Abstract
NASA'
Since the astrobiology community published its last roadmap in 2008, research in the field has focused more and more on the link between the “astro” and the “bio” in astrobiology—that is, what makes a planetary body habitable. “Habitability” has become a major buzzword in astrobiology as researchers have learned more about extraterrestrial environments in our Solar System and beyond and deepened their understanding of how and when the early Earth became habitable.
Why is Earth habitable? How, when, and why did it become habitable? Are, or were, any other bodies in our Solar System habitable? Might planets orbiting other stars be habitable? What sorts of stars are most likely to have habitable planets? These are just a few of the questions that astrobiologists are trying to answer today.
In preparing this new science strategy, hundreds of members of the astrobiology community collaborated in an intensive process of defining goals and objectives for astrobiology research moving forward. The community identified six major topics of research in the field today: • Identifying abiotic sources of organic compounds • Synthesis and function of macromolecules in the origin of life • Early life and increasing complexity • Co-evolution of life and the physical environment • Identifying, exploring, and characterizing environments for habitability and biosignatures • Constructing habitable worlds
This science strategy identifies questions to guide and inspire astrobiology research on each of these topics—in the lab, in the field, and in experiments flown on planetary science missions—over the next decade. The strategy also identifies major ongoing challenges that astrobiologists tackle as they attempt to answer these universal questions.
Progress and accomplishments in each of these areas of research over the past ten years are detailed in each of the successive chapters of this document.
What follows is a brief summary of the topics described in depth in Chapters 1–6 of this strategy.
Identifying Abiotic Sources of Organic Compounds
Where did the building blocks of life come from? A major goal of research on this topic in astrobiology is to understand how the abiotic (non-biological) production of small molecules led to the production of large and more complex molecules, prebiotic chemistry, and the origin of life on Earth. This line of research also aims to understand what roles primitive icy bodies (asteroids and comets) play in the origin of life on habitable planets and whether life or prebiotic chemistry could or did evolve on differentiated (altered) icy worlds such as Enceladus, Europa, and Titan. Understanding the production of molecules in various endogenous (planetary) environments, as well as in exogenous (space) environments with the associated delivery of extraterrestrial molecules to planetary surfaces, is critical for establishing the inventory of ingredients from which life originated on Earth.
Synthesis and Function of Macromolecules in the Origin of Life
On Earth, macromolecules—specifically, proteins and nucleic acids—form the catalytic and genetic means for life to sustain itself. Macromolecules evolve—that is, they change over time—thus meeting another criterion for recognizable life.
The macromolecules (large, complex molecules, or polymers) of Earth-based life are composed of a small subset of all potential chemical building blocks (smaller organic molecules, or monomers). It is likely that the exact components of these macromolecules are accidental. It also is possible that macromolecules formed from different selections of smaller molecules could characterize other living systems. Thus, it is crucial to characterize the overall chemical underpinnings of the processes that lead to the function and persistence of evolvable macromolecular systems. As part of this effort, it is necessary to identify interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to the diversity, selection, and replication of these systems.
These macromolecules are uniquely capable of the structural, catalytic, and genetic functions required for life. The diverse chemical alphabet of 20 amino acids found in Earth life leads to protein architectures that are capable of structural transitions essential to catalytic functioning. Catalysis can be carried out by nucleic acids and proteins. In general, protein catalysis is more efficient than nucleic acid catalysis. Nucleic acid catalysts found in life today are thought to be “living fossils” of an earlier system.
From a broader perspective, these polymers can be seen not only as the information- and function-carrying molecules in life on Earth but also as information- and function-carrying molecules for life on any planet. As such, questions of whether and how polymers transmit information and fold to generate function are of interest.
To further refine understanding of life's origins and early chemical evolution, researchers must continue to map the chemical landscape of potential primordial informational polymers. The advent of polymers that could replicate, store genetic information, and exhibit properties subject to selection likely was a critical step in the emergence of prebiotic chemical evolution. Astrobiologists thus must focus on developing an understanding of macromolecule synthesis, stability, and function in the context of plausible prebiotic conditions and environments.
