The NASA Astrobiology Roadmap · · 2008-09-25The NASA Astrobiology Roadmap provides guidance for research and technology development across the ... and what is the future of life

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

Focus Paper

The NASA Astrobiology 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 theNASA enterprises that encompass the space, Earth, and biological sciences. 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 life begin and evolve,does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? Seven ScienceGoals outline the following key domains of investigation: understanding the nature and distribution of habit-able environments in the universe, exploring for habitable environments and life in our own Solar System, 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, determiningthe principles that will shape life in the future, and recognizing signatures of life on other worlds and on earlyEarth. For each of these goals, Science Objectives outline more specific high priority efforts for the next threeto five years. These eighteen objectives are being integrated with NASA strategic planning. Astrobiology 8,xxx–xxx.

1

Introduction

ASTROBIOLOGY ADDRESSES THREE BASIC QUESTIONS that havebeen asked in various ways for generations: how does

life begin and evolve, does life exist elsewhere in the uni-verse, and what is the future of life on Earth and beyond?Accordingly, the discipline of astrobiology embraces thesearch for potentially inhabited planets beyond our SolarSystem, the exploration of Mars and the outer planets, lab-oratory and field investigations of the origins and early evo-lution of life, and studies of the potential of life to adapt tofuture challenges, both on Earth and in space. Interdiscipli-nary research is required that combines molecular biology,ecology, planetary science, astronomy, information science,

space exploration technologies, and related disciplines. Thebroad interdisciplinary character of astrobiology compels usto strive to achieve the most comprehensive and inclusiveunderstanding of biological, planetary, and cosmic phe-nomena.

The NASA Astrobiology Roadmap provides guidance forresearch and technology development across the NASA En-terprises that encompass the space, Earth, and biological sci-ences. The Roadmap is formulated in terms of seven ScienceGoals that outline key domains of investigation. These do-mains of investigation will probably require decades of ef-fort to consummate. For each of these goals, Science Objec-tives outline more specific high priority efforts for the nextthree to five years. These eighteen objectives are being inte-

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

grated with NASA strategic planning. The Roadmap also in-cludes Example Investigations, which offer examples of spe-cific research tasks that are both important and timely fortheir corresponding Science Objective. It is important to em-phasize that these investigations are intended principally tobe illustrative of relevant tasks, and that additional equallyimportant example investigations can be envisioned.

The following four basic principles are fundamental to theimplementation of NASA’s astrobiology program:

(1) Astrobiology is multidisciplinary in its content and in-terdisciplinary in its execution. Its success dependscritically upon the close coordination of diverse scien-tific disciplines and programs, including space mis-sions.

(2) Astrobiology encourages planetary stewardship throughan emphasis on protection against forward and back bi-ological contamination and recognition of ethical issuesassociated with exploration.

(3) Astrobiology recognizes a broad societal interest in itsendeavors, especially in areas such as achieving a deeperunderstanding of life, searching for extraterrestrial bio-spheres, assessing the societal implications of discover-ing other examples of life, and envisioning the future oflife on Earth and in space.

(4) The intrinsic public interest in astrobiology offers a cru-cial opportunity to educate and inspire the next genera-tion of scientists, technologists, and informed citizens;thus a strong emphasis upon education and public out-reach is essential.

DES MARAIS ET AL.2

Goals and Objectives

Goal 1—Understand the nature and distribution of habitable environments inthe universe. Determine the potential for habitable planets beyond the SolarSystem, and characterize those that are observable.

A planet or planetary satellite is habitable if it can sustainlife that originates there or if it sustains life that is carried tothe object. The Astrobiology program seeks to expand ourunderstanding of the most fundamental environmental re-quirements for habitability. However, in the near term, wemust proceed with our current concepts regarding the re-quirements for habitability. That is, habitable environmentsmust provide extended regions of liquid water, conditionsfavorable for the assembly of complex organic molecules,and energy sources to sustain metabolism. Habitability is notnecessarily associated with a single specific environment; itcan embrace a suite of environments that communicatethrough exchange of materials. The processes by which cru-cial biologically useful chemicals are carried to a planet andchange its level of habitability can be explored through thefields of prebiotic chemistry and chemical evolution. A ma-jor long-range goal for astrobiology is to recognize habit-ability beyond the Solar System, independent of the presenceof life, or to recognize habitability by detecting the presenceof life (see Goal 7: Biosignatures). This goal directly ad-dresses the call for searches for Earth-like planets and forhabitable environments around other stars that is includedin the Vision for Space Exploration.

Background. Research in astrobiology supports NASA inits attempt to search for habitable or inhabited environmentsbeyond the Solar System. Humans have pondered for mil-lennia whether other inhabitable worlds exist. Now, for thefirst time, they have an opportunity to look and see. Ofcourse it is not possible to examine the �1010 Earth-like plan-ets that simple statistical models predict to exist in ourgalaxy, much less the �1021 such planets expected to be inthe universe. Still, it should be possible to determine whether

terrestrial planets are indeed as common as predicted above,whether a substantial fraction of them show signs of habit-ability, and whether an appreciable fraction of these showbiosignatures.

