Introduction to the Atmosphere: Background Material Introduction to the Atmosphere This section provides a brief overview of the properties associated with the atmosphere. The general concepts found in this section are: The earth's atmosphere is a very thin layer wrapped around a very large planet. Two gases make up the bulk of the earth's atmosphere: nitrogen ( ), which comprises 78% of the atmosphere, and oxygen ( ), which accounts for 21%. Various trace gases make up the remainder. Based on temperature, the atmosphere is divided into four layers: the troposphere, stratosphere, mesosphere, and thermosphere. Energy is transferred between the earth's surface and the atmosphere via conduction, convection, and radiation. Ocean currents play a significant role in transferring this heat poleward. Major currents, such as the northward flowing Gulf Stream, transport tremendous amounts of heat poleward and contribute to the development of many types of weather phenomena. This section includes seven classroom activities. Atmospheric Properties The thin envelope of air that surrounds our planet is a mixture of gases, each with its own physical properties. The mixture is far from evenly divided. Two elements, nitrogen and oxygen, make up 99% of the volume of air.

The other 1% is composed of "trace" gases, the most prevalent of which is the inert gaseous element argon. The rest of the trace gases, although present in only minute amounts, are very important to life on earth. Two in particular, carbon dioxide and ozone, can have a large impact on atmospheric processes. Another gas, water vapor, also exists in small amounts. It varies in concentration from being almost non-existent over desert regions to about 4% over the oceans. Water vapor is important to weather production since it exists in gaseous, liquid, and solid phases and absorbs radiant energy from the earth. Structure of the Atmosphere The atmosphere is divided vertically into four layers based on temperature: the troposphere, stratosphere, mesosphere, and thermosphere. Throughout the Cycles unit, we'll focus primarily on the layer in which we live - the troposphere. Troposphere The word troposphere comes from tropein, meaning to turn or change. All of the earth's weather occurs in the troposphere. The troposphere has the following characteristics. It extends from the earth's surface to an average of 12 km (7 miles).

The pressure ranges from 1000 to 200 millibars (29.92 in. to 5.92 in.). The temperature generally decreases with increasing height up to the tropopause (top of the troposphere); this is near 200 millibars or 36,000 ft. The temperature averages 15°C (59°F) near the surface and -57°C (-71°F) at the tropopause. The layer ends at the point where temperature no longer varies with height. This area, known as the tropopause, marks the transition to the stratosphere. Winds increase with height up to the jet stream. The moisture concentration decreases with height up to the tropopause. The air is much drier above the tropopause, in the stratosphere. The sun's heat that warms the earth's surface is transported upwards largely by convection and is mixed by updrafts and downdrafts. The troposphere is 70% Atmospheric Processes Interactions - Atmosphere and Ocean In the Cycles overview, we learned that water is an essential part of the earth's system.

The oceans cover nearly three-quarters of the earth's surface and play an important role in exchanging and transporting heat and moisture in the atmosphere. Most of the water vapor in the atmosphere comes from the oceans. Most of the precipitation falling over land finds its way back to oceans. About two-thirds returns to the atmosphere via the water cycle. You may have figured out by now that the oceans and atmosphere interact extensively. Oceans not only act as an abundant moisture source for the atmosphere but also as a heat source and sink (storage). The exchange of heat and moisture has profound effects on atmospheric processes near and over the oceans. Ocean currents play a significant role in transferring this heat poleward. Major currents, such as the northward flowing Gulf Stream, transport tremendous amounts of heat poleward and contribute to the development of many types of weather phenomena. They also warm the climate of nearby locations. Conversely, cold southward flowing currents, such as the California current, cool the climate of nearby locations. Energy Heat Transfer Practically all of the energy that reaches the earth comes from the sun. Intercepted first by the atmosphere, a small part is directly absorbed, particularly by certain gases such as ozone and water vapor.

Some energy is also reflected back to space by clouds and the earth's surface. Energy is transferred between the earth's surface and the atmosphere via conduction, convection, and radiation. Conduction is the process by which heat energy is transmitted through contact with neighboring molecules. Some solids, such as metals, are good conductors of heat while others, such as wood, are poor conductors. Air and water are relatively poor conductors. Since air is a poor conductor, most energy transfer by conduction occurs right at the earth's surface. At night, the ground cools and the cold ground conducts heat away from the adjacent air. During the day, solar radiation heats the ground, which heats the air next to it by conduction. Convection transmits heat by transporting groups of molecules from place to place within a substance. Convection occurs in fluids such as water and air, which move freely. In the atmosphere, convection includes large- and small-scale rising and sinking of air masses and smaller air parcels. These vertical motions effectively distribute heat and moisture throughout the atmospheric column and contribute to cloud and storm development (where rising motion occurs) and dissipation (where sinking motion occurs).

To understand the convection cells that distribute heat over the whole earth, let's consider a simplified, smooth earth with no land/sea interactions and a slow rotation. Under these conditions, the equator is warmed by the sun more than the poles. The warm, light air at the equator rises and spreads northward and southward, and the cool dense air at the poles sinks and spreads toward the equator. As a result, two convection cells are formed. Meanwhile, the slow rotation of the earth toward the east causes the air to be deflected toward the right in the northern hemisphere and toward the left in the southern hemisphere. This deflection of the wind by the earth's rotation is known as the Coriolis effect. Radiation is the transfer of heat energy without the involvement of a physical substance in the transmission. Radiation can transmit heat through a vacuum. Energy travels from the sun to the earth by means of electromagnetic waves. The shorter the wavelength, the higher the energy associated with it. This is demonstrated in the animation below. As the drill's revolutions per minute (RPMs) increase, the number of waves generated on the string increases, as does the oscillation rate.

