Astrobiology

Astrobiology is the study of the origins, evolution, distribution, and future of life in the Universe. Its goals include the study of life on Earth and the search for life beyond Earth. It addresses three fundamental questions: How does life begin and evolve? Is there life beyond Earth and how can we detect it? What is the future of life on Earth and the Universe?

Astrobiology requires fundamental concepts of life and habitable environments that will help scientists to recognize biospheres that might be quite different from the Earth's one. It embraces the search for potentially inhabited planets beyond the Solar System, the exploration of Mars and the outer planets, laboratory and field investigations of the origins and early evolution of life, and studies of the potential of life to adapt to future challenges, both on Earth and in space.

Astrobiology is multidisciplinary in its content and interdisciplinary in its execution. Its success depends critically upon the close coordination of diverse scientific disciplines and programs, including space missions. Interdisciplinary research is needed that combines Molecular Biology, Ecology, Planetary Science, Astronomy, Information Science, Space Exploration, and related disciplines. The broad character of Astrobiology compels scientists to strive for the most comprehensive and inclusive understanding of biological, planetary and cosmic phenomena.

Domains of Investigation and Goals of Research

Astrobiology encompass seven key domains of investigation:

• the nature and distribution of habitable environments in the Universe;

• the past or present habitable environments, prebiotic chemistry and signs of life elsewhere in our Solar System;

• how life originates from cosmic and planetary precursors;

• how past life on Earth interacted with its changing planetary and Solar System environment;

• the evolutionary mechanisms and environmental limits of life;

• the principles that will shape the future of life, both on Earth and beyond;

• how to recognize signatures of life on other worlds and on early Earth.

To these domains corresponds respectively the following research goals:

• determine the potential for habitable planets beyond the Solar System, and characterize those that are observable;

• determine any chemical precursors of life and any ancient habitable climates in the Solar System, and characterize any extinct life, potential habitats, and any extant life on Mars and in the outer Solar System;

• perform observational, experimental and theoretical investigations to understand the general physical and chemical principles underlying the origins of life;

• investigate the historical relationship between Earth and its biota by integrating evidence from both the geologic and biomolecular records of ancient life and its environments;

• determine the molecular, genetic, and biochemical mechanisms that control and limit evolution, metabolic diversity, and acclimatization of life;

• elucidate the drivers and effects of ecosystem change as a basis for projecting likely future changes on time scales ranging from decades to millions of years, and explore the potential for microbial life to adapt and evolve in environments beyond its planet of origin;

• identify biosignatures that can reveal and characterize past or present life in ancient samples from Earth, extraterrestrial samples measured in situ, samples returned to Earth, remotely measured planetary atmospheres and surfaces, and other cosmic phenomena.

Habitability Questions

A planet or planetary satellite is habitable if it can sustain life that originates there or if it sustains life that is carried to the object. Astrobiology seeks to expand our understanding of the most fundamental environmental requirements for habitability. However, in the near term, it must proceed with the current concepts regarding the requirements for habitability. That is, habitable environments must provide extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism. Habitability is not necessarily associated with a single specific environment; it can embrace a suite of environments that communicate through exchange of materials. The processes by which crucial biologically useful chemicals are carried to a planet and change its level of habitability can be explored through the fields of Prebiotic Chemistry and Chemical Evolution. A major long-range goal for Astrobiology is to recognize habitability beyond the Solar System, independent of the presence of life, or to recognize habitability by detecting the presence of life.

Research in Astrobiology attempts to search for habitable or inhabited environments beyond the Solar System. Humans have pondered for millenia whether other inhabitable worlds exist. Now, for the first time, they have an opportunity to look and see. Of course it is not possible to examine the ~1010 Earth-like planets that simple statistical models predict to exist in our galaxy, much less the ~1021 such planets expected to be in the Universe. Still, it should be possible to determine whether terrestrial planets are indeed as common as predicted above, whether a substantial fraction of them show signs of habitability, and whether an appreciable fraction of these show biosignatures.

