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ASTROBIOLOGY Volume 8, Number 4, 2008 © Mary Ann Liebert, Inc. DOI: 10.1089/ast.2008.0819

Focus Paper

The NASA Roadmap

David J. Des Marais (Co-Lead),1 Joseph A. Nuth III (Co-Lead),2 Louis J. Allamandola,1 Alan P. Boss,3 Jack D. Farmer,4 Tori M. Hoehler,1 Bruce M. Jakosky,5 Victoria S. Meadows,6 Andrew Pohorille,1 Bruce Runnegar,7 and Alfred M. Spormann8

Abstract

The NASA Astrobiology Roadmap provides guidance for research and technology development across the NASA enterprises that encompass the , , and biological . The ongoing development of as- trobiology roadmaps embodies the contributions of diverse scientists and technologists from government, uni- versities, and private institutions. The Roadmap addresses three basic questions: how does begin and evolve, does life exist elsewhere in the , and what is the future of life on Earth and beyond? Seven Goals outline the following key domains of investigation: understanding the and distribution of habit- able environments in the universe, exploring for habitable environments and life in our own , un-

derstanding the emergence of life, determining how early life on Earth interacted and evolved with its chang- ing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth. For each of these goals, Science Objectives outline more specific high priority efforts for the next three to five years. These eighteen objectives are being integrated with NASA strategic planning. Astrobiology 8, 715–730.

Introduction technologies, and related disciplines. The broad interdisciplinary character of astrobiology compels us STROBIOLOGY ADDRESSES THREE BASIC QUESTIONS that have to strive to achieve the most comprehensive and inclusive Abeen asked in various ways for generations: how does understanding of biological, planetary, and cosmic phe- life begin and evolve, does life exist elsewhere in the uni- nomena. verse, and what is the future of life on Earth and beyond? The NASA Astrobiology Roadmap provides guidance for Accordingly, the discipline of astrobiology embraces the research and technology development across the NASA En- search for potentially inhabited beyond our Solar terprises that encompass the space, Earth, and biological sci- System, the exploration of and the outer planets, lab- ences. The Roadmap is formulated in terms of seven Science oratory and investigations of the origins and early evo- Goals that outline key domains of investigation. These do- lution of life, and studies of the potential of life to adapt to mains of investigation will probably require decades of ef- future challenges, both on Earth and in space. Interdiscipli- fort to consummate. For each of these goals, Science Objec- nary research is required that combines molecular , tives outline more specific high priority efforts for the next , , , information science, three to five years. These eighteen objectives are being inte-

1Space Science Division, NASA , Moffett Field, California. 2Goddard Space Flight Center, Greenbelt, Maryland. 3Department of Terrestrial , Carnegie Institution of Washington, Washington, DC. 4Department of Exploration, , Tempe, Arizona. 5Laboratory for Atmospheric and and Department of Geological Sciences, University of Colorado, Boulder, Colorado. 6Department of Astronomy, University of Washington, Seattle, Washington. 7Institute of , U.C.L.A., Los Angeles, California. 8Department of Civil and Environmental Engineering, Stanford University, Stanford, California.

715 716 DES MARAIS ET AL. grated with NASA strategic planning. The Roadmap also in- (2) Astrobiology encourages planetary stewardship through cludes Example Investigations, which offer examples of spe- an emphasis on protection against forward and back bi- cific research tasks that are both important and timely for ological contamination and recognition of ethical issues their corresponding Science Objective. It is important to em- associated with exploration. phasize that these investigations are intended principally to (3) Astrobiology recognizes a broad societal interest in its be illustrative of relevant tasks, and that additional equally endeavors, especially in areas such as achieving a deeper important example investigations can be envisioned. understanding of life, searching for extraterrestrial bio- The following four basic principles are fundamental to the spheres, assessing the societal implications of discover- implementation of NASA’s astrobiology program: ing other examples of life, and envisioning the future of life on Earth and in space. (1) Astrobiology is multidisciplinary in its content and in- (4) The intrinsic public interest in astrobiology offers a cru- terdisciplinary in its execution. Its success depends cial opportunity to educate and inspire the next genera- critically upon the close coordination of diverse scien- tion of scientists, technologists, and informed citizens; tific disciplines and programs, including space mis- thus a strong emphasis upon education and public out- sions. reach is essential.

Goals and Objectives Goal 1—Understand the nature and distribution of habitable environments in the universe. Determine the potential for habitable planets beyond the Solar System, and characterize those that are observable.

A or planetary satellite is habitable if it can sustain terrestrial planets are indeed as common as predicted above, life that originates there or if it sustains life that is carried to whether a substantial fraction of them show signs of habit- the object. The Astrobiology program seeks to expand our ability, and whether an appreciable fraction of these show understanding of the most fundamental environmental re- . quirements for habitability. However, in the near term, we A key difference between the search for life in the Solar must proceed with our current concepts regarding the re- System and the search in external planetary systems is that, quirements for habitability. That is, habitable environments within the Solar System, interplanetary transfer of viable mi- must provide extended regions of liquid , conditions crobes seems a plausible process, and therefore the discov- favorable for the assembly of complex organic , ery of life elsewhere in the Solar System seems plausible. and sources to sustain . Habitability is not While this is indeed of extraordinary interest, it may not cast necessarily associated with a single specific environment; it light on whether it is easy or difficult for life to begin. On can embrace a suite of environments that communicate the other hand, the fact that dispersion between through exchange of materials. The processes by which cru- are 105 to 106 times longer than for dispersion within the cial biologically useful chemicals are carried to a planet and Solar System makes independent origination of life forms change its level of habitability can be explored through the outside the Solar System more probable. fields of prebiotic and chemical . A ma- The research objectives under this goal address three key jor long-range goal for astrobiology is to recognize habit- questions. First, do terrestrial planets and large satellites tend ability beyond the Solar System, independent of the presence to form in a state where they are likely to become habitable, of life, or to recognize habitability by detecting the presence or do habitable environments emerge only after a sequence of life (see Goal 7: Biosignatures). This goal directly ad- of less probable events? Second, how frequently do habit- dresses the call for searches for Earth-like planets and for able environments arise on solid planets, including large habitable environments around other stars that is included satellites? Third, what are the specific signs of habitability in the Vision for Space Exploration. and habitation, and how do such signs change with the cir- cumstances of the planet (e.g., mass, distance from its , Background. Research in astrobiology supports NASA in history and relative abundance of volatile compounds)? To its attempt to search for habitable or inhabited environments address these questions effectively, we must investigate how beyond the Solar System. have pondered for mil- habitable planetary systems form and evolve (Goals 1 and lennia whether other inhabitable worlds exist. Now, for the 2), and we must understand the ultimate environmental lim- first , they have an opportunity to look and see. Of its of life (Goal 5). course it is not possible to examine the 1010 Earth-like plan- Much of this effort focuses upon the presence or absence ets that simple statistical models predict to exist in our of liquid water in bulk form. Water is made from the two , much less the 1021 such planets expected to be in most abundant chemically reactive elements in the universe, the universe. Still, it should be possible to determine whether and it is the necessary ingredient for Earth’s type of life. Liq- NASA ASTROBIOLOGY ROADMAP 717