The same principle applies to electromagnetic waves from the sun, where shorter wavelength radiation has higher energy than longer wavelength radiation. Most of the sun's radiant energy is concentrated in the visible and near-visible portions of the spectrum. Shorter-than-visible wavelengths account for a small percentage of the total but are extremely important because they have much higher energy. These are known as ultraviolet wavelengths. Concluding Thoughts The physical and chemical structure of the atmosphere, the way that the gases interact with solar energy, and the physical and chemical interactions between the atmosphere, land, and oceans all combine to make the atmosphere an integral part of the global biosphere. For students to truly understand the nature and importance of the atmosphere, they should understand the answers to these questions: What is the structure and composition of the atmosphere? How does solar energy influence the atmosphere? How does the atmosphere interact with land and oceans? How is heat transferred throughout the earth system? Activities Atmosphere A planet's climate is decided by its mass, its distance from the sun and the composition of its atmosphere. Earth's atmosphere is 78% nitrogen, 21% oxygen, and 1% other gases. Carbon dioxide accounts for just 0.03 - 0.04%.

Water vapour, varying in amount from 0 to 2%, carbon dioxide and some other minor gases present in the atmosphere absorb some of the thermal radiation leaving the surface and emit radiation from much higher and colder levels out to space. These radiatively active gases are known as greenhouse gases because they act as a partial blanket for the thermal radiation from the surface and enable it to be substantially warmer than it would otherwise be, analogous to the effect of a greenhouse. This blanketing is known as the natural greenhouse effect. Without the greenhouse gases, Earth's average temperature would be roughly -20°C. T The atmosphere is a cloud of gas and suspended solids extending from the Earth's surface out many thousands of miles, becoming increasingly thinner with distance but always held by the Earth's gravitational pull. The atmosphere is made up of layers. it surrounds the Earth and holds the air we breathe; it protects us from outer space; and holds moisture (clouds), gases, and tiny particles. In short, the atmosphere is the protective bubble we live in. This protective bubble consists of several gases (listed in the table to the right) with the top four making up 99.998% of all gases.

Of the dry composition of the atmosphere nitrogen, by far, is the most common. Nitrogen dilutes oxygen and prevents rapid burning at the Earth's surface. Living things need it to make proteins. Oxygen is used by all living things and is essential for respiration. It is also necessary for combustion or burning. Argon is used in light bulbs, in double-pane windows, and to preserve the original Declaration of Independence and the Constitution. Plants use carbon dioxide to make oxygen. Carbon dioxide also acts as a blanket that prevents the escape of heat into outer space. These percentages of atmospheric gases are for a completely dry atmosphere. The atmosphere is rarely, if ever, dry. Water vapor (water in a 'gas' state) is nearly always present up to about 4% of the total volume. In the Earth's desert regions (30°N/S) when dry winds are blowing, the water vapor contribution to the composition of the atmosphere will be near zero. Water vapor contribution climbs to near 3% on extremely hot/humid days. The upper limit, approaching 4%, is found in tropical climates. The table below shows the changes in atmospheric composition with the inclusion of different amounts of water vapor.

The earth's atmosphere is a very thin layer wrapped around a very large planet. Two gases make up the bulk of the earth's atmosphere: nitrogen ( ), which comprises 78% of the atmosphere, and oxygen ( ), which accounts for 21%. Various trace gases make up the remainder. Based on temperature, the atmosphere is divided into four layers: the troposphere, stratosphere, mesosphere, and thermosphere. Energy is transferred between the earth's surface and the atmosphere via conduction, convection, and radiation. Ocean currents play a significant role in transferring this heat poleward. Major currents, such as the northward flowing Gulf Stream, transport tremendous amounts of heat poleward and contribute to the development of many types of weather phenomena. Cambridge University Graphic The atmosphere is divided into five layers. It is thickest near the surface and thins out with height until it eventually merges with space. 1) The troposphere is the first layer above the surface and contains 70 to 80 per cent of the Earth's atmosphere. Weather occurs in this layer. 2) Many jet aircrafts fly in the stratosphere because it is very stable. Also, the ozone layer absorbs harmful rays from the Sun.

3) Meteors or rock fragments burn up in the mesosphere. 4) The thermosphere is a layer with auroras. It is also where the space shuttle orbits. 5) The atmosphere merges into space in the extremely thin exosphere. This is the upper limit of our atmosphere. National Geographic Video U.S. Weather Service Graphic The inhabitants of our planet live in the Troposphere. Earth's atmosphere varies in density and composition as the altitude increases above the surface. The lowest part of the atmosphere is called the troposphere (and it extends from the surface up to about 10 km (6 miles). The gases in this region are predominantly molecular Oxygen ( O2 ) and molecular Nitrogen ( N2 ). All weather is confined to this lower region and it contains 70 to 80 per cent of the Earth's atmosphere and 99% of the water vapor. The highest mountains are still within the troposphere and all of our normal day-to-day activities occur here. The high altitude jet stream is found near the tropopause at the the upper end of this region. The layer above this is the Stratosphere, this is where the Ozone Layer is formed. The atmosphere above 10 km is called the stratosphere.

The gas is still dense enough that hot air balloons can ascend to altitudes of 15 - 20 km and Helium balloons to nearly 35 km, but the air thins rapidly and the gas composition changes slightly as the altitude increases. Within the stratosphere, incoming solar radiation at wavelengths below 240 nm. is able to break up (or dissociate) molecular Oxygen ( O2 ) into individual Oxygen atoms, each of which, in turn, may combine with an Oxygen molecule ( O2 ), to form ozone, a molecule of Oxygen consisting of three Oxygen atoms ( O3 ). This gas reaches a peak density of a few parts per million at an altitude of about 25 km (16 miles). The Ozone Layer absorbs ultra-violet radiation from the Sun. Without the Ozone Layer life as we know would cease to exist on our planet. Ozone is important because it is the only atmospheric gas which absorbs light in the B region of UVB rays. The Ozone layer extends from a height of 20 kilometers to 60 kilometers above the Earth's surface. The air is very thin at these altitudes.

