INTRODUCTION

Having dealt with some of the tools and key concepts to which we will return as we develop the history of Earth and the other planets, we are ready now to consider that history. Five centuries after the beginning of the European Renaissance, humanity's explorations of Earth and the cosmos have exposed an intriguing, perhaps profound, paradox. Earth and the other planets of the solar system seem to be explainable as manifestations of common physical processes that have operated over very small and very large scales, to produce a range of cosmic phenomena. In this sense we are neither special nor particularly important in the grand scheme of things.

On the other hand, in our own solar system, we now understand Earth as the one planet with a uniquely stable climate, equable for liquid water over the billions of years required to bring forth intelligent life. Although Mars may have come close to this state at one time, no other planet seems to have the combination of characteristics needed to sustain life. Other solar systems may be common and life may flourish elsewhere, but it is also possible, with what we know today, that we are a rare or even unique speck in the cosmos. We will know more over the decades to come, but for now we seek to understand how this planet came to be, and how physical processes have operated to make it habitable for billions of years.

INTRODUCTION

Earth's evolutionary divergence from the neighboring planets of the solar system, beginning with the stabilization of liquid water, culminates in the appearance of sentient organisms sometime within the past 1 million to 2 million years. The fossil record is abundant in its yield of creatures intermediate in form and function between the great apes and modern humans; new discoveries seem to be made with increasing pace. But hidden between and among the fossil finds are the details of how and why we came to be. Even as we acknowledge our common origins with the life around us, the singular results of sentience – art, writing, technology, civilization – are surprising and enigmatic.

The story of human origins is not simple, and this chapter attempts only a sketch of the evidence and the lines of thought current in today's anthropological research. It begins with a broad view of the climatological stage on which these events took place. It ends with a focus on the closing act of human evolution, the coexistence of modern humans with a similar but probably separate sentient species in Europe and the Middle East – the Neanderthals.

PLEISTOCENE SETTING

The earliest fossils along the lineage toward humanity exist in the Pliocene epoch, prior to the Pleistocene, during a time of relative climate stability.

It has been observed under certain conditions that most mediums in the space environment can experience abrupt changes of macroscopic parameters. In a broad sense shocks and discontinuities are defined as transition layers where the state of the fluid changes from one that is near an equilibrium state to a different one. Examples involve detonation waves in the atmosphere, shocks, and transition layers in the magnetosphere, the interplanetary medium, and in the Sun. In all these cases the transition layer is very narrow compared to the characteristic scale of the problem.

In this chapter we consider the fundamental theory of shocks and discontinuities in neutral gases and quasineutral plasmas.

Normal Shock Waves in Perfect Gases

In the perfect gas approximation shock waves are discontinuity surfaces separating two distinct gas states. In higher order approximations (such as the Navier—Stokes equations) the shock wave comprises a region where physical quantities change smoothly but rapidly. In this case the shock has a finite thickness, generally of the order of the mean free path.

Because the shock wave is a more or less instantaneous compression of the gas, it cannot be a reversible process. The energy for compressing the gas flowing through the shock wave is derived from the kinetic energy of the bulk flow upstream of the shock wave.

Most space plasmas are quasi-neutral statistical systems containing mobile charged particles. On the average, the potential energy of a mobile particle due to its nearest neighbor is much smaller than its kinetic energy. This definition excludes high density plasmas (such as solid states or stellar interiors), but the description of these forms of matter goes far beyond the scope of this book.

Owing to the long-range nature of electromagnetic forces, each charged particle in the plasma interacts simultaneously with a large number of other charged particles. This process results in collective behavior of the plasma particles: In some respect even a low density space plasma behaves as a continuous medium.

In gaseous, nonrelativistic plasmas the motion of individual particles is governed by electromagnetic fields, which are a combination of internally generated (due to the presence and motion of charged particles) and externally imposed fields. The motion and interaction of plasma particles can be described by nonrelativistic classical mechanics and by electrodynamics, quantum mechanical effects are usually neglected.

The interaction of charged particles with electromagnetic fields is described by the force (Eq. 1.16), whereas the electromagnetic field itself obeys Maxwell's equations (Eqs. 1.1, 1.2, 1.3, and 1.4). It should be noted that one must include the contributions of plasma particles to the charge and electric current densities.

Sidney Chapman introduced the nomenclature used to describe the various regions of the upper atmosphere. The classification is primarily based on the variation of temperature with altitude. In this system the regions are called “spheres” and the boundaries between the regions are called “pauses.”

The troposphere (in Greek it means “turning sphere”) is the lowest atmospheric region. It begins at the surface (which provides the major heat source for the atmosphere) and extends to about 10–12 km. This region is mainly characterized by a negative temperature gradient (≈ -10 K/km). The troposphere is bounded by the tropopause, which separates the troposphere from the stratosphere (Greek word for “layered sphere”). The temperature at the tropopause is about 200 K. Originally the stratosphere was thought to be isothermal, but in fact, in this region the temperature increases about 2 K/km due to the absorption of solar UV radiation by stratospheric ozone. Stratospheric ozone is particularly important because it absorbs UV radiation harmful to life.

The maximum temperature (≈270 K) is reached at the stratopause, which is located at around 50 km altitude. Above the stratosphere lies the mesosphere (middle atmosphere), where the temperature again decreases with altitude. The temperature reaches its minimum (≈180 K) at the mesopause, located at an altitude of about 85 km.

INTRODUCTION

The absolute dating techniques of chapter 5 rely on very precise laboratory analyses of rock samples. For Earth, an abundance of accessible samples exists. However, with respect to the rest of the solar system, only meteorites, small bits of asteroidal and cometary debris – interplanetary dust particles (IDP), and samples from the Moon have been delivered to terrestrial laboratories for age analyses. One class of meteorites, the Shergottites – Nakhlites – Chassigny (SNC), may have been ejected from Mars by collision with one or several asteroids. Aside from these cases, we have no known samples of material from large bodies in the solar system and thus cannot date major geologic events on the surfaces of the bodies in an absolute fashion.

Instead, scientists use relative dating techniques to infer time histories of the moons and planets in the solar system, and they rely primarily on the record of bombardment, or cratering, of the surfaces of these bodies. We describe this technique and the physics of cratering in the present chapter. In addition to providing a foundation for inferring key aspects of the solar system's history, this discussion provides a good foundation for the presentation in chapter 8 of relative age dating on Earth, which relies on geologic processes other than cratering but for which the principles are much the same.

PROCESS OF IMPACT CRATERING

Impact cratering is a process in which a high-speed projectile collides with a solid surface, forming an excavated region called a crater.

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.