Earth: Human Impact

For eons the global environment of the planet has shaped the biosphere, and has been in turn shaped by the biosphere. According to the Gaia hypothesis, the overall impact of the biosphere on the global environment has been beneficial for the development and sustenance of life. However, this harmonious relationship between the biosphere and the environment has been disturbed with the emergence of one species, Homo sapiens, in the biosphere in the last million years. Our species has the potential to cause major disruptions in the global environment, thus threatening the integrity of the biosphere and its own survival. Some of these adverse effects are already known. The crucial challenge facing the future of humanity is to achieve a fundamental understanding of the global environment and to arrive at a new harmony between ourselves and nature. The adverse impact of humans on the local environment has been known for some time in human history. Urban pollution is an ancient problem. Land degradation and destruction of natural habitats were the probable causes of the earliest recorded migration of the Chinese people during the Shang Dynasty around 1500 B.C. (about the time of Moses) in the valleys of the Yellow River. However, until recently there has been relatively little anthropogenic impact on the global environment. There are at least two major global environmental problems that have been identified to date: the CO2 greenhouse effect and the global ozone depletion. These problems lie at the heart of what makes Earth a habitable planet. As discussed in chapter 9, Earth's atmosphere is responsible for a greenhouse effect of about 30°C, without which the surface of the planet would be too cold to allow water to flow. A doubling of atmospheric CO2 would increase the mean surface temperature of the planet by 2-3 °C. There would also be major shifts in the patterns of precipitation. Although the anticipated climatic changes caused by CO2 are small compared to the variations in climate in the geological history of the planet, the rate of change is unprecedented and could result in major social and economical disruptions. As discussed in chapter 9, life on land became possible only after an ozone shield had developed. As far as we know, no advanced living organism can survive the harsh radiation environment on Earth's surface in the absence of this ultraviolet screen.

Earth: Imprint of Life

Earth is the largest of the four terrestrial planets, three of which have substantial atmospheres. The astronomical and orbital parameters are summarized in table 9.1. Our planet has an obliquity of 23.5°, giving rise to well-known seasonal variations in solar insolation. The orbital elements are slightly perturbed by other planets in the solar system (primarily Jupiter), with time scales from 20 to 100 kyr, and these changes are believed to cause the advance and retreat of ice sheets. The last glacial maximum (LGM) occurred 18 kyr ago, at which time the planet was colder by several degrees centigrade on average. At present Earth is in an interglacial warm period. The origin of Earth may not be very different from that of the other terrestrial bodies. However, three properties may be unique to this planet. One is the formation of the Moon, probably via collision between Earth and a Mars-sized body. Second is the release of a huge amount of water from the interior (see discussion in section 8.5). Third, Earth is endowed with a large magnetic field that protects it from direct impact by the solar wind. Seventy percent of Earth's surface is covered by oceans, which have a mean depth of 3 km. There is so much water that Arthur C. Clarke proposed that "Ocean" might be a better name for our planet than "Earth." The enormous body of water became the cradle of life as early as 3.85 Gyr ago. The present terrestrial environment is the end-product of billions of years of evolution driven by the hydrological cycle and global biogeochemical cycles, in addition to the slower forces of geodynamics and geochemistry. The massive hydrological cycle and the biogeochemical cycles that operate on Earth are absent from other planets in the solar system. Mars in the remote past might have had a milder climate with liquid water on the surface, but the planet dried up a few eons ago. There is to date no observational evidence for the hypothetical oceans (composed of liquid hydrocarbons) on Titan. Life on a planetary scale equivalent to the terrestrial biosphere does not exist elsewhere in the solar system.

