Man and His Cosmic Environment

The Cosmos (Universe) The word “cosmos” is used to refer to the universe, regarded as one orderly system with a structure, whose parts are linked together in an orderly manner. Both philosophy and astronomy study the structure of the universe. Cosmology is the area of study (in both disciplines) concerned with the structure of the universe. Cosmology therefore is,

1. The branch of philosophy, which studies the structure of the universe. It deals with its origin and general structure, its parts, elements, laws. It focuses on such characteristics of the universe as space, time, causality and freedom.

2. Also the branch of astronomy, which deals with the general structure and evolution of the universe. It studies the composition extent and origins of the universe and its various components. The branch of philosophy which deals with the evolution and origin of the universe is called cosmogony.

The Cosmic Environment

The cosmic environment is the entire universe in which we live, especially those of its aspects or parts that are connected with human life, survival and interests. The structure of the cosmos as revealed by astronomy show that it includes the earth (and other planets), planetary satellites, the sun and other stars, the groups of stars called Galaxies, etc. A remarkable fact about the universe we find ourselves in is that it is capable of sustaining a planet like the Earth and the complex chemistry of life. On this planet we have achieved an understanding of the vast universe we inhabit, but this has been achieved only in the past century. Man depends on air, heat, water and other natural resources from the entire cosmos, particularly his own earth and the sun for survival.

The Solar System

The solar system consists of the Sun, the major planetary bodies plus the minor planets (about 2,000 in number) called asteroids. This is Earth’s cosmic home, the place of its origin and development. The sun is a star; one of the innumerable stars in the universe. The solar system is held together by the gravitational force of the sun. The nine planets (which revolve around the sun in their different orbits) according to their proximity to the sun are;

1. Mercury (no satellite) — smallest planet.

2. Venus (no satellite) — most brilliant planet in the solar system.

3. Earth (one satellite i.e. the moon)

4. Mars (2 satellites i.e. phobos and deimos)

5. Jupiter (12 satellites) — largest planet in the solar system.

6. Saturn (10 satellites) — second largest planet

7. Uranus (5 satellites)

8. Neptune (2 satellites)

9. Pluto (no satellite) — the outermost planet in the entire solar system.

Some scientists believe that the planets were formed from the sun, from which they broke off as gaseous elements and gradually became solid bodies in space. Some others believe that the planets in the solar system were created at the same time and from the same general material. The massive Sun, a star that generates heat by nuclear fusion, is the center of the system. Because of the Sun’s vast gravitational influence, all of the planets orbit around it. As seen from above their north poles, the planets move counterclockwise about the Sun in slightly elliptical orbits. Moreover, all orbit in the same plane as the Sun’s equator, except Pluto. A simple analogy may help Saturn Jupiter Venus Moon Earth Mercury Uranus Neptune Pluto Mars convey the size and structure of the solar system. If the Sun were the size of an orange, Earth would be roughly the size of a grain of sand orbiting 9 m (30 ft) away. Jupiter would be the size of a pea revolving 60 m (200 ft) away. Pluto would be like a grain of silt 10 city blocks away. The nearest star would be the size of another orange more than 1600 km (1000 mi) away. Until recently, the planets and their moons were mute astronomical bodies, only small specks viewed in a telescope. But today, they are new worlds as real as our own, because we have landed on their surfaces and studied them with remotely controlled probes. One of the most fundamental facts revealed by our exploration of the solar system is that the sizes and compositions of the planets vary systematically with distance from the Sun. The inner planets include Mercury and the planet like Moon, with their cratered surfaces; Venus, with its extremely hot, thick atmosphere of carbon dioxide and numerous volcanoes; Earth, with cool blue seas, swirling clouds and multicolored lands; and Mars, with huge canyons, giant extinct volcanoes, frigid polar ice caps, and long, dry river beds. Among the inner planets (Mercury, Venus, Earth, the Moon, and Mars), Earth is unique because of its size and distance from the Sun. It is large enough to develop and retain an atmosphere and a hydrosphere. Temperature ranges are moderate, such that water can exist on its surface as liquid, solid, and gas. The large outer planets—Jupiter, Saturn, Uranus and Neptune—are giant balls of gas, with majestic rings and dozens of small satellites composed mostly of ice. The most distant planet, Pluto is small and similar to these icy moons. Indeed, water ice is the most common “rock” in the outer solar system. The density of a planet or moon reveals these dramatic differences in composition. (Density is a measure of mass per unit volume: g/cm3). For example, the densities of the rocky inner planets are quite high (over 3 g/cm3) compared to the gas- and ice-rich outer planets which have densities less than about 1.6 g/cm3. Our best evidence tells us that Earth formed, along with the rest of the solar system, about 4.6 billion years ago. Nonetheless, only the inner planets are even vaguely like the Earth. The sun itself is in motion in space and at the same time the planets revolve around it. The planets and their satellites are held in their relative positions around the sun by the gravitational power of the sun. This magnetic power holds them together as a common system. The powerful energy from the sun, called solar energy is responsible for all the energy and the light in the whole solar system.