A key difference between the search for life in the SolarSystem and the search in external planetary systems is that,within the Solar System, interplanetary transfer of viable mi-crobes seems a plausible process, and therefore the discov-ery of life elsewhere in the Solar System seems plausible.While this is indeed of extraordinary interest, it may not castlight on whether it is easy or difficult for life to begin. Onthe other hand, the fact that dispersion times between starsare �105 to 106 times longer than for dispersion within theSolar System makes independent origination of life formsoutside the Solar System more probable.

The research objectives under this goal address three keyquestions. First, do terrestrial planets and large satellites tendto form in a state where they are likely to become habitable,or do habitable environments emerge only after a sequenceof less probable events? Second, how frequently do habit-able environments arise on solid planets, including largesatellites? Third, what are the specific signs of habitabilityand habitation, and how do such signs change with the cir-cumstances of the planet (e.g., mass, distance from its star,history and relative abundance of volatile compounds)? Toaddress these questions effectively, we must investigate howhabitable planetary systems form and evolve (Goals 1 and2), and we must understand the ultimate environmental lim-its of life (Goal 5).

Much of this effort focuses upon the presence or absenceof liquid water in bulk form. Water is made from the twomost abundant chemically reactive elements in the universe,and it is the necessary ingredient for Earth’s type of life. Liq-

uid water has played an intimate, if not fully understood,role in the origin and development of life on Earth. Watercontributes to the dynamic properties of an Earth-sizedplanet, permitting convection within the planetary crust thatmight be essential to supporting Earth-like life by creatinglocal chemical disequilibria that provide energy for life. Wa-ter maintains a strong polar-nonpolar dichotomy with cer-tain organic substances. This dichotomy has allowed life onEarth to form independent stable cellular structures. Thusthe primary focus of Goal 1 is concerned with planets hav-ing a liquid water boundary layer, although the focus mayexpand to include other planets or satellites as astrobiologymatures as a discipline.

There is also a focus—though not exclusive—on molecu-lar oxygen and ozone as biosignatures (see also Goal 7), andtherefore on dealing with the interface between under-standing the geological and biological aspects of oxygen, anddetermining the details of the spectral features that can beobserved and interpreted remotely. Oxygen is a very com-mon element that has provided Earth with its most distinc-tive biosignature. The chemical state of an Earth-like planet,as well as the geological activity that delivers reduced speciesto its surface environment, will cause virtually all of the mo-lecular oxygen to be consumed unless it is produced rapidly(e.g., by oxygen-producing photosynthesis). Also, the rela-

tively modest ultraviolet fluxes of many stars prevent rapidproduction of oxygen from photo-dissociation of water.These factors will help to minimize the possibility of falsepositive detections of oxygen biosignatures.

The challenge of remotely detecting life on a planet thathas not developed a biogenic source of oxygen is fraughtwith unknowns. What chemical species and spectral sig-natures should be sought? What metabolic processes mightbe operating? How does one guard against a false positivedetection? Research that is guided both by our knowledgeof Earth’s early biosphere (i.e., before the rise of an oxy-genated atmosphere) and by studies of alternative biologi-cal systems can help address these questions and provideguidance to astronomers seeking evidence of life elsewhere(Goal 7).

• Objective 1.1—Formation and evolution of habitable planets.Investigate how solid planets form, how they acquire liq-uid water and other volatile species and organic com-pounds, and how processes in planetary systems andgalaxies affect their environments and their habitability.Use theoretical and observational studies of the formationand evolution of planetary systems and their habitablezones to predict where water-dependent life is likely to befound in such systems.

NASA ASTROBIOLOGY ROADMAP 3

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

http://www.liebertonline.com/action/showImage?doi=10.1089/ast.2008.0819&iName=master.img-000.jpg&w=359&h=336

Example investigations

Understand how the processes and time scales of planet for-mation vary with the mass of the central star, for exam-ple, whether low mass M dwarf stars or very low metal-licity stars are likely to host habitable planets or satellites.

Model the potential diversity of habitable planets that mightemerge from the evolution of protoplanetary disks. Forcomparison, measure key time scales for planet-formingevents in our own Solar System based on studies of iso-topic chronometers in meteorites and samples returnedfrom primitive Solar System bodies.

Investigate how the delivery of water and other volatiles toand their loss from terrestrial-like planets of various sizeand mass can affect climate, surface, and interior pro-cesses, and how these changes affect habitability.

Develop comprehensive models of the environments of ter-restrial-like planets (including the recently discovered “Su-per Earths,” planets with masses in the range of �1 to �10Earth-masses) to investigate the evolution of habitability.

• Objective 1.2—Indirect and direct astronomical observations ofextrasolar habitable planets. Conduct astronomical, theoret-ical, and laboratory spectroscopic investigations to sup-port planning for and interpretation of data from missionsdesigned to detect and characterize extrasolar planets.


Use ground-based telescopes to detect “Super Earths” thatmay offer habitable environments around low mass, Mdwarf stars.

Investigate novel methods for detecting and characterizingextrasolar planets, particularly those that might lead to animproved understanding of the frequency of habitableEarth-like planets.

Use atmospheric models to understand the range of plane-tary conditions that can be determined from low resolu-tion, full-disk spectra at visible, near-IR, and thermalwavelengths. Use data for Venus, Earth, and Mars to val-idate these models.

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GOAL 2—Determine any past or present habitable environments, prebioticchemistry, and signs of life elsewhere in our Solar System. Determine thehistory of any environments having liquid water, chemical ingredients, andenergy sources that might have sustained living systems. Explore crustalmaterials and planetary atmospheres for any evidence of past and/orpresent life.

