The Earth and the Moon

The third planet from the Sun, the Earth, and its satellite, the Moon, form almost a double planet. The relative size of the Moon is larger than that of any other satellite, excluding the moon of Pluto. Usually satellites are much smaller than their parent planets.

The Earth is a unique body, since a considerable amount of free water is found on its surface. This is possible only because the temperature is above the freezing point and below the boiling point of water and the atmosphere is thick enough. The Earth is also the only planet where life is known to exist. (Whether it is intelligent or not is yet to be resolved...). The moderate temperature and the water are essential for terrestrial life, although some life forms can be found in extreme conditions.

The diameter of the Earth is 12,000 km. At the centre, there is an iron-nickel core where the temperature is



Moon Temperature With Depth

Depth [km]

Fig. 7.27. Internal structure of the Earth. The speed of the seismic waves, density, pressure, and temperature are shown as a function of depth

Depth [km]

Fig. 7.27. Internal structure of the Earth. The speed of the seismic waves, density, pressure, and temperature are shown as a function of depth

5000 K, the pressure 3 x 1011 Nm 2 and the density 12,000 kg m-3 (Fig. 7.27).

The core is divided into two layers, inner and outer core. The inner core, below 5150 km comprises only of 1.7% of the mass of the Earth. It is solid because of high pressure. The nonexistence of the seismic transverse S waves below a depth of 2890 km indicates that the outer core is molten. However, the speed of the longitudinal P waves change rapidly at a depth of 5150 km showing an obvious phase transition. It has been discovered that the solid inner core rotates with respect to the outer core and mantle.

The outer core comprises about 31% of the mass of the Earth. It is a hot, electrically conducting layer of liquid Fe-Ni where the convective motions take place. There are strong currents in the conductive layer that are responsible for the magnetic field.

Between the outer core and the lower mantle there is a 200 km thick transition layer. Although this D" layer is often included as a part of the lower mantle, seismic discontinuities suggest that it might differ chemically from the lower mantle.

A silicate mantle extends from 2890 km upward up to a depth of few tens of kilometres. The part below 650 km is often identified as the lower mantle. It contains about 49% of the mass and is composed mainly of silicon, magnesium, and oxygen but some iron, calcium, and aluminium may also exist. The major minerals are olivine (Mg, Fe)2SiO4 and pyroxene (Mg, Fe)SiO3. Under pressure the material behaves like a viscous liquid or an amorphous medium, resulting in slow vertical flows.

Between the lower and upper mantle there is a 250 km thick transition region or mesosphere. It is the source of basaltic magmas and is rich in calcium and aluminium. The upper mantle, between some tens of kilometres down to 400 km contains about 10% of the mass. Part of the upper mantle, called the asthenosphere, might be partially molten.

A thin crust floats on the mantle. The thickness of the crust is only 10-70 km; it is thickest below high mountain ranges such as the Himalayas and thinnest below the mid-ocean basins. The seismic discontinuity showing the border between the crust and mantle was discovered in 1909 by the Croatian scientist An-drija Mohorovicic, and it is now known as the Moho discontinuity.

The basaltic oceanic crust is very young, mostly less than 100 million years and nowhere more than 200 Ma. It is made through tectonic activity at the mid-ocean ridges. The continental crust is mainly composed of crystalline rocks that are dominated by quartz (SiO2) and feldspars (metal-poor silicates). Because the continental crust is lighter than the oceanic crust (average densities are about 2700 kg m-3 and 3000 kg m-3, respectively), the continents are floating on top of other layers, and currently they are neither created nor destroyed.

The lithosphere is the rigid outer part of the Earth (crust and the topmost part of the upper mantle). Below that is the partially molten asthenosphere where the damping of seismic waves is stronger than in the rigid lithosphere.

The lithosphere is not a single rigid and seamless layer; instead it is divided into more than 20 individual plates. The plate tectonics ("continental drift") is powered by the motion of the material in the mantle. New material is flowing up at the mid-ocean ridges, pushing the tectonic plates apart. New oceanic crust is generated at the rate of 17 km3 per year. The Earth is the only planet that shows any large-scale tectonic activity. The history of the motion can be studied by using e. g. the paleomagnetic data of magnetic orientation of crystallised rocks.