Galaxies

 The sun, plus its nine planets and their satellites form the solar system. The solar system along with other stars and their satellites form a collection or group called the Galaxy. Galaxies are a large system of stars held together by mutual gravitation and isolated from similar systems by vast regions of space. Precisely, our Galaxy is called the Milky Way Galaxy. The Milky Way Galaxy contains about 100 billion stars. There are several other galaxies (or nebulae) with different shapes in the universe. It was only as recently as the 1920s that we began to get a glimpse of the vastness of the universe of galaxies. The discovery of the microwave background radiation and the realization that the universe began in a hot Big Bang dates back only to 1965. Interestingly, the true origin of the universe and it galaxies is still unknown. And it is only since the beginning of the new millennium that cosmology has become a precision science, with a strong consensus emerging about what kind of universe we inhabit.

Measurement of distance in space

The distances between celestial objects, especially stars and galaxies, are so great that we can’t express them with ordinary numerical notation. The unit of measurement which is used to measure such astronomical distances is called a light year. A light year is therefore, a unit of measurement of distant objects such as stellar (i.e. star) distances. A light year is the distance traversed by light in one mean solar year. One light year is about 9 zillion km (5,880,000,000,000 miles) (abbreviated as Ltyr). So some distances are so great that they look clumsy when written in plain figures. Hence, the use of Lt-year (or light year) as a unit. One set of such stellar objects is Quasirs or Black Holes which are powerful sources of radio energy. Hence, they are called Quasir-Stellar Radio Source. Some are as far as 14 billion light years.

THE EARTH

The earth on which man lives is a planet or satellite of the sun. We do not as yet have evidence of human habitation in any other celestial body/planet. The earth is one of the nine planets, which rotate on their axis and at the same time revolve around the sun. The earth is spherical with continuous motion within it and in space. The earth is also made up of several spheres, layers or zones.

Rotation of the earth

It takes the earth approximately 24 hours (around the equator) to rotate i.e. make a complete 360o turn on its axis. This rotation gives rise to day and night as the earth faces or turns away from the sun. The earth derives its light from the sun. When it faces the sun, it is day for that part of the earth, when it turns away, it is night for that part of the earth. From this explanation, it is easy to deduce that when it is day in one part of the earth, it will be night in another part. Revolution of the earth

It takes the earth about 365 days (i.e. one year) to complete one revolution around the sun. The sun is at the centre around which the earth and all the other planets (or satellites of the sun) revolve. The earth is approximately 149,668,992 km (93 million miles) away from the sun and it is approximately 6,437 km (4,000 miles) in radius. Because of its spherical shape and its flattered shape around the poles, its diameter is approximately 21 km (13 miles) shorter at the poles than the equatorial radius. The equatorial diameter is approximately 63,779 km (39,630.5 miles), while the polar diameter is approximately 12,713 km (7,900 miles). The earth has a surface area of approximately 316,316,563 square km (196,550,000 square miles). Approximately 89,318,592 sq. km (55,500,000 sq. mile) of the earth’s surface is land while the rest is water. The greatest known height is Mount Everest (located in South Asia between Nepal and Tibet), which is 29,028 feet high while the greatest known oceanic depth is the Swire deep, which is 34,430 feet below sea level.