This goal seeks to understand the distribution of the ba-sic requirements for life elsewhere in our Solar System andto understand how any habitable environments and life per-sisted over geological time. Although possible life elsewherein the Solar System may have developed differently from lifeon Earth, our understanding of the nature of terrestrial lifeprovides an important starting point to guide our explo-ration strategies. For example, studies of microorganisms inextreme environments, including the biota sustained bychemical energy in the deep subsurface, have revolutionizedour assessment of the potential for life on Mars and the icymoons in the outer Solar System. Surface environments onthose bodies are presently hostile to life as we know it; how-ever, subsurface environments might be habitable. Ancientsurface environments also might have been habitable, at leastintermittently. By mapping the distribution of any past andpresent habitable environments, robotic missions will guidethe selection of samples to be returned to state-of-the-art lab-oratories on Earth to conduct life detection experiments.

Background. During the past decade, dramatic progresshas been made in our understanding of the potential for pastand present habitable environments elsewhere in the SolarSystem. For example, the Mars Global Surveyor orbiter,Mars Exploration Rovers, and the Odyssey and Mars Re-connaissance orbiters revealed that, early in martian history,surface water environments were widespread over the sur-face and shallow subsurface and were sustained by both

atmospheric precipitation and outflows from aquifers. The-oretical models and surface geomorphology have indicatedthe potential for a global groundwater system capable ofsustaining an extant subsurface biosphere. The recent dis-covery of atmospheric methane indicates that subsurfacehabitable environments might exist even today. The Galileomission found strong evidence of subsurface brines on threeof the four Galilean satellites of Jupiter (Europa, Ganymede,and Callisto), maintained by internal tidal heating. TheCassini-Huygens mission has confirmed the presence onSaturn’s moon, Titan, of an atmosphere and surface en-riched in prebiotic organic compounds. Titan can be viewedas a natural laboratory where we might gain important newinsights into the origins of life here on Earth. The existenceof lakes of liquid hydrocarbons on Titan opens up the pos-sibility for solvents and energy sources that are alternativesto those in our biosphere and that might support novel lifeforms altogether different from those on Earth. The Cassinispacecraft has imaged plumes of water vapor that eruptepisodically from the subsurface of Enceladus, an icy moonof Saturn. This indicates the possibility that interior habit-able zones with liquid water might exist on yet another bodyin the outer Solar System. Recent biological research hasdeepened our understanding of the nature, environmentallimits, and evolutionary history of life on Earth. Such dis-coveries have contributed to strategic planning activitiesthat broadly support NASA missions, including the speci-fication of scientific goals, objectives and measurement re-

quirements, new instrument designs, and integrated pay-load concepts.

Building upon such discoveries, logical next steps in theexploration for extraterrestrial life in the Solar System in-clude the following: (1) continuing to refine our under-standing of the nature and distribution of past and/or pres-ent habitable environments in the Solar System, (2)identifying the most favorable landing sites for future in situlife detection missions, (3) developing experiments for theunambiguous detection of extant and fossil biosignatures insurface/subsurface rocks, soils, and ices on other planetarybodies, like Mars and Europa, (4) developing robotic drillingsystems capable of accessing subsurface environments onMars and possibly on other bodies in the Solar System, (5)collecting and returning samples to Earth from high prior-ity sites on Mars and possibly other bodies in the Solar Sys-tem to explore for prebiotic chemistry and biosignatures uti-lizing the advanced capabilities of terrestrial laboratories.Key questions include the following: If life ever arose else-where, is it related to terrestrial life, or did other bodies inthe Solar System sustain independent origins of life? If lifenever developed elsewhere in our Solar System, is there aprebiotic chemical record preserved in ancient rocks thatmight contain clues about how life began on Earth?

Renewed lunar exploration offers opportunities to explorethe earliest history of habitable environments on Earth. TheMoon might harbor an inventory of crustal materials thatwere delivered from Earth before, during, and after the in-terval of heavy bombardment. The discovery of terrestrialmeteorites on the Moon could provide important new in-sights about the evolution of planetary habitability duringthe birth of our biosphere.

Sample return(s) from important astrobiology targets in theSolar System (e.g., Mars, icy satellites, asteroids, and comets)will greatly expand opportunities to explore for prebioticchemistry and life using the most sophisticated tools availablein terrestrial laboratories. Logical steps toward planning futuresample returns include the development of reliable robotic sys-tems for the in situ targeting, collection, and storage of sam-ples, the safe transport and delivery of samples to Earth, theconstruction of sample containment facilities on Earth for thesafe handling and storage of extraterrestrial materials, and thedistribution of samples to members of the scientific commu-nity for detailed laboratory analyses.

• Objective 2.1—Mars exploration. Through orbital and sur-face missions, explore for potentially habitable environ-ments and evidence of life, as indicated by water, organic


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


matter, atmospheric gases, and/or minerals. Study martianmeteorites to guide Mars exploration. To support both ex-ploration at Mars and the first sample return mission, up-date astrobiology measurement requirements, supportsite selection studies, and develop/improve relevant tech-nologies and analytical methods.


Target a well-instrumented robotic rover to a site of pastaqueous activity and analyze rocks for their geochemistry,minerals formed in association with water (e.g., sulfates,silica, phyllosilicates, etc.), organic chemistry, and fossilbiosignatures (including organic biomarker compounds,stable isotopes, atmospheric gases, biosedimentary struc-tures, and microtextures).