At the end of the Precambrian era, about 700 million years ago, more than half of the continents were together forming the continent known as Gondwana, containing Africa, South America, Australia and Antarctica. About 350 million years ago Gondwana was on the South Pole but it moved toward the equator before the final breakup. Mutual collisions formed new mountains and finally in the beginning of the Mesozoic era, about 200 million years ago, all the continents were joined into one supercontinent, Pangaea.

Quite soon the flow pattern in the mantle changed and the Pangaea broke up. The Atlantic Ocean is still growing and new material is flowing up at the mid-Atlantic ridge. North America is drifting away from Europe at the rate of a few centimetres per year (your fingernails are growing at the same speed). At the same time, parts of the Pacific oceanic plate are disappearing below other plates. When an oceanic crust is pushed below a continental crust, a zone of active volcanoes is created. The earthquakes in the subduction zones can even originate 600 km below the surface. In the mid-ocean ridges, the depth is only tens of kilometres (Fig. 7.28).

Mountains are formed when two plates collide. The push of the African plate toward the Eurasian plate formed the Alps about 45 million years ago. The collision of the Indian plate created the Himalayas some 40 million years ago, and they are still growing.

Most of the surface is covered with water which condensed from the water vapour released in volcanic eruptions. The primordial atmosphere of the Earth was

5 cm/year

Fig. 7.28. The tectonic plates. The dots on the map indicate with permanent GPS (Global Positioning System) tracking the location of earthquakes with magnitudes greater than 5 stations. The velocity scale is shown at lower left in the years 1980-1989. Arrows show the velocities observed very different from the modern one; there was, for example, no oxygen. When organic chemical processes started in the oceans more than 2 x 109 years ago, the amount of oxygen rapidly increased (and was poison to the first forms of life!). The original carbon dioxide is now mainly concentrated in carbonate rocks, such as limestone, and the methane was dissociated by solar UV radiation.

The Earth's main atmospheric constituents are nitrogen (77% by volume) and oxygen (21%). Other gases, such as argon, carbon dioxide, and water vapour are present in minor amounts. The chemical composition is unchanged in the lower part of the atmosphere, called the troposphere. Most of the climatic phenomena occur in the troposphere, which reaches up to 8-10 km. The height of the layer is variable, being lowest at the poles, and highest at the equator, where it can extend up to 18 km.

The layer above the troposphere is the stratosphere, extending up to 60 km. The boundary between the troposphere and the stratosphere is called the tropopause. In the troposphere, the temperature decreases 5-7 K/km, but in the stratosphere it begins to rise, due to the absorption of solar radiation by carbon dioxide, water vapour and ozone. The ozone layer, which shields the Earth from the solar UV radiation, is at a height of 20-25 km.

A total of 99% of air is in the troposphere and stratosphere. The stratopause at a height of 50-60 km separates the stratosphere from the mesosphere.

The mesosphere extends up to 85 km. In this layer, the temperature decreases again, reaching the minimum of about -90 °C at the height of 80-90 km in the mesopause. Chemicals in the mesosphere are mostly in an excited state, as they absorb energy from the Sun.

Above the mesopause is the thermosphere that extends up to 500 kilometres. The temperatures increases with altitude and can be above 1200 °C at the height of 500 km. The gas is in the form of a fully ionised plasma. Therefore, the layer above the mesopause is also called the ionosphere.

The density of air below a height of 150 km is high enough to cause colliding meteoroids to burn into ashes due to friction. It also plays an important role in radio

7. The Solar System


Fig. 7.29. Hurricane Katrina in the Gulf of Mexico, before the GOES-12 weather satellite on August 28, 2005. Compare devastating the city of New Orleans. It was photographed from this to the Great Red Spot of Jupiter in Fig. 7.57. (NOAA)

communications, since radio waves are reflected by the ionosphere. Auroras are phenomena of the upper part of the ionosphere.

