The origin of the terrestrial noble-gas signature : Nature : Nature Research The origin of the terrestrial noble-gas signature Article metrics In the atmospheres of Earth and Mars, xenon is strongly depleted relative to argon, when compared to the abundances in chondritic meteorites 1 , 2 . The origin of this depletion is poorly understood 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 . Here we show that more than one weight per cent of argon may be dissolved in MgSiO3 perovskite, the most abundant phase of Earth’s lower mantle, whereas the xenon solubility in MgSiO3 perovskite is orders of magnitude lower. We therefore suggest that crystallization of perovskite from a magma ocean in the very early stages of Earth’s history concentrated argon in the lower mantle. After most of the primordial atmosphere had been lost, degassing of the lower mantle replenished argon and krypton, but not xenon, in the atmosphere.

Our model implies that the depletion of xenon relative to argon indicates that perovskite crystallized from a magma ocean in the early history of Earth and perhaps also Mars. Subject terms: View all figures Figure 1: Scanning electron microscope images of MgSiO3 perovskite samples saturated with noble gases. a, Perovskite crystallized in the presence of excess xenon. The bright spots in the image are xenon-filled bubbles in perovskite below the sample surface. However, in those parts of the perovskite crystals that are far away from bubbles, no xenon can be detected, implying very low xenon solubility (<100 p.p.m.). b, Argon-saturated sample with perovskite (Pv, arrows) coexisting with MgSiO3 majorite (garnet structure, Maj). Dark spots are holes previously filled by argon gas. No argon can be detected in majorite (detection limit 150 p.p.m.), but the perovskite contains 3,000 p.p.m. of argon.

c, Another sample, containing perovskite (lighter areas) coexisting with Mg2SiO4 ringwoodite (Rw, dark) and MgSiO3 ilmenite (Ilm, dark). d, X-ray map of argon distribution in the same sample as shown in c. Argon is only found in the perovskite phase. The concentration scale shown corresponds to the range from 0 (blue) to approximately 0.8 wt% (green) of Ar. View in article Figure 2: A lattice-strain model 29 of noble-gas solubility in MgSiO3 perovskite. The fit is based on Ar, Kr and Xe solubility data for 25 GPa at 1,600 °C to 1,800 °C. Estimated values for the solubilities of He and Ne are also shown. Fit parameters are the effective Young’s modulus Em = 35 GPa; radius of the oxygen site ri = 1.4 Å, and strain-free solubility c0 = 1.53 wt%. Noble-gas radii are from ref. 22 .

Error bars are one standard deviation. For further details, see Supplementary Information . View in article Figure 3: A sample containing phase X (phX) coexisting with MgSiO3 ilmenite (Ilm), SiO2 stishovite (St) and K2CO3 carbonate. a, Scanning electron microscope image. b, X-ray map of argon distribution; argon is only present in phase X. The concentration scale shown corresponds to the range from 0 (blue) to approximately 1 wt% (red) of Ar. Supplementary information The relative abundance of noble gases in Earth’s atmosphere is largely controlled by the early evolution of our planet. Most of the argon in Earth’s atmosphere is the isotope 40Ar, which is a radioactive decay product of 40K. However, the abundances of ‘primordial’ (non-radiogenic) noble-gas isotopes also differ among the Sun, the terrestrial planets and chondritic meteorites 1 , 2 . Both chondritic meteorites and terrestrial planets are strongly depleted in the light noble gases helium and neon compared to the Sun.

This feature can readily be explained by the loss of light noble gases to space in a relatively weak gravitational field. However, compared to chondrites, which are believed to be similar to the material from which the terrestrial planets formed, xenon is strongly depleted on Earth and Mars relative to argon and krypton. This depletion cannot be explained by loss to space, because xenon, the heaviest of all noble gases, should be least affected by this process. Therefore, a number of alternative hypotheses has been proposed to account for the ‘missing xenon’. One class of models assumes that xenon is present on Earth in its expected chondritic ratio relative to argon and krypton and that the xenon is hidden in some near-surface reservoir, such as glacier ices, clathrates, sediments or silicates 3 , 4 , 5 , 6 . However, such a xenon-rich reservoir has never been found in nature. Another class of models has suggested that xenon may be concentrated in Earth’s core, either in solid solution with iron or possibly as a separate phase 7 , 8 .

Recent experiments, however, did not find any evidence for dissolution of xenon in iron at core pressures, although the full range of pressures and temperature relevant to core formation has not yet been explored 9 . A third class of models explains the terrestrial noble-gas abundance by a combination of hydrodynamic escape to space and mantle degassing, but these models do not provide a simple physical explanation for the depletion of xenon relative to argon in the atmosphere either 2 , 10 . Alternative hypotheses include the delivery of volatiles to Earth by comets that may be depleted in xenon 11 or the preferential loss of xenon to space owing to its low ionization potential 12 , 13 . The lower mantle is the largest reservoir on Earth, with a mass exceeding 50% of the mass of the entire planet. Surprisingly, the potential role of the lower mantle in producing the terrestrial noble-gas signature has never been investigated. We have therefore studied the solubility of noble gases (argon, krypton, and xenon) in MgSiO3 perovskite, the major phase of the lower mantle.

Although it is widely believed that noble gases are very poorly soluble in upper-mantle minerals 14 , there are structural considerations suggesting that this may be different for the perovskite phase. MgSiO3 perovskite may contain abundant oxygen vacancies 15 , 16 , 17 , 18 , 19 , 20 , 21 . These oxygen vacancies may occur charge-balanced by Si vacancies at low silica activity, such as in equilibrium with the ferropericlase (Mg,Fe)O phase in the lower mantle. The substitution of Al3+ for Si4+ may also produce oxygen vacancies. Given that the atomic radius of argon is not far away from the ionic radius of the O2− ion, oxygen vacancies in perovskite may provide suitable storage sites for noble-gas atoms of appropriate radius. Experimental results on noble-gas solubility in MgSiO3 perovskite are compiled in Supplementary Table 1 .

The experiments with argon yielded MgSiO3 perovskite with up to more than 1 wt% of argon, whereas the xenon solubility was nearly 300 parts per million (p.p.m.), even if excess xenon was clearly present during the experiment ( Fig. 1a ). Data for krypton overlap with the argon data, but tend to be somewhat lower. The high argon concentrations are a characteristic property of the perovskite phase. This can clearly be seen in Fig. 1b , which shows a sample in which argon-saturated MgSiO3 majorite (with garnet structure) coexists with MgSiO3 perovskite. No argon is found in the majorite (detection limit 150 p.p.m.), while the coexisting perovskite contains about 3,000 p.p.m. of argon. In some samples argon solubility is so high that the distribution can be visualized by X-ray mapping ( Fig. 1c and d ), although the instability of perovskite under the electron beam limits the quality of the images that may be obtained.

Again, these maps show that the high argon solubility is restricted to the perovskite phase. In the course of this study, numerous phases were produced by accident because the experiments did not reach the perovskite stability field. In none of these phases, which included MgSiO3 ilmenite, SiO2 stishovite, Mg2SiO4 ringwoodite, and MgO periclase was any argon ever detected. Careful study of the perovskite samples by scanning and transmission electron microscopy did not reveal any impurities or foreign phases, such as gas bubbles, that may account for the observed argon content. We therefore conclude that the argon must be incorporated into the crystal lattice of the perovskite phase. This is consistent with some preliminary single-crystal X-ray diffraction data. The Ar-bearing perovskite shows some additional, weak reflections that are consistent with the symmetry of the phase, but do not occur in argon-free samples. Subtle, but systematic differences in reflection intensity were also observed. A detailed structure refinement was not possible owing to the large width of the X-ray reflections.

