) than on the nearside (20–30 km). This means the Moon is slightly egg-shaped, with the small end pointing toward Earth. As a result, the Moon’s centre of mass is 2 km closer to Earth than its geometric centre. The difference in thickness also helps explain why most of the mare basalt lavas are confined to the near side of the Moon. On the near side of the Moon the lavas would have reached the surface more easily.
Seismometers placed on the Moon by the Apollo astronauts found moonquakes occur on the Moon on a fairly regular cycle of about 2 weeks. They apparently result from the tidal stresses induced in the Moon as it rotates about Earth. Most moonquakes measure less than three on the Richter scale.
Below the crust is the Moon’s mantle. The Moon’s mantle probably makes up most of its interior. Unlike the Earth’s mantle however, the Moon’s mantle is only partially molten.
The mantle is solid down to a depth of about 800–1000 km. The composition of the upper mantle may be deduced from the composition of the mare lavas, which came from these regions. Below about 1000 km the mantle becomes partially molten. Evidence for this came mainly from seismic data collected when a large meteorite weighing about one tonne hit the far side of the Moon in July 1972. At the centre of the Moon there may be a small iron-rich core perhaps only 800 km in diameter, but its existence is uncertain.
Analysis of Moon rocks shows no evidence for formation in a different part of the solar system from Earth. There is some evidence to support the theory that the Moon may have been once part of the Earth. The bulk density of the Moon is close to the silicate mantle of the Earth; however the bulk composition of the Moon is different to that of Earth’s mantle. The Moon as a whole contains a higher proportion of iron and a lower proportion of magnesium than the Earth’s mantle. The Moon also has a lower proportion of lead but a higher proportion of calcium, aluminium and uranium than Earth.
Information from the Moon rocks support an ‘impact theory’ for the formation of the Moon. In this theory a large object, about the size of Mars, hit Earth in its first 100 million years of life. This collision literally ejected a lot of rock material from the Earth’s surface into orbit forming a ‘debris ring’. This ring gradually condensed into the Moon. Such an impact could also have tipped the Earth off its axis and so created the seasons.
The Surface
There are two main types of terrain on the Moon and these can be identified by the naked eye—the heavily cratered and very old highlands or terrae, and the relatively smooth and younger plains or maria (singular mare). Most of the surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. This layer ranges in thickness from 1 to 20 m. Unlike Earth’s soil, which has decayed biological matter in it, the Moon’s regolith does not have any biological matter.
The maria make up about 16 % of the Moon’s surface and are sometimes called ‘seas’ even though they contain no water (mare means ‘sea’ in Latin). Maria are huge impact basins that have been covered by molten lava. Most maria exist on the side of the Moon facing Earth. The more prominent mare are Mare Tranquillitatis (Sea of Tranquillity), Mare Nubium (Sea of Clouds), Mare Nectaris (Sea of Nectar), and Mare Serenitatis (Sea of Serenity). The largest mare, Mare Imbrium (Sea of Showers) is circular and measures 1100 km in diameter. Like most maria, it is 2–5 km below the average lunar elevation.
Rocks brought back to Earth from the maria are solidified lava (mainly basalt), which suggest the Moon’s surface was once molten. These rocks have a composition similar to those found in volcanic rocks on Hawaii or Iceland—they contain heavy elements like iron, manganese and titanium. The molten lava has come from inside the Moon and has risen to the surface through large impact fractures in the crust.
There is also some tectonic activity in the maria caused by the weight of basalts pushing on the crust. At the edges of the maria, the basalts are stretched, causing fracturing and faulting. In the interior of maria, the basalts are compressed, resulting in folding that produces wrinkle ridges. Most mare also contain small craters and occasional cracks (lava tunnels or channels) called rilles. In the highlands, tectonic activity has produced small
scarps.
Lunar probes have shown that the far side of the Moon contains one prominent mare, Mare Moscoviense, and is heavily cratered. The cratered area on the far side is 4–5 km above the average lunar elevation.
The Moon’s surface is covered with meteorite impact craters that vary in size from tiny pits to huge craters hundreds of kilometres in diameter. Virtually all the craters are round and the result of meteorite impact. Some of the craters have rays or streaks extending outwards from their centre, while others have raised peaks at their centre. These peaks occur because the impact compresses the crater floor so much that afterwards the crater rebounds and pushes the peak upwards. As the peak goes up, the crater walls collapse and form terraces. One of the most striking craters with a central peak is Copernicus (see Fig. 6.10). Copernicus crater is about 92 km across and 800 million years old. Rays are often formed when material is ejected and scattered across the surface during large impacts. The most striking crater with rays is Tycho formed about 109 million years ago (see top crater in Fig. 6.5).
Fig. 6.9The Apennines are the largest mountain range on the Moon. Notice the wrinkle ridges and the large crater Archimedes with the lava flooded bottom. (Credit: J. Wilkinson)
Fig. 6.10Copernicus crater (large crater on left) and Kepler crater (shaded one on right) (Credit: J. Wilkinson).
The Solar System in Close-Up Page 11