There are ions and electrons at all altitudes of the terrestrial atmosphere. Below about 60 km thermal charged particles (which have comparable energies to the neutral gas constituents) do not play any significant role in determining the chemical or physical properties of the atmosphere. Above ≈60 km, however, the presence of electrons and ions becomes increasingly important. This region of the upper atmosphere is called the ionosphere. Note that the ionosphere overlaps with the upper mesosphere, the thermosphere, and the geocorona.

The typical vertical structure of the ionosphere is shown in Figure 10.1 (Hargreaves 1992). Inspection of Figure 10.1 reveals that the ionosphere exhibits a strong diurnal variation and it also varies with the solar cycle. The identification of the atmospheric layers is usually related to inflection points in the vertical density profile: The main regions are local minimums. The primary ionospheric regions are the following:

• D region (≈60–90 km, peaks around 90 km);

• E region (≈90–140 km, peaks around 110 km);

• F1 region (≈140–200 km, peaks around 200 km);

• F2 region (≈200–500 km, peaks around 300 km);

• Topside ionosphere (above the F2 region).

It can be seen that the D and F1 regions disappear at night, while the E and F2 regions become much weaker.

The Sun is an ordinary star of spectral type G2V with magnitude of 4.8. However, it is the only star we have in our immediate vicinity and it is the source of most of the energy that controls physical phenomena in our space environment. The Sun is also a living, dynamic star with varying activity as demonstrated in Figure 11.1. Changes in solar activity result in many important phenomena in the space environment, ranging from flares, to coronal mass ejections, to geomagnetic storms. The fundamental physical properties of the Sun are given in Table 11.1.

The Sun consists primarily of hydrogen (90%) and helium (10%). Elements such as C, N, and O constitute about 0.1% of its mass. The interior can be divided into four zones (see Figure 11.2):

1. The core. This is the high density, high temperature region at the center of the Sun, where thermonuclear energy production takes place. The core extends from the center to about R⊙/4 (1/64-th of the Sun's volume), but it contains about half of the solar mass. Practically all of the Sun's energy production takes place in this region.

2. The radiative zone. The energy produced in the core is transported through the core and the radiative zone by gamma ray diffusion. The gamma rays are scattered, absorbed, and reemitted many times before they reach the outer edge of the radiative zone.

3. […]

INTRODUCTION

The beginning of the Proterozoic eon is set formally by geologists at 2.5 billion years before present. However, the transition between the Archean and the Proterozoic is not a sharp one. From about 3.2 billion to 2.5 billion years ago, rocks with a modern granitic composition made a widespread appearance on Earth. Prior to this time, rocks making up the Archean continents had a composition different from modern granites in several important respects. Beginning around 3.2 billion years ago in what is now Africa, and extending to 2.6 billion years ago on the Canadian shield, large quantities of modern-type granites were produced. We can collect these rocks today and date them by use of radioisotopes. How did the original Archean continents form? Why was there a transition in chemical composition of the rocks roughly halfway through the Archean? What might Earth have been like today if this eruption of new rock types had not occurred? As we see, the transformation wrought on Earth's primitive continents may have been an inevitable consequence of their increasing coverage of Earth's surface.

What might have been inevitable on Earth was apparently difficult or impossible on the other terrestrial planets. No evidence for large granitic masses exists on any other planet. Venus bears two crustal masses that resemble continents, but the details of their geology suggest that they are more similar to primitive Archean continents than to our modern ones and, even then, the connection is a weak one.

INTRODUCTION

The preceding chapter focused on singular events in the later history of the Earth—the flowering of multicellular complex organisms at the start of the Phanerozoic eon and the widespread extinction of species some 65 million years ago at the close of the Cretaceous period. Although these events stand out in their drama and the mystery of their causes, any understanding of the interactive history of life and Earth's environment cannot rest on their study. Throughout the Phanerozoic, and before, the relatively steady rhythms of plate tectonics brought continental masses together and then moved them apart, creating new seafloor and destroying old. The process of moving great landmasses around the planet must have had profound effects on the environment, and indeed this is seen to be the case in the geologic record.

This chapter begins by reconsidering plate tectonics with an eye to understanding the apparently cyclical creation and breakup of multicontinent landmasses, or supercontinents. We consider the effects of such supercontinent cycles on the amount of volcanic activity, and hence atmospheric chemistry, on the ocean circulation patterns, on mountain building, and hence on the available area for storage of continental snow and ice deposits. Such considerations touch on a major theme of the latter portion of Earth history, the comings and goings of great ice ages. Finally, we draw our attention in detail to a particularly warm time in recent Earth history, the Cretaceous period.

INTRODUCTION

Having covered in the previous chapter some of the basics of present-day living organisms and considered the limitations of life in terms of terrestrial and extraterrestrial environments, the present chapter addresses some of the issues surrounding the origin of life. We begin with general considerations about living processes and their relationship to the natural laws that govern the workings of the universe. In particular, self-organization seems to be a property of complicated physical systems, and computer simulations of such systems suggest the kind of bootstrapping necessary to build well-controlled biochemical processes from simpler suites of chemical reactions. We then move to more specific ideas about how life might have begun and examine the issue from two somewhat different points of view: that the origin of life lay in the primitive mimicking of cellular processes (the vesicle model) or that the essential point of origin lay in an RNA or slightly more primitive genetic-coding molecule (the RNA world).