A key difference between the search for life in the Solar System and the search in external planetary systems is that, within the Solar System, interplanetary transfer of viable microbes seems a plausible process, and therefore the discovery of life elsewhere in the Solar System seems plausible. While this is indeed of extraordinary interest, it may not cast light on whether it is easy or difficult for life to begin. On the other hand, the fact that dispersion times between stars are ~ 105 to 106 times longer than for dispersion within the Solar System makes independent origination of life-forms outside the Solar System more probable.

The research objectives address three key questions. First, do terrestrial planets and large satellites tend to form in a state where they are likely to become habitable, or do habitable environments emerge only after a sequence of less probable events? Second, how frequently do habitable environments arise on solid planets, including large satellites? Third, what are the specific signs of habitability and habitation, and how do such signs change with the circumstances of the planet (e.g., mass, distance from its star, history and relative abundance of volatile compounds)? To address these questions effectively, we must investigate how habitable planetary systems form and evolve, and we must understand the ultimate environmental limits of life.

Much of this effort focuses upon the presence or absence of liquid water in bulk form. Water is made from the two most abundant chemically reactive elements in the Universe, and it is the necessary ingredient for Earth's type of life. Liquid water has played an intimate, if not fully understood, role in the origin and development of life on Earth. Water contributes to the dynamic properties of an Earth-sized planet, permitting convection within the planetary crust that might be essential to supporting Earth-like life by creating local chemical disequilibria that provide energy for life. Water maintains a strong polar-nonpolar dichotomy with certain organic substances. This dichotomy has allowed life on Earth to form independent stable cellular structures. Thus the primary focus is concerned with planets having a liquid water boundary layer, although the focus may expand to include other planets or satellites as Astrobiology matures as a discipline.

There is also a focus – though not exclusive – on molecular oxygen and ozone as biosignatures, and therefore on dealing with the interface between the understanding of the geological and biological aspects of oxygen, and the details of the spectral features that can be observed and interpreted remotely. Oxygen is a very common element that has provided Earth with its most distinctive biosignature. The chemical state of an Earth-like planet, as well as the geological activity that delivers reduced species to the surface environment, will cause virtually all of the molecular oxygen to be consumed unless it is produced rapidly (e.g., by oxygen-producing photosynthesis). Also, the relatively modest ultraviolet fluxes of many stars prevent rapid production of oxygen from photo-dissociation of water. These factors will help to prevent the possibility of false positive detections of oxygen biosignatures.

The challenge of remotely detecting life on a planet that has not developed a biogenic source of oxygen is fraught with unknowns. What chemical species and spectral signatures should be sought? What metabolic processes might be operating? How does one guard against a false positive detection? Research that is guided both by our knowledge of Earth's early biosphere (i.e., before the rise of an oxygenated atmosphere) and by studies of alternative biological systems can help address these questions and provide guidance to astronomers seeking evidence of life elsewhere.

Biosignatures

Astrobiological exploration is founded upon the premise that signatures of life (biosignatures) encountered in space will be recognizable. A biosignature is an object, substance and/or pattern whose origin specifically requires a biological agent. The usefulness of a biosignature is determined, not only by the probability of life creating it, but also by the improbability of nonbiological processes producing it. An example of such a biosignature might be complex organic molecules and/or structures whose formation is virtually unachievable in the absence of life. A potential biosignature is a feature that is consistent with biological processes and that, when it is encountered, challenges the researcher to attribute it either to inanimate or to biological processes. Such detection might compel investigators to gather more data before reaching a conclusion as to the presence or absence of life.

The concepts of life and biosignatures are inextricably linked. To be useful for exploration, biosignatures must be defined in terms that can be measured and quantified. Measurable attributes of life include its complex physical and chemical structures and also its utilization of free energy and the production of biomass and wastes; phenomena that can be sustained through self-replication and evolution. A strategy is needed for recognizing novel biosignatures. This strategy ultimately should accommodate a diversity of habitable conditions, biota and technologies in the Universe that probably exceeds the diversity observed on Earth.

