EARTH AS AN EVOLVING PLANETARY SYSTEM Part 5

174 The Core boundary, and can investigators better resolve this interaction with seismic-wave studies? Although it is clear that the inner core is anisotropic, the cause of this anisotropy remains problematic. Another area about which scientists know little is the rate at which the inner core is crystallizing and how it crystallizes. Is crystallization episodic, resulting in sudden bursts of heat loss, or is it uniform and gradual? This could be important in under-standing mantle-plume events, which may be triggered by sudden losses of core heat. Although investigators are beginning to understand the geodynamo in more detail, to make significant progress on this question, three-dimensional simulations are needed, which will require significant time on high-speed computers at great expense. So unlike our understanding of the crust and mantle, which have been significantly enhanced in the last decade, the information highway for the core is just beginning to open. Further Reading Buffett, B. A., 2000. Earth’s core and the geodynamo. Science, 288: 2007–2012. Dehant, V., Creager, K. C., Karato, S., and Zatman, S., 2003. Earth’s Core: Dynamics, Structure, Rotation. American Geophysical Union, Washington D. C., Geodynamic Series Vol. 31. Jacobs, J. A., 1992. Deep Interior of the Earth. Chapman & Hall, London, 167 pp. Merrill, R. T., McElhinney, M. W., and McFadden, P. L., 1996. The Magnetic Field of the Earth. Academic Press, New York, 531 pp. Newsome, H. E., and Jones, J. H. (eds.), 1990. Origin of the Earth. Oxford University Press, Oxford, UK, 378 pp. Tromp, J., 2001. Inner core anisotropy and rotation. Ann. Rev. Earth Planet. Sci. 29: 47–69. The Atmosphere and 6 Oceans Introduction Not only in terms of plate tectonics is the Earth a unique planet in the solar system; it also is the only planet with oceans and with an oxygen-bearing atmosphere capable of sustaining higher forms of life. How did such an atmosphere–ocean system arise, and why only on the Earth? Related questions are: once formed, how did the atmosphere and oceans evolve with time, and in particular when and how did free oxygen enter the system? How have climates changed with time, what are the controlling factors, and when and how was life created? What are the roles of plate tectonics, mantle plumes, and extraterrestrial impact in the evolution of atmosphere and oceans? These and related questions are addressed in this chapter. General Features of the Atmosphere Atmospheres are the gaseous carapaces that surround some planets and satellites, and because of gravitational forces, they increase in density toward planetary surfaces. The Earth’s atmosphere is divided into six regions as a function of height (Fig. 6.1). The mag-netosphere, the outermost region, is composed of high-energy nuclear particles trapped in the Earth’s magnetic field. This overlays the exosphere in which lightweight molecules (such as H2) occur in extremely low concentrations and escape from the Earth’s gravita-tional field. Temperature decreases rapidly in the ionosphere (to about –90° C) and then increases to near 0° C at the base of the mesosphere. It drops again in the stratosphere and then rises gradually in the troposphere toward the Earth’s surface. Because warm air overlies cool air in the stratosphere, this layer is relatively stable and undergoes little ver-tical mixing. The temperature maximum at the top of the stratosphere is caused by absorption of ultraviolet radiation in the ozone layer. The troposphere is a turbulent region that contains about 80% of the mass of the atmosphere and most of its water vapor. 175 176 The Atmosphere and Oceans Figure 6.1 Major divi-sions of the Earth’s atmo-sphere showing average temperature distribution. 5000 MAGNETOSPHERE 2000 1000 EXOSPHERE 500 IONOSPHERE 200 100 MESOSPHERE 50 20 STRATOSPHERE Cold 10 5 2 TROPOSPHERE Ozone Layer Trap 1 −120 −100 −80 −60 −40 −20 0 Temperature (°C) Tropospheric temperature decreases toward the poles, which with vertical temperature change causes continual convective overturn in the troposphere. The Earth’s atmosphere is composed chiefly of nitrogen (78%) and oxygen (21%) with small amounts of other gases such as argon and CO2. In this respect, the atmosphere is unique among planetary atmospheres (Table 6.1). Venus and Mars have atmospheres composed largely of CO2; the surface pressure on Venus is up to 100 times that on the Earth, and the surface pressure of Mars is less than 10–2 of that of the Earth. The surface Table 6.1 Composition of Planetary Atmospheres Earth (early Archean) Earth Venus Mars Jupiter Saturn Uranus Neptune Pluto Surface Temperature (°C) ~85 −20 to 40 400 to 550 −130 to 25 −160 to –90 −180 to –120 −220 to –120 −220 to –120 −235 to –210 Surface Pressure (bars) ~11 0.1–1 10–100 −0.01 −2 −2 −5 −10 −0.005 Principal Gases CO2 (N2, CO, CH4) N2, O2 CO2 (N2) CO2 (N2) H2, He H2, He H2, CH4 H2 CH4 The Primitive Atmosphere 177 temperatures of the Earth, Venus, and Mars are also different (Table 6.1). The outer plan-ets are composed largely of hydrogen and helium, and their atmospheres consist chiefly of hydrogen and, in some cases, helium and methane. The concentrations of minor gases such as CO2, H2, and ozone (O3) in the Earth’s atmo-sphere are controlled primarily by reactions in the stratosphere caused by solar radiation. Solar photons fragment gaseous molecules (such as oxygen, H2, and CO2) in the upper atmosphere, producing free radicals (C, H, and O) in a process called photolysis. One important reaction produces free oxygen atoms that are unstable and recombine to form ozone. This reaction occurs at heights of 30 to 60 km, with most ozone collecting in a relatively narrow band from about 25 to 30 km (Fig. 6.1). Ozone, however, is unstable