Spheres of the earth

The major spheres of the earth include the biosphere, hydrosphere, atmosphere and the lithosphere.

The Biosphere

The biosphere refers to the part of the earth where life exists. This includes all parts of the earth’s crust, water and atmosphere where living organisms can subsist. It includes the forests, grasslands, and familiar animals of the land, together with the numerous creatures that inhabit the sea and atmosphere. Microorganisms such as bacteria are too small to be seen, but they are probably the most common form of life in the biosphere. As a terrestrial covering, the biosphere is discontinuous and irregular; it is an interwoven web of life existing within and reacting with the atmosphere, hydrosphere, and lithosphere. It consists of more than 1.6 million described species and perhaps as many as 3 million more not yet described. Each species lives within its own limited environmental setting. Almost the entire biosphere exists in a narrow zone extending from the depth to which sunlight penetrates the oceans (about 200 m) to the snow line in the tropical and subtropical mountain ranges (about 6000 m above sea level). Certainly one of the most interesting questions about the biosphere concerns the number and variety of organisms that compose it. Surprisingly, the truth is that no one knows the answer. Despite more than 250 years of systematic research, estimates of the total number of plant and animal species vary from 3 million to more than 30 million. Of this number, only 1.6 million species have been recorded. The diversity is stranger than you may think. Insects account for more than one-half of all known species, whereas there are only 4000 species of mammals, or about 0.025% of all species. Observation shows that there are more species of small animals than of large ones. The smallest living creatures—those invisible to the unaided eye, such as protozoa, bacteria, and viruses— contribute greatly to the variety of species. The biosphere is a truly remarkable part of Earth’s systems. The main factors controlling the distribution of life on our planet are temperature, pressure, and chemistry of the local environment. However, the range of environmental conditions in which life is possible is truly amazing, especially the range of environments in which microorganisms can exist.

The Hydrosphere

The hydrosphere is the total mass of water on the surface of our planet. Water covers about 71% of the surface. About 98% of this water is in the oceans. Only 2% is in streams, lakes, groundwater, and glaciers. Thus, it is for good reason that Earth has been called “the water planet.” It has been estimated that if all the irregularities of Earth’s surface were smoothed out to form a perfect sphere, a global ocean would cover Earth to a depth of 2.25 km. Again, it is this great mass of water that makes Earth unique. Water permitted life to evolve and flourish; every inhabitant on Earth is directly or indirectly controlled by it. All of Earth’s weather patterns, climate, rainfall and even the amount of carbon dioxide in the atmosphere are influenced by the water in the oceans. The hydrosphere is in constant motion; water evaporates from the oceans and moves through the atmosphere, precipitating as rain and snow and returning to the sea in rivers, glaciers, and groundwater. As water moves over Earth’s surface, it erodes and transports weathered rock material and deposits it. These actions constantly modify Earth’s landscape. Many of Earth’s distinctive surface features are formed by action of the hydrosphere.
The Atmosphere

This refers to the gaseous envelope (or air) surrounding the earth. It is of mixed gases consisting of; Nitrogen (78.09%), Oxygen (20.95 %), Argon (0.93%), Carbon Dioxide (0.035%) and water vapor. The atmosphere is particularly significant because it moves easily and is constantly interacting with the ocean and land. It plays a part in the evolution of most features of the landscape and is essential for life. The atmosphere’s circulation patterns are shown by the shape and orientation of the clouds. At first glance, the patterns may appear confusing, but upon close examination we find that they are well organized. If we ignore the details of local weather systems, the global atmospheric circulation becomes apparent. Solar heat, the driving force of atmospheric circulation, is greatest in the equatorial regions. The heat causes water in the oceans to evaporate, and the heat makes the moist air less dense, causing it to rise. The warm, humid air forms an equatorial belt of spotty clouds, bordered on the north and south by zones that are cloud-free, where air descends. To the north and south, cyclonic storm systems develop where warm air from low latitudes confronts cold air around the poles. The earliest atmosphere was much different. It was essentially oxygen-free and consisted largely of carbon dioxide and water vapor. The present carbon dioxide-poor atmosphere developed as soon as limestone began to form in the oceans, tying up the carbon dioxide. Oxygen was added to the atmosphere later, when plants evolved. As a result of photosynthesis, plants extracted carbon dioxide from the primitive atmosphere and expelled oxygen into it. Thus, the oxygen in the atmosphere is and was produced by life. The Atmosphere is divided into layers according to major changes in temperature. Gravity pushes the layers of air down on the earth's surface. This push is called air pressure. The atmosphere is made up of several layers or zones based on temperature.