However, the broadening of the reflections (0.5° Ω half width as opposed to 0.07° for pure MgSiO3 perovskite; MoKα radiation) indicates some internal strain that itself is suggestive of argon being incorporated in the crystal lattice of the perovskite. Moreover, in some samples, high argon contents appear to correlate with low Si/Mg ratios. This is consistent with argon being incorporated in oxygen vacancies, which are charge-balanced by Si vacancies. Figure 1: Scanning electron microscope images of MgSiO3 perovskite samples saturated with noble gases. a, Perovskite crystallized in the presence of excess xenon. The bright spots in the image are xenon-filled bubbles in perovskite below the sample surface. However, in those parts of the perovskite crystals that are far away from bubbles, no xenon can be detected, implying very low xenon solubility (<100 p.p.m.). b, Argon-saturated sample with perovskite (Pv, arrows) coexisting with MgSiO3 majorite (garnet structure, Maj). Dark spots are holes previously filled by argon gas.

No argon can be detected in majorite (detection limit 150 p.p.m.), but the perovskite contains 3,000 p.p.m. of argon. c, Another sample, containing perovskite (lighter areas) coexisting with Mg2SiO4 ringwoodite (Rw, dark) and MgSiO3 ilmenite (Ilm, dark). d, X-ray map of argon distribution in the same sample as shown in c. Argon is only found in the perovskite phase. The concentration scale shown corresponds to the range from 0 (blue) to approximately 0.8 wt% (green) of Ar. Next If argon primarily substitutes for oxygen vacancies in perovskite, the low solubility of xenon is probably related to the size mismatch between xenon and the oxygen vacancies. The atomic radius of argon (1.64 Å; ref. 22 ) is close to the radius of O2− (1.28–1.4 Å), but xenon with an atomic radius of 1.96 Å is probably too large to fit into an oxygen vacancy.

Our data therefore suggest that noble gases of suitable atomic radius can easily be incorporated in minerals that have appropriate vacancies. Such vacancies exist in lower-mantle perovskite 15 , 16 , 17 , 18 , 19 , 20 , 21 , but not in the minerals of the upper mantle; the behaviour of noble gases in the lower mantle therefore fundamentally differs from that observed in upper-mantle minerals. Noble-gas solubilities in silicate perovskite observed in this study reach the level of weight per cent, implying that these phases are actually a new kind of noble-gas compound. The dependence of noble-gas solubility on the size of the noble gas atom can be well described by a lattice strain model ( Fig. 2 ). The model predicts that helium and particularly neon should also be highly soluble in perovskite. Figure 2: A lattice-strain model 29 of noble-gas solubility in MgSiO3 perovskite. The fit is based on Ar, Kr and Xe solubility data for 25 GPa at 1,600 °C to 1,800 °C.

Estimated values for the solubilities of He and Ne are also shown. Fit parameters are the effective Young’s modulus Em = 35 GPa; radius of the oxygen site ri = 1.4 Å, and strain-free solubility c0 = 1.53 wt%. Noble-gas radii are from ref. 22 . Error bars are one standard deviation. For further details, see Supplementary Information . Next The idea that noble gases are quite soluble in some minerals with appropriate vacancies is supported by an observation that was accidentally made during the course of this study. In an attempt to obtain better single crystals of perovskite for X-ray diffraction studies, some experiments were carried out with K2CO3 as the fluxing agent added to the starting material. In these experiments, a K-bearing phase was observed that also dissolved up to 0.5 wt% of argon ( Fig. 3 ). Further investigations showed that this phase had been previously described as ‘phase X’. Structure refinements of phase X (ref. 23 ) suggest that it always contains vacancies on the K site, with a general formula of K2 − xMg2Si2O7Hx.

Given that the ionic radius of K+ is almost identical to the radius of Ar, the high argon solubility observed in this phase is not surprising. Another observation in these experiments is that the argon solubility in perovskite coexisting with phase X is low. This may be due to preferred partitioning of Ar into phase X or to high silica activity, because in some of these experiments SiO2 stishovite was detected as a separate phase. The perovskite in these experiments is therefore silica-saturated and no vacancies on the Si4+ and O2− sites are expected. Figure 3: A sample containing phase X (phX) coexisting with MgSiO3 ilmenite (Ilm), SiO2 stishovite (St) and K2CO3 carbonate. a, Scanning electron microscope image. b, X-ray map of argon distribution; argon is only present in phase X. The concentration scale shown corresponds to the range from 0 (blue) to approximately 1 wt% (red) of Ar.

Editor's summary in العربية The noble gas xenon is strongly depleted relative to argon in the atmospheres of both Earth and Mars, when compared with the abundances in chondritic meteorites, which are generally considered to be indicators of early Solar System geochemistry. Here, Svyatoslav Shcheka and Hans Keppler offer a possible explanation for this apparent anomaly. They show that more than 1 wt% of argon can be dissolved in the abundant lower-mantle mineral perovskite (MgSiO3), but that xenon solubility in perovskite is orders of magnitude lower. Thus the crystallization of perovskite from a magma ocean in the very early stages of Earth's history could have concentrated argon in the lower mantle, with xenon being lost along with the early Earth's atmosphere. The argon from the lower mantle was then gradually released into Earth's atmosphere, explaining its abundance relative to xenon. Editors' pick Image credit: David White/REX/Shutterstock What Are the Three Most Abundant Gases in the Earth's Atmosphere? | The Classroom | Synonym What Are the Three Most Abundant Gases in the Earth's Atmosphere?

by Chris Deziel There is no discrete boundary between Earth's atmosphere and space. Related Articles Life on the Planet Saturn The layer of gases that surrounds Earth and forms its atmosphere is paper-thin when compared with the diameter of the planet itself, and without it, life probably wouldn't exist. Besides providing the necessities for respiration, the atmosphere filters dangerous rays from the sun while trapping warmth and preventing it from radiating into space. Two molecules form the bulk of the atmosphere, while trace amounts of several others make up the rest. The 2 Main Components As Earth was forming, volcanoes spewed ammonia, carbon dioxide and water vapor into the atmosphere. Much of the carbon dioxide dissolved in seawater, and nitrogen, which is a component of ammonia, now forms 78 percent of the atmosphere. As the planet matured, organisms in the seawater learned to live on carbon dioxide, expelling oxygen gas -- which comprises two oxygen molecules bonded together -- as a waste product. Consequently, oxygen forms 21 percent of the atmosphere. Oxygen is actually 10,000 times more abundant on Earth than nitrogen because it forms stable lattices with other elements and is incorporated into the ground.