FIG. 1. Artist’s representation of a habitable planet orbiting another star. Courtesy of Cheryse Triano, Topspin Design Works.

uid water has played an intimate, if not fully understood, tively modest fluxes of many stars prevent rapid role in the origin and development of life on Earth. Water production of from photo-dissociation of water. contributes to the dynamic properties of an Earth-sized These factors will help to minimize the possibility of false planet, permitting convection within the planetary crust that positive detections of oxygen biosignatures. might be essential to supporting Earth-like life by creating The challenge of remotely detecting life on a planet that local chemical disequilibria that provide energy for life. Wa- has not developed a biogenic source of oxygen is fraught ter maintains a strong polar-nonpolar dichotomy with cer- with unknowns. What chemical species and spectral sig- tain organic substances. This dichotomy has allowed life on should be sought? What metabolic processes might Earth to form independent stable cellular structures. Thus be operating? How does one guard against a false positive the primary focus of Goal 1 is concerned with planets hav- detection? Research that is guided both by our knowledge ing a liquid water boundary layer, although the focus may of Earth’s early (i.e., before the rise of an oxy- expand to include other planets or satellites as astrobiology genated ) and by studies of alternative biologi- matures as a discipline. cal systems can help address these questions and provide There is also a focus—though not exclusive—on molecu- guidance to seeking evidence of life elsewhere lar oxygen and as biosignatures (see also Goal 7), and (Goal 7). therefore on dealing with the interface between under- standing the geological and biological aspects of oxygen, and • Objective 1.1—Formation and evolution of habitable planets. determining the details of the spectral features that can be Investigate how solid planets form, how they acquire liq- observed and interpreted remotely. Oxygen is a very com- uid water and other volatile species and organic com- mon element that has provided Earth with its most distinc- pounds, and how processes in planetary systems and tive . The chemical state of an Earth-like planet, affect their environments and their habitability. as well as the geological activity that delivers reduced species Use theoretical and observational studies of the formation to its surface environment, will cause virtually all of the mo- and evolution of planetary systems and their habitable lecular oxygen to be consumed unless it is produced rapidly zones to predict where water-dependent life is likely to be (e.g., by oxygen-producing ). Also, the rela- found in such systems. 718 DES MARAIS ET AL.

Example investigations • Objective 1.2—Indirect and direct astronomical observations of extrasolar habitable planets. Conduct astronomical, theoret- Understand how the processes and time scales of planet for- ical, and laboratory spectroscopic investigations to sup- mation vary with the mass of the central star, for exam- port planning for and interpretation of data from missions ple, whether low mass M dwarf stars or very low metal- designed to detect and characterize extrasolar planets. licity stars are likely to host habitable planets or satellites. Model the potential diversity of habitable planets that might Example investigations emerge from the evolution of protoplanetary disks. For comparison, measure key time scales for planet-forming Use ground-based telescopes to detect “Super ” that events in our own Solar System based on studies of iso- may offer habitable environments around low mass, M topic chronometers in and samples returned dwarf stars. from primitive Solar System bodies. Investigate novel methods for detecting and characterizing Investigate how the delivery of water and other volatiles to extrasolar planets, particularly those that might lead to an and their loss from terrestrial-like planets of various size improved understanding of the frequency of habitable and mass can affect , surface, and interior pro- Earth-like planets. cesses, and how these changes affect habitability. Use atmospheric models to understand the range of plane- Develop comprehensive models of the environments of ter- tary conditions that can be determined from low resolu- restrial-like planets (including the recently discovered “Su- tion, full-disk spectra at visible, near-IR, and thermal per Earths,” planets with masses in the range of 1 to 10 wavelengths. Use data for , Earth, and Mars to val- Earth-masses) to investigate the evolution of habitability. idate these models.

GOAL 2—Determine any past or present habitable environments, prebiotic chemistry, and signs of life elsewhere in our Solar System. Determine the history of any environments having liquid water, chemical ingredients, and energy sources that might have sustained living systems. Explore crustal materials and planetary for any evidence of past and/or present life.

This goal seeks to understand the distribution of the ba- atmospheric precipitation and outflows from aquifers. The- sic requirements for life elsewhere in our Solar System and oretical models and surface geomorphology have indicated to understand how any habitable environments and life per- the potential for a global groundwater system capable of sisted over geological time. Although possible life elsewhere sustaining an extant subsurface biosphere. The recent dis- in the Solar System may have developed differently from life covery of atmospheric indicates that subsurface on Earth, our understanding of the nature of terrestrial life habitable environments might exist even today. The provides an important starting point to guide our explo- mission found strong evidence of subsurface brines on three ration strategies. For example, studies of in of the four Galilean satellites of (, , extreme environments, including the biota sustained by and ), maintained by internal . The chemical energy in the deep subsurface, have revolutionized Cassini-Huygens mission has confirmed the presence on our assessment of the potential for and the icy ’s moon, , of an atmosphere and surface en- moons in the outer Solar System. Surface environments on riched in prebiotic organic compounds. Titan can be viewed those bodies are presently hostile to life as we know it; how- as a natural laboratory where we might gain important new ever, subsurface environments might be habitable. Ancient insights into the origins of life here on Earth. The existence surface environments also might have been habitable, at least of of liquid on Titan opens up the pos- intermittently. By mapping the distribution of any past and sibility for and energy sources that are alternatives present habitable environments, robotic missions will guide to those in our biosphere and that might support novel life the selection of samples to be returned to state-of-the-art lab- forms altogether different from those on Earth. The Cassini oratories on Earth to conduct life detection experiments. has imaged plumes of water vapor that erupt episodically from the subsurface of , an icy moon Background. During the past decade, dramatic progress of Saturn. This indicates the possibility that interior habit- has been made in our understanding of the potential for past able zones with liquid water might exist on yet another body and present habitable environments elsewhere in the Solar in the outer Solar System. Recent biological research has System. For example, the Mars Global Surveyor orbiter, deepened our understanding of the nature, environmental Mars Exploration Rovers, and the Odyssey and Mars Re- limits, and evolutionary on Earth. Such dis- connaissance orbiters revealed that, early in martian history, coveries have contributed to strategic planning activities surface water environments were widespread over the sur- that broadly support NASA missions, including the speci- face and shallow subsurface and were sustained by both fication of scientific goals, objectives and measurement re- NASA ASTROBIOLOGY ROADMAP 719