Solar Flux and Molecular Absorption

In this book we are concerned primarily with disequilibrium chemistry, of which the sun is the principal driving force. The sun is not, however, the only source of disequilibrium chemistry in the solar system. We briefly discuss other minor energy sources such as the solar wind, starlight, precipitation of energetic particles, and lightning. Note that these sources are not independent. For example, the ultimate energy source of the magnetospheric particles is the solar wind and planetary rotation; the energy source for lightning is atmospheric winds powered by solar irradiance. Only starlight and galactic cosmic rays are completely independent of the sun. While the sun is the energy source, the atoms and molecules in the planetary atmospheres are the receivers of this energy. For atoms the interaction with radiation results in three possibilities: (a) resonance scattering, (b) absorption followed by fluorescence, and (c) ionization. lonization usually requires photons in the extreme ultraviolet. The interaction between molecules and the radiation field is more complicated. In addition to the above (including Rayleigh and Raman scattering) we can have (d) dissociation, (e) intramolecular conversion, and (f) vibrational and rotational excitation. Note that processes (a)-(e) involve electronic excitation; process (f) usually involves infrared radiation that is not energetic enough to cause electronic excitation. The last process is important for the thermal budget of the atmosphere, a subject that is not pursued in this book. Scattering and fluorescence are a source of airglow and aurorae and provide valuable tools for monitoring detailed atomic and molecular processes in the atmosphere. Processes (c) and (d) are most important for determining the chemical composition of planetary atmospheres. Interesting chemical reactions are initiated when the absorption of solar energy leads to ionization or the breaking of chemical bonds. In this chapter we provide a survey of the absorption cross sections of selected atoms and molecules. The selection is based on the likely importance of these species in planetary atmospheres.

Chemical Kinetics

It is convenient to distinguish two types of chemistry in the solar system. The first is thermochemical chemistry driven by the thermal energy of the atmosphere. This type of chemistry is important, for example, in the interior of the giant planets where pressures and temperatures exceed 1000 bar and 1000 K, respectively. The second type is disequilibrium chemistry, driven by an external energy source, of which solar radiation is the most important in the solar system. Chemical reactions between stable molecules are very slow at pressures less than 1 bar in planetary atmospheres. Sunlight is the ultimate source of greater chemical activity in the middle and upper atmospheres of planets. As shown in chapter 2, the absorption of solar ultraviolet radiation by atmospheric gases leads to the production of radical species (e.g., atoms, ions, excited molecules) that are extremely reactive. The bulk of atmospheric chemistry involves the reaction between the radicals themselves and between the radicals and stable molecules. In this chapter we briefly survey the chemical kinetics that are important for understanding the chemistry of planetary atmospheres.

Mars

Mars has been extensively studied by a series of spacecraft since the dawn of the space age: by Mariners 4, 6, 7, and 9 (1965-1972), Mars 2 through 6 (1971-1974), and the two Viking Landers and Orbiters in 1976. The knowledge from spacecraft is supplemented by ground-based observations. The essential aspects of Mars are summarized in table 7.1. It is a smaller planet than Earth; the radius and mass are, respectively, 53% and 11% of Earth. The surface gravity is 3.71 m s~2, compared with the terrestrial value of 9.82 m s~2. The physical properties and composition of the Martian atmosphere are summarized in tables 7.1 and 7.2; isotopic composition is given in table 7.3. An example of how this knowledge is obtained is illustrated in figure 7.1, showing the mass spectrum obtained by the mass spectrometer experiment on Viking. The bulk atmosphere is composed of CO2, with small amounts of N2 and Ar and a trace amount of water vapor. Located at 1.52 AU from the sun, the mean insolation at Mars is about half that of Earth. As a result, it is a colder planet, with mean surface temperature of 220 K, too cold for water to flow on the surface in the current epoch. The lack of an ocean results in an arid and dusty climate. The obliquity of Mars is 25.2°, close to the terrestrial value of 23.5°; however, Mars has an eccentric orbit, with eccentricity of 0.093. The ratio of incident solar radiation at perihelion to aphelion is 1.45. The large seasonal variation in heating is believed to be responsible for the spectacular global dust storms that can be observed from Earth and have inspired imaginative but erroneous theories about their origin. The polar regions of Mars can be as cold as 125 K, so CO2 will condense as frost on the surface. In fact, according to the Leighton-Murray model, this is what determines the pressure of the atmosphere. Figure 7.2 shows the seasonal pressure variations at the Viking lander sites for 3.3 Mars years from 1976. Note that the magnitude of the pressure changes is of the order of 20%, compared to the maximum change of 1% on the surface of Earth.