and continually breaks down to form molecular oxygen. The production rate of ozone is approximately equal to the rate of loss; thus, the ozone layer maintains a relatively con-stant thickness in the stratosphere. Ozone is an important constituent in the atmosphere because it absorbs ultraviolet radiation from the Sun, which is lethal to most forms of life. Hence, the ozone layer provides an effective shield that permits a large diversity of living organisms to survive on the Earth. It is for this reason we must be concerned about the release of synthetic chemicals into the atmosphere that destroy the ozone layer. The dis-tributions of N2, O2, and CO2 in the atmosphere are controlled by volcanic eruptions and by interactions among these gases and the solid Earth, oceans, and living organisms. The Primitive Atmosphere Three possible sources have been considered for the Earth’s atmosphere: residual gases remaining after Earth accretion, extraterrestrial sources, and degassing of the Earth by volcanism. Of these, only degassing accommodates a variety of geochemical and isotopic constraints. One line of evidence supporting a degassing origin for the atmosphere is the large amount of 40Ar in the atmosphere (99.6%) compared with the amount in the Sun or a group of primitive meteorites known as carbonaceous chondrites (both of which contain <0.1% 40Ar). 40Ar is produced by the radioactive decay of 40K in the solid Earth and escapes into the atmosphere chiefly by volcanism. The relatively large amount of this iso-tope in the terrestrial atmosphere indicates that the Earth is extensively degassed of argon and, because of a similar behavior, of other rare gases. Although most investigators agree that the present atmosphere, except for oxygen, is chiefly the product of degassing, whether a primitive atmosphere existed and was lost before extensive degassing began is a subject of controversy. One line of evidence supporting the existence of an early atmosphere is that volatile elements should collect around planets during their late stages of accretion. This follows from the low temperatures at which volatile elements condense from the solar nebula (Chapter 10). A significant depletion in rare gases in the Earth compared with carbonaceous chondrites and the Sun indicates that if a primitive atmosphere collected during accretion, it must have been lost (Pepin, 1997). The reason for this is that gases with low atomic weights (CO2, CH4, NH3, H2, etc.) that probably composed this early atmosphere should be lost even more readily than rare 178 The Atmosphere and Oceans gases with high atomic weights (Ar, Ne, Kr, and Xe) and greater gravitational attraction. Just how such a primitive atmosphere may have been lost is not clear. One possibility is by a T-Tauri solar wind (Chapter 10). If the Sun evolved through a T-Tauri stage during or soon after (<100 My) planetary accretion, this wind of high-energy particles could readily blow volatile elements out of the inner solar system. Another way an early atmo-sphere could have been lost is by impact with a Mars-size body during the late stages of planetary accretion, a model also popular for the origin of the Moon (Chapter 10). Calculations indicate, however, that less than 30% of a primordial atmosphere could be lost during the collision of the two planets (Genda and Abe, 2003). Two models have been proposed for the composition of a primitive atmosphere. The Oparin-Urey model (Oparin, 1953) suggests that the atmosphere was reduced and composed dominantly of CH4 with smaller amounts of NH3, H2, He, and water; the Abelson model (Abelson, 1966) is based on an early atmosphere composed of CO2, CO, water, and N2. Neither atmosphere allows significant amounts of free oxygen, and exper-imental studies indicate that reactions may occur in either atmosphere that could produce the first life. By analogy with the composition of the Sun and the compositions of the atmospheres of the outer planets and of volatile-rich meteorites, an early terrestrial atmosphere may have been rich in such gases as CH4, NH3, and H2 and would have been a reducing atmo-sphere. One of the major problems with an atmosphere in which NH3 is important is that this species is destroyed directly or indirectly by photolysis in as little as 10 years (Cogley and Henderson-Sellers, 1984). In addition, NH3 is highly soluble in water and should be removed rapidly from the atmosphere by rain and solution at the ocean surface. Although CH4 is more stable against photolysis, OH, which forms as an intermediary in the methane oxidation chain, is destroyed by photolysis at the Earth’s surface in less than 50 years. H2 rapidly escapes from the top of the atmosphere; therefore, it also is an unlikely major constituent in an early atmosphere. Models suggest that the earliest atmosphere may have been composed dominantly of CO2 and CH4, both important greenhouse gases (Pavlov et al., 2000; Catling et al., 2001). The Secondary Atmosphere Excess Volatiles The Earth’s present atmosphere appears to have formed largely by degassing of the mantle and crust and is commonly referred to as a secondary atmosphere (Kershaw, 1990). Degassing is the liberation of gases from within a planet, and it may occur directly during volcanism or indirectly by the weathering of igneous rocks on a planetary surface. For the Earth, volcanism appears to be most important both in terms of current degassing rates and calculated past rates. The volatiles in the atmosphere, hydrosphere, biosphere, and sediments that cannot be explained by weathering of the crust are known as excess volatiles (Rubey, 1951). These include most of the water, CO2, and N2 in these near-surface reservoirs. The similarity in the distribution of excess volatiles in volcanic gases ... - tailieumienphi.vn