FIG. 2. Hemisphere of Mars showing (from left to lower right) Olympus Mons, Tharsis volcanos and Vallis Marineris. Courtesy of NASA.

quirements, new instrument designs, and integrated pay- Renewed lunar exploration offers opportunities to explore load concepts. the earliest history of habitable environments on Earth. The Building upon such discoveries, logical next steps in the Moon might harbor an inventory of crustal materials that exploration for in the Solar System in- were delivered from Earth before, during, and after the in- clude the following: (1) continuing to refine our under- terval of heavy bombardment. The discovery of terrestrial standing of the nature and distribution of past and/or pres- meteorites on the Moon could provide important new in- ent habitable environments in the Solar System, (2) sights about the evolution of during identifying the most favorable landing sites for future in situ the birth of our biosphere. life detection missions, (3) developing experiments for the Sample return(s) from important astrobiology targets in the unambiguous detection of extant and biosignatures in Solar System (e.g., Mars, icy satellites, , and ) surface/subsurface rocks, , and ices on other planetary will greatly expand opportunities to explore for prebiotic bodies, like Mars and Europa, (4) developing robotic drilling chemistry and life using the most sophisticated tools available systems capable of accessing subsurface environments on in terrestrial laboratories. Logical steps toward planning future Mars and possibly on other bodies in the Solar System, (5) sample returns include the development of reliable robotic sys- collecting and returning samples to Earth from high prior- tems for the in situ targeting, collection, and storage of sam- ity sites on Mars and possibly other bodies in the Solar Sys- ples, the safe transport and delivery of samples to Earth, the tem to explore for prebiotic chemistry and biosignatures uti- construction of sample containment facilities on Earth for the lizing the advanced capabilities of terrestrial laboratories. safe handling and storage of extraterrestrial materials, and the Key questions include the following: If life ever arose else- distribution of samples to members of the scientific commu- where, is it related to terrestrial life, or did other bodies in nity for detailed laboratory analyses. the Solar System sustain independent origins of life? If life never developed elsewhere in our Solar System, is there a • Objective 2.1—Mars exploration. Through orbital and sur- prebiotic chemical record preserved in ancient rocks that face missions, explore for potentially habitable environ- might contain clues about how life began on Earth? ments and evidence of life, as indicated by water, organic 720 DES MARAIS ET AL.

, atmospheric gases, and/or . Study martian • Objective 2.2—Outer Solar System exploration. Conduct ba- meteorites to guide Mars exploration. To support both ex- sic research, develop instrumentation to support astro- ploration at Mars and the first sample return mission, up- biological exploration, and provide scientific guidance date astrobiology measurement requirements, support for outer Solar System missions. Model the potential for site selection studies, and develop/improve relevant tech- subsurface habitable environments on icy moons such nologies and analytical methods. as Europa, Titan, and Enceladus. Support the develop- ment of missions to explore the surface ices and thin Example investigations atmospheres of these bodies for evidence of subsurface habitable environments, organic chemistry, and/or Target a well-instrumented robotic rover to a site of past biosignatures. aqueous activity and analyze rocks for their , minerals formed in association with water (e.g., sulfates, Example investigations silica, phyllosilicates, etc.), organic chemistry, and fossil biosignatures (including organic compounds, Develop flight instrumentation that can survive the surface stable , atmospheric gases, biosedimentary struc- environments of Europa and Titan and that can determine tures, and microtextures). the composition (including any organic compounds) of Develop flight instruments to characterize samples in situ, to surface ices and atmospheres, and explore for cryo-pre- select samples for return to Earth, and to analyze atmo- served biosignatures. spheric composition from orbit. Develop numerical models to constrain the nature of deep Develop universal approaches for detecting evidence of ex- subsurface environments on Europa, Titan, and Ence- tant life forms that can be applied over a broad range of ladus (presence of life-sustaining solvents, biologically surface environments to distinguish between forward con- essential elements, and energy sources) that might sus- tamination and in situ extraterrestrial life. tain life. Analyze results from previous and current missions in the Analyze results from the Cassini mission in the context of context of their significance for astrobiology. their significance for astrobiology.

GOAL 3—Understand how life emerges from cosmic and planetary precursors. Perform observational, experimental, and theoretical investigations to understand the general physical and chemical principles underlying the origins of life.

How life begins remains a fundamental unsolved mystery. discipline, “Universal Biology.” Although this discipline will The origin of life on Earth may well represent only one path- benefit from an understanding of the origins and limits of way among many along which life can emerge. This belief terrestrial life, it also requires that we define the environ- forms the intellectual foundations for observations and mis- mental conditions and the chemical structures and processes sions aimed at searching for extant or extinct life elsewhere that could support life on other habitable planets. Thus we in the universe. Thus the universal principles must be un- need to exploit universal laws of physics and chemistry to derstood that underlie not only the origins of life on Earth, understand polymer formation, self-organization processes, but also its possible origins elsewhere. The starting point for energy utilization, information transfer, and Darwinian evo- establishing these principles is to determine what raw ma- lution that might lead to the emergence of life in planetary terials of life can be produced by chemical evolution in space environments other than Earth. Clearly an inventory of mol- and on planets. Further, it should be understood how or- ecules must exist that is capable of gaining chemical, struc- ganic compounds are assembled into more complex molec- tural, and functional complexity and eventually assembling ular structures and how the functions of these biomolecular into living systems. This is strongly conditioned on temper- structures become coordinated to form complex evolving ature, , energy sources, etc. Some conditions that sup- systems that accompany life’s origins. Such systems must port chemistry that is sufficiently rich to seed life might be have the capabilities to perform and coordinate capture of detrimental to self-organization of biological structures. Con- energy and nutrients from the environment, catalyze the versely, conditions that promote the emergence of biologi- chemical reactions needed for maintenance and growth of cal complexity might be unfavorable to organic chemistry. these systems, and manufacture copies of key . Thus an integrative, interdisciplinary approach is necessary Clues from the biomolecular and fossil records, as well as to formulate the principles underlying universal biology. The from diverse microorganisms, should be explored in order perspectives gained from understanding these principles to define better the fundamental properties of the living state. will markedly improve our ability to define habitability and recognize biosignatures. This, in turn, will guide missions Background. We must move beyond the circumstances and observational projects aimed at finding life in the Solar of our own particular origins in order to develop a broader System and beyond. NASA ASTROBIOLOGY ROADMAP 721

FIG. 3. Vesicles formed spontaneously from an aqueous solution of decanoic acid and de- canol. Courtesy of D. Deamer, University of California, Santa Cruz.