The 1 Percent The last 1 percent of the gases in the atmosphere form two groups: those whose quantities fluctuate and those that remain constant. The most abundant of these trace components is argon, which forms a constant 0.93 percent of the atmosphere. Other noble gases -- helium, neon and krypton -- also exist in trace amounts. Among the elements whose quantities vary are water vapor, ozone, methane, sulfur dioxide, nitrogen oxides and -- most abundantly -- carbon dioxide. The atmosphere also contains particulate matter, such as snow, dust and volcanic ash, whose quantities are highly variable. Importance of Variable Components The main components of the atmosphere -- nitrogen and oxygen -- don't have much effect on the meteorological patterns or climate, but the variable elements do. For example, variations in water vapor reflect relative humidity. The so-called greenhouse gases -- carbon dioxide, sulfur dioxide, methane and nitrogen oxide -- absorb heat from the Earth and warm the planet. Without these gases in the atmosphere, the average temperature of the earth would be 30 degrees Celsius (54 degrees Fahrenheit) cooler than it is now, and current life forms probably wouldn't exist.

Learning from Other Planets Saturn's moon Titan has an atmosphere that resembles what scientists believe once existed on Earth. It is composed almost entirely of nitrogen, but has small amounts of methane and other hydrocarbons. By studying this atmosphere and the effect that sunlight has upon it, scientists hope to gain an understanding of how life developed on primordial Earth. The planet Venus, on the other hand, offers a dramatic demonstration of the effects of greenhouse gases. The presence of large amounts of carbon dioxide and sulfur dioxide in the atmosphere of that planet has produced a runaway warming effect that has raised the surface temperature to 460 degrees Celsius (860 degrees Fahrenheit). References What are Noble Gases? : Noble Gases in the Sea : Woods Hole Oceanographic Institution (WHOI) Text Size: What are Noble Gases? Noble gases are chemically unreactive gases (at least under normal earth-like conditions) that exist in trace quantities in our atmosphere and to varying extents dissolved in the ocean. They are unreactive because their outermost electron shells are completely filled, so they don't form chemical bonds very easily. If you look at the table of elements, you will find them in the far right hand column.

The first 5 noble gases are Helium: This is the lightest of the gases, and has the elemental symbol He. It is the second most abundant element in the universe, but exists in our atmosphere at only 5 parts per million. The reason why it is so rare in our atmosphere is because it is so light: a lot of it has been lost to outer space over geologic time. It is very insoluble in water compared to most other gases. Neon: The second lightest noble gas has the elemental symbol Ne. It is much less abundant in our universe than He, but it is actually about 4 times more abundant (18 parts per million) than He in our atmosphere. Ne is also very insoluble in water, but a little more than He. Argon: Argon's elemental symbol is Ar, and it is the most abundant noble gas in the atmosphere. There's so much Ar in the atmosphere because most of it was produced by a long-lived radioactive isotope of potassium in the earth's crust and released over geologic time. Ar is more soluble in seawater than He and Ne. Krypton: This has nothing to do with superman! This is a heavier noble gas with the elemetnal symbol Kr.

It is rarer than He in the atmosphere, having an abundance slightly more than 1 part per million. It is also more soluble than Ar in water. Xenon: This is the heaviest of the stable noble gases, with the elemental symbol Xe. It is also the least abundant in our atmosphere, being 10 times less common than Kr. It is the most soluble of the 5 stable noble gases in water. Last updated December 28, 2007 © Woods Hole Oceanographic Institution. All rights reserved Noble Gases | G-WADI Noble Gases ( return to top ) The noble gases are any of the chemically inert elements found on the far right of the periodic table in the helium group. The noble gas family consists of six different elements, five of which have stable isotopes (helium [He], neon [Ne], argon [Ar], krypton [Kr], and xenon [Xe]), and one that only has radioactive isotopes (radon [Ra]). Table 1 lists the stable isotopes in the noble gas family and their abundances per element. The radioactive isotopes of argon and krypton and the stable isotopes of helium are described separately on this website.

( return to top ) Gas Chromatography/Mass Spectrometry: Price ranges for noble gas analysis are not currently posted. Many research labs perform this analysis but on an in-house basis only. For more information, visit web sites for individual labs such as: ( return to top ) Atmospheric Composition Noble gases are naturally occurring and present in the earth's atmosphere. Table 2 displays the abundances for each of these gases within the atmosphere: Source: Most of the noble gases have been in the earth's atmosphere since the earth formed. Two exceptions are radon and helium. Radon is continuously produced as a daughter product during uranium decay. It is radioactive and decays quickly to other elements. Due to its light weight, most helium originally present in the atmosphere has escaped to space. The helium currently present in the earth's atmosphere is largely the product of alpha decay of heavy isotopes (to 4He) and the beta decay of cosmogenic tritium (to 3He). Measurement techniques ( return to top ) Sampling Samples are typically taken from wells through a plastic hose into a copper tube (~1cm in diameter, <1m long). Water is run through the tube for several minutes before it is sealed using stainless steel clamps or special pliers.

A regulator valve at one end of the copper tube is typically used to increase the pressure in order to minimize bubble formation. Detailed instructions for sampling are outlined at the USGS Reston Chlorofluorocarbon Laboratory FAQ page . Measurement Noble gas concentrations in groundwater samples can currently be measured in several ways: gas chromatography, mass spectrometry, or a combination of both. The precision of mass spectrometry leads to a reproducibility (including all steps in the sampling and measurement procedures) of ± 1-2% for the abundance of each gas and ±0.1-1% for ratios of more abundant isotopes. Gas chromatography is slightly less precise. Noble Gas Behavior ( return to top ) The behavior of noble gases is governed by three factors: diffusion, partition between phases, and nuclear transformation. Diffusion is the process by which the gases move from areas of higher concentration to areas of lower concentration until equilibrium is reached. Lighter isotopes are more mobile and thus more affected by diffusion than heavier isotopes. In gases, the ratio of diffusion coefficients is equal to the square root of their masses. Partitioning between phases is an important process in bodies of water in contact with the atmosphere.

Lakes, rivers, oceans, and groundwater at the water table are all generally assumed to be in equilibrium with the atmosphere with respect to their concentrations of noble gases. The atmosphere is the greatest source of noble gases for these bodies of water. The equilibrium relationship between the atmosphere and the body of water is described in Henry's Law: pi = ki(T,S)xi where pi is the partial pressure of the noble gas in the gas phase, ki is a proportionality coefficient (Henry's coefficient, dependent on temperature [T] and salinity [S]), and xi is the equilibrium concentration of the dissolved gas in water (or other liquid) as a mole fraction. In other words, the equilibrium concentration of the noble gases depends on three parameters: temperature, salinity, and atmospheric pressure. Nuclear transformation through radioactive decay may be important for some isosopes. For example, 4 He is a stable product of many U-series decay paths. In contrast, 222Rn gas is a radioactive intermediate.

Hydrological Applications ( return to top ) Assuming that salinity and atmospheric pressure are known, water that is at equilibrium with the atmosphere at the time of recharge and is subsequently sealed off from the atmosphere may be used to reconstruct the temperature at the time of the recharge. Since noble gases tend to not react with other elements, the concentrations of noble gases in groundwater remain fairly constant once sealed off from the atmosphere. Two additional parameters need to be considered in the reconstruction of these past or paleo-temperatures: amount of excess air (A) and degree of re-equilibration (R). Excess air is dissolved gas in the groundwater that does not originate with the initial equilibration with the atmosphere. One possible scenario for the formation of this excess air is as follows: the water table rises, quickly trapping air bubbles in the groundwater; the air bubbles partially dissolve in the water (with the heavier noble gases partially dissolving first, leading to fractionation); the gas slowly exchanges with the atmosphere (re-equilibration) until new recharge seals off the exchange (Stute and Schlosser 2000). The resulting gas component in the water is considered excess air.