If all of the Ozone in the Earth's atmosphere were compressed into a single layer at the Earth's surface, it would only be 3 millimeters thick-basically two stacked pennies! The gas becomes increasingly rarefied at higher altitudes. At heights of 80 km (50 miles), the gas is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The existence of charged particles at this altitude and above, signals the beginning of the ionosphere a region having the properties of a gas and of a plasma. Spheres of Influence Spheres of Influence: The Earth's Differentiation Into Solid, Liquid, and Gas Layers by Neil Sitron and Dan Goldstein Introduction Any person examining their surroundings immediately realizes the vastness of what envelopes him. However, the visible objects of the Earth pale in comparison to what is going on, unseen and unnoticed. The Earth around us is divided into three layers, the gas layer (atmosphere), liquid layer (hydrosphere), and solid layer (land). These layers developed and evolved over the course of billions of years and are still changing today. More importantly, these layers perform constant mutations of the planet and interact with each other.

The three layers, solid, liquid, and gas form the basis of what we call Planet Earth. History of the Earth's Layers The Earth, as we know it today, consists of three separate states of matter: solid, liquid, and gas. They are better known as land, sea, and air. Initially though, during its infancy, the Earth was a part-molten planet with very primitive and less clearly defined forms of each layer. The differentiation that occurred to bring about the modern day Earth took billions of years and involved several complex processes. One must first examine the formation of land. At first, Earth was a large melt, from which a nickel and iron core developed. Eventually, a solid silicate mantle evolved. The mantle became the foundation from which the crust, or modern day land, was formed. To explain the crust's evolution, it was proposed that the crust of the continents originated from a particular mantle. The average continental crust has properties quite similar to those of granite, but the mantle has properties closer to ocean-floor basalt and chondritic meteorites. Through a process called "chemical fractionation", a concentration of relatively light elements distilled from the basalt mantle to form a granitic residue.

This residue comprised approximately 7-8% of the original mantle. As the mantle underwent this process, numerous amounts of light elements including silicon, oxygen, potassium, sodium, aluminum, carbon, nitrogen, hydrogen, helium, and calcium rose to the surface to form a primitive crust. Through these elements, the most familiar rock-producing minerals of today were formed including alkali feldspar (KAlSi3O8 and NaAlSi3O8), quartz (SiO2), and pyroxenes (Ca [Mg, Fe] Si2O6). A complete list of these minerals can be found in Table 1. Table 1 (Batten & Dott, 126) This original continental crust did not become stable until nearly 1 billion years after the separation of the mantle from the core. During its period of instability, Earth was much hotter and saw several times more radioactive decay than today. As a result, some crust would form, melt, and re-form again. As the Earth cooled down, however, this process ceased, and the current crust has been stable for roughly 3.5 billion years. The evolution of the Earth's atmosphere poses more of a mystery than that of the crust.

In an effort to develop an accurate explanation, geologists have decided to compare our present atmosphere with that of large planets, such as Jupiter and the meteorites. Planetologists believe that the larger planets, Jupiter especially, retained the greatest percentage of original gases from the initial atmospheres of the planets. Table 2 lists the components of Earth's atmosphere and compares them to Jupiter's and the gases found on meteorites. The meteorites are widely believed to resemble the early protoplanets, so studying their various gases provides important clues to the Earth's past. Table 2 (Batten & Dott, 128) In analyzing the atmosphere's evolution, two popular hypotheses exist, both assuming that considerable free hydrogen and helium have drifted into space over time because of their small atomic masses. The great mystery that both of these hypotheses attempt to explain is the emergence of free oxygen, something that has not occurred on either Jupiter or the meteorites. The first hypothesis is called the Photochemical Dissociation Hypothesis. Based on studies of Jupiter, it assumes that the original atmosphere contained the same primary gases that Jupiter does- ammonia, methane and water vapor. The original atmosphere contained no free oxygen nor did it contain ozone.

As a result, high-energy, ultraviolet radiation could easily penetrate the atmosphere at will. One property of such intense radiation is its ability to trigger photochemical, or light-induced, reactions. The Photochemical Dissociation Hypothesis states that these reactions caused the evolution of the atmosphere through the following four step process: Step 1: The ultraviolet lights combined with the water vapor to set the hydrogen off into space and free the oxygen. 2H2O + uv light energy ----> 2H2 (freed into space) + O2 Step 2: The newly freed oxygen reacted with methane, forming carbon dioxide and additional water vapor. CH4 + 2O2 ----> CO2 + 2H2O Step 3: The oxygen also reacted with ammonia, producing nitrogen and water. 4NH3 + 3O2 ----> 2N2 + 6H2O Step 4: After converting the ammonia and methane to carbon dioxide and nitrogen, free oxygen began to accumulate as further dissociation of water vapor continued. This hypothesis states that once the water vapor reacted with the intense radiation, oxygen was freed, allowing the creation of nitrogen and carbon dioxide.

Together, these three elements form the main consituents of the modern day atmosphere. The Outgassing Hypothesis presents an alternative to the Photochemical Dissociation Hypothesis. It states that most of the atmospheric gases arose from the interior by a process called "outgassing". Through outgassing, a gaseous transfer occurred between the Earth's interior and its surface, mainly through igneous activity. A perfect example of outgassing today comes from volcanoes. When volcanoes erupt, they expel steam, carbon dioxide, nitrogen, hydrogen and carbon monoxide into the air. So, according to the hypothesis, before volcanoes even existed, some other process occurred which launched gases such as nitrogen, carbon dioxide and water vapor into the atmosphere. In contrast to the Photochemical Dissociation Hypothesis, the Outgassing Hypothesis argues that the free oxygen came from the photosynthesis of primitive organisms which existed 1.5 - 3.5 billion years ago. The oxygen took approximately 2 billion years to become free, but when it did, it formed the ozone layer, eliminating the dangerous radiation and setting up the foundation for a habitable planet.