Develop flight instruments to characterize samples in situ, toselect samples for return to Earth, and to analyze atmo-spheric composition from orbit.

Develop universal approaches for detecting evidence of ex-tant life forms that can be applied over a broad range ofsurface environments to distinguish between forward con-tamination and in situ extraterrestrial life.

Analyze results from previous and current missions in thecontext of their significance for astrobiology.

• Objective 2.2—Outer Solar System exploration. Conduct ba-sic research, develop instrumentation to support astro-biological exploration, and provide scientific guidancefor outer Solar System missions. Model the potential forsubsurface habitable environments on icy moons suchas Europa, Titan, and Enceladus. Support the develop-ment of missions to explore the surface ices and thinatmospheres of these bodies for evidence of subsurfacehabitable environments, organic chemistry, and/orbiosignatures.


Develop flight instrumentation that can survive the surfaceenvironments of Europa and Titan and that can determinethe composition (including any organic compounds) ofsurface ices and atmospheres, and explore for cryo-pre-served biosignatures.

Develop numerical models to constrain the nature of deepsubsurface environments on Europa, Titan, and Ence-ladus (presence of life-sustaining solvents, biologicallyessential elements, and energy sources) that might sus-tain life.

Analyze results from the Cassini mission in the context oftheir significance for astrobiology.

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GOAL 3—Understand how life emerges from cosmic and planetaryprecursors. Perform observational, experimental, and theoreticalinvestigations to understand the general physical and chemical principlesunderlying the origins of life.

How life begins remains a fundamental unsolved mystery.The origin of life on Earth may well represent only one path-way among many along which life can emerge. This beliefforms the intellectual foundations for observations and mis-sions aimed at searching for extant or extinct life elsewherein the universe. Thus the universal principles must be un-derstood that underlie not only the origins of life on Earth,but also its possible origins elsewhere. The starting point forestablishing these principles is to determine what raw ma-terials of life can be produced by chemical evolution in spaceand on planets. Further, it should be understood how or-ganic compounds are assembled into more complex molec-ular structures and how the functions of these biomolecularstructures become coordinated to form complex evolvingsystems that accompany life’s origins. Such systems musthave the capabilities to perform and coordinate capture ofenergy and nutrients from the environment, catalyze thechemical reactions needed for maintenance and growth ofthese systems, and manufacture copies of key biomolecules.Clues from the biomolecular and fossil records, as well asfrom diverse microorganisms, should be explored in orderto define better the fundamental properties of the living state.

Background. We must move beyond the circumstancesof our own particular origins in order to develop a broader

discipline, “Universal Biology.” Although this discipline willbenefit from an understanding of the origins and limits ofterrestrial life, it also requires that we define the environ-mental conditions and the chemical structures and processesthat could support life on other habitable planets. Thus weneed to exploit universal laws of physics and chemistry tounderstand polymer formation, self-organization processes,energy utilization, information transfer, and Darwinian evo-lution that might lead to the emergence of life in planetaryenvironments other than Earth. Clearly an inventory of mol-ecules must exist that is capable of gaining chemical, struc-tural, and functional complexity and eventually assemblinginto living systems. This is strongly conditioned on temper-ature, solvent, energy sources, etc. Some conditions that sup-port chemistry that is sufficiently rich to seed life might bedetrimental to self-organization of biological structures. Con-versely, conditions that promote the emergence of biologi-cal complexity might be unfavorable to organic chemistry.Thus an integrative, interdisciplinary approach is necessaryto formulate the principles underlying universal biology. Theperspectives gained from understanding these principleswill markedly improve our ability to define habitability andrecognize biosignatures. This, in turn, will guide missionsand observational projects aimed at finding life in the SolarSystem and beyond.

Sources of organic compounds required for the ori-gin of life. To understand how life can begin on a habitableplanet such as Earth, it is essential to know what organiccompounds were likely to have been available and how theyinteracted with the planetary environment. Chemical syn-theses that occur within the solid crust, hydrosphere, andatmosphere are likely to be important sources of biogeniccompounds. Interesting organic compounds are also presentin interstellar clouds, the birthplace of planetary systems.Laboratory simulations of interstellar ices have recentlydemonstrated that key prebiotic molecules can also be syn-thesized under interstellar cloud conditions, and such mate-rials are incorporated into nascent solar systems. More recent astronomical observations and analyses of extrater-restrial materials have shown that many compounds rele-vant to life processes are also present in meteorites, inter-planetary dust particles, and comets. It is likely thatsubstantial amounts of such organic material were deliveredto Earth during late accretion, providing organic compoundsthat could serve as a feedstock for chemical evolution or thatcould be incorporated directly into early forms of life. Animportant research objective within this goal is to establishsources of prebiotic organic compounds and to understandtheir history in terms of universal processes that would takeplace on any newly formed planet. This will require an in-tegrated program of pan-spectral astronomical observations,sample return missions, laboratory studies of extraterrestrialmaterials, and realistic laboratory simulations of inaccessi-ble cosmic environments.

