Technology, not intelligence

SETI (Search for Extra Terrestrial Intelligence) can be defined as the branch of astrobiology looking for inhabited worlds by taking advantage of the deliberate technological actions of extraterrestrial organisms. This definition usually draws a chuckle during public lectures, but it underscores why this chapter is somewhat different from the preceding ones. As in other parts of astrobiology, one must consider the diversity of physical environments in the cosmos, and the limitations imposed by them. But with SETI one must also consider modifications to the environment that are not just the byproduct of life, but the result of deliberate actions by intelligent organisms intended to achieve some result.

For millennia people have speculated about the existence of other habitable worlds, and their inhabitants (Chapter 1), but the rules of the game underwent a profound change in the second half of the twentieth century. The publication of the initial scientific paper on SETI (Cocconi and Morrison, 1959) and Drake's (1961) first radio search (Project Ozma, described in Section 1.9) turned speculation into an observational science. No longer were priests and philosophers the sole respondents to the “Are we alone?” question; scientists and engineers could work on finding an answer empirically. Following the first flurry of observing programs in the US and the Soviet Union (Chapter 2), the acronym SETI became the accepted name for this new exploratory activity. But, in fact, SETI is a misnomer because there is no known way to detect intelligence directly across interstellar distances.

People have wondered for centuries whether we are alone or share this Universe with extraterrestrial beings. Questions about the origin of life and the possible existence of extraterrestrial life have deep roots in the history of both science and culture (Chapters 1 and 2; Dick 1997, 1998). In ancient times, the debate centered mainly on our uniqueness versus the plurality of worlds. Following the advances of the Copernican era, scientists gradually accepted notions about the plurality of solar systems and recognized the large-scale nature of the Universe. Today, cosmic evolution, from the Big Bang to the evolution of intelligence, has become a working hypothesis for astrobiology, one that combines characteristics of both biological and physical universes into a “biophysical cosmology.” This new world view recognizes the enormity of the Universe and hypothesizes that life is one of its basic and essential properties, a cosmic imperative rather than an accidental or incidental property found only on Earth. The current key questions in astrobiology are whether biological laws reign throughout the Universe, whether Darwinian natural selection is a universal phenomenon rather than simply a terrestrial one, and consequently whether there may be other biologies, histories, cultures, religions, and philosophies beyond Earth. In short, is the ultimate outcome of cosmic evolution merely planets, stars, and galaxies – or life, mind, and intelligence? The answers to these questions raise a multitude of issues in the realms of both science and society.

Introduction to cellular metabolism

While it may be impossible to derive a satisfactory definition of life in a global or astrobiological context, we can circumvent such metaphysical questions if we define life not by what it is, but rather by what it does. Every function of life requires metabolism, the coupled chemical processes that generate and exploit biochemical energy. This chapter discusses what can be deduced from examination of extant organisms about the origin and early history of life. First, however, in this section we lay out the basic principles of cellular metabolism.

It is constructive to divide metabolism into functional categories and then consider the various biochemical details. The primary distinction between metabolic functions is whether they generate biologically useful energy or, instead, use this energy. Processes that use energy from the environment for the production of biological energy are referred to as catabolic metabolic processes. Anabolic metabolic processes, on the other hand, use stored biological energy to do biosynthesis, i.e., to synthesize the required building blocks for cellular structure. In any organism, the coordination (or regulation) of anabolic and catabolic processes is the essence of cellular metabolism (Fig. 8.1). At the core of metabolism lies the flow of carbon within an organism – its chemical pathways govern all essential cellular function and are the basis of what is called intermediary metabolism.

For biological energy conversion and structural biosynthesis, it is fundamental that life exploits chemical gradients and thermodynamic potentials that naturally exist in the external environment.

Introduction

The fossil record conjures up images of dinosaurs, trilobites, and extinct humans. This reflects not only our anthropocentric (or better, metazoan-centric) bias, but also a visual bias. Starting about 542 Ma, macroscopic remains of animals, traces, shells, and later bones and plants, become quite evident in the fossil record, hence the geological term Phanerozoic, which literally means the eon of “visible life.” These 542 Myr of the fossil record demonstrate that changes have occurred in organisms and have provided compelling evidence for the theory of evolution. This chapter addresses the history of life in the two youngest eons of geological time, the Proterozoic and the Phanerozoic, which span the last 2500 Myr. It is a record of contrasts. Whereas the Proterozoic was mainly a microbial world, the Phanerozoic is a world of macroscopic animals and plants. The transition from the Proterozoic to the Phanerozoic marks what may be one of the most significant evolutionary events in the history of life, when many of the modern animal phyla evolved. Highlights of this 2500 Myr record include the rise to dominance of cyanobacteria in shallow marine environments, the early evolution and diversification of eukaryotes, the first appearance of multicellular algae, the appearance of animals and their subsequent rapid diversification (the Cambrian explosion), the first land plants and their subsequent diversification, and the appearance and dominance of intelligent life (humans).

