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 on its surface, 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, the surface appears lifeless today. Europa may have a habitable oceanic environment beneath its icy crust. An intriguing possibility is that Saturn's moon Titan may have had a stable “hydrosphere” over its history, but one in which methane substitutes for water: whether such an environment could be habitable for a very exotic form of life is not known (Chapter 12). 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 Earth came to be, and how physical processes have operated to make it habitable for billions of years.

Introduction

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 13.7 billion years ago. In this chapter we discuss rather precise techniques that enable us to determine the age of the Earth and other solid matter in the solar system with even higher accuracy and perhaps more confidence: some 4.5682 billion years ago, the planet we live on began to take shape in the form of tiny solids condensed from a hot, gaseous disk.

Overview of age dating

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 order in which a series of objects is found – and then assuming that the series represents a temporal ordering. In sediments on Earth, older layers of soil, sand, and rock are by definition those which are deposited first, hence they lie at the bottom of a sequence of layers progressing upward from oldest to youngest. If there is no disturbance, one can reasonably assume that the layers have been preserved in the order in which they were deposited. 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 and hence their inversion by some geological event. We discuss relative geologic dating in Chapter 8.

The enormous impact of the Caltech Two Micron Survey in 1969 stimulated interest in developing specialized ground-based infrared telescopes. While far-infrared wavelengths are accessible only from space, atmospheric windows can be used in the near-infrared and submillimetre wavelengths from high mountaintop sites. From the late 1970s, large near-infrared and submillimetre telescopes began to be built on high-altitude sites, especially on the 4200-metre dormant volcanic peak of Mauna Kea, Hawaii.

The advent of these new infrared telescopes and the dramatic impact of the new infrared-array detectors in the 1980s generated a profusion of scientific discoveries. These ranged from high-redshift galaxies to studies of luminous dusty galaxies and the effects of gravitational lensing, the black hole in the heart of our Milky Way Galaxy, brown dwarf stars, protostars and protoplanetary systems.

LARGE GROUND-BASED NEAR-INFRARED TELESCOPES: UKIRT, IRTF AND CFHT

The first of the large, specialized infrared telescopes was the United Kingdom’s 3.8-metre UK Infrared Telescope (UKIRT), constructed between 1975 and 1978 on Mauna Kea, Hawaii. UKIRT was designed to be lightweight and cheap. It started work in 1979 and had an immediate impact on near-infrared astronomy. At about the same time, NASA’s 3-metre Infrared Telescope Facility (IRTF), with Eric Becklin as its first director, and the Canada-France-Hawaii 3.6-metre optical/infrared telescope also began work on Mauna Kea.

As we begin the study of the universe, we have to confront a fundamental question: Are the laws of physics that hold on or near the Earth also valid in distant regions of the universe? And are these laws of physics also valid at all times? When cosmologists look at distant regions of the universe, they see them as they were a long time ago – they see a galaxy at a distance of, say, 1010 light-years as it was 1010 year ago. To interpret the data collected in such observations, we need to know the laws of physics that govern these regions, far away in space and in time.

Newton set a precedent for the universal validity of physical laws in his cosmological speculations. He conjectured that the (apparently) static distribution of the “fixed” stars was the result of an equilibrium of their mutual gravitational forces. He assumed that his inverse-square law for the gravitational force was also valid for distant stars, and that “the fixed stars, being equally spread out in all points of the heavens, cancel out their mutual pulls by opposite attractions” (Newton, 1713).

Einstein intended to follow Newton’s precedent by applying to the entire universe the field equation he had posited for the Solar System. But when he found that with these field equations he could not obtain a static solution for the mass distribution of the universe, he introduced new physics, in the form of a cosmological term added to the field equations. After the discovery of the expansion of the universe, Einstein promptly retracted his adoption of the cosmological term, and he thereafter strictly followed the example of Newton in treating the universe by the same laws as apply within the Solar System.

The linear tensor theory of gravitation that we developed by analogy with electrodynamics started out as the theory of a tensor field in a flat spacetime background. The geometric interpretation of this tensor field emerged only as an afterthought. However, the analysis of spacetime measurements (with clocks for time measurements and also for distance, by means of the radar-ranging procedure) has shown us that the flat spacetime background is purely fictitious – in a gravitational field, the real geometry measured by our instruments is the geometry of a curved spacetime, that is, the geometry of a Riemannian spacetime.