Sources of organic compounds required for the ori- property of organic molecules—the ability to undergo spon- gin of life. To understand how life can begin on a habitable taneous chemical transformation—with the uncommon planet such as Earth, it is essential to know what organic property of synthesizing a copy of that system. This process, compounds were likely to have been available and how they unique to life, allows Darwinian-like selection and evolution interacted with the planetary environment. Chemical syn- to occur. At the core of the life process are polymers com- theses that occur within the solid crust, , and posed of monomeric species such as amino acids, carbohy- atmosphere are likely to be important sources of biogenic drates, and . The pathways by which monomers compounds. Interesting organic compounds are also present were first incorporated into primitive polymers on early in interstellar , the birthplace of planetary systems. Earth remain unknown, and physical properties of the prod- Laboratory simulations of interstellar ices have recently ucts are largely unexplored. A primary goal of research on demonstrated that key prebiotic molecules can also be syn- the origin of life must be to understand better the sources thesized under interstellar conditions, and such mate- and properties of primitive polymers on early Earth, their rials are incorporated into nascent solar systems. More initial evolution towards acquiring increased fidelity, effi- recent astronomical observations and analyses of extrater- ciency, and diversity of functions, and the pathway by which restrial materials have shown that many compounds rele- polymerization reactions of peptides and oligonucleotides vant to life processes are also present in meteorites, inter- became genetically linked. As a parallel effort, the catalytic planetary dust , and comets. It is likely that and informational potential of other polymers that can form substantial amounts of such organic material were delivered in different planetary conditions should be explored. These to Earth during late , providing organic compounds specific lines of inquiry should be complemented with the- that could serve as a feedstock for chemical evolution or that oretical work, based on a “general principles” approach, to could be incorporated directly into early forms of life. An elucidate how the various protobiological properties of mol- important research objective within this goal is to establish ecules and systems, including emergent behavior and infor- sources of prebiotic organic compounds and to understand mation transfer, depend on and scale with complexity. their history in terms of universal processes that would take place on any newly formed planet. This will require an in- Origins of energy transduction. Life requires energy to tegrated program of pan-spectral astronomical observations, yield, in a reproducible fashion, specific, very low probabil- sample return missions, laboratory studies of extraterrestrial ity outcomes, such as the generation and maintenance of lo- materials, and realistic laboratory simulations of inaccessi- cally ordered states of the , the production ble cosmic environments. of thermodynamically unfavorable species, and the synthe- sis of polymers having specific sequences from among large Origins and evolution of functional biomolecules. Life numbers of alternative sequence possibilities. Thus the emer- can be understood as a chemical system that links a common gence of complex life depends upon the development of re- 722 DES MARAIS ET AL. liable mechanisms for capturing energy from the surround- nature of extraterrestrial organic compounds. Conduct ings and channeling it, in appropriate forms, into specific laboratory experiments and simulations to provide a processes. Overarching areas for focus are the following: (i) framework for interpreting observations of meteorites, Foundational—understanding the relationship between en- samples returned from comets and asteroids, and spectra ergy, information, and complexity; understanding what of interstellar clouds. types of energy can be captured and utilized by biological Identify the organic compounds and complexes produced systems; and understanding how threshold requirements for under primordial planetary conditions through laboratory complexity and information processing place upper and simulation experiments. lower bounds on the biological utilization of energy. (ii) Mechanistic—elucidating mechanisms by which the princi- • Objective 3.2—Origins and evolution of functional biomole- pal elements of energy transduction—capture of environ- cules. Identify plausible pathways for the synthesis of pre- mental energy, storage by membrane-based or molecular biotic monomers and their condensation into polymers. means, and coupling to key energy-requiring processes— Identify the potential for creating catalytic and genetic may have functioned in primordial environments and com- functions, investigate their protobiological evolution, and bined into dedicated energy-transducing systems. (iii) Evo- explore primitive mechanisms for linking these two func- lutionary—understanding how the systems for energy tions. capture and utilization evolved from a complete lack of en- ergy transduction, through a partially developed state, to Example investigations fully elaborated and dedicated systems for collecting and di- Investigate polymers other than nucleic acids that have the recting energy, and examining how these systems co- potential to have been precursor molecules capable of con- evolved with the capabilities for functionality, information taining genetic information. processing, and replication. Conduct laboratory experiments and computational studies on the emergence of functional . Investigate mech- Origins of cellularity and protobiological systems. For anisms by which primordial proteins evolved towards in- life to begin in a natural setting such as a planetary surface, creased efficiency and selectivity, and acquired new struc- mechanisms must exist that concentrate and maintain inter- tures and functions. acting molecular species in a microenvironment. A bounded Investigate ancestral translation mechanisms and the emer- system of replicating and catalytic molecules capable of un- gence of the genetic code using laboratory experiments dergoing Darwinian evolution is by definition a . At some and clues from and . point life on Earth became cellular, either from its inception or soon thereafter. Boundary membranes divide complex • Objective 3.3—Origins of energy transduction. Investigate, molecular mixtures into large numbers of individual struc- conceptually and quantitatively, the relationship between tures that can undergo selective processes required for bio- energy, complexity, and information as applied to the ori- logical evolution. They also have the capacity to develop sub- gin of biological systems. Understand how the evolution stantial ion gradients that represent a central energy source of molecules, metabolic cycles, linked systems, and or- for virtually all life today. A primary research objective is to ganisms is enabled and constrained by the development understand self-organizing and evolutionary processes that of energy transduction. Identify prebiotic mechanisms by can lead to the emergence of cellular structures and to test which energy can be captured, stored, and coupled to en- this understanding by creating laboratory models of primi- ergy-requiring processes. tive cells. These models are systems of interacting molecules within bounded environments capable of working in concert Example investigations to capture energy and nutrients from the surroundings, transduce environmental signals, form metabolic networks Quantify the theoretical amount of energy required to spec- that allow for growth through polymerization, and repro- ify the sequence of amino or polymers as a duce some of their polymeric components. Approaching this of chain length; compare to the actual energy in- challenging problem will lead to a more refined definition put required based on known biological mechanisms. of the living state and will clarify the hurdles faced by self- Identify the classes of reactions available for prebiotic syn- assembled systems of organic molecules as they evolved to- thesis under a range of plausible Solar System environ- ward life. mental conditions, based on single-random inputs of en- ergy or no inputs of energy. • Objective 3.1—Sources of prebiotic materials and catalysts. Investigate mechanisms by which the energy available in ion Characterize the exogenous and endogenous sources of gradients across boundary membranes could couple to the matter (organic and inorganic) for potentially habitable synthesis of high-energy compounds such as pyrophos- environments in the Solar System and in other planetary phate. and protoplanetary systems. • Objective 3.4—Origins of cellularity and protobiological sys- tems. Investigate both the origins of membranous bound- Example investigations aries on early Earth and the associated properties of en- Emphasizing important prebiotic species, trace the forma- ergy transduction, transport of nutrients, growth, and tion, chemical evolution, and processing of interstellar division. Investigate the origins and early coordination of molecules and solids into more complex compounds. key cellular processes such as metabolism, energy trans- Analyze meteorites and returned samples to understand the duction, translation, and transcription. Without regard to NASA ASTROBIOLOGY ROADMAP 723