The rise of exobiology, the study of the origin of life and of possible life outside the Earth, was intimately related to the birth of the Space Age, and particularly to the birth of the US National Aeronautics and Space Administration (NASA) in 1958. By providing the means to enter space, NASA placed exobiology into the arena where an age-old problem could be empirically tested with in situ observations and experiments. Moreover, in pursuit of exobiology, on the ground NASA funded experiments on the origin of life, revived planetary science, sponsored theoretical and observational studies on planetary systems, and assembled the flagship program in the Search for Extraterrestrial Intelligence (SETI) – all conceptual elements of the budding discipline. Over four decades, at a relatively small but steady level of funding punctuated by the landmark Viking mission to Mars, the American space agency put into place the conceptual, institutional, and community structures necessary for a new discipline, leaving no doubt of NASA's status as the primary patron of exobiology. The interest in the search for life, however, knew no national boundaries. Especially in the post-Viking era, international involvement grew in the field that also became known as “bioastronomy,” and that was transformed at the end of the century into a broadened “astrobiology” effort.

The ability to search for life beyond Earth did not guarantee its adoption as a program within NASA or any other space agency.

Astrobiology: a new discipline?

The controversial assertion in 1996 that the martian meteorite ALH84001 contained evidence for ancient microbial life stoked public, political, and scientific interest in astrobiology. Two years later, the NASA Astrobiology Institute (NAI) was launched, its mission to “study the origin, evolution, distribution, and future of life on Earth and in the Universe.” This mission, though ambitious, is not really new. Indeed, according to this definition, astrobiology has been studied for decades if not centuries (Chapter 1). Astrobiological research would include James Watson and Francis Crick's decipherment half a century ago of the double helix and with it (according to Crick) the “secret of life.” It also would include Stanley Miller and Harold Urey's in vitro synthesis of amino acids by electric discharge, the Viking Mission of 1976 to search for life on Mars, the discovery of hydrothermal vents, and the recognition that a wayward impactor probably drove the dinosaurs extinct. Yet none of this research was carried out under the auspices of astrobiology or required a specific astrobiological framework. Recently, however, during the so-called “Astrobiology Revolution” of the 1990s (Section 2.3; Ward and Brownlee, 2000), increasing attention has been devoted to developing precisely such a framework, as implied by the formation of NAI. Programs oriented to astrobiology research or training have acquired various degrees of formalization throughout the world. Astrobiology journals have been created. You are reading a textbook devoted to astrobiology. These milestones both represent and fuel ambitions to transform astrobiology into a new discipline.

Mars and astrobiology

Mars is at the center of the field of astrobiology in many ways. Today's vigorous program of ongoing exploration is largely motivated by Mars's potential to harbor life at present or at some time in the past. But astrobiology as a discipline is about more than just finding out whether there is or ever was life on Mars or on other planets and satellites. It is about determining the governing principles behind whether life will originate on a given planet or whether it can survive if transplanted there; about what the mechanisms are by which planets and biota can and do interact with each other; and about determining which fundamental factors distinguish a planet that is habitable from one that is not.

We know that the Earth meets the environmental requirements for being habitable. In Mars, we have an example of a planet that evolved under different physical and chemical conditions, allowing us to see what effects these differences can have. Today, Earth has global-scale plate tectonics, and Mars does not. Earth has global oceans at its surface, and Mars does not. Earth has a climate conducive to the coexistence of the solid, liquid, and vapor phases of water, each of which affects the geology and geochemistry of the planet, and Mars does not. In finding out whether Mars has or ever had life, we obtain a second example of whether and how life can occur on a planetary body.

Introduction

Common among the many definitions of life (Chapter 5) is mention of sets of chemical reactions that allow metabolism, replication, and evolution. The specifics of those reactions are generally not part of these definitions, although a century of study of the metabolisms that support life on Earth has given us a rich repertoire of illustrative biochemical examples. Unfortunately, because all known life on Earth is descended from a single common ancestor, our study of terran biochemistry, no matter how extensive, cannot provide a comprehensive view of the full range of possible reactions that might generally support life.