In either example, the formation of the atmosphere no doubt included the presence of water vapor. This water vapor, in turn, accumulated and liquefied to form the seas and oceans that today comprise over two thirds of the Earth's surface. Geologists do not concern themselves so much with how the water vapor liquefied; instead, they tend to focus on the rate at which the water accumulated over time. Initially, it was assumed that the water grew at a constant rate over time. Others believed that the rate of accumulation of sea water would depend exclusively on atmospheric production. The atmospheric production, in turn, depended on chemical fractionation, based on the hypothesis' link between the Earth's interior and atmospheric gases of nitrogen, carbon dioxide and water vapor. Thus, it was reasoned that the rate of accumulation grew (and still does grow) in direct proportion to an inferred increase of the volume of the continental crust over time. Geologists have compared estimates analyzing the rate of releases of helium and argon from the interior into the atmosphere. It is believed that if water was released at a comparable rate, then the water would accumulate over a long period of time, but not at a constant rate.

Most likely, crustal growth and outgassing were far more rapid earlier in the Earth's history as a result of the immense radioactivity that existed. Table 3 shows a graph of the water accumulating at a constant rate and plots it against the proposed accumulation rate. As a result, the water accumulated at a faster rate as well and has gradually increased at a decreasing rate over the past 2 billion years. SOLID LAYER A computer-generated model isolating the Geosphere, picturing the world as if the Hydrosphere did not exist ( The exterior of the Earth represents the planet's solid layer. Its base is formed by the uppermost part of the mantle, known as the lithosphere which contains the rigid material of the outer portion of the Earth. The lithosphere possesses a thin, rocky outer layer called the crust. The crust ranges in thickness, from 30-60 kilometers thick beneath the continents to only 5-10 kilometers thick beneath the oceans. Bedrock of the crust is covered by loose rock particles called the regolith that forms when bedrock reacts with the atmosphere and hydrosphere. When nature transports regolith to a different location, the regolith is re-named sediment, which means "settling".

The upper portion of regolith creates soil, which permits the growth of plants and crops that are crucial for human survival. The regions on the Earth where life can evolve belong to the biosphere. The biosphere is defined as all the Earth's living matter as well as all dead plants and animals that have not yet been completely decomposed. The biosphere has a maximum range of 20 kilometers in thickness and ranges from the depths of the oceans to the peaks of the highest mountains. It is primarily composed of various compounds of carbon, hydrogen and oxygen. Continents, or continental masses, refer to the major land areas of the world. Continental masses comprise roughly 30% of the surface of the Earth and average approximately 800 meters in altitude. The continents are underlain by ancient rocks, dating as far back in time as 3.9 billion years. An interesting point, however, is that the materials which comprise the continents all resulted from the geological cycle that transformed the Earth from its primitive state to its modern one. No remnants of any materials from the Earth's infant stage have ever been found in the continents.

Research has shown that the most accurate way to analyze the continents is to organize them into several large regions where the surface morphology and the various rocks display similar traits. These regions are called physiographic provinces. Examples include mountains, plains, plateaus, and shields. A mountain is a physiographic province defined as a landmass that stands 400 meters higher or more than its surroundings. Cordilleras, a Spanish term meaning "strings", is a pseudonym for the various mountain belts of the Earth. A cordilleras has several individual parts based on morphology, including mountain ranges, mountain systems, and mountain chains. Plains represent a second type of physiographic province. Because of the minimal variations between their lowest and highest points, plains are quite flat. Beneath the plains are sediments, and the addition of new layers of these sediments causes the surface area of plains to grow. Plains are most often found in coastal regions or large regions within the interiors of the continents. A high-standing area within which the rock strata are generally horizontal is a third physiographic province called a plateau. Plateaus can be viewed as elevated plains. Usually, they result when the sediments underlying a former plains area cement together and subsequently elevate.

Not all plateaus are underlain by sediments; some are formed by layers of volcanic rocks. Shields, the fourth physiographic province, are low-lying regions which form when several ancient mountain belts join together. Shield areas are relatively thick and rigid continental blocks. The regions on the Earth where life can evolve belong to the biosphere. The biosphere is defined as "the totality of the Earth's living matter and, in addition, dead plants and animals that have not yet been completely decomposed." The biosphere has a maximum range of 20 kilometers in thickness and ranges from the depths of the oceans to the peaks of the highest mountains. It is primarily composed of various compounds of carbon, hydrogen, and oxygen. The Liquid Layer A computer-generated model of the Hydrosphere detailing the average daily temperature of the Earth's water supply ( The liquid layer of the Earth is called the hydrosphere and it represents the entire conglomeration of water bodies enveloping the earth's surface. The hydrosphere consists of the rivers, streams, lakes and oceans, while its sub-category, the cryosphere, is made up of the temporarily frozen, solid water bodies such as ice, snow and glaciers.

The liquid layer of the earth covers approximately three-quarters of the earth's surface, of which oceans comprise ninety-seven percent. The distribution of the liquid layer of the earth is shown on Table 4. The daily changes in the liquid layer is explained by the "hydrologic cycle." The Sun's heat evaporates the water of the hydrosphere. The newly-created water vapor rises into the atmosphere, cools, condenses, and becomes precipitation, falling back onto the land and into the oceans, lakes, and rivers. The hydrologic cycle describes the movement of water in its three states: solid, liquid and gas. The evaporation and precipitation of the water, the hydrosphere of planet Earth, recycles and rejuvenates the Earth and its water supply each and every day. Evaporation and precipitation across the Earth is in total balance, though evaporation is greater than precipitation over the oceans and the reverse is true over the land. Water vapor usually averages twelve days in the atmosphere before coming back down to Earth again as rain or snow. This difference is corrected by the flow of rivers into the oceans and balance is maintained.