Habitable planets create nonbiological features that mimic biosignatures and therefore must be understood in order to clarify our interpretations. A library of biosignatures and their nonbiological mimics of life as we know it must be created. Catalogs of biosignatures must be developed that reflect fundamental and universal characteristics of life, and thus are not restricted solely to those attributes that represent local solutions to the challenges of survival. For example, certain examples of our biosphere's specific molecular machinery, e.g., DNA and proteins, might not necessarily be mimicked by other examples of life elsewhere in the Cosmos. On the other hand, basic principles of biological evolution might indeed be universal.

However, not all of the universal attributes of life will be expressed in ancient planetary materials or detectable remotely (e.g., by astronomical methods). For example, the processes of biological evolution are highly diagnostic for life, but evidence of biological evolution might not be readily detected as such in a sample returned from Mars. However, better-preserved evidence of life might include complex structures that are often retained in aquatic sediments or can be preserved in large quantities in the environment. Thus, for example, categories of biosignatures can include the following: cellular and extracellular morphologies, biogenic fabrics in rocks, bio-organic molecular structures, chirality, biogenic minerals, biogenic stable isotope patterns in minerals and organic compounds, atmospheric gases, and remotely detectable features on planetary surfaces (photosynthetic pigments, etc.).

On Earth, biosignatures also include those key minerals, atmospheric gases and crustal reservoirs of carbon, sulfur and other elements that collectively have recorded the enduring global impact of the utilization of free energy and the production of biomass and wastes. Oxygen-producing photosynthesis has simultaneously created large reservoirs of atmospheric oxygen, marine sulfates and sedimentary ferric iron and sulfates (its oxidized products), as well as large sedimentary reservoirs of biogenic organic matter and sulfides (its corresponding reduced products). Again, such features must be sufficiently complex and/or abundant so that they retain a diagnostic expression of some of life's universal attributes. Also, their formation by nonbiological processes should be highly improbable.

As more complex biological features eventually evolved, as evidenced by plants and animals, the associated biosignatures became easier to distinguish from the abiotic world. Human technology continues this trend, with the added benefit that it might be detected remotely. Thus, although technology is probably much more rare than life in the universe, its associated biosignatures perhaps enjoy a much higher "signal-to-noise" ratio. Accordingly, current methods should be further developed and novel methods should be identified for detecting electromagnetic radiation or other diagnostic artifacts that indicate remote technological civilizations.

Search for Life in the Solar System

The exploration for habitable environments, life and/or prebiotic chemistry in the Solar System directly links basic research in Astrobiology to space missions. Because little is presently known about habitable environments within our Solar System, the distribution and nature of potentially habitable environments should be determined on Mars, Titan, Europa, and other promising objects. As a corollary, we should understand the mechanisms of evolution of habitable environments throughout the Solar System. Although life elsewhere could have developed in ways different from life on Earth, our current knowledge of life and habitable environments serves as the starting point for our exploration strategy. Research in such widely divergent areas as planetary and Solar System evolution, the Biology of extreme environments, and Precambrian Paleontology has been instrumental in guiding the search for evidence of life elsewhere in the Solar System. Earth-based analog studies and theoretical investigations, informed by data from previous Solar System missions, will assist astrobiologists to refine exploration strategies and scientific priorities for future missions.

Understanding planetary habitability and the relationship between the occurrence of life and the evolution of planets must be a primary organizing theme of any Solar System exploration program. In the most basic sense, the strategy for the astrobiological exploration of the Solar System involves exploring for environments regarded as necessary for life to begin and/or persist, namely those having liquid water, energy sources that can sustain metabolism, conditions that promote the synthesis of complex organic molecules, and understanding the evolution of habitable environments on Solar System objects.

Advances in our understanding of the environmental limits of life on Earth have provided crucial information for refining the strategies to explore for life elsewhere in the Solar System. For example, a deep subsurface biosphere was discovered that included non-photosynthetic organisms that make organic compounds from hydrogen and other simple byproducts of aqueous weathering. This discovery has revolutionized the thinking about the potential for life on other planets like Mars or Europa, where surface conditions are fundamentally inhospitable to life. The necessity to explore the deep subsurface of other Solar System bodies has identified the need to develop robotic drilling systems that can penetrate 100's to 1000's of meters below the surface, where interior habitable zones of liquid water and a life-sustaining redox chemistry might exist. Of course, geological activity or meteorite impacts might have brought evidence of subsurface life to the surface, therefore the ability to identify and reach key sites with landers and rovers is also a high priority.