Troposphere: The troposphere is the lowest layer of Earth's atmosphere. Most of the mass (about 75- 80%) of the atmosphere is in the troposphere. Most types of clouds are found in the troposphere, and almost all weather occurs within this layer. The bottom of the troposphere is at Earth's surface. The troposphere extends upward to about 10 km (6.2 miles or about 33,000 feet) above sea level. The height of the top of the troposphere varies with latitude (it is lowest over the poles and highest at the equator) and by season (it is lower in winter and higher in summer). It can be as high as 20 km (12 miles or 65,000 feet) near the equator, and as low as 7 km (4 miles or 23,000 feet) over the poles in winter. Air is warmest at the bottom of the troposphere near ground level. Air gets colder as one rises through the troposphere. That's why the peaks of tall mountains can be snow-covered even in the summertime. Air pressure and the density of the air also decrease with altitude. That's why the cabins of high-flying jet aircraft are pressurized. The layer immediately above the troposphere is called the stratosphere. The boundary between the troposphere and the stratosphere is called the "tropopause".

Stratosphere: The stratosphere is the second layer of the atmosphere as you go upward. The troposphere, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the mesosphere. The bottom of the stratosphere is around 10 km (6.2 miles or about 33,000 feet) above the ground at middle latitudes. The top of the stratosphere occurs at an altitude of 50 km (31 miles). The height of the bottom of the stratosphere varies with latitude and with the seasons. The lower boundary of the stratosphere can be as high as 20 km (12 miles or 65,000 feet) near the equator and as low as 7 km (4 miles or 23,000 feet) at the poles in winter. The lower boundary of the stratosphere is called the tropopause; the upper boundary is called the stratopause. Ozone, an unusual type of oxygen molecule that is relatively abundant in the stratosphere, heats this layer as it absorbs energy from incoming ultraviolet radiation from the Sun. Temperatures rise as one moves upward through the stratosphere. This is exactly the opposite of the behavior in the troposphere in which we live, where temperatures drop with increasing altitude. Because of this temperature stratification, there is little convection and mixing in the stratosphere, so the layers of air there are quite stable. Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below. The stratosphere is very dry; air there contains little water vapor. Because of this, few clouds are found in this layer; almost all clouds occur in the lower, more humid troposphere. Polar stratospheric clouds (PSCs) are the exception. PSCs appear in the lower stratosphere near the poles in winter. They are found at altitudes of 15 to 25 km (9.3 to 15.5 miles) and form only when temperatures at those heights dip below -78° C. They appear to help cause the formation of the infamous holes in the ozone layer by "encouraging" certain chemical reactions that destroy ozone. PSCs are also called nacreous clouds. Air is roughly a thousand times thinner at the top of the stratosphere than it is at sea level. Because of this, jet aircraft and weather balloons reach their maximum operational altitudes within the stratosphere. Due to the lack of vertical convection in the stratosphere, materials that get into the stratosphere can stay there for long times. Such is the case for the ozone-destroying chemicals called CFCs (chlorofluorocarbons). Large volcanic eruptions and major meteorite impacts can fling aerosol particles up into the stratosphere where they may linger for months or years, sometimes altering Earth's global climate. Rocket launches inject exhaust gases into the stratosphere, producing uncertain consequences. Various types of waves and tides in the atmosphere influence the stratosphere. Some of these waves and tides carry energy from the troposphere upward into the stratosphere; others convey energy from the stratosphere up into the mesosphere. The waves and tides influence the flows of air in the stratosphere and can also cause regional heating of this layer of the atmosphere. A rare type of electrical discharge, somewhat similar to lightning, occurs in the stratosphere. These "blue jets" appear above thunderstorms, and extend from the bottom of the stratosphere up to altitudes of 40 or 50 km (25 to 31 miles).

