Introduction

The appeal of astrobiology lies in two primary features, the first being the important questions astrobiologists ask: “Are we alone? How did life begin? How will it all end?” The second attractor is the opportunity to work across many disciplines. Astrobiology challenges us to draw from many intellectual resources in the attempt to answer these questions – biology, chemistry, physics, astronomy, geology, and engineering are all required. Life detection, in particular, requires a strong interdisciplinary approach. In this chapter we focus on life detection within the context of Solar System exploration; techniques for detecting planets and possible associated life beyond the Solar System are discussed in Chapters 21 and 26. Within our Solar System life detection efforts have been and still are primarily focused on Mars (Chapter 18), and so we will use Mars exploration as a model for discussion, though our approach is applicable to any potential habitat for life.

The success of a life detection mission to another planet should not be focused solely upon whether or not live organisms are detected, but rather it must be able to correctly classify observations as evidence of (a) life, (b) non-life, (c) once-alive-but-now-dead, or (d) made-by-life (biogenic). It is essential that the scientific community agree on which measurements should be made, and then how to interpret those measurements.

Communication across the many disciplines involved in astrobiology is fraught with difficulty on many levels, including even the seemingly simple matter of units and usage of terms and abbreviations. When first introduced in any chapter, unusual units often not known to those outside the field are defined. In this appendix we give conventions and conversions for various units used throughout the book.

Astrobiologists hope to understand the origin, history, extent, and future of life in the Universe. This is a huge task, considering that two of the terms in this mission statement are difficult to define. The definition of “life” is itself worth a chapter in this book (Chapter 5). Here in this chapter, we must not only concern ourselves with a definition of life, but also with the concept of “understanding.” What does it mean to say that we “understand life”?

Any attempt to understand life soon engages organic chemistry. Biology today is increasingly focused on the molecular scale. Indeed, it is difficult to find a biologist today who is not attempting to put a molecular structure on the phenomenon that they are studying, so much so that biology can be (provocatively!) viewed as the subfield of chemistry dealing with chemical systems capable of Darwinian evolution.

Some illustrations make this point. The human genome is nothing more (and nothing less) than a collection of chemical structures, recording how carbon, oxygen, nitrogen, hydrogen, and phosphorus atoms are bonded in the natural products directly responsible for heritance. Molecular evolution uses organic chemistry to describe the Darwinian evolution of species, the process that drives biology. Neurobiologists are attempting to describe the inventory of molecules, including messenger RNA, that allow neurons to learn and remember.

Nowhere is this more evident than in the segment of astrobiology that investigates the origin of life.

Earth is not the only body in the Solar System that is habitable. Life as we know it requires liquid water and free energy gradients, both of which probably also exist on Mars and Europa, although liquid water on those bodies is restricted to the subsurface. Earth is, however, the only planet in the Solar System that has liquid water at its surface. Similar planets may exist around other stars (Chapter 21) and would be of profound interest for two reasons. First, biology on such planets might resemble life on Earth. Second, the biosphere on such planets would interact with the planet's atmosphere and could modify it in a way that may be detectable remotely. Today, life may be thriving on Mars or Europa but its discovery will require subsurface exploration. In contrast, we might be able to tell whether a distant Earth-like planet is inhabited by measuring the spectrum of its atmosphere.

Thus, from an astrobiological standpoint, one of the most fundamental characteristics of a planet is its surface temperature Ts. If Ts is not within the range in which liquid water can exist, remotely detectable life will probably not exist there. Consequently, the first part of this chapter is concerned with planetary surface temperatures. The constraint on temperature is not as obvious as 0 < Ts < 100 °C.

Introduction

The search for extraterrestrial life is intimately linked with our understanding of the distributions, activities, and physiologies of Earth-life. This is not to say that only Earth-life could exist on other planets and moons but it is important to know the extent of environmental conditions that can support terrestrial organisms as a first-order set of criteria for the identification of potential extraterrestrial habitats. Even though the life forms may have different biochemistries and in fact may have had different origins, the limits of life on Earth may help define the potential for habitability elsewhere. It is also likely that many of the limits of Earth-life could extend out of the bounds of extreme conditions found on modern-day Earth. This is the case for the bacterium Deinococcus radiodurans that can tolerate levels of radiation beyond those found naturally on Earth, and also for the apparent tolerance by Escherichia coli to hydrostatic pressures that exceed by more than ten times the pressures in the deepest ocean trenches (Cox and Battista, 2005; Sharma et al., 2002).

Since Earth is the only planet that unequivocally supports modern, living ecosystems, it is logical to first look for life elsewhere that resembles Earth-life. Earth-life requires either light or a chemical energy source, and other nutrients including nitrogen, phosphorus, sulfur, iron, and a large number of elements in trace concentration; 70 elements in all are either required or are targets of interaction by various species of Earth-life (Wackett et al., 2004).