Table 3 (Batten & Dott, 131) The hydrosphere has three main functions which make it so important to the planet Earth. First, water shapes the earth's surface by eroding certain areas and relocating those materials. Ocean waves are a primary instrument of erosion. Surf zone abrasion is an example of this. The sediment carried by the waves smashes into the beach and the constant action wears down the surf zone, smoothing and flattening the land. Second, the oceans serve as basins for the accumulation of almost all of the substances available on Earth. The rivers of the hydrosphere collect materials and sediment and bring them out to the interconnected oceans. As a result, fine-grained, course-grained and evaporite sediments can all be found in the oceans. Also, terrigenous sediment, materials eroded by waves, carried by rivers, and blown by the wind, combine with pelagic sediment--marine organic materials referred to as deep-sea ooze--dissolve into the ocean waters and collect on the ocean floor. However, the hydrosphere is not just pure water. Through erosion and movement of sediment, several compounds have become dissolved in the water.

Three to four percent of ocean water is mineral matter in solution, of which seventy-five percent is sodium chloride (NaCl--common table salt). Most of this mineral matter is contained in the oceans and only a minimal amount exists in the lakes and streams. Most importantly, the hydrosphere's third function is to be used as an essential building block and sustainer of virtually all life forms. However, one must not ignore the fact that although most people think of this as virtually the only important function of the liquid layer, the importance is actually in the proper operation of all of the functions and the successful interaction of the hydrosphere and the solid and gas layers. The Gas Layer The outermost layer of Earth is the gas layer, better known as the atmosphere. The atmosphere is composed primarily of nitrogen and oxygen, but is also comprised of several trace gases such as carbon dioxide (CO2) and water vapor. All of these gases, which together form what is commonly called "air", are held to the Earth by the pull of gravity and each serves an important function.

Carbon dioxide gas provides a barrier which keeps heat from escaping the Earth; this is known as the "greenhouse effect." The "greenhouse effect" caused by carbon dioxide in the atmosphere serves to maintain a temperature on Earth great enough to sustain life. Yet, if the amount of carbon dioxide gas were to increase drastically, too much heat would be trapped in the atmosphere, making it too hot for many organisms to survive. This causes a frightful problem wherein a certain balance of CO2 gas must be maintained to ensure that life may be maintained on the planet. Carbon dioxide also serves as the lifeline to most plants, allowing them to breath by taking in the carbon dioxide and emitting oxygen gas. Another gas, ozone (O3) screens out the harmful ultraviolet rays emanating from the sun. One of the most abundant gases in the atmosphere, oxygen, is needed for animals to breath and live. This graph shows the range of predictions made as to the global-mean surface temperature during the next eighty years ( The gas layer is constantly moving around the Earth in the form of wind and filling the openings of the solid layer of Earth that have not yet been penetrated by water.

The movement of wind is created by the uneven pattern of the Sun's heat on Earth, wherein the heat is greater near the equator than at the poles. This causes the equatorial air to expand, become less dense, and move toward the poles, where it will cool, contract, become more dense and flow toward the equator. However, the Coriolis effect modifies this pattern by forcing the air to clump into wind belts. The Coriolis effect causes any body that moves freely with respect to the rotating solid of the Earth to veer toward the right in the Northern Hemisphere and towards the left in the Southern Hemisphere. The Coriolis effect manipulates the movement of the Earth's gaseous layer and the flow of air becomes curved east in the north and west in the south instead of merely moving north and south between the poles. A satellite image of cloud movements and formations over Europe, with the political boundaries artificially superimposed ( The atmosphere works as a geological agent of movement and erosion in two ways: wind and rock weathering. Winds are often extremely violent and can do extensive damage and erode land in a short period of time. Rock weathering, though, also erodes land but in a much slower process.

Rock weathering slowly loosens material so that streams, gravity, and other instruments may carry it to the oceans. The atmosphere's interaction with the hydrosphere, the liquid layer of the Earth, is extremely important, as the gas layer holds water vapor (water in a gaseous state, caused by heat from the sun) and redistributes it back to the Earth's surface. The gas layer's role in the hydrologic cycle is to carry the evaporated water and cool it back down, then release it as precipitation. Without an atmosphere, water vapor would merely escape into space and the Earth's water supply would deplete. The atmosphere is the reason why rain is possible on the planet Earth. Conclusion The differentiation process that occurred on Earth was both lengthy and complicated. Over the course of nearly 4.5 billion years, the Earth has transformed from a molten protoplanet to a world with clear, beautiful oceans, majestic mountains, and an atmosphere that can sustain human, animal, and plant life. While the process was tedious, the results are undeniable. Had the Earth not differentiated into solid, liquid, and gas layers, life never would have evolved. But will life as we know it continue forever on Earth? Today's research suggests otherwise.

Most scientists believe that several billion years from now the sun will burn out, killing any and all life forms remaining on the Earth. A more immediate problem faces the life-forms on Earth. Because of human negligence, (mainly in the form of pollution), the solid, liquid, and gas layers of the Earth are being threatened. The extent of the damage is unknown at this time. Should humans continue, however, to pollute the oceans, destroy the land and erode the ozone layer, the consequences will be frightening. The differentiation process took billions of years; reversing it may not take as long. Glossary of Terms Atmosphere : The gas layer of the Earth, primarily consisting of nitrogen, oxygen, carbon dioxide, and water vapor. Bedrock : Large quantities of solid rock which form the primary composition of the crust. Chemical Fractionation : A concentration of relatively light elements which distilled to form a granitic residue. Continental Masses : The major land areas of the world. Cordilleras : Spanish for "strings". Refers to the Earth's mountain belts.