Mesosphere: The mesosphere as a layer of Earth's atmosphere is directly above the stratosphere and below the thermosphere. It extends from about 50 to 85 km (31 to 53 miles) above our planet. Temperature decreases with height throughout the mesosphere. The coldest temperatures in Earth's atmosphere, about -90° C (-130° F), are found near the top of this layer. The boundary between the mesosphere and the thermosphere above it is called the mesopause. At the bottom of the mesosphere is the stratopause, the boundary between the mesosphere and the stratosphere below. The mesosphere is difficult to study, so less is known about this layer of the atmosphere than other layers. Weather balloons and other aircraft cannot fly high enough to reach the mesosphere. Satellites orbit above the mesosphere and cannot directly measure traits of this layer. Scientists use instruments on sounding rockets to sample the mesosphere directly, but such flights are brief and infrequent. Since it is difficult to take measurements of the mesosphere directly using instruments, much about the mesosphere is still mysterious. Most meteors vaporize in the mesosphere. Some material from meteors lingers in the mesosphere, causing this layer to have a relatively high concentration of iron and other metal atoms. Very strange, high altitude clouds called "noctilucent clouds" or "polar mesospheric clouds" sometime form in the mesosphere near the poles. These peculiar clouds form much, much higher up than other types of clouds. Odd electrical discharges akin to lightning, called "sprites" and "ELVES", occasionally appear in the mesosphere dozens of kilometers (miles) above thunderclouds in the troposphere below. The stratosphere and mesosphere together are sometimes referred to as the middle atmosphere. At the mesopause (the top of the mesosphere) and below, gases made of different types of atoms and molecules are thoroughly mixed together by turbulence in the atmosphere. Above the mesosphere, in the thermosphere and beyond, gas particles collide so infrequently that the gases become somewhat separated based on the types of chemical elements they contain. Various types of waves and tides in the atmosphere influence the mesosphere. These waves and tides carry energy from the troposphere and the stratosphere upward into the mesosphere, driving most of its global circulation.

Thermosphere: The thermosphere is directly above the mesosphere and below the exosphere. It extends from about 90 km (56 miles) to between 500 and 1,000 km (311 to 621 miles) above our planet. Temperatures climb sharply in the lower thermosphere (below 200 to 300 km altitude), then level off and hold fairly steady with increasing altitude above that height. Solar activity strongly influences temperature in the thermosphere. The thermosphere is typically about 200° C (360° F) hotter in the daytime than at night, and roughly 500° C (900° F) hotter when the Sun is very active than at other times. Temperatures in the upper thermosphere can range from about 500° C (932° F) to 2,000° C (3,632° F) or higher. The boundary between the thermosphere and the exosphere above it is called the thermopause. At the bottom of the thermosphere is the mesopause, the boundary between the thermosphere and the mesosphere below. Although the thermosphere is considered part of Earth's atmosphere, the air density is so low in this layer that most of the thermosphere is what we normally think of as outer space. In fact, the most common definition says that space begins at an altitude of 100 km (62 miles), slightly above the mesopause at the bottom of the thermosphere. The space shuttle and the International Space Station both orbit Earth within the thermosphere! Below the thermosphere, gases made of different types of atoms and molecules are thoroughly mixed together by turbulence in the atmosphere. Air in the lower atmosphere is mainly composed of the familiar blend of about 80% nitrogen molecules (N2) and about 20% oxygen molecules (O2). In the thermosphere and above, gas particles collide so infrequently that the gases become somewhat separated based on the types of chemical elements they contain. Energetic ultraviolet and X-ray photons from the Sun also break apart molecules in the thermosphere. In the upper thermosphere, atomic oxygen (O), atomic nitrogen (N), and helium (He) are the main components of air. Much of the X-ray and UV radiation from the Sun is absorbed in the thermosphere. When the Sun is very active and emitting more high energy radiation, the thermosphere gets hotter and expands or "puffs up". Because of this, the height of the top of the thermosphere (the thermopause) varies. The thermopause is found at an altitude between 500 km and 1,000 km or higher. Since many satellites orbit within the thermosphere, changes in the density of (the very, very thin) air at orbital altitudes brought on by heating and expansion of the thermosphere generates a drag force on satellites. Engineers must take this varying drag into account when calculating orbits, and satellites occasionally need to be boosted higher to offset the effects of the drag force. High-energy solar photons also tear electrons away from gas particles in the thermosphere, creating electrically-charged ions of atoms and molecules. Earth's ionosphere, composed of several regions of such ionized particles in the atmosphere, overlaps with and shares the same space with the electrically neutral thermosphere. Like the oceans, Earth's atmosphere has waves and tides within it. These waves and tides help move energy around within the atmosphere, including the thermosphere. Winds and the overall circulation in the thermosphere are largely driven by these tides and waves. Moving ions, dragged along by collisions with the electrically neutral gases, produce powerful electrical currents in some parts of the thermosphere. Finally, the aurora (the Southern and Northern Lights) primarily occur in the thermosphere. Charged particles (electrons, protons, and other ions) from space collide with atoms and molecules in the thermosphere at high latitudes, exciting them into higher energy states. Those atoms and molecules shed this excess energy by emitting photons of light, which we see as colorful auroral displays.