In preparing for future missions to explore for life and/or prebiotic chemistry in the Solar System, an important precursor activity for Astrobiology is to identify in situ instrumentation to support the search for complex organic molecules and life. As a starting point, there is a critical need for research to define unambiguous approaches to life detection over a broad range of environmental conditions that represent other planetary environments. Such research will also help address planetary protection issues, such as the effects of forward contamination of other planetary surfaces and the risks of back contamination associated with samples returned to Earth.

In pursuing the question of extraterrestrial life, humans have long held a fascination with Mars. Indeed, the robotic exploration of the red planet has provided compelling evidence for surface environments that could have supported life early in the planet's history. More recently, arguments have been made for the existence of a Martian groundwater system that could harbor an extant subsurface biota. Key questions include the following. If ever life arose on Mars, is it related to terrestrial life, or did Mars sustain an independent origin of life? If life never developed on Mars, is there a prebiotic chemical record preserved in ancient martian rock sequences that might contain clues about how life began on Earth?

Possibilities for subsurface habitable zones of liquid water have also been recognized in the outer Solar System. Induced magnetism, as well as surface geomorphology and chemistry, have provided compelling evidence for an ocean of liquid water (brine) beneath the icy crust of Europa. Similar conditions may also exist on two other Galilean satellites –Ganymede and Callisto. In addition, a complex prebiotic chemistry and zones of liquid water might exist on Titan. Some of these environments might resemble aspects of early Earth and thus they can teach us about our own origins. However, other environments could be quite different, and these might have hosted a prebiotic chemical evolution that led to an altogether different form of Biology.

As the nature of the potentially habitable environments in our Solar System becomes better defined, the Astrobiology program must interact with both observational Astronomy and mission scientists to consider also the possibility of life in non-aqueous environments. Such a possibility can be explored during missions to places (like Titan) where liquid water is not predominant, and by developing the ability to recognize the biosignatures of life in non-aqueous environments.

Emergence of Life Elsewhere

How life begins remains a fundamental unsolved mystery. The origin of life on Earth is likely to represent only one pathway among many along which life can emerge. Thus the universal principles must be understood that underlie not only the origins of life on Earth, but also the possible origins of life elsewhere. These principles will be sought by determining what raw materials of life can be produced by chemical evolution in space and on planets. It should be understood how organic compounds are assembled into more complex molecular systems and the processes by which complex systems evolve those basic properties that are critical to life's origins. Such properties include capturing energy and nutrients from the environment, and manufacturing copies of key biomolecules. Clues from the biomolecular and fossil records, as well as from diverse microorganisms, should be explored in order to define better the fundamental properties of the living state.

Astrobiology must move beyond the circumstances of Earth's own particular origins in order to develop a broader discipline, a "Universal Biology." Although this discipline will benefit from an understanding of the origins and limits of terrestrial life, it also requires that the environmental conditions and the chemical structures and reactions that could support life on other habitable planets be defined. These may be very different than what scientists have learned to expect from the Biology of Earth. For example, liquid water is essential for all life on Earth, however, at least under laboratory conditions, certain chemical systems can undergo a form of replication in non-aqueous solvents. Furthermore, laboratory experiments that involve analogs of the nucleic acids, proteins, sugars, and lipids indicate that the particular molecular structures found in Earth-based life would not be essential in life forms having a genesis independent of life on Earth.

The perspectives gained from such research will improve both the search for habitable environments in the Solar System and the recognition of biosignatures within those environments. The invention of translation, the creation of new metabolic pathways, the adaptation of organisms to extreme environments, and the emergence of multi-cellular life forms and other higher order functions, are all constrained by the intrinsic chemistry of the molecules that supported the particular example of life that achieved these innovations. Given this abundance of chemical opportunity, it seems likely that an expanded research effort will lead to novel molecular systems having the combination of properties that we associate with life processes. Such research will help to understand better the link between molecular evolution and Chemistry that is central to Astrobiology.