Corlolis Effect : Effect that causes any body of air that moves freely with respect to the rotating solid of Earth to move towards the right in the Northern Hemisphere and towards the left in the Southern Hemisphere Cryosphere : A sub-category of the hydrosphere consisting of the temporarily frozen, solid water bodies including snow, ice, and glaciers. Greenhouse Effect : Occurs when carbon dioxide gas provides a barrier which keeps heat from escaping the Earth. It serves to maintain a temperature on Earth great enough to sustain life. Hydrologic Cycle : The cyclic movement of water through evaporation, precipitation, wind transport, stream flow, percolation, and related processes. Hydrosphere : The liquid layer of the Earth. The entire conglomeration of water bodies enveloping the Earth's surface. Lithosphere : The uppermost portion of the Earth's mantle. Outgassing Hypothesis : Hypothesis which states that atmosphere gases arose from the interior of the Earth through igneous activity, such as volcanoes. Photochemical Dissociation Hypothesis : Hypothesis which states that intense ultraviolet radiation triggered photochemical reactions which caused the evolution of the modern-day atmosphere on Earth.

Physiographic Province : A large continental region where the surface morphology and various rocks display similar traits to each other. Plains : A physiographic province with little variation between its lowest and highest points. Plateau : A physiographic province. It is a high standing area whose rock strata are horizontal. Regolith : A sheet of loose particles covering the bedrock. It plays a vital role in plant growth by creating soil. Rock Weathering : A form of erosion which slowly loosens material so that streams, gravity, and other instruments carry it to the oceans. Sediment : Regolith which is transported to a different location by nature. Surf Zone Abrasion : One method of shaping the Earth's surface caused by ocean waves which erode certain areas. Resources Batten, Roger L. and Robert H. Dott. Evolution of the Earth. New York: McGraw-Hill Book Company, 1971. Landes, Kenneth K. Physical Geology and Man. New York: Prentice-Hall, Inc., 1948. Menard, H.W. Geology, Resources, and Society. San Francisco: W.H. Freeman and Company, 1974.

Porter, Steven C. and Brian J. Skinner. The Dynamic Earth. New York: John Wiley and Sons, 1989. Sanders, John E. Principles of Physical Geology. New York: John Wiley and Sons, 1981. Molecules in the Atmosphere Molecules in the Atmosphere Elements and Molecules Earth's primordial atmosphere was probably similar to the gas cloud that created the sun and planets. It consisted of hydrogen and helium, along with methane, ammonia, and water. This was a reducing atmosphere. There was no molecular oxygen or other reactive oxides. Over time, some of this first atmosphere, particularly the lighter gases, outgassed and was lost. More water may have arrived with comets colliding on the surface of the planet. Volcanic activity in the early, Earth created major changes with release of water vapor, carbon dioxide, and ammonia along with small quantities of SO2, H2S, HCl, N2, NO2, He, Ar, and other noble gases. This produced the second atmosphere. Comet impacts may have increased the amount of water. Water vapor formed clouds. These produced rain.

Over a period of thousands of years, the liquid water accumulated as rivers, lakes, and oceans on the Earth's surface. Bodies of liquid water acted as sinks for carbon dioxide. Chemical and biological processes transformed CO2 gas to carbonate rocks. The nitrogen and argon accumulated in the atmosphere. They do not react with water or other atmospheric components. Oxygen existed in only trace quantities before life began. Living things created much of the third atmosphere, the one that now exists on Earth. Cyanobacteria were responsible for the rise in the atmospheric concentration of oxygen beginning 2.3 billion years ago. These bacteria, algae, and other plants produce oxygen by photosynthesis. Although most of this oxygen is used in respiration (biological oxidation) or in the atmospheric oxidation of the carbon-containing products, approximately 0.1 % of the organic matter is sequestered in sediments and that quantity of oxygen is added to the atmosphere. Over time, the excess oxygen has built up so that it is now makes up nearly 20% of the gases close to Earth. Composition of Earth's Atmosphere Carbon dioxide, Methane, Rare (inert) gases 0.1% Nitrogen and oxygen are the most common gases in today's atmosphere.

Others are present in small concentrations. The other more common gases are shown in the table below. There is a remarkable difference between the original, reducing atmosphere and the current oxidizing atmosphere. Nitrogen Nitrogen is present in the atmosphere mainly as the dimeric molecule N2. See below for a summary of information on atomic nitrogen. There are two primary isotopes of atomic nitrogen with a ratio of 14N/ 15N = 272. Dinitrogen is extremely robust with a very high heat of dissociation (944 kJ mol-1 ). It is a colorless gas with a boiling point of -196 deg C. The Haber Process is a method for producing ammonia developed by Germany during World War I. The Germans used the ammonia as a source of nitrogen for making explosives. The process is still used by industrial chemists. N2 + 3 H2 2 NH3 Certain plants are able to transform dinitrogen into ammonia at ambient temperature through a biochemical process involving the enzyme nitrogenase. Molecular nitrogen also reacts with molecular oxygen during lightning storms to produce nitric oxide, NO. Other than that, there is virtually no reaction chemistry of N2 in the atmosphere.

It is an unreactive gas. Bonding in Molecular Nitrogen Why is N2 so unreactive? This is due to the strong, triple bond between the nitrogen atoms. Oxygen Living things created much of the atmosphere that now exists on Earth. Cyanobacteria were responsible for the rise in the atmospheric concentration of oxygen beginning 2.3 billion years ago. These bacteria, algae, and other plants produce oxygen by photosynthesis. Although most of this oxygen is used in respiration (biological oxidation) or in the atmospheric oxidation of the carbon-containing products, approximately 0.1 % of the organic matter is sequestered in sediments and that quantity of oxygen is added to the atmosphere. Over time, the excess oxygen has built up so that it is now makes up nearly 20% of the gases close to Earth. Information on oxygen is summarized in the chart below. Atomic oxygen has three isotopes, 16O (99.759 %), 17O (0.0374 %), 18O (0.2039 %).