It can be recognized when the ratio of N2 to Ar in the sample is different from the equilibrium solubility ratio of N2 to Ar in water. High concentrations of excess air are caused by a rapid rise in the water table and are indicative of fractured rock aquifers and aquifers in semi-arid lands. Because the solubility of gases varies as a function of temperature, noble gas concentrations can be used to determine recharge temperature of a groundwater sample. (Studies generally use stable neon, argon, krypton, and xenon gases in reconstructing the temperature of recharge from years to millions of years ago; due to high radiogenic production, helium is not often considered). This may provide a clue as to where the recharge occurred. Likewise, if combined with an age-dating isotope such as 3H, 36Cl, or 14C, a record of temperature can be reconstructed for the aquifer system. Such information is useful for determining change in groundwater recharge and the relationship to climate change. This technique deals with total gas concentration, not one particular isotope. Several noble gases must be measured for the best temperature determination.

The solubilities of noble gases are highly temperature dependent but each isotope has a unique relationship to temperature. Therefore, by determining the ratios of several noble gas concentrations in groundwater samples, recharge temperature can be determined: Variation in noble gas solubilities with temperature relative to their solubility at 0ºC (modified from Benson and Krause 1976 and Andrews 1992). References and Further Reading Aeschbach-Hertig, W., et al, Interpretation of dissolved atmospheric noble gases in natural waters, Water Resour. Res. 35(9), 2779-2792, 1999. Andrews, J.N., Mechanisms for noble gas dissolution by groundwaters, in Isotopes of Noble Gases as Tracers in Environmental Studies, pp. 87-110, International Atomic Energy Agency, Vienna, Austria, 1992. Attendorn, H.G., and R.N.C. Bowen, Radioactive and Stable Isotope Geology, Chapman and Hall, New York, 1997. Benson, B.B., and D.

Krause, Empirical laws for dilute aqueous solutions of nonpolar gases, J. Chem. Phys. 64, 639-709, 1976. evolution of the atmosphere | atmosphere | Britannica.com Evolution of the atmosphere halo Evolution of the atmosphere, the development of Earth’s atmosphere across geologic time . The process by which the current atmosphere arose from earlier conditions is complex; however, evidence related to the evolution of Earth’s atmosphere, though indirect, is abundant. Ancient sediments and rocks record past changes in atmospheric composition due to chemical reactions with Earth’s crust and, in particular, to biochemical processes associated with life. Figure 2: A “best guess” reconstruction of the abundance of O2 in the … Earth’s original atmosphere was rich in methane , ammonia , water vapour, and the noble gas neon , but it lacked free oxygen . It is likely that hundreds of millions of years separated the first biological production of oxygen by unicellular organisms and its eventual accumulation in the atmosphere. The composition of the atmosphere encodes a great deal of information bearing on its origin. Furthermore, the nature and variations of the minor components reveal extensive interactions between the atmosphere, terrestrial environment, and biota.

The development of the atmosphere and such interactions are discussed in this article, with particular attention given to the rise of biologically produced molecular oxygen, O2, as a major component of air . For modern atmospheric chemistry and physics, see atmosphere . Concepts related to atmospheric development A complete reconstruction of the origin and development of the atmosphere would include details of its size and composition at all times during the 4.5 billion years since Earth’s formation. This goal could not be achieved without knowledge of the pathways and rates of supply and consumption of all atmospheric constituents at all times. Information regarding these particular processes, however, is incomplete even for the present atmosphere, and there is almost no direct evidence regarding atmospheric constituents and their rates of supply and consumption in the past. The contrast with related fields of Earth’s history is notable. Fossils and other structural and chemical details of ancient rocks provide information useful to evolutionary biologists and historical geologists, but ancient atmospheres, “mere vapours,” have not left such substantial remnants. These vapours are, however, the stuff of stars and the moving force of storms and erosion .

The atmosphere as part of the crust Similar Topics oceanic trough To the Earth scientist, the crust includes not only the top layer of solid material (soil and rocks to a depth of 6 to 70 km [4 to 44 miles], separated from the underlying mantle by differences in density and by susceptibility to surficial geologic processes) but also the hydrosphere (oceans, surface waters on land, and groundwater beneath the land surface) and the atmosphere. Interactions among these solid, liquid, and gaseous portions of the crust are so frequent and thorough that considering them separately introduces more complexities than it eliminates. As a result, a description of the history of the atmosphere must concern itself with all volatile components of the crust.

Materials Volatile compounds as well as elements important in present and past atmospheres or in interactions between the atmosphere, biosphere , and other portions of the crust include the following: Present major components: molecular nitrogen (N2) and molecular oxygen (O2) Noble gases : helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe) Abundant variable components: water vapour (H2O) and carbon dioxide (CO2) Other components: molecular hydrogen (H2), methane (CH4), carbon monoxide (CO), ammonia (NH3), nitrous oxide (N2O), nitrogen dioxide (NO2), hydrogen sulfide (H2S), dimethyl sulfide [(CH3)2S], sulfur dioxide (SO2), and hydrogen chloride (HCl). Some elements appear in multiple form—for example, carbon as carbon dioxide, methane , or dimethyl sulfide. It is useful to consider the occurrence of the elements before focusing on the more specific aspects of atmospheric chemistry (the forms in which the elements are present).

One can speak of Earth’s “inventory of volatiles ,” recognizing that the components of the inventory may be reorganized from time to time, but also that it is always composed primarily of the compounds of hydrogen, carbon, nitrogen, and oxygen, along with the noble gases. Processes Scientists Ponder Menopause in Killer Whales A process that delivers a gas to the atmosphere is termed a source for the gas. Depending on the question under consideration, it can make sense to speak in terms of either an ultimate source—the process that delivered a component of the volatile inventory to Earth—or an immediate source—the process that sustains the abundance of a component of the present atmosphere. Any process that removes gas either chemically, as in the consumption of oxygen during the process of combustion, or physically, as in the loss of hydrogen to space at the top of the atmosphere, is called a sink . Trees: Giants Holding the Sky Throughout the history of the atmosphere, sources and sinks have often been simultaneously present. While one process consumes a particular component, another produces it, and the concentration of that component in the atmosphere will rise or fall depending on the relative strengths of the sources and sinks.

If those strengths are balanced (or nearly so), the composition of the atmosphere will not change (or will change only very slowly, perhaps imperceptibly); however, the molecules of the gas in question are passing through the atmosphere and are not permanently resident. The rate of the resulting turnover of molecules in the atmosphere is expressed in terms of the residence time , the average time spent by a molecule in the atmosphere after it leaves a source and before it encounters a sink. Processes affecting the composition of the early atmosphere Ultimate sources The material from which the solar system formed is often described as a gas cloud or, at a later stage, a solar nebula . The cloud was rich in volatiles (termed primordial gases) and must have been the ultimate source of the atoms in the present atmosphere. What is of primary concern, however, is the sequence of events and processes by which the volatiles present in the initial gas cloud were transferred to Earth’s inventory and the efficiency with which this was accomplished. Centre of the Orion Nebula (M42).