how life actually emerged on Earth, create in the labora- Investigate polymerase reactions that can take place in mem- tory and study artificial chemical systems that undergo brane-bounded microenvironments using external sources and . of monomers and chemical energy, and determine self- replication capabilities of such simple systems. Considering clues from prebiotic chemistry and contem- Example investigations porary , investigate the organization of Determine how ionic and polar nutrients could permeate chemical reactions into a primitive cellular metabolism membrane boundaries to supply monomers and energy and establish properties of the emerging metabolic net- for intracellular metabolism and biosynthesis. works.

GOAL 4—Understand how life on Earth and its planetary environment have co-evolved through geological time. Investigate the evolving relationships between Earth and its biota by integrating evidence from the geosciences and biosciences that shows how life evolved, responded to environmental change, and modified environmental conditions on a planetary scale.

Understand how extraterrestrial processes and the plane- key steps in the development of life will enable a deeper un- tary environment have influenced the evolution of life and derstanding of the pathways leading to biological complex- how biological processes have transformed conditions on lo- ity and thus inform the search for living systems on distant cal to planetary scales. An improved knowledge of how life worlds. operated under diverse and extreme conditions will improve our ability to define, detect, and interpret biosignatures of Background. As in other areas of astrobiology, integra- living and extinct , here and elsewhere. Studies of tion of knowledge from many fields of science is needed to

FIG. 4. Banded chert, more than 3.4 billion years old, near Marble Bar, . Ancient cherts continue to re- veal key insights about Earth’s early biosphere, thus they underscore the importance of recently discovered silica-rich de- posits on Mars. Photograph by D. Des Marais, NASA Ames Research Center. 724 DES MARAIS ET AL. understand the evolution of life in a planetary context. Earth • Objective 4.1—Earth’s early biosphere. Investigate the de- system models that relate processes operating on vastly dif- velopment of key biological processes and their environ- ferent temporal and spatial scales, from the core of Earth to mental consequences during the early the atmosphere and beyond, are an integral part of this en- through biogeochemical, paleobiological, geological, and deavor. Comparative genomic studies, often based on genomic studies. environmental samples, including those from subsurface , will provide a fuller understanding of microbial Example investigations within communities that control biogeochemi- Use information obtained from microbial genomes to un- cal cycles, such as those involving the use of , sulfur, derstand the early evolution of key microbial processes iron, and compounds. Studies of the metabolic re- (e.g., , sulfate reduction, aerobic metabo- quirements for life, the geochemical effects of those meta- lism, etc.). bolic activities in the form of proxy records, and the pre- Develop timelines for the evolution of different microbial served biomolecular signatures of ancient organisms will processes by linking observations of biogeochemical prox- provide a deep time dimension to the evolution of life in a ies to detailed, high-resolution geochronological measure- global context and show how Earth’s planetary profile, as ments of ancient rocks. observed remotely, has changed throughout time. Investi- Develop complex numerical models for the performance of gations that explore the response of life to environmental biogeochemical cycles under conditions that existed on perturbations on ecological to geological time scales will en- early Earth and apply such knowledge to the interpreta- hance our understanding of planetary habitability and con- tion of early Mars. tribute to the search for life beyond the Solar System. And research based on the fossil record of life and its activities, • Objective 4.2—Production of complex life. Investigate the coupled with knowledge gained from living organisms, will pathways and mechanisms that increased the complexity enable key steps in the pathways to biological complexity to of life from microbial communities through successive lev- serve as guideposts in the search for life beyond the Solar els of multicellularity. System. Using these tools and methods, astrobiologists can study Example investigations the reciprocal interactions between organisms and their planetary environment and address the following questions: Study detailed records of biogeochemical proxies to deter- What microbial existed at the time of the old- mine if global environmental events were driven by bio- est sedimentary rocks, and what were their environmental logical innovation. effects? What might we learn about the earlier history of life Use comparative in conjunction with the fossil and on Earth if sedimentary samples that are even older can be biomolecular fossil records to better understand key steps found on the Moon? What can we learn from the early his- in the evolution of and sources of their tory of life on Earth that will inform the search for biosig- genomes. natures on ancient Mars? How was the evolution of Earth’s Apply knowledge gained from studies of the evolution of de- atmosphere and during the first half of Earth history velopment to understand the sudden evolution of complex related to the development of life? How have life’s course during the Neoproterozoic and early Cambrian. and Earth’s habitability been changed by major environ- • Objective 4.3—Effects of extraterrestrial events upon the bio- mental perturbations such as those resulting from extrater- sphere. Study the short- and long-term effects of extrater- restrial impacts, the eruption of large igneous complexes, the restrial phenomena such as secular changes in the mag- oxygenation of the atmosphere, global glaciations, and sig- nitude and quality of solar and cosmic , the nificant biological innovations? How does biological com- evolution of the Earth-Moon system, nearby supernovae, plexity arise, and what are its effects on planetary-scale en- and infalling comets and asteroids on life and the envi- vironments? Is there a predictable series of biological ronments in which it exists. innovations that will inform searches for life on planets or- biting other stars? These and other questions are tied to this Example investigations goal that seeks to understand the historical interconnections between Earth and its biota to help guide our search for life Use data obtained from the geological record to determine elsewhere. All of this research requires a deeper under- any effects of extraterrestrial environmental perturbations standing of evolutionary mechanisms at the levels of mole- upon the sustainability and/or diversity of life. cules, cells, organisms, and (Goal 5). The results Investigate the mechanisms that enable small changes in the contribute directly to the identification of biosignatures amount and quality of solar radiation received by Earth to (Goal 7). influence global climate and biological productivity. NASA ASTROBIOLOGY ROADMAP 725 GOAL 5—Understand the evolutionary mechanisms and environmental limits of life. Determine the molecular, genetic, and biochemical mechanisms that control and limit evolution, metabolic diversity, and acclimatization of life.