Early Life and Increasing Complexity
Understanding the history of life on Earth is key to a full understanding of what life is and how it works. Over four billion years, life on Earth has generated an extraordinary range of organizational plans, creating the immense variety that operates on Earth today. Astrobiologists face the challenge of deciphering overarching rules for evolutionary processes, drawing on theory and observation to make a general model of life.
Recognizing life on other planets depends on how scientists define life. However, defining life has proved problematic because it is unclear where to draw the boundary between living and non-living entities, or whether drawing such a boundary is the best way to frame the issue. For example, self-replicating RNA, viruses, and prions are alive by some definitions but not by others. The lack of a precise boundary between living and non-living entities today mirrors a similarly fuzzy divide at the origin of life. Identifying which attributes of life are likely to be common to all origins, and which are context-dependent, will enable better predictions about the possible nature of life on other planets.
Co-Evolution of Life and the Physical Environment
Life affects its environment. At the same time, the environment affects life. Astrobiologists are focused on understanding the relationship between life and environment well enough to inform the search for potentially habitable environments beyond Earth. Examples of major transitions in biological evolution that affected our planet include the origins of photosynthesis, multicellularity, and intelligent life. Major changes in the physical state of the planet that have affected biology include the emergence of plate tectonics and continents, as well as climatic transitions such as “Snowball Earth” episodes.
Studying the co-evolution of life and environment informs other lines of research in astrobiology in three major ways. First, the delivery of abiotic organic compounds to Earth and the development of prebiotic chemistry on Earth can be thought of as the first environmental influences on life. Second, as early life evolved increasing complexity, its interactions with the planet would have increased in diversity, eventually developing into complex feedback systems. Studying Earth's co-evolutionary past can improve understanding of habitability on Earth and Earth-like planets. Third, studies of other planets—both real and hypothetical—inform and benefit from work on the intimate interactions between life and its physical environment.
Identifying, Exploring, and Characterizing Environments for Habitability and Biosignatures
Identifying and characterizing habitable or inhabited environments requires the synthesis of information from a large range of spatial scales. Astrobiologists are focused on the goal of determining whether a particular environment was or is presently habitable and whether it was or is able to generate and support life. Habitability indicators, including biosignatures, must be interpreted within a planetary and environmental context. The aim is to understand how habitable worlds and environments form and evolve, better understand the range of parameters that influence habitability, and determine how to detect, confirm, and characterize habitable environments.
The development of new tools for determining the relative habitability of either present or ancient environments within the Solar System will facilitate target selection for future planetary missions. These tools also will enable researchers to prioritize exoplanets for targeted follow-up observations of potential habitability.
Constructing Habitable Worlds
Earth is the only inhabited world we know thus far. Missions to explore other worlds are searching for others. In addition to Solar System bodies, astrobiologists now have a growing catalogue of planets orbiting other stars to explore as potentially habitable, all with diverse and potentially exotic chemistries and environments. They now face the challenge of determining whether limited experience with habitability, on Earth alone, has limited understanding of the basic set of requirements for a habitable world or whether this experience serves as a helpful guide for the search for life beyond Earth.
Habitability has been defined as the potential of an environment (past or present) to support life of any kind. Liquid water is a necessary but not sufficient condition for life as we know it. Habitability is a function of a multitude of environmental parameters whose study is biased by the effects that biology has on these parameters. Habitability may be a matter of degrees, depending on how much diversity, productivity, or spatial cover of life an environment supports.
A habitable environment is one with the ability to generate life endogenously—solely using available resources—or support the survival of life that may arrive from elsewhere. Whether a planet will emerge as habitable depends on the sequence of events that led to its formation—which could include the production of organic molecules in molecular clouds and protoplanetary disks, delivery of materials during and after planetary accretion, and the orbital location in the planetary system. Habitability provides the context for understanding possible signs for life. A deeper understanding of habitability provides context for interpreting the significance of presumed biosignatures, or their absence.
Conclusion
Given NASA's focus on the search for planets and life, astrobiology will be the focus of a growing number of solar systems exploration missions. Astrobiology research sponsored by NASA will continue pushing science closer to answering the Big Questions in space science: Where did we come from? Where are we going? And are we alone?