Ionosphere: Earth's atmosphere contains a series of regions that have a relatively large number of electrically charged atoms and molecules. As a group, these regions are collectively called the ionosphere. High-energy X-rays and ultraviolet (UV) "light" from the Sun are constantly colliding with gas molecules and atoms in Earth's upper atmosphere. Some of these collisions knock electrons free from the atoms and molecules, creating electrically charged ions (atoms or molecules with missing electrons) and free electrons. These electrically charged ions and electrons move and behave differently than normal, electrically neutral atoms and molecules. Regions with higher concentrations of ions and free electrons occur at several different altitudes and are known, as a group, as the ionosphere. There are three main regions of the ionosphere, called the D layer, the E layer, and the F layer. These regions do not have sharp boundaries, and the altitudes at which they occur vary during the course of a day and from season to season. The D region is the lowest, starting about 60 or 70 km (37 or 43 miles) above the ground and extending upward to about 90 km (56 miles). Next higher is the E region, starting at about 90 or 100 km (56 or 62 miles) up and extending to 120 or 150 km (75 or 93 miles). The uppermost part of the ionosphere, the F region, starts about 150 km (93 miles) and extends far upward, sometimes as high as 500 km (311 miles) above the surface of our home planet. The regions of the ionosphere are not considered separate layers, such as the more familiar troposphere and stratosphere. Instead, they are ionized regions embedded within the standard atmospheric layers. The D region usually forms in the upper part of the mesosphere, while the E region typically appears in the lower thermosphere and the F region is found in the upper reaches of the thermosphere. The height, fraction of ionized particles, and even the existence of the different regions of the ionosphere varies over time. The ionosphere is very different in the daytime versus night. During the day, X-rays (an UV light from the Sun) continuously provides the energy that knocks electrons free from atoms and molecules, producing a continuous supply of ions and free electrons. At the same time, some of the ions and electrons collide and re-combine to form normal, electrically neutral atoms and molecules. During the day, more ions are created than are destroyed, so the number of ions in the three regions increases. At night, the recombination process takes over in the absence of sunlight, and the number of ions drops. Over the course of most nights, the D region disappears entirely and the E region weakens as the number of ions in that layer plummets. Each morning, as solar X-rays and UV light return, the D and E regions are repopulated with ions. The highest altitude F region sticks around throughout the night, but generally splits into an upper F2 layer and a lower F1 layer during the day. Before communication via satellites became common, the operators of radio communication systems often used the ionosphere to extend the range of their transmissions. Radio waves generally travel in straight lines, so unless a tall transmission tower can "see" the top of a receiver tower, the curvature of the Earth limits the range of radio transmissions to stations that are not over the horizon. However, some frequencies of radio waves bounce or reflect off of the electrically charged particles in certain ionosphere layers. Pre-satellite radio communications often took advantage of this phenomenon, bouncing radio waves off of the "sky" to extend the range of the signals. Radio operators had to account for the constant changes in the ionosphere, particularly the shifts or disappearance of the layers between day and night, to effectively take advantage of these mirror-like reflections of radio waves. The ionosphere regions can absorb or dampen radio signals, or they can bend radio waves, as well as reflecting the signals as described above. The specific behavior depends on both the frequency of the radio signal as well as the characteristics of the ionosphere region involved. Since Global Positioning System (GPS) satellites use radio signals to determine locations, the accuracy of GPS can be severely reduced when those signals bend as they pass through ionosphere regions. Similarly, some radio communications can be disrupted if the frequency used is one that an ionosphere layer dampens or absorbs entirely, resulting in a weakened signal or even total loss of communications. Scientists constantly measure and produce computer models of the ever-changing ionosphere so that people in charge of radio communications can anticipate disruptions. Scientists use radio waves in various ways to probe and monitor the otherwise invisible ionosphere. Various radio antennas and radar systems, on the ground and on satellites, are used to monitor the constantly evolving ionosphere. Radio antennas "listen" for radio signals generated by the ionosphere itself, radar systems bounce signals of the different layers, and pairs of transmitters and receivers shoot signals through the ionosphere to determine how much those signals are dampened or redirected. Along with the daily fluctuations in the ionosphere, there are also seasonal and longer-term variations in this complex set of regions. Different latitudes warm and cool with the seasons as the intensity of sunlight varies from place to place due to the tilt of Earth's axis. Similarly, the ionosphere varies seasonally as the location of the peak intensity of solar X-rays and UV light, which drive the rate of formation of ions, moves around on the globe. Seasonal changes in the chemistry of the atmosphere also play a role, influencing the rate of recombination events which remove ions from the atmosphere. Longer term, the 11-year sunspot cycle has a strong influence on the upper reaches of the atmosphere, including the ionosphere. The brightness of the Sun, in visible light wavelengths that we can see, varies by less than 1/10th of one percent between the high point and the low point of the sunspot cycle. However, the X-ray and UV output of the Sun varies much more throughout the solar cycle, fluctuating by a factor of 10 or more. Since these X-rays and UV radiation control the rate of ion formation that produces the ionosphere, large variations in these types of radiation lead to big changes in the ion densities in the ionosphere regions. Also, large geomagnetic storms triggered by solar flares and coronal mass ejections from the Sun can create severe temporary disruptions in the ionosphere.