Photo AURA/STScI/NASA/JPL (NASA photo # STScI-PRC95-45a) The formation of the solar system began when one portion of the gas cloud became dense enough due to compression by some external force—a shock wave from the explosion of a nearby supernova , perhaps—to gravitationally attract the material around it. This material “fell” into the region of higher density, making it even denser and attracting other material from still further away. As gravitational collapse continued, the centre of the cloud became very dense and hot, because the kinetic energy of the incoming material was released as heat. Thermonuclear reactions began at the core of the central object, the Sun . Capture and retention of primordial gases Connect with Britannica Facebook Twitter YouTube Instagram Pinterest Far from the central point, the material in the gas cloud tended to settle to an extensive equatorial plane around the Sun. As the material in this disk cooled, chunks of rock grew and accreted to form the planets. The planets are much less massive than the Sun, but if they grew large enough and if the gases around them were cool enough, they could accumulate an atmosphere from the volatile components of the gas cloud. .

This direct capture is the first of three source mechanisms that can be described. Abundances of elements 0.0000036 0.0000036 *Abundances indicate how many atoms of each element (or, in the case of the noble gases, isotopes) would accompany one million silicon (Si) atoms. For example, the abundance of nitrogen (N) in the solar system is 2.5 times greater than that of Si, whereas its abundance on Earth is less than that of Si by a factor of 0.000079. The table includes the eight most abundant volatile elements, together with others. **See text. A planetary atmosphere accumulated in this way would consist of primordial gases, but the relative abundances of the individual components would differ from those in the gas cloud if the gravitational field of the new planet were strong enough to hold some, but not all, of the gases around it. It is convenient to express the strength of a gravitational field in terms of escape velocity , the speed at which any particle (a molecule or spacecraft) must be traveling in order to overcome the force of gravity.

For Earth, this velocity is 11.3 km (7.0 miles) per second, and it follows that, once the solid material had accumulated, gas molecules passing Earth at lower speeds would have been captured and accumulated to form an atmosphere. The speed at which a gas molecule moves is proportional to (T/M)1/2, where T is absolute temperature in kelvins (K) and M is molecular mass. The uppermost layers of the present atmosphere are still very hot and might have been much hotter early in Earth’s history. At temperatures below 2,000 K, however, molecules of any compound with a molecular weight greater than about 10 will have an average velocity of less than 11.3 km per second (7.0 miles per second). On this basis, it has long been thought that Earth’s earliest atmosphere must have been a mixture of the primordial gases with molecular weights greater than 10. Hydrogen and helium, with molecular weights of 2 and 4, should have been able to escape. Because hydrogen is the most abundant element in the solar system, it is thought that the most abundant forms of the other volatile elements were their compounds with hydrogen.

If so, methane , ammonia , and water vapour, together with the noble gas neon , would have been the most abundant volatiles with molecular weights greater than 10 and, thus, the major constituents of Earth’s primordial atmosphere. The atmospheres of the four giant outer planets ( Jupiter , Saturn , Uranus , and Neptune ) are rich in such components, as well as in molecular hydrogen and, presumably, helium, which those more massive and colder bodies were apparently able to retain. Outgassing of the solid planet Britannica Lists & Quizzes Editor Picks: Exploring 10 Types of Basketball Movies The release of gases during volcanic eruptions is one example of outgassing; releases at submarine hydrothermal vents are another. Although the gas in modern volcanic emanations commonly derives from rocks that have picked up volatiles at Earth’s surface and then have been buried to depths at which high temperatures remobilize the volatile material, a very different situation must have prevailed at the earliest stages of Earth’s history. A column of gas and ash rising from Mount Pinatubo in the Philippines on June 12, 1991, just days … David H.

Harlow/U.S.Geological Survey The planet accreted from solid particles that formed as the primordial gas cloud cooled. Long before the volatile components of the cloud began to condense to form massive solid phases (that is, long before water vapour condensed to form ice), their molecules would have coated the surfaces of the solid particles of rocky material that were forming. As these solid particles continued to grow, a portion of the volatiles coating their surfaces would have been trapped and carried thereafter by the particles. If the solids were not remelted by impact as they collected to form the planet, the volatiles they carried would have been incorporated in the solid planet. In this way, even without collecting an enveloping gaseous atmosphere, a newly formed planet could include—as material occluded in its constituent grains—a substantial inventory of volatiles. Trending Topics Open Door policy At some point in its early history, Earth became so hot that much of the iron dispersed among the solid particles melted, became mobile, and collected to form the core. Related events led to the formation of rocky layers that were the precursors of Earth’s present-day mantle and crust.

As part of this process of differentiation, volatiles present in the particles would have been released through outgassing. The outgassing must have occurred on a colossal scale if the accreting particles had retained their volatiles right up to the time of differentiation. An atmosphere created by retention of these outgassing products would derive ultimately from nebular gases. Its chemical composition, however, would be expected to differ in two principal respects from that of an atmosphere formed by the capture of primordial gases: (1) whereas the captured atmosphere would contain all gases that were moving slowly enough (that is, that were sufficiently cold and/or of sufficient molecular weight) so that it was possible for the planet to retain them gravitationally, the outgassed atmosphere would contain only those gases “sticky” enough to have been significantly retained in the rocky particles from which the planet formed; and (2) methane and ammonia, two presumed components of a captured atmosphere, would probably not be stable under the conditions involved in outgassing. Thus, the noble gases, which would be poorly held by particles, would be of low abundance relative to gases derived from chemically active elements.

Further, the principal forms of carbon and nitrogen in an outgassed atmosphere would be carbon monoxide or carbon dioxide together with molecular nitrogen. Importation A compromise between the extremes of direct capture and outgassing proposes that Earth’s inventory of volatiles was delivered to the planet late in its accretionary history—possibly after differentiation was nearly complete—by impact of a “last-minute” crop of solid bodies that were very strongly enriched in volatile materials (these were the last substances to condense as the solar nebula cooled). Such bodies might have had compositions similar to those of comets that still can be observed in the solar system. These last-minute condensates may have coated the planet as a surface veneer that yielded gases only when heated during differentiation, or they may have released their volatiles on impact. Because such bodies would have been relatively small, they would not have been able to retain primordial gases by means of a substantial gravitational field.

Their complement of volatiles, retained by cold trapping in ices and on particle surfaces, would be expected to resemble the “sticky” (that is, polar and reactive) gases occluded by solid particles at earlier stages of cooling of the gas cloud but possibly lost during earlier higher temperature phases of Earth’s accretion. Sinks The dominant pathways by which gases are removed from the present atmosphere are discussed below in the section Biogeochemical cycles . Apart from those processes, three other sinks merit attention and are described here. Photochemical reactions Sunlight can provide the energy required to drive chemical reactions that consume some gases. Due to a rapid and efficient photochemical consumption of methane (CH4) and ammonia (NH3), a methane–ammonia atmosphere, for example, would have a maximum lifetime of about 1 million years. This finding is of interest because it has been suggested that life originated from mixtures of organic compounds synthesized by nonbiological reactions starting from methane and ammonia. Recognition of the short atmospheric lifetimes of these materials poses grave difficulties for such a theory. Water, too, is not stable against sunlight that has not been filtered by overlying layers containing ozone or molecular oxygen, which very strongly absorb much of the Sun’s ultraviolet radiation.