This leaves open an important question. If life exists elsewhere in the Cosmos, will its chemistry be similar to the chemistry of life on Earth? Recent articles addressing this issue are by Irwin and Schulze-Makuch (2001), Crawford (2001), Bains (2004), and Benner et al. (2004), and several popular books are listed under “Further reading” at chapter's end. As in many areas in contemporary astrobiology, no clear methodology exists to address the question of “weird life.” Chemistry, however, including the skills outlined in Chapter 7, provides one set of tools for constructing hypotheses about possible alternative biological chemistries.

The emerging field of synthetic biology (Benner and Sismour, 2005) provides another set of tools. Here, chemists attempt to give substance to concepts about alternative life forms by synthesizing molecules that might support alternative genetic systems or alternative metabolisms.

The geological timescale is one of science's great triumphs. It represents “deep time” (McPhee, 1982), millions and billions of years ago, time beyond human comprehension. Geologists use two different ways to discuss geological time: (a) absolute time, and (b) relative time.

Relative time was developed first, and is based on relative age relationships among rock units as determined by geometric relationships, fossils, and other distinctive attributes of the rock. Layered, sedimentary rocks (strata) are the most widely used, and during the nineteenth century allowed geologists to establish a relative geological timescale. The scale used strata with their contained fossils, and applied four fundamental principles of relative time: (1) original horizontality, (2) superposition, (3) original lateral continuity, and (4) fossil succession. Fossil succession refers to the particular vertical (stacked) order that fossils occur in strata. William Smith recognized this in 1799 and employed it to great effect in his geological map of England and Wales published in 1815. The other three principles were established much earlier by the Danish scholar Nicolaus Steno (1669). Original horizontality states that sediments are originally laid down in a horizontal or nearly horizontal manner. Superposition means that the oldest stratum is at the bottom and the youngest at the top. Original lateral continuity refers to the way strata extend laterally in all directions. The geometric relationship of bodies of rock to one another is also used; for example, an intrusion that cuts layered rock is younger than the layered rock it cuts.

The emerging field of astrobiology encompasses a daunting variety of specialties, from astronomy to microbiology, from biochemistry to geology, from planetary sciences to phylogenetics. This is both exciting and frustrating – exciting because the potential astrobiologist is continually exposed to entirely new ways to look at the world, and frustrating because it is difficult to understand new results when venturing outside the confines of one's own specialty. There are now many excellent popular books on astrobiology, but a scientist wants more details and more sophistication than these afford. Where can an astronomer without any formal biology since high school learn the basics of cellular metabolism? Or the principles of evolution? Or notions about alternative forms of life? And where can a microbiologist with little physics and no astronomy learn the basics of how a planetary atmosphere works? Or how the Earth formed? Or how planets are detected around other stars? This book is designed to fill these needs.

We have endeavored to cover all the important aspects of astrobiology at an advanced level, yet such that most of the contents in every chapter should be understandable to anyone versed in any relevant science discipline. We envision our youngest readers to be science majors near the end of undergraduate study or the beginning of graduate study. And at the other extreme, we aim to serve scientists who haven't taken an academic course for forty years, but are intrigued by the nascent field of astrobiology.

From Lamarck to Darwin to the central dogma

The basic notion of evolution is that inherited changes in populations of organisms result in expressed differences over time – these differences are at the gene level (the genotype) and/or expression of the gene into an identifiable characteristic (the phenotype). The important underlying fact of evolution is that all organisms share a common inheritance, or, put more dramatically, all extant organisms on Earth evolved from a common ancestor. We see this in the universal nature of the genetic code and in the unity of biochemistry: (a) all organisms share the same biochemical tools to translate the universal information code from genes to proteins, (b) all proteins are composed of the same twenty essential amino acids, and (c) all organisms derive energy for metabolic, catalytic, and biosynthetic processes from the same high-energy organic compounds such as adenosine triphosphate (ATP).

In On the Origin of Species Charles Darwin (1859) (Fig. 10.1) built his theory of evolution using evidence that included an ancient Earth thought at the time by many geologists to have an age in millions of years. He also took the extinction of species to be a real phenomenon since fossils existed that were without living representatives.