Exosphere: The exosphere is the uppermost region of Earth's atmosphere as it gradually fades into the vacuum of space. Air in the exosphere is extremely thin - in many ways it is almost the same as the airless void of outer space. The layer directly below the exosphere is the thermosphere; the boundary between the two is called the thermopause. The bottom of the exosphere is sometimes also referred to as the exobase. The altitude of the lower boundary of the exosphere varies. When the Sun is active around the peak of the sunspot cycle, X-rays and ultraviolet radiation from the Sun heat and "puff up" the thermosphere - raising the altitude of the thermopause to heights around 1,000 km (620 miles) above Earth's surface. When the Sun is less active during the low point of the sunspot cycle, solar radiation is less intense and the thermopause recedes to within about 500 km (310 miles) of Earth's surface. Not all scientists agree that the exosphere is really a part of the atmosphere. Some scientists consider the thermosphere the uppermost part of Earth's atmosphere, and think that the exosphere is really just part of space. However, other scientists do consider the exosphere part of our planet's atmosphere. Since the exosphere gradually fades into outer space, there is no clear upper boundary of this layer. One definition of the outermost limit of the exosphere places the uppermost edge of Earth's atmosphere around 190,000 km (120,000 miles), about halfway to the Moon. At this distance, radiation pressure from sunlight exerts more force on hydrogen atoms than does the pull of Earth's gravity. A faint glow of ultraviolet radiation scattered by hydrogen atoms in the uppermost atmosphere has been detected at heights of 100,000 km (62,000 miles) by satellites. This region of UV glow is called the geocorona. Below the exosphere, molecules and atoms of atmospheric gases constantly collide with each other. However, air in the exosphere is so thin that such collisions are very rare. Gas atoms and molecules in the exosphere move along "ballistic trajectories", reminiscent of the arcing flight of a thrown ball (or shot cannonball!) as it gradually curves back towards Earth under the pull of gravity. Most gas particles in the exosphere zoom along curved paths without ever hitting another atom or molecule, eventually arcing back down into the lower atmosphere due to the pull of gravity. However, some of the faster-moving particles don't return to Earth - they fly off into space instead! A small portion of our atmosphere "leaks" away into space each year in this way. Although the exosphere is technically part of Earth's atmosphere, in many ways it is part of outer space. Many satellites, including the International Space Station (ISS), orbit within the exosphere or below. For example, the average altitude of the ISS is about 330 km (205 miles), placing it in the thermosphere below the exosphere! Although the atmosphere is very, very thin in the thermosphere and exosphere, there is still enough air to cause a slight amount of drag force on satellites that orbit within these layers. This drag force gradually slows the spacecraft in their orbits, so that they eventually would fall out of orbit and burn up as they re-entered the atmosphere unless something is done to boost them back upwards. The ISS loses about 2 km (1.2 miles) in altitude each month to such "orbital decay", and must periodically be given an upward boost by rocket engines to keep it in orbit.