Water molecules that rise above these layers are degraded to yield, among other products, hydrogen atoms (H·). Escape Hydrogen molecules (H2) and helium , or products like H·, tend to have velocities high enough so that they are not bound by Earth’s gravitational field and are lost to space from the top of the atmosphere. The importance of this process extends beyond the very earliest stages of Earth’s history because continuous sources exist for these light gases. Helium is continually lost as it is produced by the decay of radioactive elements in the crust. A combination of photochemical reactions and the subsequent escape of products can serve as a source for molecular oxygen (O2), a major component of the modern atmosphere that, because of its reactivity, cannot possibly have derived from any of the other sources so far discussed. In this process, water vapour is broken up by ultraviolet light and the resulting hydrogen is lost from the top of the atmosphere, so that the products of the photochemical reaction cannot recombine. The residual oxygen-containing products then couple to form O2. Solar-wind stripping The Sun emits not only visible light but also a continuous flow of particles known as the solar wind .

Most of these particles are electrically charged and interact only weakly with the atmosphere, because the Earth’s magnetic field tends to steer them around the planet. Prior to the formation of Earth’s iron core and consequent development of the geomagnetic field, however, the solar wind must have struck the top layers of the atmosphere with full force. It is postulated that the solar wind was much more intense at that time than it is today and, further, that the young Sun emitted a powerful flux of extreme ultraviolet radiation. In such circumstances, much gas may have been carried away by a kind of atomic sandblasting that may have had a marked effect on the earliest phases of atmospheric development. Biogeochemical cycles Interactions with the crust and, in particular, with living things—the biosphere—can strongly affect the composition of the atmosphere. These interactions, which form the most important sources and sinks for atmospheric constituents, are viewed in terms of biogeochemical cycles, the most prominent and central being that of carbon . The carbon cycle includes two major sets of processes: biologic and geologic.

The carbon cycle organic carbon in metamorphic rocks 3,480,000 *One Pg (abbreviation for petagram) equals one quadrillion (1015) grams. Entries refer to amounts of carbon. The carbon dioxide in the atmosphere is in equilibrium with, and far less abundant than, the oceanic inventory of carbon dioxide, bicarbonate ions (HCO3-), and carbonate ions (CO32-). If all carbon dioxide were somehow suddenly removed from the atmosphere, the ocean would replenish the supply within a few thousand years (the so-called stirring time of the ocean). Likewise, any change in the concentration of CO2 in the atmosphere is accompanied by a quantitatively far larger change in the amount of CO2, HCO3-, and CO32- in the ocean. Similar equilibriums prevail for molecular nitrogen (N2) and molecular oxygen (O2). The atmosphere contains about 3,940,000 petagrams (Pg; one petagram equals 1015 grams) of nitrogen as N2, with about 22,000 Pg being dissolved in the ocean.

Oxygen is distributed in such a way that 1,200,000 Pg of O2 are in the atmosphere while 12,390 Pg are in the ocean. Weathering reactions No matter what their origins, reactive gases in the atmosphere are likely to interact with other parts of the crust through what are termed weathering reactions. Not just carbonic acid associated with the carbon cycle but any acid becomes involved in acidic dissolution of susceptible rocks. As it does so, its concentration in the atmosphere declines, eventually reaching zero unless some process keeps replenishing the supply. Even if respiration were suddenly to cease, oxygen produced by photosynthesis, or any oxidant in the atmosphere, would be consumed if oxidizable materials were present. The corrosion of metals is the most familiar example of this process in the modern world, but there are other examples involving natural forms of iron, sulfur, and carbon as well. Much of the iron bound in minerals is in the ferrous form (Fe2+). As this material is exposed by uplift and erosion, it consumes atmospheric oxidants to form ferric iron (Fe3+), the red, fully oxidized form of iron commonly identified as rust (Fe2O3).

Sulfide minerals ( pyrite , or fool’s gold, being the most familiar example) also consume oxidants as the sulfur is oxidized to produce sulfate. Finally, natural exposure of sedimentary organic matter, including coal beds or oil seeps, results in the consumption of atmospheric oxidants as the organic carbon is oxidized to produce carbon dioxide. Page 1 of 2 Noble Gases - body, used, Earth, life, chemical, form, energy, reaction, oxygen Noble Gases Noble gases Photo by: photo-dave The noble gases are the six elements that make up Group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). At one time, this family of elements was also known as the rare gases. Their present name comes from the fact that the six gases are highly unreactive; they appear almost "noble"—above interacting with other members of the periodic table. This lack of reactivity has also led to a second name by which they are sometimes known—the inert gases. (Inert means inactive.) Abundance and production As their former name suggests, the noble gases are rather uncommon on Earth.

Collectively, they make up about 1 percent of Earth's atmosphere. Most of the noble gases have been detected in small amounts in minerals found in Earth's crust and in meteorites. They are thought to have been released into the atmosphere long ago as by-products of the decay of radioactive elements in Earth's crust. (Radioactivity is the property that some elements have of spontaneously giving off energy in the form of particles or waves when their nuclei disintegrate.) Of all the rare gases, argon is present in the greatest amount. It makes up about 0.9 percent by volume of Earth's atmosphere. The other noble gases are present in such small amounts that it is usually more convenient to express their concentrations in terms of parts per million (ppm). The concentrations of neon, helium, krypton, and xenon are, respectively, 18 ppm, 5 ppm, 1 ppm, and 0.09 ppm. For example, there are only 5 liters of helium in every million liters of air. By contrast, helium is much more abundant in the Sun, stars, and outer space. In fact, next to hydrogen, helium is the most abundant element in the universe.

About 23 percent of all atoms found in the universe are helium atoms. Radon is present in the atmosphere in only trace amounts. However, higher levels of radon have been measured in homes around the United States. Radon can be released from soils containing high concentrations of uranium, and they can be trapped in homes that have been weather sealed to make heating and cooling systems more efficient. Radon testing Lead canisters used to store xenon for medical diagnostic purposes. (Reproduced by permission of Photo Researchers, Inc. ) kits are commercially available for testing the radon content of household air. Most of the rare gases are obtained commercially from liquid air. As the temperature of liquid air is raised, the rare gases boil off from the mixture at specific temperatures and can be separated and purified. Although present in air, helium is obtained commercially from natural gas wells where it occurs in concentrations of between 1 and 7 percent of the natural gas. Most of the world's helium supplies come from wells located in Texas, Oklahoma, and Kansas. Radon is isolated as a product of the radioactive decay of radium compounds. Properties The noble gases are all colorless, odorless, and tasteless.

They exist as monatomic gases, which means that their molecules consist of a single atom apiece. The boiling points of the noble gases increase in moving down the periodic table. Helium has the lowest boiling point of any element. It boils at 4.215 K (−268.93°C). It has no melting point because it cannot be frozen at any temperature. The most important chemical property of the noble gases is their lack of reactivity. Helium, neon, and argon do not combine with any other elements to form compounds. It has been only in the last few decades that compounds of the other rare gases have been prepared. In 1962 English chemist Neil Bartlett (1932–) succeeded in preparing the first compound of a noble gas, a compound of xenon. The compound was xenon platinofluoride (XePtF 6 ). Since then, many xenon compounds containing mostly fluorine or oxygen atoms have also been prepared. Krypton and radon have also been combined with fluorine to form simple compounds. Because some noble gas compounds have powerful oxidizing properties, they have been used to synthesize other compounds.