The diversity of life on Earth today is a result of the dy- cope with a broad range of physical and chemical conditions. namic interplay between genetic opportunity, metabolic ca- What are the features that enable some microbes to thrive pability, and environmental change. For most of their exis- under extreme conditions that are lethal to many others? An tence, Earth’s habitable environments have been dominated understanding of the tenacity and versatility of life on Earth, by microorganisms and subjected to their metabolism and as well as an understanding of the molecular systems that evolution. As a consequence of geological, climatologic, and some organisms utilize to survive such extremes, will pro- microbial processes acting across geological time scales, the vide a critical foundation for the search for life beyond Earth. physical-chemical environments on Earth have been chang- These insights will help us understand the molecular adap- ing, thereby determining the path of evolution of subsequent tations that define the physical and chemical limits for life life. For example, the release of molecular oxygen by on Earth. They will provide a baseline for developing pre- as a by-product of photosynthesis as well as dictions and hypotheses about life on other worlds. the colonization of Earth’s surface by metazoan life contrib- uted to fundamental, global environmental changes. The al- Background. The evolution of biogeochemical processes, tered environments, in turn, posed novel evolutionary op- genomes, and microbial communities has resulted in the portunities to the organisms present, which ultimately led to complexity and robustness of the modern biosphere. How- the formation of our planet’s major and species. ever, we lack an understanding of how mutational events on Therefore, this “co-evolution” between organisms and their the single gene level scale upward to altered macroscopic environment is an intrinsic feature of living systems. ecological performances of entire . We can ex- Life survives and sometimes thrives under what seem to amine the reciprocal interactions between biosphere and be harsh conditions on Earth. For example, some microbes geosphere that can shape genes, genomes, organisms, and thrive at temperatures of 113°C. Others exist only in highly species interactions. Accordingly we will develop an under- acidic environments or survive exposures to intense radia- standing of the evolution of biochemical and metabolic ma- tion. While all organisms are composed of nearly identical chinery that drives the global cycles of the elements, as well , evolution has enabled such microbes to as the potential of such evolution and its limits. Furthermore

FIG. 5. Cyanobacteria and red phototrophic in a gypsum-hosted microbial com- munity in a solar saltern, Exportadora de Sal, S. A., Guerrero Negro, Baja California Sur, Mex- ico. Photograph by D. Des Marais, NASA Ames Research Center. 726 DES MARAIS ET AL. we must observe their coordination into genetic circuitries the evolution of genes, metabolic pathways, genomes, mi- and their integration into more complex biological entities crobial species, and . Experimentally investigate such as whole cells and microbial communities. the forces and mechanisms that shape the structure, or- While co-evolution of Earth’s physical-chemical environ- ganization, and plasticity of microbial genomes. Examine ment and its life is dynamic and proceeds at all organismic how these forces control the genotype-to-phenotype rela- levels, microorganisms have played a critical role in shaping tionship. Conduct environmental perturbation experi- our planet. Microbes and viruses can serve as highly ad- ments on single microbial species to observe and quantify vanced experimental systems for biochemical, genetic, and adaptive evolution to astrobiologically relevant environ- genomic studies. More than 500 microbial genomes have ments. been sequenced as of this writing. This unprecedented wealth of information, together with the experimental tools Example investigations now available for single cell studies, provides a tremendous opportunity for experimental studies to be conducted on the Experimentally observe the expansion and contraction of mi- evolution of microbial genes, genomes, microbial communi- crobial genomes within the context of different environ- ties, and viruses. Such studies will uncover fundamental ments and selective forces. principles of molecular, cellular, and level evo- Examine the molecular basis of the robustness and resilience lution with relevance to Earth and other planets. Of specific of metabolic pathways in response to environmental interest is observing or simulating the evolution of those mo- changes. lecular properties that facilitate the metabolic coupling of the Develop an understanding of the link between single gene oxidation/reduction cycles of elements and the evolution and the ecological consequences on the level of to novel environments, especially extreme environments, microbial populations. created by simulated perturbations. Hypothesis-driven ex- perimentation on microbial ecosystems using single species • Objective 5.2—Co-evolution of microbial communities. Exper- with known genome sequences can be employed to predict imentally examine the metabolic and genetic interactions environmental changes and evolutionary solutions. Such in microbial communities, including viruses, which have studies can be extended to defined mixed communities to determined major geochemical processes and changes on study the plasticity and adaptation of the “metagenome,” Earth. Investigate how these interactions shape the evo- which comprises the genomes of all members of a microbial lution and maintenance of metabolic diversity in micro- community, when it is subjected to environmental changes bial communities. Investigate how novel microbial species and genetic flux. The evolved genotypes and phenotypes establish and adapt into existing communities. should be correlated to the specific changes they induce in the physical-chemical environment. Example investigations Our ongoing exploration of Earth has led to continued dis- Investigate small interactions and their role in coveries of life in environments that have been previously coordinating energy flow in mixed phototrophic/ considered uninhabitable. For example, we find thriving chemotrophic microbial communities. communities in the boiling hot springs of Yellowstone, the Examine the molecular basis of , e.g., the evolution frozen deserts of , the concentrated of a generalist to a specialist, and, conversely, of a spe- in acid-mine drainages, and the fields in cialist to a generalist. nuclear reactors. We find some microbes that grow only in Examine the deterministic and stochastic processes leading brine and require saturated salts to live, and we find others to the resilience of established microbial communities. that grow in the deepest parts of the oceans and require 500 to 1000 bars of hydrostatic pressure. Life has evolved strate- • Objective 5.3—Biochemical adaptation to extreme environ- gies that allow it to survive even beyond the daunting phys- ments. Document life that survives or thrives under the ical and chemical limits to which it has adapted to grow. To most extreme conditions on Earth. Characterize and elu- survive, organisms can assume forms that enable them to cidate the biochemical capabilities that define the limits withstand freezing, complete desiccation, starvation, high for cellular life. Explore the biochemical and evolutionary levels of radiation exposure, and other physical or chemical strategies that push the physical-chemical limits of life by challenges. Furthermore, they can survive exposure to such reinforcing, replacing, or repairing critical biomolecules conditions for weeks, months, years, or even centuries. We (e.g., spore formation, resting stages, replacement need to identify the limits for growth and survival and to rates, or DNA repair). Characterize the structure and understand the molecular mechanisms that define these lim- metabolic diversity of microbial communities in such ex- its. Biochemical studies will also reveal inherent features of treme environments. biomolecules and biopolymers that define the physical- chemical limits of life under extreme conditions. Broadening Example investigations our knowledge both of the range of environments on Earth that are inhabitable by microbes and of their adaptation to Investigate the physiological and molecular basis of the abil- these habitats will be critical for understanding how life ity of microorganisms to persist at low water activity. might have established itself and survived in habitats be- Biochemically characterize the diversity of DNA-repair yond Earth. mechanisms that allow microorganisms to recover from radiation damage. • Objective 5.1—Environment-dependent, in Study microbial strategies that permit life in stationary microorganisms. Experimentally investigate and observe phase. NASA ASTROBIOLOGY ROADMAP 727 GOAL 6—Understand the principles that will shape the future of life, both on Earth and beyond. Elucidate the drivers and effects of microbial change as a basis for forecasting future changes on time scales ranging from decades to millions of years, and explore the potential for microbial life to survive and evolve in environments beyond Earth, especially regarding aspects relevant to US Space Policy.