Origins and evolution of functional biomolecules. Lifecan be understood as a chemical system that links a common

property of organic molecules—the ability to undergo spon-taneous chemical transformation—with the uncommonproperty of synthesizing a copy of that system. This process,unique to life, allows Darwinian-like selection and evolutionto occur. At the core of the life process are polymers com-posed of monomeric species such as amino acids, carbohy-drates, and nucleotides. The pathways by which monomerswere first incorporated into primitive polymers on earlyEarth remain unknown, and physical properties of the prod-ucts are largely unexplored. A primary goal of research onthe origin of life must be to understand better the sourcesand properties of primitive polymers on early Earth, theirinitial evolution towards acquiring increased fidelity, effi-ciency, and diversity of functions, and the pathway by whichpolymerization reactions of peptides and oligonucleotidesbecame genetically linked. As a parallel effort, the catalyticand informational potential of other polymers that can formin different planetary conditions should be explored. Thesespecific lines of inquiry should be complemented with the-oretical work, based on a “general principles” approach, toelucidate how the various protobiological properties of mol-ecules and systems, including emergent behavior and infor-mation transfer, depend on and scale with complexity.

Origins of energy transduction. Life requires energy toyield, in a reproducible fashion, specific, very low probabil-ity outcomes, such as the generation and maintenance of lo-cally ordered states of the biological system, the productionof thermodynamically unfavorable species, and the synthe-sis of polymers having specific sequences from among largenumbers of alternative sequence possibilities. Thus the emer-gence of complex life depends upon the development of re-


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


liable mechanisms for capturing energy from the surround-ings and channeling it, in appropriate forms, into specificprocesses. Overarching areas for focus are the following: (i)Foundational—understanding the relationship between en-ergy, information, and complexity; understanding whattypes of energy can be captured and utilized by biologicalsystems; and understanding how threshold requirements forcomplexity and information processing place upper andlower bounds on the biological utilization of energy. (ii)Mechanistic—elucidating mechanisms by which the princi-pal elements of energy transduction—capture of environ-mental energy, storage by membrane-based or molecularmeans, and coupling to key energy-requiring processes—may have functioned in primordial environments and com-bined into dedicated energy-transducing systems. (iii) Evo-lutionary—understanding how the systems for energycapture and utilization evolved from a complete lack of en-ergy transduction, through a partially developed state, tofully elaborated and dedicated systems for collecting and di-recting energy, and examining how these systems co-evolved with the capabilities for functionality, informationprocessing, and replication.

Origins of cellularity and protobiological systems. Forlife to begin in a natural setting such as a planetary surface,mechanisms must exist that concentrate and maintain inter-acting molecular species in a microenvironment. A boundedsystem of replicating and catalytic molecules capable of un-dergoing Darwinian evolution is by definition a cell. At somepoint life on Earth became cellular, either from its inceptionor soon thereafter. Boundary membranes divide complexmolecular mixtures into large numbers of individual struc-tures that can undergo selective processes required for bio-logical evolution. They also have the capacity to develop sub-stantial ion gradients that represent a central energy sourcefor virtually all life today. A primary research objective is tounderstand self-organizing and evolutionary processes thatcan lead to the emergence of cellular structures and to testthis understanding by creating laboratory models of primi-tive cells. These models are systems of interacting moleculeswithin bounded environments capable of working in concertto capture energy and nutrients from the surroundings,transduce environmental signals, form metabolic networksthat allow for growth through polymerization, and repro-duce some of their polymeric components. Approaching thischallenging problem will lead to a more refined definitionof the living state and will clarify the hurdles faced by self-assembled systems of organic molecules as they evolved to-ward life.

• Objective 3.1—Sources of prebiotic materials and catalysts.Characterize the exogenous and endogenous sources ofmatter (organic and inorganic) for potentially habitableenvironments in the Solar System and in other planetaryand protoplanetary systems.


Emphasizing important prebiotic species, trace the forma-tion, chemical evolution, and processing of interstellarmolecules and solids into more complex compounds.

Analyze meteorites and returned samples to understand the

nature of extraterrestrial organic compounds. Conductlaboratory experiments and simulations to provide aframework for interpreting observations of meteorites,samples returned from comets and asteroids, and spectraof interstellar clouds.

Identify the organic compounds and complexes producedunder primordial planetary conditions through laboratorysimulation experiments.

• Objective 3.2—Origins and evolution of functional biomole-cules. Identify plausible pathways for the synthesis of pre-biotic monomers and their condensation into polymers.Identify the potential for creating catalytic and geneticfunctions, investigate their protobiological evolution, andexplore primitive mechanisms for linking these two func-tions.


Investigate polymers other than nucleic acids that have thepotential to have been precursor molecules capable of con-taining genetic information.

Conduct laboratory experiments and computational studieson the emergence of functional proteins. Investigate mech-anisms by which primordial proteins evolved towards in-creased efficiency and selectivity, and acquired new struc-tures and functions.

Investigate ancestral translation mechanisms and the emer-gence of the genetic code using laboratory experimentsand clues from structural biology and bioinformatics.

• Objective 3.3—Origins of energy transduction. Investigate,conceptually and quantitatively, the relationship betweenenergy, complexity, and information as applied to the ori-gin of biological systems. Understand how the evolutionof molecules, metabolic cycles, linked systems, and or-ganisms is enabled and constrained by the developmentof energy transduction. Identify prebiotic mechanisms bywhich energy can be captured, stored, and coupled to en-ergy-requiring processes.