The Lithosphere

The solid, strong, and rigid outer layer of a planet is the lithosphere (“rock sphere”). The lithosphere includes the crust and the uppermost part of the mantle. Earth’s lithosphere varies greatly in thickness, from as little as 10 km in some oceanic areas to as much as 300 km in some continental areas.

The Crust: Studies of earthquake waves, meteorites that fall to Earth, magnetic fields, and other physical properties show that Earth’s interior consists of a series of shells of different compositions and mechanical properties. Earth is called a differentiated planet because of this separation into layers. Earth consists of internal layers of increasing density toward the center. The internal layers were produced as different materials rose or sank so that the least-dense materials were at the surface and the most dense were in the center of the planet. Thus, gravity is the motive force behind Earth’s differentiated structure. Geologists use the term crust for the outermost compositional layer. The base of the crust heralds a definite change in the proportions of the various elements that compose the rock but not a strong change in its mechanical behavior or physical properties. Moreover, the crust of the continents is distinctly different from the crust beneath the ocean basins Continental crust is much thicker (as much as 75 km), is composed of less-dense “granitic” rock (about 2.7 g/cm3), is strongly deformed, and includes the planet’s oldest rocks (billions of years in age). By contrast, the oceanic crust is only about 8 km thick, is composed of denser volcanic rock called basalt (about 3.0 g/cm3), is comparatively undeformed by folding, and is geologically young (200 million years or less in age). These differences between the continental and oceanic crusts, as you shall see, are fundamental to understanding Earth.

The Mantle: The next major compositional layer of Earth, the mantle, surrounds or covers the core (Figure 1.5, left). This zone is about 2900 km thick and constitutes the great bulk of Earth (82% of its volume and 68% of its mass).The mantle is composed of silicate rocks (compounds of silicon and oxygen) that also contain abundant iron and magnesium. Within the upper mantle, there is a major zone where temperature and pressure are just right so that part of the material melts, or nearly melts. Under these conditions, rocks lose much of their strength and become soft and plastic and flow slowly. Fragments of the mantle have been brought to the surface by volcanic eruptions. Because of the pressure of overlying rocks, the mantle’s density increases with depth from about 3.2 g/cm3 in its upper part to nearly 5 g/cm3 near its contact with the core. This zone of easily deformed mantle is known as the asthenosphere (“weak sphere”).

The Core

Earth’s core is a central mass about 7000 km in diameter. Its density increases with depth but averages about 10.8 g/cm3. The core makes up only 16% of Earth’s volume, but, because of its high density, it accounts for 32% of Earth’s mass. Earth’s core marks a change in both chemical composition and mechanical properties. On the basis of mechanical behavior alone, the core has two distinct parts: a solid inner core and a liquid outer core. The outer core has a thickness of about 2270 km compared with the much smaller inner core, with a radius of only about 1200 km. The core is extremely hot, and heat loss from the core and the rotation of Earth probably cause the liquid outer core to flow. This circulation generates Earth’s magnetic field.