Astrobiology is a field of rapid change. In the 3 years since publication of the National Aeronautics and Space Administration’s (NASA’s) Astrobiology Strategy 2015,1 significant scientific, technological, and programmatic advances in the quest for life beyond Earth have taken place. Scientific advances have revolutionized fields of astrobiological study, ranging from results from missions focused on exoplanets, such as Kepler, to continuing discoveries from existing planetary missions. Returned results have changed how problems are thought of and integrated across astrobiological disciplines. From biology (e.g., miniaturized nucleic acid detection devices) to astronomy (e.g., continued improvement of starlight suppression technologies), technological advances in life detection instrumentation have continued, but will need to accelerate to match the rate of scientific advancement. Simultaneously, programmatic advances—for example, the creation of research coordination networks—have begun to break down traditional disciplinary boundaries and resulted in greater communication across the broad fields of astrobiological research.
Against the backdrop of these changes, increasing public interest in astrobiology, and the approaching decadal surveys in astronomy and astrophysics and planetary sciences, which will guide agency scientific priorities for the coming decade, NASA’s request for this assessment of advances and future directions in the field of astrobiology is timely. The committee’s statement of task was to build on the foundation of the 2015 NASA Astrobiology Strategy, emphasizing key scientific discoveries, conceptual developments, and technology advances since its publication. Rather than revisiting aspects that were already well covered in that document, the committee’s work focused on additional insights from recent advances in the field—intellectual (e.g., conceptual insights and frameworks, modeling), empirical (e.g., observations, discoveries, novel technologies), and programmatic. This approach highlights areas of rapid scientific and technological growth and advancement that have occurred since the 2015 publication, raising key scientific questions and identifying technologies that are emerging and likely to shape the field in the coming two decades. Further, the committee identifies the roles that near-term space missions and ground-telescope projects will play and highlights increasing opportunities for private, interagency, and international partnerships. The NASA Astrobiology Program’s history and continuing success in engineering cross-divisional collaborations between Earth science, astronomy, heliophysics, and planetary science (to break down disciplinary entrenchments) bodes well for its ability to leverage such partnerships to advance the search for life.
Strong collaboration between diverse scientific communities is at the core of astrobiology. Astrobiology
1 NASA, NASA Astrobiology Strategy 2015, https://nai.nasa.gov/media/medialibrary/2016/04/NASA_Astrobiology_Strategy_2015_FINAL_041216.pdf.
is inherently a systems-level science requiring contributions from a wide range of disciplines. For example, in astrobiology the “system” under study is frequently a planet with a potential (or in the case of Earth, realized) biosphere. Astrobiology seeks to understand the web of interrelationships and feedbacks between time-variable planetary processes—both physical and chemical—and the proto-biological chemical and organizational dynamics that lead to the emergence and persistence of life. Systems science provides a holistic, transdisciplinary paradigm for addressing this complexity. Although detailed mathematical modeling is not (and may never be) applied to many problems in astrobiology, most notably the emergence of life, integration across diverse and sometimes seemingly disparate disciplines is key to major progress on astrobiology’s fundamental questions.
Astrobiology is usually defined as the study of the origin, evolution, distribution, and future of life in the universe. However, adopting a systems approach suggests that astrobiology system science can be defined as the integrative study of the interactions within and between the physical, chemical, biological, geologic, planetary, and astrophysical systems as they relate to understanding how an environment transforms from nonliving to living and how life and its host environment coevolve.
As the above definition suggests, a systems-level view of the emergence of life that includes its environmental context, and how life and its environment subsequently changed together to maintain a habitable Earth, is leading to a new view of habitability. The concept of dynamic habitability drives the insight that habitability is more appropriately thought of as a continuum—that an environment may transition from inhabitable to habitable over different spatial and temporal scales as a function of planetary and environmental evolution, the presence of life, and the feedbacks between related complex physical, chemical, and biological parameters and processes. Planetary environments that may be habitable today or in the past are not necessarily the same as those that could have fostered the emergence of life. Evidence from major transitions in environmental conditions from early Earth to today, and an understanding of how they occurred, is critical for the search for life.