Mathematically, a Riemannian space is a differentiable manifold endowed with a topological structure and a geometric structure. In the discussion of the geometric structure of a curved space we must make a distinction between the affine geometry and the metric geometry. These two kinds of geometries correspond to two different ways in which we can ascertain the curvature of a space. One way is by examination of the behavior of parallel line segments, or parallel vectors. For example, on the surface of a sphere, we can readily detect the curvature by transporting a vector around a closed path, always keeping the vector as parallel to itself as possible. Figure 6.1 shows what happens if we parallel-transport a vector around a “triangular” path on the sphere. The final vector differs in direction from the initial vector, whereas on a flat surface the final vector would not differ. Such changes in a vector produced by parallel transport characterize the affine geometry (the word affine means connected and refers to how parallels at different places are connected, or related).

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 electro-magnetism 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). Thus the elements that make up life today (carbon, nitrogen, oxygen, phosphorus, etc.), with the exception of hydrogen, were manufactured by the very same process that today provides the energy source sustaining life on the planet. This chapter sets us on an evolutionary course that joins up eventually with the history of Earth and life, as we consider the processes by which elements are made.

The preceding chapter dealt with the kinematics of the curved spacetime geometry, that is, the description of the geometry and its curvature. We now come to the dynamics of the geometry, that is, the interaction of the geometry and matter. This interaction is the essence of Einstein’s equations for the gravitational field.

There are several routes that lead to Einstein’s equations; they differ in their starting points. One route begins with the equations of the linear approximation of Chapter 3 and adds the assumption that the exact, nonlinear equations are of second differential order and are endowed with general invariance. These assumptions suffice to completely determine the exact, nonlinear equations for the gravitational field.

That the linear equations imply the full nonlinear equations is a quite remarkable feature of Einstein’s theory of gravitation. Given some complicated set of nonlinear equations, it is always easy to derive the corresponding linear approximation; but in general, if we know only the linear approximation, we cannot reconstruct the nonlinear equations. What permits us to perform this feat in gravitational theory is the requirement of general invariance. As we will see later, this requirement states that the form and the content of the equations must remain unchanged under all coordinate transformations.

In this chapter we will obtain some simple time-independent solutions of the linear field equations, such as the solution for the field produced by a static spherical mass and by a steadily rotating mass. For a given solution of the field equations, the equation of motion then permits us to predict the trajectories of particles or of light signals, which we can compare with experimental or observational data.

Most of the experimental or observational tests that have so far been performed on the relativistic theory of gravitation involve only the linear approximation. For instance, the gravitational time dilation predicted by the linear approximation has been tested and confirmed by experiments with clocks in the gravitational field of the Earth. Some other tests exploit the motion of light signals in the gravitational field of the Sun. Since light signals are necessarily relativistic, their motion provides a direct test of the relativistic features of our equations.

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 the formation of planetary systems is a common result of star formation. Planets are extremely common throughout the universe, and the technology to detect and characterize them continues to improve. Almost 900 planets had been discovered as of the end of 2012, with even more candidates awaiting confirmation. Both the sizes and masses are becoming known for an increasing number of planets, allowing a preliminary division into rocky, icy, and gaseous classes.

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 super-greenhouse warming: perpetually too hot to ever permit liquid water to exist.

Pioneers Working from Mountaintops

In this chapter, I describe how pioneers working from mountaintops discovered new types of infrared stars, which turned out to be either dying stars shrouded in dust or stars in the process of being born. The leaders of this infrared revolution were the Americans Frank Low (1933–2009) and Gerry Neugebauer (b.1932). First efforts to map the sky with telescopes on balloons and rockets began the push to observe at longer wavelengths inaccessible from the ground. And theorists began to use the observed distributions of brightness with wavelength in the new infrared sources to model the distribution and properties of the interstellar dust in clouds around stars and spread through interstellar space.

The first pioneer of the modern age of infrared astronomy was Harold Johnson (1921–1980) (Figure 4.1). He and his group, then at the University of Texas at Austin, began in the late 1950s to work with lead sulphide detectors, which they cooled with liquid nitrogen to 77 K (−196 °C). Cooling made the detectors much more sensitive, and over the next few years Johnson’s group observed several thousand stars. Johnson had earlier defined the standard optical observing bands, which are denoted U (ultraviolet), B (blue), V (visual), R (red) and I (infrared).