THERMODYNAMICS AND LIFE

Thermodynamics, introduced in chapter 3, is the study of energy transfer in macroscopic systems. A fundamental principle that governs the transfer of matter and energy in both natural and artificial systems is called the second law of thermodynamics. It is most precisely expressed mathematically but, in words, it says that the capacity for a system to do useful work (move something) decreases with time, unless usable energy is pumped into it. Looked at another way, systems tend to become more disordered with time.

INTRODUCTION

Perhaps the most fundamental shift in the evolution of Earth's surface and atmosphere was the oxygen “revolution,” an event stretching over the Proterozoic eon when molecular oxygen levels in the atmosphere rose and carbon dioxide levels decreased. (Hereinafter, for brevity, we refer to molecular oxygen, which is O2, simply as oxygen.) In consequence, the fundamental chemical nature of the atmosphere and its interactions with life changed drastically. Life helped precipitate the drastic increase in oxygen levels and, as a result, was set on a radical new course. Earth's atmosphere today is not the sedate carbon dioxide atmosphere in chemical equilibrium with the surface, as on Mars and Venus. Instead, it is an atmosphere far from equilibrium, held in a precarious chemical state by the biosphere. As Margulis and Sagan (1986) express it, the modern biosphere hums “with the thrill and danger of free oxygen.”

In this chapter we explore how this change came about on the Proterozoic Earth, by first examining the presentday oxygen cycle and the evidence in the rock record for an oxygen-poor Archean and early Proterozoic Earth. We then consider a model that, although not necessarily fully correct, illustrates very well how the change might have taken place. Such models often have critical utility in science, in that they point the way toward new observations and investigations that will yield deeper insight into a particular process (even while proving the model itself to be incomplete or incorrect).

INTRODUCTION

Humankind's present-day dilemma with respect to global warming often is viewed with virtually no temporal perspective at all. The decade of the 1980s was among the hottest in the short century that reliable weather records have been kept. But how does this century compare to other centuries, or this millennium to others? In the third part of this book, we explored extremes of Earth climate far more profound than those experienced in modern times, or even through the short span of human history.

To really put global warming in perspective, however, we need to understand how the climate has varied during the most recent geologic epoch, the Pleistocene, a time when all of Earth's geologic processes, and the chemistry of the atmosphere, are fully modern in all respects. The time since the last interglacial, through the last ice age to the present interglacial, is recent enough that evidence is available by which very detailed records of climate can be constructed. The most thorough records can be assembled for the past 10,000 years of Earth history, the Holocene. In this chapter, techniques for assembling detailed climate information are summarized, and we compare the climate in this interglacial with that in the last, a kind of “Jekyll and Hyde” story.

THE RECORD IN ICE CORES

As discussed in chapter 6, the stable heavy isotopes of both hydrogen and oxygen exist in ocean water, and the resulting heavy water tends preferentially to exist in liquid form as opposed to vapor.

In its most general form the Boltzmann equation is a seven-dimensional nonlinear integro-differential equation. The solutions of the Boltzmann equation provide a full description of the phase-space distribution function at all times. In most cases, however, it is next to impossible to solve the full Boltzmann equation and one has to resort to various approximate methods to describe the spatial and temporal evolution of macroscopic quantities characterizing the gas.

Transport equations for macroscopic molecular averages are obtained by taking velocity moments of the Boltzmann equation. This seemingly straightforward technique runs into considerable difficulties because the governing equations for the components of the n-th velocity moment also depend on components of the (n + 1)-th moment. In order to get a closed transport equation system, one has to use closing relations (expressing a higher-order velocity moment of the distribution function in terms of the components of lower moments) and thus make implicit assumptions about the distribution function.

Moment Equations

Velocity Moments

We start by examining the physical interpretation of the various velocity moments of the phase-space distribution function.

Macroscopic variables, such as number density, average flow velocity, kinetic pressure, and so on, can be considered as average values of molecular properties.

INTRODUCTION

In chapter 1, we became acquainted with the scale of the solar system – the stage upon which planetary evolution is set. However, the formation of elements out of which planets and life came into being involved the universe of stars and galaxies – a scale much larger than the solar system – and the microscopic world of atoms, which involves size scales much smaller than that of our ordinary experiences. In this chapter we explore how cosmic distances are gauged, and then begin to acquaint ourselves with the basic building blocks of matter.

SCIENTIFIC NOTATION

Although the book is written with the nonmathematically inclined reader in mind, the discussion of numbers, both large and small, cannot be avoided if we are to gain a true understanding of Earth and its place in the cosmos. Numbers of interest in science range over enormous magnitudes (figure 2.1). The number of protons contained in a single star, our Sun, is of order 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000; the size of an individual proton (itself made up of smaller elementary units) is of order 0.0000000000001 cm. (The term of order refers to how many powers of 10 a number contains, rather than the specific numerical value it has; hence 200 is of order 100, 40 is of order 10, etc.) These numbers are inconvenient to write down and manipulate in even the simplest mathematical expressions.