A better understanding of the emerging concept of dynamic habitability will come from studying the one inhabited planet currently known—Earth. The planetary environments of early Earth that gave rise to life remain poorly constrained. A better understanding of these environments entails a “mission to early Earth.” Such a “mission” will, in the near term, integrate prebiotic chemistry, origins of life research, and early Earth planetary conditions to understand their coevolution in the context of multiple parameters (including, e.g., temperature, pressure, and pH conditions) evolving over a range of spatial and temporal scales. Projecting forward, increased understanding of dynamic habitability and how life and its environment evolved together on Earth will allow questions to be addressed concerning which elements of planetary evolution are predictable and independent of biosphere evolution; what feedbacks exist between the biosphere and geosphere, including during long periods of quiescence; and how periods of catastrophic change affect the balance of influence between planetary dynamics and the biosphere. Although the research for these basic questions is most easily carried out on Earth, the far-reaching questions to be addressed in the next two decades demonstrate that dynamic habitability and the coevolution of planets and life provide a powerful comparative foundation upon which to integrate diverse astrobiology communities focusing on Earth, the solar system, stellar astronomy, and exoplanetary systems.
Recommendation: NASA and other relevant agencies should catalyze research focused on emerging systems-level thinking about dynamic habitability and the coevolution of planets and life, with a focus on problems and not disciplines—that is, using and expanding successful programmatic mechanisms that foster interdisciplinary and cross-divisional collaboration. (Chapter 2)
Understanding dynamic habitability has been furthered by recent advances in investigations of extreme life and how it interacts with its environment on Earth. Identifying life in isolated refugia or ephemeral habitats on Earth (e.g., in the Atacama Desert) has emphasized that habitability, rather than being a binary state, is a continuum defined over varying time and spatial scales. Increasing understanding of the habitability of saline and hypersaline environments, life’s limits in extreme environments, concurrent with the discovery of potential brines on Mars, has led to a resurgence in interest in adaptations of life to saline fluids. The recent discovery of communities existing in the subsurface of the ocean floor and continental lithosphere, away from the influence of the Sun’s energy, has provided new models for rock-hosted, chemosynthetic life that may exist on other worlds. Such subsurface com-
munities, which often live in energy-limited environments, contrast starkly to life in energy-rich environments. Whereas “slow” life that is barely able to survive in an austere environment may be detectable because the noise level is low, “fast” life in a rich environment may be detectable because the signal is high. Assessing the relative signal-to-noise ratio of each type of population in its given environmental context would help identify corresponding biosignatures that are most relevant and distinctive. Discoveries of the role of water-rock interactions producing essential electron donors and electron acceptors (e.g., hydrogen, methane sulphate), both at high rates in high-temperature vents, and at slow rates in lower-temperature continental settings, has generated a renewed focus on how to seek for signs of subsurface life—thereby informing astrobiology investigations of the subsurface of other rocky planets (e.g., Mars), ocean or icy worlds, and beyond to exoplanets.
In sum, expanded understanding of habitability of subsurface environments, brine stability of chemosynthetic organisms, and adaptations of life to saline fluids, have widespread implications for the search for life in the solar system. In the next two decades, continued field, laboratory, and modeling studies of these communities will address the following questions:
- How does subsurface life adapt to extreme environments and energetic spectrums?
- How do marine and continental subsurface terrestrial communities inform what chemosynthetic or rock-hosted communities on other worlds might look like?
- What is the spatial and temporal distribution of potentially habitable environments on Mars, especially in the subsurface?
- What are the chemical inventories and physical processes sustaining rock-hosted life on ocean worlds?
Recommendation: NASA’s programs and missions should reflect a dedicated focus on research and exploration of subsurface habitability in light of recent advances demonstrating the breadth and diversity of life in Earth’s subsurface, the history and nature of subsurface fluids on Mars, and potential habitats for life on ocean worlds. (Chapter 2)
The search for life beyond the solar system has seen substantial changes in the last 3 years. Since 2015, the Kepler spacecraft more than doubled the catalog of confirmed exoplanets. Thanks to extended observations during Kepler’s so-called K2 mission and improvements in data analysis, its results continue to refine our knowledge of exoplanet statistics. Some of the Kepler planets fall within what is commonly considered the “habitable zone”—traditionally defined as that region around a star where an Earth-like exoplanet could support liquid water on its surface—of their host star. This discovery, coupled with estimates of the fraction of stars with rocky, habitable-zone planets, has matured the search for evidence of life beyond the solar system enough to warrant taking the next steps toward the discovery of life on exoplanets. That search will be greatly aided by future missions and the implementation of technologies currently in development. For instance, in the near to midterm, the Transiting Exoplanet Survey Satellite (TESS), the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (Ariel), and the James Webb Space Telescope (JWST) will focus on identifying and characterizing potentially habitable, transiting exoplanets. In addition, the Wide Field Infrared Survey Telescope (WFIRST) may demonstrate the coronograph technology needed for direct imaging of exoEarths. From the ground, new instruments for direct imaging (e.g., the Gemini Planet Imager) and high-resolution spectroscopy (e.g., the Magellan Planet Finding Spectrograph) and telescopes (e.g., the Thirty Meter Telescope or Giant Magellan Telescope) will complement the observations of space-based missions using direct imaging, particularly with radial velocity measurements, and atmospheric spectra. In fact, ground-based telescopes have already detected small, potentially rocky planets in the habitable-zones of M-dwarf stars.