The most common oxygen species in the atmosphere is triplet dioxygen, a diradical (boiling point = -183 deg C). Singlet dioxygen is diamagnetic with two electrons in one of the pi antibonding orbitals. This is higher in energy than triplet dioxygen and acts as an electrophile in its chemical reactions. The singlet/triplet designations arise from the number of transitions between spin states when the molecule is placed in an external magnetic field. Where S is the sum of all electron spins, the spin state = (2S + 1): 0 unpaired electrons, singlet state 1 unpaired electron, doublet state 2 unpaired electrons, triplet state Molecules with no upaired spins are diamagnetic and are weakly repelled by magnetic fields. Molecules with one or more unpaired spins are paramagnetic and are attracted by magnetic fields. Molecular oxygen, O2, is much more reactive than molecular nitrogen, N2. The bond between the atoms of O2 is significantly weaker than that between the atoms of N2, so bond homolysis can occur at a lower energy.

Also, while dinitrogen is diamagnetic, dioxygen is paramagnetic and reacts with other radical species in the atmosphere. Ozone or O3 is the other allotrope of oxygen (boiling point = -112 deg C). This is a bent, diamagnetic species. Ozone is a reactive molecule and plays an important role in the chemistry and photochemistry of the upper atmosphere. Photolysis of ozone in the stratosphere helps to protect life on Earth from highly energetic UV radiation and its depletion in the ozone layer by reactions with halogenated compounds is a serious concern. The reactivity of ozone makes it toxic to humans. The reactions of nitrogen oxides, hydrocarbons, and oxygen near the Earth's surface produce ozone and the concentration of this compound is frequently used as a measure of air pollution. Bonding in O2 Lewis structures give us an approximation of the bonding in molecules. This approximation isn't very good for molecular oxygen. The best Lewis structure doesn't indicate the ground state properties of the molecule: its diradical nature. We can better predict the properties of triplet dioxygen by examining the molecular orbital diagram than with the Lewis structure. The molecular orbital diagrams for all homonuclear diatomic molecules is similar.

The 2s atomic orbitals combine to form bonding and antibonding molecular orbitals (just like the 1s orbitals on hydrogen). Similarly, 2pz orbitals on both atoms combine to form sigma bonding and antibonding orbitals. The 2px orbitals combine to form bonding and antibonding orbitals with pi rotational symmetry about the bond axis. The 2py orbitals combine to form another set of pi orbitals at 90 degrees to the first set. The two pi bonding orbitals and the two pi antibonding orbitals form degenerate sets. Water, H2O Water is present in variable amounts in the atmosphere, from 0 % to 4 %. Unlike oxygen and nitrogen, the concentration of oxygen depends on local weather conditions and changes greatly from place to place on Earth. The water in the atmosphere makes up only a very small percentage of the total water on Earth. In the atmosphere, water exists as a gas (water vapor from evaporation), as a liquid (droplets of rain and liquid water that coats solid particles), and as a solid (snow and ice). Its structure depends on its state.

Water in the gas phase has a bent structure with an H-O-H angle of 104.5 degrees. In the liquid and solid forms, there are hydrogen bonds between the hydrogen atoms of one molecule of H2O and oxygen atoms of other molecules. This gives a 3-dimensional structure in which each oxygen atom is surrounded by a tetrahedral array of 4 hydrogen atoms. Argon, Ar Argon is one of the inert or noble gases. Because it has a filled shell electronic configuration, argon in unreactive and is present in the atmosphere as a monoatomic gas. Argon is the product of radioactive decay processes within the Earth. Atomic number: 18 Atmosphere | Atmospheric Gases | Aerosols | Greenhouse Gases Atmospheric Composition Air Air is a mixture of gases and aerosols that composes the atmosphere surrounding Earth. The primary gases of air include nitrogen (78%) and oxygen (21%). Trace gases and aerosols make up the remaining 1% of air. The trace gases include the noble gases argon, neon, helium, krypton and xenon; hydrogen; and the greenhouse gases.

The aerosols are solid or liquid particles having diameters in the region of 0.001 to 10 microns (millionth of a metre), and include dust, soot, sea salt crystals, spores, bacteria, viruses and a plethora of other microscopic particles, which may be natural or man-made. Earth maintains an atmosphere through its gravitational pull. Consequently, most air is found in the lowest 10 kilometres of the atmosphere. Experienced mountain climbers are aware of how thin the air becomes, and may carry oxygen tanks to assist breathing at high altitudes. Within the lower atmosphere, however, air remains remarkably uniform in composition, as a result of efficient recycling processes and turbulent mixing in the atmosphere. Atmospheric Gases There are a number of atmospheric gases which make up air. The main gases are nitrogen and oxygen, which make up 78% and 21% of the volume of air respectively. Oxygen is utilised primarily by animals, including humans, but also to a small degree by plants, in the process of respiration (the metabolism of food products to generate energy). The remaining 1% of the atmospheric gases is made up of trace gases.

These include the noble gases, very inert or unreactive gases, of which the most abundant is argon. Other noble gases include neon, helium, krypton and xenon. Hydrogen is also present in trace quantities in the atmosphere, but because it is so light, over time much of it has escaped Earth's gravitational pull to space. The remaining trace gases include the greenhouse gases, carbon dioxide, methane, nitrous oxide, water vapour and ozone, so-called because they are involved in the Earth natural greenhouse effect which keeps the planet warmer than it would be without an atmosphere. Oxygen The gas oxygen (O 2 ), composed of molecules of two oxygen atoms, occupies 21% of the Earth's atmosphere by volume. It is colorless, odorless, and tasteless. Oxygen also comprises 86% of the oceans and 60% of the human body, and is the third most abundant element found in the Sun. Almost all plants and animals require oxygen for respiration to maintain life. Oxygen is very reactive and oxides of most elements are known. A chemical reaction in which an oxide is formed is known as oxidation. The rate at which oxidation occurs varies with the element with which oxygen is reacting.