Quantify the theoretical amount of energy required to spec-ify the sequence of amino or nucleic acid polymers as afunction of chain length; compare to the actual energy in-put required based on known biological mechanisms.

Identify the classes of reactions available for prebiotic syn-thesis under a range of plausible Solar System environ-mental conditions, based on single-random inputs of en-ergy or no inputs of energy.

Investigate mechanisms by which the energy available in iongradients across boundary membranes could couple to thesynthesis of high-energy compounds such as pyrophos-phate.

• Objective 3.4—Origins of cellularity and protobiological sys-tems. Investigate both the origins of membranous bound-aries on early Earth and the associated properties of en-ergy transduction, transport of nutrients, growth, anddivision. Investigate the origins and early coordination ofkey cellular processes such as metabolism, energy trans-duction, translation, and transcription. Without regard to

DES MARAIS ET AL.8

how life actually emerged on Earth, create in the labora-tory and study artificial chemical systems that undergomutation and natural selection.


Determine how ionic and polar nutrients could permeatemembrane boundaries to supply monomers and energyfor intracellular metabolism and biosynthesis.

Investigate polymerase reactions that can take place in mem-brane-bounded microenvironments using external sourcesof monomers and chemical energy, and determine self-replication capabilities of such simple systems.

Considering clues from prebiotic chemistry and contem-porary biochemistry, investigate the organization ofchemical reactions into a primitive cellular metabolismand establish properties of the emerging metabolic net-works.


GOAL 4—Understand how life on Earth and its planetary environment haveco-evolved through geological time. Investigate the evolving relationshipsbetween Earth and its biota by integrating evidence from the geosciencesand biosciences that shows how life evolved, responded to environmentalchange, and modified environmental conditions on a planetary scale.

Understand how extraterrestrial processes and the plane-tary environment have influenced the evolution of life andhow biological processes have transformed conditions on lo-cal to planetary scales. An improved knowledge of how lifeoperated under diverse and extreme conditions will improveour ability to define, detect, and interpret biosignatures ofliving and extinct organisms, here and elsewhere. Studies of

key steps in the development of life will enable a deeper un-derstanding of the pathways leading to biological complex-ity and thus inform the search for living systems on distantworlds.

Background. As in other areas of astrobiology, integra-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, Western Australia. 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.


understand the evolution of life in a planetary context. Earthsystem models that relate processes operating on vastly dif-ferent temporal and spatial scales, from the core of Earth tothe atmosphere and beyond, are an integral part of this en-deavor. Comparative genomic studies, often based on environmental samples, including those from subsurfacehabitats, will provide a fuller understanding of microbialbiodiversity within communities that control biogeochemi-cal cycles, such as those involving the use of carbon, sulfur,iron, and nitrogen compounds. Studies of the metabolic re-quirements for life, the geochemical effects of those meta-bolic activities in the form of proxy records, and the pre-served biomolecular signatures of ancient organisms willprovide a deep time dimension to the evolution of life in aglobal context and show how Earth’s planetary profile, asobserved remotely, has changed throughout time. Investi-gations that explore the response of life to environmentalperturbations on ecological to geological time scales will en-hance our understanding of planetary habitability and con-tribute to the search for life beyond the Solar System. Andresearch based on the fossil record of life and its activities,coupled with knowledge gained from living organisms, willenable key steps in the pathways to biological complexity toserve as guideposts in the search for life beyond the SolarSystem.

Using these tools and methods, astrobiologists can studythe reciprocal interactions between organisms and theirplanetary environment and address the following questions:What microbial metabolisms existed at the time of the old-est sedimentary rocks, and what were their environmentaleffects? What might we learn about the earlier history of lifeon Earth if sedimentary samples that are even older can befound on the Moon? What can we learn from the early his-tory of life on Earth that will inform the search for biosig-natures on ancient Mars? How was the evolution of Earth’satmosphere and oceans during the first half of Earth historyrelated to the development of life? How have life’s courseand Earth’s habitability been changed by major environ-mental perturbations such as those resulting from extrater-restrial impacts, the eruption of large igneous complexes, theoxygenation of the atmosphere, global glaciations, and sig-nificant biological innovations? How does biological com-plexity arise, and what are its effects on planetary-scale en-vironments? Is there a predictable series of biologicalinnovations that will inform searches for life on planets or-biting other stars? These and other questions are tied to thisgoal that seeks to understand the historical interconnectionsbetween Earth and its biota to help guide our search for lifeelsewhere. All of this research requires a deeper under-standing of evolutionary mechanisms at the levels of mole-cules, cells, organisms, and ecosystems (Goal 5). The resultscontribute directly to the identification of biosignatures(Goal 7).

• Objective 4.1—Earth’s early biosphere. Investigate the de-velopment of key biological processes and their environ-mental consequences during the early history of Earththrough biogeochemical, paleobiological, geological, andgenomic studies.


Use information obtained from microbial genomes to un-derstand the early evolution of key microbial processes(e.g., methanogenesis, sulfate reduction, aerobic metabo-lism, etc.).

Develop timelines for the evolution of different microbialprocesses by linking observations of biogeochemical prox-ies to detailed, high-resolution geochronological measure-ments of ancient rocks.

Develop complex numerical models for the performance ofbiogeochemical cycles under conditions that existed onearly Earth and apply such knowledge to the interpreta-tion of early Mars.