Life on Earth depends upon networks of biogeochemical stabilizing feedbacks that permit the continuity of ecosys- reactions that interact with the crust, oceans, and atmosphere tems in the face of rapidly changing physical conditions, and to maintain a biosphere that has been remarkably resilient to understand the limits of these stabilizing feedbacks. Ide- to environmental challenges. These reaction networks de- ally this consideration will provide insight into the potential veloped within self-organized microbial ecosystems that col- effects of environmental changes that are abrupt as well as lectively responded to environmental conditions and those changes that unfold over time scales ranging from sea- changes. Evolutionary are working to understand sonal cycles to millions of years. how such biological and environmental processes have The ability of life to move beyond Earth will depend shaped specific ecosystems in Earth’s history. It is highly upon the potential for microorganisms to utilize resources challenging yet critically important to employ such princi- and to adapt and evolve in extraterrestrial environments. ples to formulate accurate predictions about the state of fu- Viable microorganisms might be transported by natural ture ecosystems, especially when environments change events such as impacts or by robotic spacecraft, but they faster than the tempo of biological evolution. Predictions of most certainly will accompany missions. Life forms this nature will require improved models of the biogeo- will be challenged by extremes in temperature, pressure, chemical cycling of critical elements that links biological pro- radiation and the availability of nutrients. Studies of adap- cesses with the surrounding physical world. tation and survival will indicate not only whether micro- Viewing Earth’s ecosystems in the context of astrobiology bial life can expand its evolutionary trajectories beyond challenges us to consider how “resilient” life really is on a Earth but also how it can play key supporting roles in hu- planetary scale, to develop mathematical representations of man exploration.

FIG. 6. Earthrise over the Moon. Courtesy of NASA. 728 DES MARAIS ET AL.

Background. Humans are increasingly perturbing Earth’s • Objective 6.1—Effects of environmental changes on microbial biogeochemical cycles. In addition to impacting the carbon ecosystems. Conduct remote sensing, laboratory, and field cycle, humans have doubled the natural global sulfur emis- studies that relate the effects of present-day environmen- sions to the atmosphere, doubled the global rate of nitrogen tal changes to the biogeochemical cycling of key elements fixation, enhanced levels of phosphorus loading to the , by microorganisms. Relate changes in elemental cycling altered the , and perhaps, most critically, altered to effects on the structure and functioning of microorgan- the hydrological cycle. When compared to many natural per- isms, their ecosystems, and the surrounding environment. turbations, the effects of human activities have been ex- Develop predictive models that integrate biogeochemical tremely rapid. Understanding how these changes will affect cycles with adaptation by microbial ecosystems and with planetary climate, ecosystem structure, and human habitats environmental change (both those driven by anthro- is an urgent research priority in which astrobiology can play pogenic and non-anthropogenic forces, including ex- an important role. traterrestrial events [see Objective 4.3]). A conceptual continuum embraces the development of biogeochemical cycles, the evolution to the modern bio- sphere, and ongoing human effects. Studies of processes over Example investigations long time scales (millennia to millions of years) offer an ob- Document the ecological impact of changes in climate, habi- servational context that extends and strengthens the inter- tat complexity, and nutrient availability upon the struc- pretation of shorter time scale (annual to century) phenom- ture and function of a selected ecosystem, as a guide to ena. While longer-term changes in Earth’s ecosystems are understanding changes that might occur over time scales strongly affected by processes such as tectonics and evolu- ranging from abrupt events (a few years or less) to mil- tion, the relatively rapid rates of recent change, influenced lions of years. by anthropogenic forcing, may only have analogues in pre- Identify biosignatures associated with global change or mi- vious important events such as major extinctions. crobial stress. A key objective for elucidating the sign of the feedbacks Develop techniques for remote sensing of microbial biosig- in biogeochemical cycles and for understanding how the cy- natures at a variety of scales. cles respond to perturbations is to develop quantitative mod- els that incorporate the interactions between metabolic and geochemical processes. For example, how are the key bio- • Objective 6.2—Adaptation and evolution of life beyond Earth. geochemical cycles of the light elements (e.g., C, N, O, S, P, Explore the adaptation, survival and evolution of micro- etc.) related? What constrains these cycles on time scales of bial and other organisms under environmental conditions years to millions of years? How are these cycles altered by that simulate conditions in space or on other potentially rapid changes in climate? Does functional redundancy, as in- habitable planets. Identify survival strategies to evaluate dicated by a great diversity within microbial ecosystems, en- the potential for interplanetary transfer of viable - sure ecosystem resilience? Are specific metabolic pathways isms and to establish requirements for effective planetary more sensitive to perturbations than others? How have the protection. Identify and validate roles that microorgan- biogeochemical cycles co-evolved with terrestrial environ- isms might play in life support and acquisition ments on time scales of millions of years? Our vision of the during human missions envisioned by US Space Policy. future will be sharpened by a retrospective view offered by Develop tools to track the function and adaptation of mi- a biogeochemical model that is verified by preserved records. crobes and other organisms to extraterrestrial environ- This effort is needed in order to expand the current focus on ments during mankind’s exploration efforts. short-term changes and “what is happening” in order to per- form more hypothesis testing and thus address “why is this Example investigations happening.” Life forms that are transported beyond their planet of ori- Document the effects of the upon micro- gin will probably experience conditions more extreme than bial ecosystems. those in the most extreme habitats on Earth. Such conditions Examine the survival, genomic alteration, and adaptation of will challenge their very existence in most, if not all, cases. microbial ecosystems in a wide range of simulated Understanding survival and evolution beyond the planet of martian environments. Interpret the significance of these origin is essential for evaluating the potential for the inter- experiments regarding the potential for the forward bio- planetary transfer of viable organisms and thus the poten- logical contamination of Mars and for utilizing microor- tial that life elsewhere in the Solar System might share a com- ganisms to support the needs of human exploration. mon origin with life on Earth. Survivorship beyond Earth is Examine the effects of the space environment upon the an ultimate test of the resilience of terrestrial life and thus of biosynthesis and utilization of biomolecules that play key its potential to diversify far beyond the limits of our current roles in biogeochemical processes and also upon the via- understanding. Microbes and other organisms are destined bility of microbes that might be transferred between plan- ultimately to play critical roles in life support during ex- ets by natural processes (e.g., impact ejection). tended human missions. Their study should be an integral Develop automated tools to monitor the adaptation of component of the expansion of mankind throughout the So- organisms in lunar and martian environments, especially lar System as NASA carries out US Space Policy over the those areas most likely to be visited by human explorers next century. over the next century. NASA ASTROBIOLOGY ROADMAP 729 GOAL 7—Determine how to recognize signatures of life on other worlds and on early Earth. Identify biosignatures that can reveal and characterize past or present life in ancient samples from Earth, extraterrestrial samples measured in situ or returned to Earth, and remotely measured planetary atmospheres and surfaces. Identify biosignatures of distant technologies.