To understand how life can begin on a habitable planet such as the Earth, it is essential to know what organic compounds were likely to have been available, and how they interacted with the planetary environment. Chemical syntheses that occur within the solid crust, hydrosphere and atmosphere are potentially important sources of organic compounds, therefore they continue to be an important focus of research on this question. Prebiotic chemistry might begin in interstellar clouds. Laboratory simulations have recently demonstrated that key molecules can be synthesized in interstellar ices that are incorporated into nascent solar systems, and astronomical observations and analyses of extraterrestrial materials have shown that many compounds relevant to life processes are also present in meteorites, interplanetary dust particles and comets. It is likely that substantial amounts of such organic material were delivered to the Earth during late accretion, thereby providing organic compounds that could be directly incorporated into early forms of life or serve as a feedstock for further chemical evolution. An important research objective within this goal is to establish sources of prebiotic organic compounds and to understand their history in terms of universal processes that would take place on any newly formed planet. This will require an integrated program of pan-spectral astronomical observations, sample return missions, laboratory studies of extraterrestrial materials, and realistic laboratory simulations of inaccessible cosmic environments.

Life can be understood as a chemical system that links a common property of organic molecules – the ability to undergo spontaneous chemical transformation – with the uncommon property of synthesizing a copy of that system. This process, unique to life, allows changes in a living molecular system to be copied, thereby permitting Darwinian-like selection and evolution to occur. At the core of the life process are polymers composed of monomeric species such as amino acids, carbohydrates, and nucleotides. The pathways by which monomers were first incorporated into primitive polymers on the early Earth remain unknown, and physical properties of the products are largely unexplored. A primary goal of research on the origin of life must be to understand better the sources and properties of primitive polymers on the early Earth, and the evolutionary pathway by which polymerization reactions of peptides and oligonucleotides became genetically linked.

Axiomatically, life cannot exist in an environment at thermodynamic equilibrium. If the environment were at equilibrium, then, by the Second Law of Thermodynamics, no net chemical transformation would be possible. Thus we assume that life began in an environment that was far from thermodynamic equilibrium, so that free energy was available to drive the chemical transformations required for life processes. A fundamental question concerns the mechanisms by which this energy was captured by the earliest forms of life. The forms of available energy include light, chemical bond energy, and the energy of electron transfer reactions involving compounds with different redox potentials. It seems likely that photosynthesis appeared very early in evolutionary history, thus it is important to identify primitive pigment systems. Hydrothermal vents and other geothermal environments offer a second potential source of energy in the form of dissolved gases such as hydrogen and hydrogen sulfide, and mechanisms by which reduced gases in solution can deliver energy to living systems should be investigated. In contemporary cells, the energy present in chemical bonds is captured by metabolism, and the first forms of life must have incorporated linked chemical reactions as simple metabolic pathways. A primary research objective will be to identify mechanisms by which any of these energy sources were coupled to polymerization chemistry.

For life to begin in a natural setting such as a planetary surface there must be mechanisms that concentrate and maintain interacting molecular species in a microenvironment. From this perspective, life began as a bounded system of interacting molecules, none of which has the full property of life outside of that system. A bounded system of replicating, catalytic molecules is by definition a cell, and at some point life became cellular, either from its inception or soon thereafter. Besides separating the contents of a cell from the environment, membranes have the capacity to develop substantial ion gradients that represent a central energy source for virtually all life today. Boundary membranes also divide complex molecular mixtures into large numbers of individual structures that can undergo selective processes required to initiate biological evolution. A primary objective of research is to assemble laboratory versions of model cells. These will incorporate systems of interacting molecules within membrane-bounded environments. They will have the capability to capture energy and nutrients from the environment, grow through polymerization, and reproduce some of their polymeric components. Approaching this challenging problem will lead to a more refined definition of the living state, and will clarify the hurdles faced by self-assembled systems of organic molecules as they evolved toward the first life on the Earth.