• Objective 4.2—Production of complex life. Investigate thepathways and mechanisms that increased the complexityof life from microbial communities through successive lev-els of multicellularity.


Study detailed records of biogeochemical proxies to deter-mine if global environmental events were driven by bio-logical innovation.

Use comparative genomics in conjunction with the fossil andbiomolecular fossil records to better understand key stepsin the evolution of eukaryotes and sources of theirgenomes.

Apply knowledge gained from studies of the evolution of de-velopment to understand the sudden evolution of complexanimals during the Neoproterozoic and early Cambrian.

• Objective 4.3—Effects of extraterrestrial events upon the bio-sphere. Study the short- and long-term effects of extrater-restrial phenomena such as secular changes in the mag-nitude and quality of solar and cosmic radiation, theevolution of the Earth-Moon system, nearby supernovae,and infalling comets and asteroids on life and the envi-ronments in which it exists.


Use data obtained from the geological record to determineany effects of extraterrestrial environmental perturbationsupon the sustainability and/or diversity of life.

Investigate the mechanisms that enable small changes in theamount and quality of solar radiation received by Earth toinfluence global climate and biological productivity.

DES MARAIS ET AL.10

The diversity of life on Earth today is a result of the dy-namic interplay between genetic opportunity, metabolic ca-pability, and environmental change. For most of their exis-tence, Earth’s habitable environments have been dominatedby microorganisms and subjected to their metabolism andevolution. As a consequence of geological, climatologic, andmicrobial processes acting across geological time scales, thephysical-chemical environments on Earth have been chang-ing, thereby determining the path of evolution of subsequentlife. For example, the release of molecular oxygen bycyanobacteria as a by-product of photosynthesis as well asthe colonization of Earth’s surface by metazoan life contrib-uted to fundamental, global environmental changes. The al-tered environments, in turn, posed novel evolutionary op-portunities to the organisms present, which ultimately led tothe formation of our planet’s major animal and plant species.Therefore, this “co-evolution” between organisms and theirenvironment is an intrinsic feature of living systems.

Life survives and sometimes thrives under what seem tobe harsh conditions on Earth. For example, some microbesthrive at temperatures of 113°C. Others exist only in highlyacidic environments or survive exposures to intense radia-tion. While all organisms are composed of nearly identicalmacromolecules, evolution has enabled such microbes to

cope with a broad range of physical and chemical conditions.What are the features that enable some microbes to thriveunder extreme conditions that are lethal to many others? Anunderstanding of the tenacity and versatility of life on Earth,as well as an understanding of the molecular systems thatsome organisms utilize to survive such extremes, will pro-vide a critical foundation for the search for life beyond Earth.These insights will help us understand the molecular adap-tations that define the physical and chemical limits for lifeon Earth. They will provide a baseline for developing pre-dictions and hypotheses about life on other worlds.

Background. The evolution of biogeochemical processes,genomes, and microbial communities has resulted in thecomplexity and robustness of the modern biosphere. How-ever, we lack an understanding of how mutational events onthe single gene level scale upward to altered macroscopicecological performances of entire populations. We can ex-amine the reciprocal interactions between biosphere andgeosphere that can shape genes, genomes, organisms, andspecies interactions. Accordingly we will develop an under-standing of the evolution of biochemical and metabolic ma-chinery that drives the global cycles of the elements, as wellas the potential of such evolution and its limits. Furthermore


GOAL 5—Understand the evolutionary mechanisms and environmental limitsof life. Determine the molecular, genetic, and biochemical mechanisms thatcontrol and limit evolution, metabolic diversity, and acclimatization of life.

FIG. 5. Cyanobacteria and red phototrophic bacteria 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.


we must observe their coordination into genetic circuitriesand their integration into more complex biological entitiessuch as whole cells and microbial communities.

While co-evolution of Earth’s physical-chemical environ-ment and its life is dynamic and proceeds at all organismiclevels, microorganisms have played a critical role in shapingour planet. Microbes and viruses can serve as highly ad-vanced experimental systems for biochemical, genetic, andgenomic studies. More than 500 microbial genomes havebeen sequenced as of this writing. This unprecedentedwealth of information, together with the experimental toolsnow available for single cell studies, provides a tremendousopportunity for experimental studies to be conducted on theevolution of microbial genes, genomes, microbial communi-ties, and viruses. Such studies will uncover fundamentalprinciples of molecular, cellular, and community level evo-lution with relevance to Earth and other planets. Of specificinterest is observing or simulating the evolution of those mo-lecular properties that facilitate the metabolic coupling of theoxidation/reduction cycles of elements and the adaptationto novel environments, especially extreme environments,created by simulated perturbations. Hypothesis-driven ex-perimentation on microbial ecosystems using single specieswith known genome sequences can be employed to predictenvironmental changes and evolutionary solutions. Suchstudies can be extended to defined mixed communities tostudy the plasticity and adaptation of the “metagenome,”which comprises the genomes of all members of a microbialcommunity, when it is subjected to environmental changesand genetic flux. The evolved genotypes and phenotypesshould be correlated to the specific changes they induce inthe physical-chemical environment.

