INTRODUCTION

In addition to the dating of rocks by measuring amounts of radioactive isotopes and their decay products, isotopes can be useful as indicators of climate variations on Earth over its long history. Here, the key is to use stable isotopes of the same element. The difference in mass between the isotopes leads to separation, called fractionation, of the isotopes in natural systems; the separations in some cases are a function of the climate, specifically temperature.

To use isotopes as climate indicators, four key features are required:

1. availability of stable isotopes of the same element whose separation depends on temperature;

2. incorporation of the fractionated isotope mixture in some storage medium that is preserved for a long time;

3. ability to measure accurately the ratio of the various isotopes;

4. a means to date, in an absolute or a relative sense, the age of the stored isotope data.

STABLE ISOTOPES, SEAFLOOR SEDIMENTS, AND CLIMATE

Carbon

Three important elements for tracking climate changes are carbon, oxygen, and hydrogen. Consider the carbon first. Carbon has two stable isotopes, 13C and 12C. Recall that 14C is radioactive and used for dating relatively recent events. Certain biological processes distinguish mass differences in isotopes. We cannot survive on deuterated water (HDO or 1H2HO). Likewise, plants are observed to preferentially take up 12C in carbon dioxide (CO2), and hence preferentially enrich the atmosphere in 13C.

OVERVIEW OF AGE DATING

To understand the history of Earth in the cosmos, we must be able to establish ages of physical evidence and timescales over which processes have occurred. The task is daunting because of the enormous spans of time over which the physical universe and Earth have existed, and several different approaches must be used. In chapter 2, we discussed observations leading to the conclusion that the universe is in an overall state of expansion, which began some 10 billion to 20 billion years ago. In this chapter we discuss rather precise techniques that enable us to determine the age of Earth and other solid matter in the solar system very confidently: Some 4.56 billion years ago, the planet we live on began to take shape.

It is useful to distinguish between two kinds of chronologies that are constructed in regard to Earth's history, because the techniques and uncertainties are quite different. A relative chronology is derived by observing the relative position in which the remains of an event lie. In sediments on Earth, older layers of soil, sand, and rock are deposited first, and then overlain by subsequent layers. Geologic processes might turn a whole stack of layers upside down, but fossils present in the layers, which can be compared to those in other layers worldwide, enable us to determine the age progression of the layers. We discuss relative geologic dating in chapter 8.

The terrestrial magnetosphere comprises the region of space where the properties of naturally occurring ionized gases are controlled by the presence of Earth's magnetic field. This very broad definition means that the terrestrial magnetosphere extends from the bottom of the ionosphere to more than ten Earth radii (Re) in the sunward direction and to several hundred Re in the antisunward direction.

The magnetosphere is formed as a result of the interaction of the supersonic, superalfvénic, magnetized solar wind with the intrinsic magnetic field of the Earth. To understand this interaction, we first briefly discuss the main characteristics of the intrinsic terrestrial magnetic field and then turn our attention to the interaction between this intrinsic field and the solar wind.

The Intrinsic Magnetic Field

A couple of hundred years ago Gauss showed that the magnetic field at the surface of the Earth can be described as the gradient of a scalar potential. In general the near Earth magnetic field can be expressed as

where Φint and Φext represent scalar potentials due to intrinsic and external sources, and Φtot is the sum of these two potentials (describing the total geomagnetic field). In general, planetary magnetic potentials are expressed as an infinite series using associated Legendre polynomials:

where θ and ϕ are geographic colatitude and east longitude, respectively.

The origin and evolution of Earth involved physical processes that operate on all matter and energy in the universe. The formation of stars is a common phenomenon in galaxies, and we are on the threshold of confirming that the formation of planetary systems is a common result of star formation. Planets, most likely, are extremely common throughout the universe, and the technology to detect planets around other stars is just now available. In the year between October 1995 and September 1996, for example, planets were discovered around six other stars similar to the Sun in our galactic neighborhood.

In our solar system, three rocky planets had the potential early on for supporting life. Venus, Earth, and Mars were all endowed with carbon dioxide atmospheres, and at least Earth and Mars received large influxes of organic materials and water. The presence of a watery ocean was a key early step toward regulating and retaining the atmosphere. The absence or early demise of an ocean on Venus is causal to its present state: With no sink for carbon dioxide in the form of carbonates, all of the carbon dioxide remained as a massive atmosphere supporting a supergreenhouse warming: perpetually too hot to ever permit liquid water to exist.