Our concepts of life and biosignatures are inextricably Background. Astrobiological exploration is founded linked. To be useful for exploration, biosignatures must be upon the premise that signatures of life (biosignatures) will defined in terms that can be measured and quantified. Mea- be recognizable in the context of their environments. A surable attributes of life include its complex physical and biosignature is an object, substance and/or pattern whose chemical structures and also its utilization of free energy origin specifically requires a biological agent. The usefulness and the production of and wastes, phenomena that of a biosignature is determined, not only by the probability can be sustained through self-replication and evolution. of life creating it, but also by the improbability of nonbio- Habitable planets might create nonbiological features that logical processes producing it. An example of such a biosig- mimic biosignatures and therefore must be understood in nature might be complex organic molecules and/or struc- order to clarify our interpretations. We must create a library tures whose formation is virtually unachievable in the of biosignatures and their nonbiological mimics of life as absence of life. A potential biosignature is a feature that is we know it. A strategy is needed for recognizing novel consistent with biological processes and that, when it is en- biosignatures. This strategy ultimately should accommo- countered, challenges the researcher to attribute it either to date a diversity of habitable conditions, biota, and tech- inanimate or to biological processes. Such detection might nologies in the universe that probably exceeds the diver- compel investigators to gather more data before reaching a sity observed on Earth. conclusion as to the presence or absence of life.

FIG. 7. Some examples of biosignatures. Images courtesy of A. Knoll, Harvard University (Eoentophysalis cells); B. Runnegar, U.C.LA. (Dickinsonia Ediacara fossil); D. Des Marais, NASA Ames Research Center (), D. Blake, NASA Ames Research Center ( chain). 730 DES MARAIS ET AL.

Suites of biosignatures must be identified that reflect fun- ther developed and novel methods should be identified for damental and universal characteristics of life and thus are detecting electromagnetic radiation or other diagnostic arti- not restricted solely to those attributes that represent local facts that indicate remote technological civilizations. solutions to the challenges of survival. For example, certain examples of our biosphere’s specific molecular machinery, • Objective 7.1—Biosignatures to be sought in Solar System ma- e.g., DNA and proteins, might not necessarily be mimicked terials. Learn how to recognize and interpret any biosig- by other examples of life elsewhere in the cosmos. On the natures either in ancient rocks on Earth or in the crustal other hand, basic principles of biological evolution might in- materials and atmospheres of other Solar System bodies deed be universal. in order to characterize any ancient and/or present-day However, not all of the universal attributes of life will be life. expressed in ancient planetary materials or detectable re- motely (e.g., by astronomical methods). For example, the pro- Example Investigations cesses of biological evolution are highly diagnostic for life, Determine additional organic that will help to but evidence of biological evolution might not be readily de- chart the presence and development of photosynthetic mi- tected as such in a sample returned from Mars. However, crobiota in Precambrian rocks. better-preserved evidence of life might include complex Determine the features of sedimentary laminated textures structures that are often retained in aquatic or can that uniquely require biological processes. be preserved in large quantities in the environment. Thus, Identify examples of chemical, mineralogical and stable iso- for example, categories of biosignatures can include the fol- topic biosignatures that can indicate the presence of sub- lowing: cellular and extracellular morphologies, biogenic surface biota (e.g., microbes living in water-saturated fabrics in rocks, bio-organic molecular structures, , rocks) and that can be preserved in crustal materials. biogenic minerals, biogenic stable patterns in min- Characterize any potential gaseous biosignatures in the erals and organic compounds, atmospheric gases, remotely martian atmosphere via orbiting spacecraft and/or detectable features on planetary surfaces (photosynthetic ground-based observatories, and investigate their poten- pigments, etc.), and characteristic temporal changes in global tial sources. planetary properties (e.g., seasonally mediated respiration and biomass cycles). On Earth, biosignatures also include • Objective 7.2—Biosignatures to be sought in nearby planetary those key minerals, atmospheric gases, and crustal reservoirs systems. Learn how to identify and measure biosignatures of carbon, sulfur, and other elements that collectively have that can reveal the existence of life or technology through recorded the enduring global impact of the utilization of free remote observations. energy and the production of biomass and wastes. Oxygen- producing photosynthesis has simultaneously created large Example Investigations reservoirs of atmospheric oxygen, marine sulfates, and sed- imentary ferric iron and sulfates (its oxidized products), as Determine the nature and fate of reduced gases that are pro- well as large sedimentary reservoirs of biogenic organic mat- duced by specific microbial ecosystems in an anoxic (“pre- ter and sulfides (its corresponding reduced products). Again, oxygenated”) biosphere. in order to qualify as biosignatures, such features must be Carry out laboratory, observational and modeling studies to sufficiently complex and/or abundant so that they retain a separate false from true biosignatures (e.g., atmospheric O2 diagnostic expression of some of life’s universal attributes. in a range of planetary environments [see Objective 1.2]). Also, their formation by nonbiological processes should be Develop novel approaches to detect evidence of distant tech- highly improbable. nologies. All biosignatures are characteristic of the modification of a local or planetary environment by life. Human technology Address reprint requests to: and its resultant products and outputs can therefore also be David J. Des Marais considered as a biosignature, with the added benefit that it Mail Stop 239-4 might be detected remotely. Thus, although technology is NASA Ames Research Center probably much less common than life in the universe, its as- Moffett Field, California 94035 sociated biosignatures perhaps enjoy much higher “signal- to-noise” ratios. Accordingly, current methods should be fur- E-mail: David.J.DesMarais@.gov