The technologies utilized by these instruments and missions, and the near-term data on the atmospheres of rocky exoplanets that they would yield, have the potential to make possible the first observational tests of potential habitability or, perhaps, even biosignatures within the next two decades. To confidently assess these biosignatures, it will be important to also characterize the atmospheres and the full spectrum of incident radiation for exoplanets of different sizes, compositions, and stellar irradiances so that understanding of the physical and chemical processes that lead to false positives and negatives will be increased. In order to make this progress, starlight suppression technologies that are still in development, such as coronagraphs and starshades, will be essential.
Recommendation: NASA should implement high-contrast starlight suppression technologies in near-term space- and ground-based direct imaging missions. (Chapter 5)
Technology alone will not advance the search for habitable exoplanets. A better understanding of the contexts in which potentially habitable exoplanets formed, evolved, and currently exist will be needed to inform exoplanetary exploration and planet target selection. Because exoplanets coevolve with their host stars, just as Earth coevolved with the Sun, stellar activity and evolution are critically important for understanding the dynamic habitability of exoplanets. Further, the context of solar and planetary system architecture, including the distribution of small bodies and their potential for volatile delivery to exoplanets, and evolution of that architecture, are important for determining a planet’s history of habitability as well as the limits on its current habitability. Such investigations will benefit from comparisons between the architecture, evolution, and coevolution of stellar and planetary dynamics in the solar system. Comparative planetology between the solar system and exoplanetary systems is a powerful approach to understanding the processes and properties that impact planetary habitability and is essential for informing experiments, modeling, and mission planning in astrobiology, and fundamentally collaborative, and therefore ideally suited to research coordination networks.
In addition, methods well suited to the analysis of data on exoplanetary systems as well as comparative planetology will increasingly move the field forward. Continued theoretical modeling of planetary environments, including model inter-comparisons, will become increasingly necessary to explore processes, interactions, and environmental outcomes and to understand habitability and biosignatures in the context of their environment. Techniques based on statistical methods, scaling laws, information theory, and probabilistic approaches currently used in other branches of science will continue to gain traction in astrobiology. Furthermore, rapid progress in the development of artificial intelligence machine learning algorithms has the potential to improve analysis of the large, complex data sets, which are increasingly common to fields related to the search for life. In the coming two decades, that search will increasingly address questions concerning the formation, evolution, and architectures of planetary systems and how these interact with their host star to sustain habitable planets—aided by evolving understanding of how planetary systems are studied and by new missions, technologies, and approaches to data set analysis.
The search and discovery of life in this solar system and beyond hinges on the ability to identify and validate signs of life. Since publication of the 2015 NASA Astrobiology Strategy, the field of biosignature research has advanced in four major areas as follows:
- The search for and identification of novel biosignatures, especially those that are agnostic to life’s molecular makeup or metabolism (i.e., agnostic biosignatures).
- A concerted effort to better understanding abiosignatures (signature of abiotic processes and phenomenon), in particular those that may mimic biosignatures. Critically some (but not all) abiosignatures could be false positives and some (but not all) false positives could be abiosignatures.
- An improved understanding of which biosignatures are most likely to survive in the environment, and at what timescales of preservation.
- The first steps toward developing a comprehensive framework that could be used to interpret potential biosignatures, abiosignatures, false positives, and false negatives, and increase confidence and consensus in interpretations.