Satellites and Pluto

The presence of an atmosphere on a small planetary body the size of the Moon is surprising. Loss of material by escape would have depleted the atmosphere over the age of the solar system. Since these objects are not large enough to possess, or to sustain for long, a molten core, continued outgassing from the interior is not expected. However, it is now known that four small bodies in the outer solar system possess substantial atmospheres: lo, Titan, Triton, and Pluto. These atmospheres range from the very tenuous on lo (of the order of a nanobar) to the very massive on Titan (of the order of a bar). The atmospheric pressures on Triton and Pluto are of the order of 10 μbar. Perhaps the most interesting questions about these atmospheres concern their unusual origin and their chemical evolution. lo is the innermost of the four Galilean satellites of Jupiter, the other three being Ganymede, Europa, and Callisto. All the Galilean moons are comparable in size, but there is no appreciable atmosphere on the other moons. The first indications that lo possesses an atmosphere came in 1974 with the discovery of sodium atoms surrounding the satellite and the detection of a well-developed ionosphere from the Pioneer 10 radio occultation experiment. The Voyager encounter in 1979 established the existence of active volcanoes as well as SOa gas. These are the only extraterrestrial active volcanoes discovered to date, and they owe their existence to a curious tidal heating mechanism associated with the 2:1 resonance between the orbits of lo and Europa.

Venus

Venus has been visited by a number of spacecraft. Those most important in advancing our understanding of the atmosphere include Mariner 10 (1974), Pioneer Venus (1978- 1992), and the series of Venera probes by the former USSR (1982-1986). The Pioneer Venus orbiter conducted more than a decade of monitoring of the upper atmosphere of Venus. The spacecraft data are supplemented by telescopic and satellite (in Earth orbit) observations. The astronomical data for Venus are summarized in table 8.1. Venus is a close but slightly smaller sibling of Earth. The radius is 6051 km, as compared to Earth's 6371 km. The mass is 4.87 x 1024 kg, a little less than Earth's 5.98 x 1024 kg. The gravity of Venus is 8.87 ms-2, as compared to Earth's 9.82 ms-2. The dynamical and orbital parameters of Venus are very different from those of Earth. The rotation of Venus is retrograde, with a period of 225 days. The planet has little obliquity, and its orbit is close to being circular. Thus there is little seasonal variation in insolation over a Cytherian year. Perhaps the greatest surprise about Venus is its dense, dry, and hot atmosphere of 92 bar (see figure 8.1). This is all the more surprising because the planet is completely covered by thick clouds. Figure 8.2 shows the altitude profiles of three modes of cloud particles in the middle atmosphere. The high albedo of Venus implies that the planet receives less energy from the sun than Earth despite its closer proximity to the sun. However, the surface of Venus is hot, with a temperature of 733 K, attributed to the greenhouse effect. The bulk of our knowledge of the atmosphere of Venus is derived from observations in the middle atmosphere (60-100 km altitude). At cloud-top levels (65 km), groundbased ultraviolet (UV) observations revealed a 4-5 day period, east-to-west circulation that is 60 times faster than the solid surface. The mechanism for generating this superrotation is not well understood. The net result of this rotation is that it gives the upper atmosphere an effective diurnal cycle of 4-5 days.

Jovian Planets

The four giant planets in the outer solar system, Jupiter, Saturn, Uranus, and Neptune, are a distinct group by themselves. The essential astronomical and atmospheric aspects of these planets are summarized in table 5.1. The significance of this group in the chemistry of the solar system is briefly pointed out in chapter 4. These planets are composed primarily of the lightest elements, hydrogen and helium, which were captured from the solar nebula during formation. The planets have rocky cores made of heavier elements. In the case of Jupiter and Saturn the mass of the gas greatly exceeds that of the core, whereas for Uranus and Neptune the masses of gas and core are comparable. Due to the enormous gravity of the giant planets, little mass has escaped from their atmospheres. Hence, the bulk composition of these planets provides a good measure of the initial composition of the solar nebula from which they were derived. Of all planetary bodies in the solar system, the constituents of giant planets are the closest to the cosmic abundances of the elements. The chemistry of the atmospheres of the giant planets is interesting for the following reasons:… 1. chemistry in a dominantly reducing atmosphere 2. interplay between photochemistry and equilibrium chemistry 3. ion chemistry in polar auroral regions 4. heterogeneous chemistry of aerosols 5. chemistry of meteoritic debris 6. lack of a planetary "surface"… We briefly comment on these reasons in this section. Each topic will receive a more detailed treatment in later sections. First of all, the atmospheres of the Jovian planets are more than 90% hydrogen and helium. Since helium is inert, the atmospheric chemistry is dominated by hydrogen. Therefore, we would expect the most stable compounds of carbon, oxygen, nitrogen, and phosphorus to be CH4, H2O, NHa, and PHs. This is in fact confirmed by the available observed composition of the bulk atmospheres of these planets. However, in the upper atmospheres of these planets, the composition is controlled by photochemistry.