Rust, or iron oxide, for example forms relatively slowly, over days or weeks. Burning or combustion, however, involves a very rapid oxidation. Carbon in fossil fuels, for example, can be quickly oxidised to carbon monoxide and carbon dioxide, with a considerable amount of heat being given off. We can convert this heat into useful energy for heating, electricity and locomotion. Within the stratosphere, oxygen molecules combine with free oxygen atoms to form ozone (O 3 ). Ozone absorbs ultraviolet (UV) radiation from the Sun, and protects life on Earth from its damaging effect. Although abundant between 19 and 30 km altitude, the air at these levels in the atmosphere is thin. If all the ozone in the stratosphere was compressed to ordinary atmosphere pressure at ground level, it would occupy a layer only 3 mm thick. Nitrogen The gas nitrogen (N 2 ), composed of molecules of two nitrogen atoms, occupies 78% of the Earth's atmosphere. It is colorless, odorless, and tasteless. Nitrogen is as important as it is common. It's essential to the nutrition of plants and animals.

Nitrogen is a constituent in all proteins and in the genetic material (DNA) in all organisms. The low content of nitrogen in most soils exists in stark contrast to the abundance of nitrogen in air. This is because gaseous nitrogen molecules have very strong bonds linking the atoms together, making the gas chemically stable and unusable by most biological organisms. Some species of bacteria absorb nitrogen from the air and convert it into ammonium, which plants can use. This process, called nitrogen fixation, is the principal natural means by which atmospheric nitrogen is added to the soil. Legumes, such as beans, can fix nitrogen from the atmosphere. This is accomplished by nitrogen-fixing bacteria living in nodules on the plant roots. Nitrogen molecules in the atmosphere can also be broken by the energy generated by lightning strikes and volcanic action. Whenever lightning flashes in the atmosphere, some nitrogen combines with oxygen and forms the gas nitric oxide (NO). This nitric oxide is converted to nitric acid, which is highly soluble in water and falls to the ground in rainwater, to be absorbed by soils. Globally, however, nitrogen-fixing bacteria are a far more significant source of fixed nitrogen.

Trace Gases Most of our atmosphere is made up of nitrogen (78% by volume) and oxygen (21% by volume). The remaining 1% of the atmospheric gases are known as trace gases because they are present in such small concentrations. The most abundant of the trace gases is the noble gas argon (approximately 1% by volume). Noble gases, which also include neon, helium, krypton and xenon, are very inert and do not generally engage in any chemical transformation within the atmosphere. Hydrogen is also present in trace quantities in the atmosphere, but because it is so light, over time much of it has escaped Earth's gravitational pull to space. Despite their relative scarcity, the most important trace gases in the Earth's atmosphere are the greenhouse gases. Most abundant in the troposphere, these gases include carbon dioxide, methane, nitrous oxide, water vapour and ozone, so-called because they are involved in the Earth natural greenhouse effect which keeps the planet warmer than it would be without an atmosphere. Apart from water vapour, the most abundant greenhouse gas (by volume) is carbon dioxide.

Despite being present in only 380 parts per million by volume of air, carbon dioxide and the other greenhouse gases help to keep the Earth 33�C warmer than it would otherwise be without an atmosphere. Through emissions of greenhouse gases however, mankind has enhanced with natural greenhouse effect which may now be leading to a warming of the Earth climate. Whilst ozone behaves like a greenhouse gas in the troposphere, in the stratosphere where its abundance is most significant within the ozone layer, it helps to filter out the incoming ultraviolet radiation from the Sun, protecting life on Earth from its harmful effects. Air within the stratosphere is thin however. If all the ozone in the stratosphere was compressed to ordinary atmosphere pressure at ground level, it would occupy a layer only 3 mm thick. Other trace gases in the atmosphere arise from natural phenomena such as volcanic eruptions, lightning strikes and forest fires. Gases from these sources include nitric oxide (NO) and sulphur dioxide (SO 2 ). In addition to natural sources of nitric oxide and sulphur dioxide there are now many man-made sources, including pollutant emissions from cars, agriculture and electricity generation through the burning of fossil fuels.

During the 20th century other man-made processes have put completely new trace gases into the atmosphere, for example the chlorofluorocarbons (CFCs) which damage the ozone layer. Aerosols Aerosols are solid or liquid particles dispersed in the air, and include dust, soot, sea salt crystals, spores, bacteria, viruses and a plethora of other microscopic particles. Collectively, they are often regarded as air pollution, but many of the aerosols have a natural origin. They are conventionally defined as those particles suspended in air having diameters in the region of 0.001 to 10 microns (millionth of a metre). They are formed by the dispersal of material at the surface (primary aerosols), or by reaction of gases in the atmosphere (secondary aerosols). Primary aerosols include volcanic dust, organic materials from biomass burning, soot from combustion and mineral dust from wind-blown processes. Secondary aerosols include sulphates from the oxidation of sulphur-containing gases during the burning of fossil fuels, nitrates from gaseous nitrogen species, and products from the oxidation of volatile organic compounds (VOCs).

Although making up only 1 part in a billion of the mass of the atmosphere, they have the potential to significantly influence the amount of sunlight that reaches the Earth�s surface, and therefore the Earth's climate. Although the abundance of aerosols varies over short time scales, for example after a volcanic eruption, over the long term the atmosphere is naturally cleansed through mixing processes and rainfall. Cleansing is never complete however, and there exists a natural background level of aerosols in the atmosphere. The average time spent in the atmosphere by aerosols is dependent upon their physical and chemical characteristics, and the time and location of their release. Natural sources of aerosols are probably 4 to 5 times larger than man-made ones on a global scale, but regional variations of man-made aerosol emissions may change this ratio significantly in certain areas, particularly in the industrialised Northern Hemisphere. At certain times of the year, the natural background level of aerosols may increase, for example, during the growing season, when large quantities of pollen are released into the atmosphere.

Question: Which gas forms approximately 1% of the atmosphere?
Answer: Argon