The identification of novel and agnostic biosignatures focuses on both in situ biosignature detection and remotely sensed biosignatures. Remotely sensed agnostic biosignatures may take the form of complex chemical networks in planetary atmospheres or atmospheric disequilibria. Such potential biosignatures, although suggestive of life and worthy of follow-on investigation, may result from a wide range of abiotic and biologic processes and therefore will need to be closely evaluated in the context of their environments. Contextual information provided by exoplanet observation may include quantification of atmospheric gases, knowledge of the stellar spectral energy distribution across a broad wavelength range (including ultraviolet wavelengths), and models of gas fluxes. Having a strong characterization of exoplanet atmospheres across their range of sizes and compositions—not only those
that are potentially habitable—will aid in evaluating the potential for false positives. For in situ biosignatures, agnostic and novel approaches are benefiting from, for example, the promise of current nucleic acid sequencing technology and the commercial availability of compact, low-power, RNA and DNA sequencing devices that could contribute significantly to the robustness of the current portfolio of life detection technologies. However, while current technology for DNA amplification and sequencing may be useful for in situ detection of terrestrial contamination and lifeforms that are closely related to terran life, at present, these devices are not sufficiently agnostic to the composition of an informational polymer. Over the next two decades, improvements in these areas will help address the question of how novel and/or agnostic biosignatures are identified.
Recommendation: The search for life beyond Earth requires more sophisticated frameworks for considering the potential for non-terran life; therefore, NASA should support research on novel and/or agnostic biosignatures. (Chapter 4)
In addition to identifying novel and agnostic biosignatures, in the past few years, a greater emphasis has been placed on improving understanding of which biosignatures survive in the environment and how the environment may change surviving biosignatures. Record bias, preservational bias, false negatives, and false positives all play a role in biosignature detectability. There is increasing focus on understanding the range of signatures abiotic processes can produce, particularly those that might be confused with signatures of life. Ambiguous examples of early life from Earth’s own stratigraphic record demonstrate that the task of achieving community consensus on a biosignature, even on Earth, can be long and arduous. Such a task would be even greater on another planetary body. Re-addressing controversial biosignatures from Earth’s early sedimentary rock record can provide an important test bed for biosignature assessment frameworks. Such biosignatures occur at the microscale, and new technologies for microscale and nanoscale analyses combining optical microscopy, Raman spectroscopy, laser-induced breakdown spectroscopy, infrared, and other interrogatory methods offer promise for advancing detection of and confidence in biosignature interpretation. Over the next two decades, the foregoing lines of research will converge to give a clearer picture of preservational biases for biosignatures, how these may result in false negatives, and which biosignatures have the highest probability for preservation and detection, and on what timescales preservation is possible or probable.
Recommendation: NASA should direct the community’s focus to address important gaps in understanding the breadth, probability, and distinguishing environmental contexts of abiotic phenomena that mimic biosignatures. (Chapter 4)
The potential value of a biosignature reflects not only the intrinsic value of the biosignature, but also the associated propensity for both false positives and false negatives, which together create an uncertainty and probability for detection and reliability that is unique to each biosignature. This in itself represents a fundamental problem in attaining community consensus. Thus, in the next two decades, a growing question will be how biosignature detection and interpretation can be standardized as a probabilistic outcome such that the community can agree upon the robustness of a biosignature interpretation. Resolving this challenge before potentially controversial results from missions with potential astrobiological implications are returned is particularly important.
Recommendation: NASA should support expanding biosignature research to addressing gaps in understanding biosignature preservation and the breadth of possible false positives and false negative signatures. (Chapter 4)
As an important step toward these goals, a near-term, systematic reevaluation and increased understanding of the nature and detectability of biosignatures of chemoautotrophic and subsurface life would be immensely helpful. This follows from the increasing focus on such communities, not only on Earth, but also in the search for life in other subsurfaces—both rocky planets (e.g., Mars) and ocean worlds in the solar system. Concurrent with increasing the depth and breadth of the catalog of known biosignatures, however, it will be important to establish community consensus criteria and standards by which purported biosignatures can be evaluated and verified.
Recommendation: NASA should support the community in developing a comprehensive framework for assessment—including the potential for abiosignatures, false positives, and false negatives—to guide testing and evaluation of in situ and remote biosignatures. (Chapter 4)
In addition to developing the specific research and technologies, overarching programmatic advances will be important in advancing the detection of biosignatures on other planetary bodies in future astrobiology missions. Because of the inherent ambiguity in many known biosignatures, and the necessity of making multiple measurements on a sample, in situ detection of life is best advanced by integrated suites of instruments or single instruments that permit multiple analytical techniques, including nondestructive approaches, to be applied to the same materials. Of particular importance is that, when designing such suites, science requirements, rather than off-the-shelf engineering solutions or ease of implementation, remain the key decision drivers.