One of the reasons Herschel and his discovery of infrared light do not resonate more strongly in the history of science is that it took so long for infrared astronomy to develop. During the 150 years after Herschel, from 1800 to 1950, progress was extremely slow. There were two main reasons for this slow progress. Firstly, the night is not ‘dark’ at infrared wavelengths; in fact it is very bright. The Earth’s atmosphere radiates strongly in the infrared as part of the greenhouse process, so even if we had infrared eyes we would have great difficulty picking out the stars against this bright foreground, even at night. The second and main problem was the slow progress in developing infrared detectors. Herschel’s thermometers could detect infrared radiation from the Sun, but to detect anything else something better was needed. In this chapter I describe the slow progress in detector technology through the nineteenth and early twentieth centuries, the detection of infrared radiation from the Moon during the classic expedition of Piazzi Smyth to Tenerife, the efforts between 1870 and 1914 to detect radiation from bright stars, and the crucial discovery in the 1930s of extinction of visible light by interstellar dust.

The first step on the road to improved infrared detectors was made by a German physicist, Thomas Seebeck (1770–1831). In 1821, he made a discovery that led to the invention of the thermocouple. He found that when a metallic strip is constructed of two different metals and then heated, a small electric current is generated in the strip. The thermocouple applies this discovery to the detection of heat, or infrared radiation, by measuring the current generated from a bimetallic strip when heated. It was this device, far more sensitive than the thermometer used by Herschel, that Piazzi Smyth would use to make the next major discovery in infrared astronomy, the detection of infrared radiation from the Moon, the second brightest astronomical object in the sky.

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. The more temperate the climate, the more land area that is available for plants, and the more 12C that is taken up. In ice ages, global plant activity is reduced, and so less 12C is taken up.

Introduction

Earth at the close of the Archean, 2.5 billion years ago, was a world in which life had arisen and plate tectonics dominated the evolution of the crust and the recycling of volatiles. Yet oxygen (O2) still was not prevalent in the atmosphere, which was richer in CO2 than at present. In this last respect, Earth's atmosphere was somewhat like that of its neighbors, Mars and Venus, which today retain this more primitive kind of atmosphere.

Speculations on the nature of Mars and Venus were, prior to the space program, heavily influenced by Earth-centered biases and the poor quality of telescopic observations (Figure 15.1). Forty years of US and Soviet robotic missions to these two bodies changed that thinking drastically. The overall evolutions of Mars and Venus have been quite different from that of Earth, and very different from each other. The ability of the environment of a planet to veer in a completely different direction from that of its neighbors was not readily appreciated until the eternally hot greenhouse of Venus' surface and the cold desolation of the Martian climate were revealed by spacecraft instruments.

However, robotic missions also provided evidence that Mars once had liquid water flowing on its surface. It is tempting, then, to assume that the early Martian climate was much warmer than it is at present, warm enough perhaps to initiate life on the surface of Mars. The difficulty of sustaining a warm Martian atmosphere in the face of the faint-young-Sun problem of Chapter 14 remains a daunting puzzle, one that is highly relevant to the broader question of habitable planets beyond our solar system. What is the range of distances from any given star for which liquid water is stable on a planetary surface and life can gain a foothold?

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 changes with every new fossil find. Therefore, 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 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. The pace of human evolution picks up in the Pleistocene, and species close enough in form to us to warrant assignment to the genus Homo (Latin, man in the sense of humans) appear close to, but perhaps slightly before, the time when climate shifted into an ice-age pattern of glacial and interglacial episodes.

Throughout this book we are concerned with fields – that is, functions of space and time, such as gravitational fields, electromagnetic fields, and velocity fields – and density distributions that describe masses and charges. Spacetime is the arena in which these fields perform their joint evolutions. It is therefore clear that we must first get to know the structure and geometry of spacetime. Unfortunately, because the velocity of light is so large, everyday experience leads us to acquire various misconceptions about the geometry of spacetime. This set of misconceptions goes under the name of Newtonian, or Galilean, spacetime. The true (or more true) geometry of spacetime was discovered through the development of Einstein’s theory of special relativity, starting in 1905. The keystone of this theory is the principle of relativity, according to which the laws of physics are the same in all inertial reference frames. Einstein was led to this principle by his investigation of Maxwell’s equations. As he wrote in his autobiographical notes,