INTRODUCTION

The understanding of the origin of sunlight (and starlight in general) was a nineteenth and early twentieth century development that culminated in the release of nuclear energy in human-made devices on Earth. Beyond the implications (both negative and positive) of such developments, however, lies the profound perspective gained in the latter half of the twentieth century regarding the origin of the elements of the periodic table. The existence and abundances of the 90-odd elements that make up Earth, the planets, the solar system, and the universe beyond have an explanation that lies in natural nuclear reactions that have taken place in the several generations of stars preceding the formation of the Sun and the solar system.

STARS AND NUCLEAR FUSION

The observable cosmos around us is, by and large, made of stars. Stars are spheres primarily of hydrogen and helium gas in balance between the attractive force of gravity pulling everything inward and the pressure forces associated with the high temperatures of stars' interiors, tending to push the material outward. Most stars eventually evolve, through nuclear processes described below, into dense spheres of carbon, oxygen, or exotic neutrons; some collapse into the mysterious and incredibly dense black holes.

The copious amounts of photons coming out of stars, including the Sun, are a signature of the enormous temperatures in their interiors. The origin of these high temperatures, and hence of sunlight or starlight, was a matter of debate throughout the nineteenth century.

At the close of the second millennium A.D., we live in extraordinary times. A generation ago, human beings first ventured beyond Earth's atmosphere into the vast emptiness of space, began to unlock the remarkable secrets beneath the oceans of how Earth's geology works, and to crack in earnest the genetic code that determines the fundamental nature of all life. Today, these and other frontiers remain open to us, yet we also are consumed with a multitude of problems seemingly of our own making. Increasingly, too many people compete for too few resources and make fundamental changes to the life-giving air and oceans of our planet. Further, we find ourselves confused about science and technology: Are they the cause of, or the solution to, these daunting problems?

Regardless of which way one chooses to answer this question, of deepest concern is that science and technology are understood by few, even in the technologically advanced industrial nations. We use computers and cellular phones with ease, yet how many of us understand the basic principles by which they work? We look forward to the change of seasons and scan the weather reports for tomorrow's outlook on rain, yet remarkably few of us can explain the motions of Earth in the cosmos, and the underlying causes of the atmospheric changes we call weather and climate. We talk glibly about the promise and problems of genetic engineering, yet most such conversations are conducted in the absence of any familiarity with what the genetic code actually is and how it functions.

INTRODUCTION TO THE PHANEROZOIC

The Phanerozoic eon is a major division in the fossil record that dates radioisotopically at a bit younger than 600 million years before present. Its geologic marker is the appearance of numerous complex multicellular organisms in the fossil record. This eon has no counterpart on any other planet, even if Mars harbored simple life-forms within the first billion years of its history. On Phanerozoic Earth, life began to occupy just about every conceivable niche on land, sea, and air. Geologically, Earth was more or less modern in form as the eon opened: The total continental mass was comparable to that today, modern-style plate tectonics were operating, and oxygen levels in the atmosphere were approaching present-day values.

The Phanerozoic eon is divided into eras, eras into periods, and periods into epochs. The boundaries between most of the periods are defined by extinction episodes in which a number (sometimes very large) of species disappear and are replaced in the sedimentary fossil record above that point by new species. Although the resulting story of complex multicellular organisms is too large to tell in detail in this book, some of the highlights are shown in figure 18.1.

The presence of multicellular organisms per se was not new. Multicellular bacterial colonies had existed since the Archean; multicellular algae (for example, green seaweed) made their appearance shortly after the first unicellular eukaryotes in the fossil record.

ANCIENT ATTEMPTS TO DETERMINE THE SCALE OF THE COSMOS

The science of astronomy developed in many different cultures and from many different motivations. Because even in cities of the preindustrial world, the stars could be seen readily at night, the pageant of the sky was an inspiration for, and embodiment of, the myths and legends of almost all cultures. Some people tracked the fixed stars and moving planets with great precision, some for agricultural purposes (the ancient Egyptians needed to prepare for the annual flooding of the Nile River valley) and more universally to attempt to predict the future. The regularity of the motions of the heavens was powerfully suggestive of the notion that history itself was cyclical, and hence predictable. The idea of human history linked to celestial events remains with us today as the practice of astrology. In spite of a lack of careful experimental tests, or demonstrated physical mechanisms, this powerfully attractive belief system is pursued widely with varying amounts of seriousness, extending in the early 1980s to the level of the presidency of the United States.

Although ancient understanding of the nature of the cosmos varied widely and was usually a reflection of particular mythologies of a given culture, the classical Greeks distinguished themselves by their (often successful) attempts to use experiment and deduction to learn about the universe. Some Greek philosophers understood the spherical nature of Earth and something of the scale of nearby space.