Introduction

The question “What is life?” is foundational to biology and especially important to astrobiologists who may one day encounter utterly alien life. But how should one approach this question? One widely adopted strategy among scientists is to try to define ‘life.’ This chapter critically evaluates this strategy. Drawing from insights gained by philosophical investigations into the nature of logic and language, we argue that it is unlikely to succeed. We propose a different strategy, which may prove more fruitful in searches for extraterrestrial life.

We begin in Section 5.2 by reviewing the history of attempts to define ‘life,’ and their utility in searches for extraterrestrial life. As will become apparent, these definitions typically face serious counterexamples, and may generate as many problems as they solve.

To explain why attempts to define ‘life’ are fraught with so many difficulties, we must first develop the necessary philosophical background. Therefore, in Sections 5.3 and 5.4 we discuss the general nature of definition and of so-called theoretical identity statements. Section 5.5 then applies the material developed in these sections to the project of defining ‘life.’ We argue that the idea that one can answer the question “What is life?” by defining ‘life’ is mistaken, resting upon confusions about the nature of definition and its capacity to answer fundamental questions about natural categories (Cleland and Chyba, 2002).

To answer the question “What is life?” we require not a definition but a general theory of the nature of living systems.

Introduction

Ten times farther from the Sun than the Earth is, shrouded in an orange haze, preserved at temperatures near 100 K, Saturn's moon Titan seems an unlikely astrobiological target. In fact, its extremes suggest images of death rather than life.

Yet this planet-sized moon possesses a dense atmosphere of nitrogen and methane, which, over time and with the action of ultraviolet radiation, may have generated 1016 tons of hydrocarbons and nitriles – the constituents of life – that were then deposited on its surface in liquid and solid form. Within these vast organic deposits, at those times when water ice might liquefy because of volcanism or of impact heating, some of the organic chemical steps leading toward the origin of life might be replicated and then preserved for study on the surface of Titan.

Titan's dense atmosphere has easily won it the high status we reserve for terrestrial planets with atmospheres (Venus, Earth, and Mars) although it is consigned to the outer Solar System. It possesses methane-driven meteorology, and its large size for an ice-rock world likely means a wealth of tectonic activity in its interior. A variety of observed surface landforms produced, apparently caused by erosional processes driven by winds and surface liquid hydrocarbons. In short, other than the likely absence of extant life because of the extreme cold, Titan exhibits a broad range of atmospheric and geologic processes that rival in complexity those of Mars and perhaps of Earth.

Exploring knowledge space for microbes

Astrobiology and biology more generally are integrating new visions of biodiversity with evolutionary and ecological processes. The body of knowledge about hundreds of thousands of microbial species is huge, and involves data on ontogenetic transitions and intraspecific variation; encompasses scales of biology ranging from molecular variation in a particular kind of cell to the role of individuals in complex ecosystems; and accommodates the biology of individuals whose identity and role change as a function of time and place, as well as in response to biotic and abiotic interactions. The knowledge may be either digital or in traditional media, such as often found in libraries, museums, and herbaria. Researchers need solutions that lead to a comprehensive and evolving “knowledge space” (which includes information and its interpretation). The solutions will include tools to empower experts to transfer knowledge from traditional to contemporary media, as well as allow them to integrate old with new. An approach that is universal, inclusive, scalable, and flexible can evolve into a comprehensive Encyclopedia of Life (Wilson, 2003).

The absence of websites offering comprehensive treatments of microbial diversity is a serious impediment for students and investigators who are in need of morphological, physiological, and lifestyle information about microbes. In fact, this is a problem not only for microbial life but for all life. There are no robust standards for indexing phenotypic and biodiversity data, it is difficult to parse and recompile existing relevant data resources.

Introduction

Astrobiology is a discipline that is best enjoyed in the field. What follows is a series of short descriptions by University of Washington students and faculty of selected astrobiological destinations that our planet offers. We cannot hope to provide a comprehensive list – with more space and time we might have included the Burgess shale of Canada; the Atacama desert of Chile; the Cretacious/Tertiary boundary at Gubbio, Italy; Louis Pasteur's home and lab in Paris; Witwatersrand mine in South Africa; the channelled scabland of eastern Washington state, to name but a few. Nevertheless, the ten selected locales have played primary roles in determining how we have come to view the phenomenon of life, and how we have placed constraints on its potential occurrence both on our own planet and elsewhere.

From boiling microbial ponds in Yellowstone to frozen wastes of Greenland harboring Earth's oldest sedimentary rocks, a lifetime of exploration awaits you.

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.