A new synthesis: the Biological Universe

A remarkable shift in our scientific world picture is taking place, potentially as fundamental in its consequences as the new views put forth by Copernicus in the sixteenth century, or by Darwin in the nineteenth. Although astronomers have long been involved with the prospects for extraterrestrial life, their fundamental task since Newton has been to apply physics to a lifeless Universe. On the other hand, biologists have pursued their studies for centuries in cosmic isolation, meaning that biology considered life on Earth, with no attention paid to its cosmic context. Today, however, both camps are recognizing fruitful and exciting avenues of research created by a new synthesis. Biology is vastly enriched when attention is paid to a broader context for life as we know it, as well as the possibilities for other origins of life. And astronomy is coming to realize that the themes of cosmic, galactic, stellar, and planetary evolution, which have become central over the past century, must now also incorporate biological origin(s) and evolution(s). Historian of science Steven Dick (1996) has hailed this new synthesis as the Biological Universe. Although astronomy and biology are its two primary poles, many other disciplines are also vital components, in particular Earth and planetary sciences.

Introduction

Ice is the “natural state” or predominant form of water in our Solar System. The surfaces of most planets and moons are currently at temperatures well below the freezing point of pure water, including two of the more promising sites in the search for traces of extraterrestrial life, Mars and Europa. The amount of water-ice on Europa exceeds the volume of liquid water on Earth. Comets, considered potential vectors for precursors or early stages of life, are also icy bodies (Chapter 3). Even Earth may have undergone a series of complete (or near-complete) glaciations in its recent history, earning the title “Snowball Earth” (Section 4.2.4).

The presence of the liquid phase of water, however, is essential to the prospering of life as we know it. In order to study where liquid water can occur, we must understand scales ranging from the structure of the water molecule to the temperatures possible on a planetary surface or subsurface. In our Solar System, only Earth allows for a planetary surface with abundant liquid water (Chapter 4). Mars, however, may well have some liquid water in permafrost (perennially frozen soil) beneath its surface today (Section 18.4.1), and the evidence is strong that Europa and perhaps other moons of Jupiter have water oceans below their icy crusts (Chapter 19).

Overview

Why history?

The core questions of astrobiology are not new. They have always been asked and are central to Western intellectual history. How did life begin? How has it changed? What is the relation of humans to other species? Does life exist elsewhere? If so, where might it be and what is it like? Although these questions are ancient, what is new are the tools at hand to search for answers, ranging from robotic spacecraft to genome sequencing, from electron microscopes to radio telescopes. These tools and other factors (see the Prologue and Chapter 2) appear to have brought astrobiology to a point where it is gelling into something qualitatively different – our first sound attack on these questions. But is this so? Or is today no different from any other time in the past few centuries?

In every era, including our own, scientists can do no more than tackle questions with the best tools available, apply the best insight they can muster, and struggle to fashion a consensus as to the nature of the world. In this manner our understanding has progressed, for example, from the “animalcules” that van Leeuwenhoek described three hundred years ago to the richness of contemporary microbiology. To understand such a thread as it meanders through history, we need to document more than the accumulation of facts. When evaluating a given episode, historians of science look carefully at evidence of not only the science itself, but also of the larger enveloping context.

Introduction

Europa, one of the four large satellites of Jupiter, is nearly the size of Earth's Moon. Tidal flexing driven by Jupiter's gravity and sustained by an orbital resonance with two other jovian satellites, Io and Ganymede, results in significant heat dissipation within Europa. Calculations indicate that this tidal heating is sufficient to maintain liquid water beneath Europa's ice crust. Moreover, observational evidence suggests that it is indeed probable, but not yet completely certain, that Europa harbors a subsurface ocean of liquid water whose volume is about twice that of Earth's oceans. The likely presence of abundant liquid water places Europa among the highest priority targets for astrobiology.

To support life, Europa would also require an inventory of biogenic elements and a source of sufficient free energy. The ability to support life does not of course guarantee that the origin of life took place on Europa, or that life is present today; answering these questions will require further exploration. This chapter considers Europa in an astrobiological context, distinguishing among what is known, what is supported by evidence but still uncertain, and what remains more speculative. We conclude with a discussion of future missions that will be needed to address current geological and astrobiological questions.

Jupiter and its satellites

The planet Jupiter, orbiting the Sun at 5.2 AU, is more massive than the other planets in the Solar System combined; it is 3.3 times more massive than Saturn and 318 times more than the Earth.