Given the range of new technologies that will be implemented in biosignature detection in the coming decades, it will be increasingly important to pay particular attention to ensuring the resultant instruments and suites of instruments are successfully selected and perform to an agreed upon standard that will facilitate community consensus on results. Current NASA instrument evaluation and selection policies tend to favor low risk technologies, which in some cases adversely impacts scientific payoff. This inhibits development and selection of potentially game-changing life detection technologies. Furthermore, because of possible ambiguity in proposer-defined instrument success criteria, there is inherent risk in using these, rather than observation and measurement validation standards established by community consensus, to propose, evaluate, and select instruments designed to detect biosignatures. Most fundamental to the success in the search for life, however, is a need for dedicated focus on astrobiology. Planning, implementation, and operations of planetary exploration missions with astrobiological objectives have tended to be more strongly defined by geological perspectives than by astrobiology-focused strategies. However, biosignature detection will require increasingly specialized instrumentation specific to astrobiological objectives, such as micro- and macroscale imaging, spectral imaging, mass spectrometry, and nucleotide sequencing.
Recommendation: To advance the search for life in the universe, NASA should accelerate the development and validation, in relevant environments, of mission-ready, life detection technologies. In addition, it should integrate astrobiological expertise in all mission stages—from inception and conceptualization to planning, development, and operations. (Chapter 5)
The scientific questions and goals summarized above, the missions and technological advances that will be implemented to solve them, and those searches for life not currently engaged in by NASA present immense challenges that will require partnerships with other agencies and private and international entities to address. Opportunities for such partnerships are increasing. The existence of technologies outside of the space industry—for instance, in biomedical applications and artificial intelligence—that could be used in the search for life provide prime areas for establishing partnerships with the commercial sector. Partnership models with the commercial sector do not have to be formalized, long-term agreements, but could take the form of collaborative events bringing together industry, government agency, and individual researchers. Through such events, the agency could foster increased collaboration between individual investigators and interested corporations. Philanthropic investment in the search for life is increasing and not only on traditional award funding to individual investigators, but also to self-funded and crowd-funded missions that may be categorized as “high risk/high payoff.”
One high-risk/high-payoff area for which philanthropic and, increasingly, international investments have entirely supported the search for life is the search for technosignatures, or the signature of technologically advanced life. International and philanthropic investments in the search for technosignatures over the last few years have greatly enhanced search capabilities, and corresponding improvements to radio and optical facilities have also benefited the scientific community. Philanthropic investments have supported the Allen Telescope Array, advances to instrumentation at the Green Bank Telescope and the Murchison Widefield Array Telescope, and the design of dedicated optical and near-infrared observatories. International facilities include, among others, the European Low Frequency Array, the Australian Murchison Widefield Array, and the recently completed Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China. Such investments have led directly to discoveries and advances
in methodology for the broad scientific community, such as with the discovery of Fast Radio Bursts and with the implementation of big data analysis techniques applied to signal detection.
Sharing assets and resources for large undertakings, such as missions, is also becoming increasingly important as mission complexity increases, although barriers to effective cooperation and collaboration exist. Expensive assets and infrastructure exist within the United States but are poorly leveraged by the astrobiological community due to insufficient coordination between government agencies. Unified research strategies between relevant entities—including, but not limited to NASA, the National Science Foundation (NSF), and the National Oceanic and Atmospheric Administration (NOAA)—for conducting research in shared areas (e.g., polar regions and other difficult-to-access analog environments) and with shared infrastructure (e.g., ground- and space-based telescopes) would facilitate advances in astrobiology. Given existing government-level and international collaborative tools within NASA’s Astrobiology Program, there is potential to further catalyze coordination of international research and mission planning in this area. Although not explicitly an astrobiological mission, the multidecadal Mars Sample Return campaign to be undertaken by NASA and the European Space Agency is one such example. The nucleation of government-level astrobiological partnerships that has been initiated by NASA could have the potential to motivate formation of an international organization with a unified focus on solving the immense challenges of detecting and confirming evidence for life within and beyond the solar system. One possible example discussed by the committee would be the establishment of a new international organization dedicated to the goal of supporting the development, construction, and operation of a direct-imaging space telescope capable of searching hundreds of nearby stars for possibly habitable exoEarths. Such an organization, perhaps modeled on CERN (European Organization for Nuclear Research), the European Southern Observatory, or the International Thermonuclear Experimental Reactor (ITER) Organization, might be what is required to guarantee the sustained funding required to achieve this goal over multidecadal time scales.
In summary, the search for life beyond Earth presents many opportunities for public, private, and international partnerships, which have the potential to advance the search for life rapidly.
Recommendation: NASA should actively seek new mechanisms to reduce the barriers to collaboration with private and philanthropic entities, and with international space agencies, to achieve its objective of searching for life in the universe. (Chapter 7)