The thermosphere goes over into the exosphere at about 500 km. There the air pressure is much lower than in the best laboratory vacuums.

The magnetic field of the Earth is generated by flows in its core. The field is almost a dipole but there are considerable local and temporal variations. The mean field strength close to the equator is 3.1 x 10-5 Tesla (0.31 Gauss). The dipole is tilted 11° with respect to the Earth's axis, but the direction gradually changes with time. Moreover, the magnetic north and south poles have exchanged places several times during the past million years. More details are explained in Sect. 7.6 and in Figs. 7.16, 7.17, 7.18, and in Table 7.2.

The Moon. Our nearest neighbour in space is the Moon. Dark and light areas are visible even with the naked eye. For historical reasons, the former are called seas or maria (from Latin, mare, sea, pl. maria). The lighter areas are uplands but the maria have nothing in common with terrestrial seas, since there is no water on the Moon. Numerous craters, all meteorite impacts, can be seen, even with binoculars or a small telescope (Fig. 7.30). The lack of atmosphere, volcanism, and tectonic activity help to preserve these formations.

Fig. 7.30. A map of the Lunar surface, composed of images as compared to the almost complete absence of the maria at taken by the Clementine space probe in 1994. Note the large the Lunar far side. (US Naval Observatory) areas of maria in the Lunar near side, at the centre of the figure,

The Moon is the best-known body after the Earth. The first man landed on the Moon in 1969 during the Apollo 11 flight. A total of over 2000 samples, weighing 382 kg, were collected during the six Apollo flights (Fig. 7.31). Moreover, the unmanned SovietLuna spacecraft collected and returned about 310 grams of Lunar soil. Instruments placed on the Moon by the Apollo astronauts operated as long as eight years. These included seismometers, which detected moonquakes and meteorite impacts, and passive laser reflectors which made exact Earth-Moon distance measurements possible. The reflectors are still used for Lunar laser ranging (LLR) measurements.

Seismometric and gravimetric measurements have supplied basic information on the internal structure of the Moon. Moonquakes take place at a depth of 800-1000 km, considerably deeper than earthquakes, and they are also much weaker than on the Earth. Most of the quakes occur at the boundary of the solid mantle, the lithosphere, and the asthenosphere (Fig. 7.32). The transversal S waves cannot penetrate the asthenosphere, indicating that it is at least partially molten. Tidal forces may generate at least some of the moonquakes because most of them occur close to perigee or apogee.

Lunar orbiters have observed local mass concentrations, mascons, beneath the maria. These are large basaltic blocks, formed after the huge impacts which produced the maria. The craters were filled by lava flows during the next billion years or so in several phases. This can be seen, e.g. in the area of Mare Imbrium. Large maria were formed about 4 x 109 years ago when meteorite bombardment was much heavier than today. The last 3 x 109 years have been quite peaceful, without any major events.

The centre of mass is not at the geometric centre of the Moon but about 2.5 km away due to the 20-30 km thick basaltic plates below the large maria. Moreover, the thickness of the crust varies, being the thickest at the far side of the Moon, about 100 km. On the near side the thickness of the crust is about 60 km.

The mean density of the Moon is 3400 kg m-3, which is comparable to that of basaltic lavas on the Earth. The Moon is covered with a layer of soil with scattered rocks, regolith. It consists of the debris blasted out by meteorite impacts. The original surface is nowhere visible. The thickness of the regolith is estimated to be at least tens of metres. A special type of rock, breccia, which is a fragment of different rocks compacted and welded

7. The Solar System


Fig. 7.31. Apollo 17 astronaut Harrison Schmitt on the Moon in 1972. (NASA)

together by meteor impacts, is found everywhere on the Moon.

The maria are mostly composed of dark basalts, which form from rapid cooling of massive lava flows. The highlands are largely composed of anorthosite, an igneous rock that forms when lava cools more slowly than in the case of basalts. This implies that the rocks of the maria and highlands cooled at different rates from the molten state and were formed under different conditions.