The low reactivity of the noble gases can be explained by their electronic structure. The atoms of all six gases have outer energy levels containing eight electrons. Chemists believe that such arrangements are the most stable arrangements an atom can have. Because of these very stable arrangements, noble gas atoms have little or no tendency to gain or lose electrons, as they would have to do to take part in a chemical reaction. Uses As with all substances, the uses to which the noble gases are put reflect their physical and chemical properties. For example, helium's low density and inertness make it ideal for use in lighter-than-air craft such as balloons and dirigibles (zeppelins). Because of the element's very low boiling point, it has many applications in low-temperature research and technology. Divers breathe an artificial oxygen-helium mixture to prevent the formation of gas bubbles in the blood as they swim to the surface from great depths. Other uses for helium have been in supersonic wind tunnels, as a protective gas in growing silicon and germanium crystals and, together with neon, in the manufacture of gas lasers. Neon is well known for its use in neon signs. Glass tubes of any shape can be filled with neon.

When an electrical charge is passed through the tube, an orange-red glow is emitted. By contrast, ordinary incandescent lightbulbs are filled with argon. Because argon is so inert, it does not react with the hot metal filament and prolongs the bulb's life. Argon is also used to provide an inert atmosphere in welding and high-temperature metallurgical processes. By surrounding hot metals with inert argon, the metals are protected from potential oxidation by oxygen in the air. Krypton and xenon also find commercial lighting applications. Krypton can be used in incandescent lightbulbs and in fluorescent lamps. Both are also employed in flashing stroboscopic lights that outline commercial airport runways. And because they emit a brilliant white light when electrified, they are used in photographic flash equipment. Due to the radioactive nature of radon, it has medical applications in radiotherapy. Trace Gases 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 370 parts per million by volume of air, carbon dioxide and the other greenhouse gases help to keep the Earth 33ï¿½C 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 (SO2). 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 .

High School Earth Science/The Atmosphere - Wikibooks, open books for an open world High School Earth Science/The Atmosphere From Wikibooks, open books for an open world 8 Points to Consider Earth's atmosphere is a thin blanket of gases and tiny particles—together called air. Without air, the Earth would just be another lifeless rock orbiting the Sun. Although we are rarely aware of it, air surrounds us. We are most aware of air when it moves, creating wind. Like all gases, air takes up space. These gases that make up our air are packed closer together near the Earth's surface than at higher elevations. All living things need some of the gases in air for life support. In particular, all organisms rely on oxygen for respiration—even plants require oxygen to stay alive at night or when the Sun is obscured. Plants also require carbon dioxide in the air for photosynthesis. All weather happens in the atmosphere. The atmosphere has many other important roles as well. These include moderating Earth's temperatures and protecting living things from the Sun's most harmful rays. Describe the importance of the atmosphere to our planet and its life. Outline the role of the atmosphere in the water cycle.

List the major components of the atmosphere and know their functions. Describe how atmospheric pressure changes with altitude. Significance of the Atmosphere[ edit ] Without the atmosphere, planet Earth would be much more like the Moon than like the planet we live on today. The Earth's atmosphere, along with the abundant liquid water on the Earth's surface, are keys to our planet's unique place in the solar system. Much of what makes Earth exceptional depends on the atmosphere. Let's consider some of the many reasons we are lucky to have an atmosphere. Atmospheric Gases are Indispensable for Life on Earth[ edit ] Without the atmosphere, Earth would be lifeless. Carbon dioxide (CO2) and oxygen (O2) are the most important gases for living organisms. CO2 is vital for use by plants in photosynthesis, in which plants use CO2 and water to convert the Sun's energy into food energy. This food energy is in the form of the sugar glucose (C6H12O6). Plants also produce O2. Photosynthesis is responsible for nearly all of the oxygen currently found in the atmosphere.

The chemical reaction for photosynthesis is: 6CO2 + 6H2O + solar energy → C6H12O6 + 6O2 By creating oxygen and food, plants have made an environment that is favorable for animals. In respiration, animals use oxygen to convert sugar into food energy they can use. Plants also go through respiration and consume some of the sugars they produce. The chemical reaction for respiration is: C6H12O6 + 6O2 → 6CO2 + 6H2O + usable energy Notice that respiration looks like photosynthesis in reverse. In photosynthesis, CO2 is converted to O2 and in respiration, O2 is converted to CO2. The Atmosphere is a Crucial Part of the Water Cycle[ edit ] Water moves from the atmosphere onto the land, into soil, through organisms, to the oceans and back into the atmosphere in any order. This movement of water is called the water cycle or hydrologic cycle (Figure 15.1). Figure 15.1: The Water Cycle. Water changes from a liquid to a gas by evaporation. Water vapor is the name for water when it is a gas.

When the Sun's energy evaporates water from the ocean surface or from lakes, streams, or puddles on land, it becomes water vapor. The water vapor remains in the atmosphere until it condenses to become tiny droplets of liquid. The tiny droplets may come together to create precipitation, like rain and snow. Snow may become part of the ice in a glacier, where it may remain for hundreds or thousands of years. Eventually, the snow or ice will melt to form liquid water. A water droplet that falls as rain, could become part of a stream or a lake, or it could sink into the ground and become part of groundwater. At the surface, the water will eventually undergo evaporation and reenter the atmosphere. If the water is taken up by a plant and then evaporates from the plant, the process is called evapotranspiration. All weather takes place in the atmosphere, virtually all of it in the lower atmosphere. Weather describes what the atmosphere is like at a specific time and place, and may include temperature, wind and precipitation. It is the changes we experience from day to day. Climate is the long-term average of weather in a particular spot.

Although the weather for a particular winter day in Tucson, Arizona may include snow, the climate of Tucson is generally warm and dry. The physical and chemical changes that happen on Earth's surface due to precipitation, wind and reactions with the gases in our atmosphere are called weathering. Weathering alters rocks and minerals and shapes landforms at the Earth's surface. Without weathering, Earth's surface would not change much at all. For example, the Moon has no atmosphere, water or winds, so it does not have weathering. The footprints that astronauts made on the Moon decades ago will remain there until someone (human or alien) smooths them out! You would only need to spend a few minutes at the beach to know that Earth's surface is changing all the time. Ozone in the Upper Atmosphere Makes Life on Earth Possible[ edit ] Ozone is a molecule composed of three oxygen atoms, (O3). Ozone in the upper atmosphere absorbs high energy ultraviolet radiation (UV) coming from the Sun. This protects living things on Earth's surface from the Sun's most harmful rays. Without ozone for protection, only the simplest life forms would be able to live on Earth.

The Atmosphere Keeps Earth's Temperature Moderate[ edit ] Our atmosphere keeps Earth's temperatures within an acceptable range; the difference between the very coldest places on Earth and the very hottest is about 150°C (270°F). Without our atmosphere, Earth's temperatures would be frigid at night and scorching during the day. Our daily temperatures would resemble those seen on the Moon, where the temperature range is 310°C (560°F) because there is no atmosphere. Greenhouse gases trap heat in the atmosphere. Important greenhouse gases include carbon dioxide, methane, water vapor and ozone. Atmospheric Gases Provide the Substance for Waves to Travel Through[ edit ] The atmosphere is made of gases, mostly nitrogen and oxygen. Even though you can't see them, gases take up space and can transmit energy. Sound waves are among the types of energy that can travel though the atmosphere. Without an atmosphere, we could not hear a single sound. Earth would be as silent as outer space. Of course, no insect, bird or airplane would be able to fly since there would be no atmosphere to hold it up!