The evidence is abundant that Mars had a mild, wet climate early on; the absence of plate tectonic recycling of carbon dioxide allowed carbonate formation to permanently lock up carbon dioxide in the crust, progressively cooling the surface and atmosphere until liquid water froze completely.

INTRODUCTION

The previous chapters have touched on the scale of the universe and the nature of the smallest pieces of matter. The structure of the universe is determined not just by the matter contained within it, but by the forces that both bind matter together and compel it to move apart. These forces, which act at the macroscopic and microscopic levels, are thought to be carried by certain types of subatomic particles. In the case of electromagnetism the force-bearing particle is called the photon.

We have learned most of what we know of the universe around us by studying the light coming from objects; our most information-filled sense is that of vision, and we have augmented it through the use of devices that can measure in detail the energy distribution of the light. This energy distribution from celestial bodies reveals much about their chemical composition and physical condition. Light from one such self-luminous body, the Sun, is the primary power source for Earth's climate and for life on the planet. The light by which the Sun and other stars shine is not generated by chemical reactions, but by reactions involving the nuclei of atoms at enormous pressures and temperatures deep within these gaseous objects' interiors; these are called nuclear reactions.

The nuclear reactions powering stars have, over time, generated essentially all of the natural elements except hydrogen, the most abundant element, and some of the helium (the remainder having been made from hydrogen in the primordial Big Bang).

THE CASE FOR AN EQUABLE CLIMATE IN THE ARCHEAN

There is ample evidence that the Archean Earth possessed liquid water. The existence of metamorphosed sedimentary rocks from this period, as discussed in chapter 11, require erosion by liquid water and deposition in a lake or marine environment. The presence of life itself, recorded through isotopic signatures and fossil evidence, also implies liquid water. As discussed in chapter 12, we know of no living thing today that can get by without water. Many don't require oxygen (and are poisoned by it), but all require liquid water.

Figure 14.1 summarizes constraints arguing for Earth's mean temperatures being above the melting point of water during the Archean. In chapter 15, we explore the case for a Martian climate, at the time of Earth's Archeon eon, which was warmer than at present (either continuously or episodically). In total, the evidence on Earth and Mars points to planetary climates at least as warm as those experienced today. Surprisingly, as we now show, such climates impose rather strong constraints on the nature of the Archean atmospheres of the Earth and Mars – provided our understanding of the evolution of the Sun is correct.

THE FAINT EARLY SUN

Simple reasoning about the physics of hydrogen fusion indicates that the Sun was cooler in the past than it is at present.

INTRODUCTION

Prior to the invention of radioisotopic techniques for dating rock samples, geologists determined relative ages for rocks using simple principles of how rocks and their fragments are deposited, and using remains or records of extinct life to correlate samples from different locations. When combined later with the dating of rocks by radioisotopic techniques, a detailed history of Earth could be developed. We work with this history repeatedly throughout the rest of the book. This chapter serves as an introduction to the techniques used to assemble such a record.

CATASTROPHISM VERSUS UNIFORMITARIANISM

When we look at Earth's landforms, we are viewing a snapshot, a moment in a vast span of time during which mountains rise and fall, seas expand over land areas and contract again, and continents shift their positions and grow slowly from new rock added by volcanoes. These processes all require vast amounts of time for their completion, but most do not proceed in a smooth, gradual manner. Instead, geologic processes are a combination of gradual effects and sudden catastrophes. The earthquakes that shake California represent sudden failures of rock after the buildup of stresses over time as one portion of California slowly glides past the other, as we discuss in chapter 9.

This book provides a comprehensive introduction to the physics of the space environment for graduate students and interested researchers. The text is based on graduate level courses I taught in the Department of Aerospace Engineering and in the Department of Atmospheric, Oceanic, and Space Sciences of the University of Michigan College of Engineering. These courses were intended to provide a broad introduction to the physics of solar—planetary relations (or space weather, as we have started to call this discipline more recently).

The courses on the upper atmosphere and on the solar wind and magnetosphere have been taught for a long period of time by many of my friends and colleagues here at Michigan before I was fortunate enough to teach them. I greatly benefited from discussions with Drs. Thomas M. Donahue, Lennard A. Fisk, and Andrew F. Nagy here at the University of Michigan and Drs. Thomas E. Cravens (University of Kansas), Jack T. Gosling (Los Alamos National Laboratory), and József Kóta (University of Arizona). I am grateful for their advise, criticism, and physical insight.

I would also like to acknowledge the constructive criticism of Konstantin Kabin, my graduate student here at the University of Michigan. His mathematical rigor and helpful suggestions greatly helped me in producing the final version of the manuscript.

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.