Data returned by the Lunar Prospector and Clementine spacecraft indicated that water ice is present at both the north and south lunar poles. Data indicates that there may be nearly pure water ice buried beneath the dry re-golith. The ice is concentrated at the bottoms of deep valleys and craters that are in a permanent shadow where the temperature is below 100 K.

The Moon has no global magnetic field. Some of the rocks have remanent magnetism indicating a possible global magnetic field early in the Moon's history. Without the atmosphere and magnetic field, the solar wind can reach the Moon's surface directly. The ions from the solar wind have embedded in the regolith. Thus samples returned by the Apollo

Fig. 7.32. Structure of the Moon. The height differences of the surface are strongly exaggerated

missions proved valuable in studies of the solar wind.

The origin of the Moon is still uncertain; it has, however, not been torn off from the Earth at the Pacific Ocean, as is sometimes believed. The Pacific is less than 200 million years old and formed as a result of continental drift. Also, the chemical composition of the lunar soil is different from that of terrestrial material.

Recently it was suggested that the Moon was formed in the early stages of the formation of the Earth, when a lot of protoplanet embryos were orbiting the Sun. An off-axis collision of a Mars-size body resulted in ejection of a large amount of debris, a part of which then accreted to form the Moon. Differences in chemical compositions of the modern Earth and the Moon can be explained with the theory, as well as the orientation and evolution of the Moon's orbit and the Earth's relatively fast spin rate.

* Atmospheric Phenomena

The best-known atmospheric phenomenon is the rainbow, which is due to the refraction of light from water droplets. The radius of the arc of the rainbow is about 41° and the width, 1.7°. The centre of the arc is opposite the Sun (or any other source of light). When the light is refracted inside a water droplet, it is divided into a spectrum, where the red colour is at the outer edge and blue, at the inner edge. Light can be reflected twice inside the droplet, resulting in a secondary rainbow outside the primary one. The colours of the secondary rainbow are in reversed order and its radius is 52°. A rainbow caused by the Moon is usually very

Left: A typical halo; Right: Auroras (Photos P. Parviainen)

weak and colourless, since the human eye is incapable of resolving colours of a dim object.

A halo results when the solar or lunar light is reflected from atmospheric ice crystals. The most common halo is a 22° arc or circle around the Sun or the Moon. Usually the halo is white, but occasionally even bright colours can be seen. Another common form is the side lobes which are at the same height as the Sun but at a distance of 22° from it. All other forms of halo are less common. The best "weather" for halos is when there are cirrostratus or cirrus clouds or an icy fog in the sky.

Noctilucent clouds are thin formations of cloud, at a height of approximately 80 km. The clouds contain particles, which are less than one micron in diameter, and become visible only when the Sun (which is below the horizon) illuminates the clouds. Most favourable conditions are at the northern latitudes during the summer nights when the Sun is only a few degrees below the horizon.

The night sky is never absolutely dark. One reason (in addition to light pollution) is the airglow or light emitted by excited atmospheric molecules. Most of the radiation is in the infrared domain, but e. g. the forbidden line of oxygen at 558 nm, has also been detected.

The same greenish oxygen line is clearly seen in auroras, which are formed at a height of 80-300 km. Auroras can be seen mainly from relatively high northern or southern latitudes because the Earth's magnetic field forces charged particles, coming from the Sun, close toward the magnetic poles. Alaska and northern Scandinavia are the best places to observe auroras. Occasionally, auroras can be seen as far south as 40°. They are usually greenish or yellow-green, but red auroras have been observed, too. They most commonly appear as arcs, which are often dim and motionless, or as belts, which are more active and may contain rapidly varying vertical rays.

Meteors (also called shooting stars although they have nothing to do with stars) are small grains of sand, a few micrograms or grams in weight, which hit the Earth's atmosphere. Due to friction, the body heats up and starts to glow at a height of 100 km. Some 20-40 km lower, the whole grain has burnt to ashes. The duration of a typical meteor is less than a second. The brightest meteors are called bolides (magnitude smaller than about -2). Even larger particles may sur vive down to the Earth. Meteors are further discussed in Sect. 7.18.

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