Composition of Air[ edit ] Chlorofluorocarbons (CFCs) 0.00000002 Air is made almost entirely of two gases. The most common gas is nitrogen, and the second most common gas is oxygen (O2). Nitrogen and oxygen together make up 99% of the planet's atmosphere. All other gases together make up the remaining 1%. Although each of these trace gases are only found in tiny quantities, many such as ozone, serve important roles for the planet and its life. One very important minor gas is carbon dioxide, CO2, which is essential for photosynthesis and is also a very important greenhouse gas (Table (15.1). In nature, air is never completely dry. Up to 4% of the volume of air can be water vapor. Humidity is the amount of water vapor in air. The humidity of the air varies from place to place and season to season. This fact is obvious if you compare a summer day in Atlanta, Georgia where humidity is very high, with a winter day in Phoenix, Arizona where humidity is very low. When the air is very humid, it feels heavy or sticky.

Your hair might get really curly or frizzy when it is very humid outside. Most people feel more comfortable when the air is dry. The percentage of water vapor in our atmosphere is listed with a wide range of values in the table above because air can be both very humid or dry. Argon, neon, helium, xenon, and krypton are noble gases. They are colorless, odorless, tasteless, and they do not become part of ordinary chemical reactions because they are chemically inert. The noble gases simply exist in the atmosphere. Some of what is in the atmosphere is not a gas. Particles of dust, soil, fecal matter, metals, salt, smoke, ash and other solids make up a small percentage of the atmosphere. This percentage is variable, as anyone who has spent a windy day in the desert knows (Figure 15.2). Particles are important because they provide starting points (or nuclei) for water vapor to condense on, which then forms raindrops. Some particles are pollutants, which are discussed in the chapter on human actions and the atmosphere . Figure 15.2: A dust storm in Al Asad, Iraq.

Pressure and Density[ edit ] Figure 15.3: The difference in air pressure between 2,000 meters and sea level caused this bottle to collapse. The bottle was originally at higher elevation, where air pressure is lower. When it was brought down to sea level, the higher air pressure at sea level caused the bottle to collapse. The atmosphere has different properties at different elevations above sea level, or altitudes. The density of the atmosphere (the number of molecules in a given volume) decreases the higher you go. This is why explorers who climb tall mountains, like Mt. Everest, have to set up camp at different elevations to let their bodies get used to the changes. What the atmosphere is made of, or the composition of the first 100 kilometers of the atmosphere stays the same with altitude, with one exception: the ozone layer at about 20-40 kilometers above the Earth. In the ozone layer, there is a greater concentration of ozone than in other portions of the atmosphere (Figure 15.3). The molecules in gases are able to move freely. If no force acted on a gas at all, it would just escape or spread out forever.

Gravity pulls gas molecules in towards the Earth's surface, pulling stronger closer to sea level. This means that atmospheric gases are denser at sea level, where the gravitational pull is greater. Gases at sea level are also compressed by the weight of the atmosphere above them. The weight of the atmosphere on a person's shoulders is equal to more than one ton. The force of the air weighing down over a unit of area is known as its atmospheric pressure, or air pressure. People and animals are not crushed because molecules inside our bodies are pushing outward to compensate. Air pressure is felt from all directions, not just from above. The atmosphere has lower atmospheric pressure and is less dense at higher altitudes. There is less pull from gravity and there is less gas to push down from above. Without as much weight above them, the gases expand, so the air is lighter. For each 6 km (3.7 mile) increase in altitude, the air pressure decreases by half. At 5,500 meters (18,000 feet) above sea level, the air pressure is just less than half of what it is at sea level. This means that the weight of the air on a person’s shoulders at that altitude is only one-half ton.

At a high enough altitude, there is no gas left. The density of the atmosphere at 30 km (19 miles) above sea level is only 1% that of sea level. By 700 km (435 miles) from the planet's surface, the air pressure is almost the same as that in the vacuum of deep space. If your ears have ever 'popped', you have experienced a change in air pressure. This occurs when you go up or down in altitude quickly, such as flying in an airplane or riding in a car as it goes up or down a mountain. Gas molecules are found inside and outside your ears. When you change altitude quickly, your inner ear keeps the density of molecules at the original altitude. The popping occurs when the air molecules inside your ear suddenly move through a small tube in your ear equalizing the pressure. This sudden rush of air is felt as a popping sensation. Colder, drier places on Earth usually have higher air pressure, while warmer, more humid places usually have lower air pressure. This happens because large areas of air move up or down by convection. Air pressure also often changes over time, as low and high pressure systems change locations.

These phenomena will be discussed when we learn about weather. Lesson Summary[ edit ] Without its atmosphere, Earth would be a very different planet. Gases in the atmosphere allow plants to photosynthesize and animals and plants to engage in respiration. Water vapor, which is an atmospheric gas, is an essential part of the water cycle. All weather takes place in the atmosphere. While the amount of gases do not vary relative to each other in the atmosphere, there is one exception: the ozone layer. Ozone in the upper atmosphere protects life from the Sun's high-energy ultraviolet radiation. Air pressure varies with altitude, temperature and location. Review Questions[ edit ] What gas is used and what gas is created during photosynthesis? What gas is used and what gas is created during respiration? Describe two reasons why photosynthesis is important. Briefly describe the movement of water through the water cycle. What is evapotranspiration? What will happen if the humidity of the atmosphere increases? Is weathering more effective in a humid or a dry climate? On an unusual February day in Portland, Oregon, the temperature is 18°C (65°F) and it is dry and sunny. The winter climate in Portland is usually chilly and rainy.

How could you explain a warm, dry day in Portland in winter? What important role do greenhouse gases play in the atmosphere? Why do your ears pop when you are in an airplane and the plane descends for a landing? If air pressure at sea level is one ton and at 5,500 m (18,000 feet) is one-half ton, what is air pressure at 11,000 m (36,000 feet)? What are noble gases? | Reference.com What are noble gases? A: Quick Answer The noble gases are a group of seven elements on the Periodic Table of Elements. Together they make up Group 18 on the table and include (in increasing atomic number) helium, neon, argon, krypton, xenon, radon and element 118, which has been temporarily named ununoctium. All noble gases display similar properties under standard conditions. They are colorless, tasteless, odorless and non-flammable. Full Answer It was previously thought that the noble gases were unable to form chemical bonds with the atoms of other elements. It's now known that this is possible, and they do form chemical compounds.

The greater the atomic number of the element, the less abundant it is in the universe. Helium, with atomic number one, is the most abundant element of the noble gases and the most abundant element in general next to Hydrogen.The Earth's atmosphere contains all noble gases. With the exception of helium and radon, noble gases are extracted from the air using a process called liquefaction of gas and fractional distillation. Helium is obtained from natural gas fields using cryogenic gas separation. Radon comes from the radioactive decay of radium compounds. There are several industrial applications from the use of noble gases. Light bulbs and lamps are both manufactured with the help of noble gases. The elements also have medicinal applications. Helium is used to help asthmatics, while xenon aids in medical imaging.

Question: Which among the noble gases is the most abundant in the atmosphere?
Answer: Argon