The story so far has been of stars shrouded in dust, clouds of gas and dust where new stars are forming, and nearby star-forming galaxies. Now we shift the stage to the whole universe with the discovery of background radiation at microwave wavelengths. The cosmic microwave background (CMB) radiation is the dying whisper of the initial fireball phase of the hot Big Bang universe, and its discovery transformed our understanding of the universe. This background is the dominant form of astronomical radiation at submillimetre wavelengths outside the Milky Way. When astronomers were trying to detect submillimetre sources in the 1970s, nodding the telescope between the source position and a nearby position on the sky to subtract out the emission from the Earth’s atmosphere, each telescope beam would detect an amount of CMB radiation far brighter than the source they were trying to detect. However, because they were only interested in the difference between the two measurements, the CMB radiation exactly cancelled and did not affect the observations. In this chapter, I describe the discovery of the radiation and its impact on cosmology.

The story of the discovery of the cosmic microwave background by Arno Penzias and Bob Wilson in 1965 has been told many times. Initially this was a story of short-wavelength radio astronomy and microwaves (centimetre wavelength radiation), but gradually it became clear that the wavelength of peak energy is around 1 millimetre, so while about half the CMB is microwave, half is submillimetre. To understand the significance of this radiation, we have to understand the origin and evolution of the universe itself.

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 made primarily of hydrogen and helium gas; the size of the spheres is determined by a balance between the attractive force of gravity pulling everything inward and the pressure associated with the high temperatures of stars' interiors, which is a force 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. A hypothesis by the British physicist Lord Kelvin, that the Sun was radiating away the energy associated with its initial collapse from clouds of interstellar gas and dust, met with a timescale problem: the Sun would cool in several tens of millions of years, but various lines of evidence suggested that terrestrial rocks were older by at least a factor of 10.

Introduction

The period from the formation of Earth, some 4.56 billion years ago, to the time when the oldest rocks still in existence today were formed, roughly 3.8 billion to 4.0 billion years ago, is called both the Hadean eon and Priscoan eon of Earth. The term Hadean, referring to the classical Greek version of hell, is well chosen, because all evidence that we have is that the Hadean Earth was very hot and extremely active, with widespread vol-canism and frequent impacts of debris left over from planetary formation. This time encompasses the assemblage of Earth from the smaller planetesimals, dramatic internal rearrangements such as core formation, the creation of the ocean and earliest atmosphere, and the origin of Earth's Moon. Forces that acted on Earth were essentially the same as those acting on Mars and Venus, and a traveler visiting Earth would have seen little to distinguish it from the two neighboring terrestrial planets.

Each planet initially had a molten, or nearly molten, silicate surface, followed by cooling and establishment of a solid crust. Each had an atmosphere dominated by carbon dioxide (CO2), with little free molecular oxygen (O2). Evidence exists that each planet had liquid water on its surface during a portion of the Hadean eon. Most important, no sign of life could be seen on any of these three planets - conditions were too severe and variable to allow life-forms to survive except near the end of the Hadean on Earth, and perhaps at about the same time on Mars.

The enormous success of IRAS stimulated both the European Space Agency (ESA) and NASA to develop new space infrared observatories that would follow up the wealth of discoveries about the infrared universe made with IRAS.

Early in February 1983, the European Space Agency met to select a new medium-sized astronomy space mission. Peter Clegg was able to place on the table at the meeting the first scan around the sky from IRAS, and its quality was sufficient to convince the European Space Agency to select the Infrared Space Observatory (ISO). The idea for a European infrared space observatory had been first proposed in 1979. ISO was finally launched in November 1995 with a planned life of 18 months (Figure 10.1). In fact, its helium coolant lasted until April 1998, almost a year longer than expected.

ISO had a camera, ISOCAM, led by Catherine Cesarsky and a spectrometer, SWS, led by Thijs de Graauw, working at the near- and mid-infrared wavelengths (3–20 microns); and a camera, ISOPHOT, led by Dietrich Lemke and a spectrometer, LWS, led by Peter Clegg, working at far-infrared (40–160 micron) wavelengths. The two cameras also had smaller low-resolution spectrometers as part of their capability. The spectrometers of ISO were especially powerful in unravelling the nature of the dust around stars and in interstellar space, and in probing young stars in the process of formation.

Introduction

We close this part of the book on techniques for discerning Earth's history with a conceptual tool. The concept of plate tectonics, whereby the outer layer of Earth is divided into a small number of distinct segments called plates, which move relative to each other, represents a breakthrough in explaining a diverse range of geologic phenomena across our planet. Although the basic ideas are now 30 years old or more, this picture or concept of how Earth's geology works, in a unified way, continues to provide fresh insights into evolution of Earth, the stability of the gross climate of our planet, and the distinctions between Earth and the other planets. Because of its importance, we introduce the concept early to allow the reader to gain an understanding of the basic ideas. We come back to plate tectonics again and again as a fundamental process on Earth driving climate change, erosional processes, atmospheric chemistry, and even the nature of life.

Early evidence for and historical development of plate tectonics

Revolutions in scientific thinking often take place when increasing numbers of observations challenge existing theories, which in many cases have become dogmatic over time in the face of conflicting data. Particularly satisfying is the synthesis of widely diverse data into a single framework that explains well all of the data.

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 was responsible for, or at least helped to, 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, relatively unreactive carbon dioxide atmosphere 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 present-day 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 approximate and based on mechanisms that are still debated, 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).

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.