Origins

Cosmology is a subject that borders on and sometimes merges with philosophy and religion. Since antiquity, the deep mysteries of the universe have intrigued mankind. Who are we? Where do we come from? What are we made of? Is the development of advanced intelligence capable of comprehending the grand design of the cosmos, the ultimate purpose of the universe? Is there life elsewhere in the universe? Is ours the only advanced intelligence or the most advanced intelligence in the universe? These questions have motivated great thinkers to pursue what Einstein called "the highest wisdom and the most radiant beauty." In the fourth century B.C., the essence of the cosmological question was formulated by the philosopher Chuang Tzu:… If there was a beginning, then there was a time before that beginning. And a time before the time which was before the time of that beginning. If there is existence, there must have been non-existence. And if there was a time when nothing existed, then there must be a time before that—when even nothing did not exist. Suddenly, when nothing came into existence, could one really say whether it belonged to the category of existence or of nonexistence? Even the very words I have just uttered, I cannot say whether they have really been uttered or not. There is nothing under the canopy of heaven greater than the tip of an autumn hair. A vast mountain is a small thing. Neither is there any age greater than that of a child cut off in infancy. P'eng Tsu [a Chinese Methuselah] himself died young. The universe and I came into being together; and I, and everything therein, are one. … Fortunately, our subject matter, solar system chemistry, is less esoteric than the questions asked by Chuang Tzu. A schematic diagram showing the principal pathways by which our solar system is formed is given in figure 4.1. The great triumphs of modern science have been summarized in this figure as fundamental contributions to the five "origins": (a) origin of the universe, (b) origin of the elements, (c) origin of the solar system, (d) origin of life, and (e) origin of advanced intelligence.

Introduction

It is usual in the study of planets to consider the Earth first, and then the other planets, so that we can better understand how and why the rest of the solar system is different from us. In this book the order of study will be reversed: we shall first try to understand the solar system, and then we will ask why Earth is unique. We adopt this unconventional approach for two reasons. First, Earth's atmosphere today is the end-point of an evolution that started about 4.6 billion years ago. The pristine materials have all been drastically altered. However, by examining other parts of the solar system that have evolved to a lesser degree, we may deduce what the early Earth might have been like. Second, Earth's atmosphere today is largely determined by the complex biosphere, whose evolution has been intimately coupled to that of the atmosphere. In other words, ours is the only atmosphere in the solar system that supports life, and it is in turn modified by life. Therefore, to appreciate the beauty and the intricacy of our planet, we must start with simpler objects without life. Chemical composition is intimately connected to evolution, which in turn is driven by chemical change. In this book we attempt to provide a coherent basis for understanding the planetary atmospheres, to identify the principal chemical cycles that control their present composition and past history. Figure 1.1 gives an illustration of the intellectual framework in which our field of study is embedded. The unifying theme that connects the planets in the solar system is "origin"; that is, all planets share a common origin about 4.6 billion years ago. The subsequent divergence in the solar system may be partly attributed to evolution, driven primarily by solar radiation. The bulk of solar radiation consists of photons in the visible spectrum with a mean blackbody radiation temperature of 5800 K. The part that is responsible for direct atmospheric chemistry is a tiny portion (less than 1% of the total flux) in the ultraviolet. In addition, the sun emits a steady stream of corpuscular particles, known as the solar wind. While the sun provides the principal source of energy for change, the time rate of change is crucial, and that is where chemical kinetics and chemical cycles play pivotal roles.