Our ongoing exploration of Earth has led to continued dis-coveries of life in environments that have been previouslyconsidered uninhabitable. For example, we find thrivingcommunities in the boiling hot springs of Yellowstone, thefrozen deserts of Antarctica, the concentrated sulfuric acidin acid-mine drainages, and the ionizing radiation fields innuclear reactors. We find some microbes that grow only inbrine and require saturated salts to live, and we find othersthat grow in the deepest parts of the oceans and require 500to 1000 bars of hydrostatic pressure. Life has evolved strate-gies that allow it to survive even beyond the daunting phys-ical and chemical limits to which it has adapted to grow. Tosurvive, organisms can assume forms that enable them towithstand freezing, complete desiccation, starvation, highlevels of radiation exposure, and other physical or chemicalchallenges. Furthermore, they can survive exposure to suchconditions for weeks, months, years, or even centuries. Weneed to identify the limits for growth and survival and tounderstand the molecular mechanisms that define these lim-its. Biochemical studies will also reveal inherent features ofbiomolecules and biopolymers that define the physical-chemical limits of life under extreme conditions. Broadeningour knowledge both of the range of environments on Earththat are inhabitable by microbes and of their adaptation tothese habitats will be critical for understanding how lifemight have established itself and survived in habitats be-yond Earth.

• Objective 5.1—Environment-dependent, molecular evolution inmicroorganisms. Experimentally investigate and observe

the evolution of genes, metabolic pathways, genomes, mi-crobial species, and viruses. Experimentally investigatethe forces and mechanisms that shape the structure, or-ganization, and plasticity of microbial genomes. Examinehow these forces control the genotype-to-phenotype rela-tionship. Conduct environmental perturbation experi-ments on single microbial species to observe and quantifyadaptive evolution to astrobiologically relevant environ-ments.


Experimentally observe the expansion and contraction of mi-crobial genomes within the context of different environ-ments and selective forces.

Examine the molecular basis of the robustness and resilienceof metabolic pathways in response to environmentalchanges.

Develop an understanding of the link between single geneevolution and the ecological consequences on the level ofmicrobial populations.

• Objective 5.2—Co-evolution of microbial communities. Exper-imentally examine the metabolic and genetic interactionsin microbial communities, including viruses, which havedetermined major geochemical processes and changes onEarth. Investigate how these interactions shape the evo-lution and maintenance of metabolic diversity in micro-bial communities. Investigate how novel microbial speciesestablish and adapt into existing communities.


Investigate small molecule interactions and their role in coordinating energy flow in mixed phototrophic/chemotrophic microbial communities.

Examine the molecular basis of speciation, e.g., the evolutionof a generalist to a specialist, and, conversely, of a spe-cialist to a generalist.

Examine the deterministic and stochastic processes leadingto the resilience of established microbial communities.

• Objective 5.3—Biochemical adaptation to extreme environ-ments. Document life that survives or thrives under themost extreme conditions on Earth. Characterize and elu-cidate the biochemical capabilities that define the limitsfor cellular life. Explore the biochemical and evolutionarystrategies that push the physical-chemical limits of life byreinforcing, replacing, or repairing critical biomolecules(e.g., spore formation, resting stages, protein replacementrates, or DNA repair). Characterize the structure andmetabolic diversity of microbial communities in such ex-treme environments.


Investigate the physiological and molecular basis of the abil-ity of microorganisms to persist at low water activity.

Biochemically characterize the diversity of DNA-repairmechanisms that allow microorganisms to recover fromradiation damage.

Study microbial strategies that permit life in stationaryphase.

DES MARAIS ET AL.12

Life on Earth depends upon networks of biogeochemicalreactions that interact with the crust, oceans, and atmosphereto maintain a biosphere that has been remarkably resilientto environmental challenges. These reaction networks de-veloped within self-organized microbial ecosystems that col-lectively responded to environmental conditions andchanges. Evolutionary biologists are working to understandhow such biological and environmental processes haveshaped specific ecosystems in Earth’s history. It is highlychallenging yet critically important to employ such princi-ples to formulate accurate predictions about the state of fu-ture ecosystems, especially when environments changefaster than the tempo of biological evolution. Predictions ofthis nature will require improved models of the biogeo-chemical cycling of critical elements that links biological pro-cesses with the surrounding physical world.

Viewing Earth’s ecosystems in the context of astrobiologychallenges us to consider how “resilient” life really is on aplanetary scale, to develop mathematical representations of

stabilizing feedbacks that permit the continuity of ecosys-tems in the face of rapidly changing physical conditions, andto understand the limits of these stabilizing feedbacks. Ide-ally this consideration will provide insight into the potentialeffects of environmental changes that are abrupt as well asthose changes that unfold over time scales ranging from sea-sonal cycles to millions of years.

The ability of life to move beyond Earth will dependupon the potential for microorganisms to utilize resourcesand to adapt and evolve in extraterrestrial environments.Viable microorganisms might be transported by naturalevents such as impacts or by robotic spacecraft, but theymost certainly will accompany human missions. Life formswill be challenged by extremes in temperature, pressure,radiation and the availability of nutrients. Studies of adap-tation and survival will indicate not only whether micro-bial life can expand its evolutionary trajectories beyondEarth but also how it can play key supporting roles in hu-man exploration.


GOAL 6—Understand the principles that will shape the future of life, both onEarth and beyond. Elucidate the drivers and effects of microbial ecosystemchange as a basis for forecasting future changes on time scales rangingfrom decades to millions of years, and explore the potential for microbial lifeto survive and evolve in environments beyond Earth, especially regardingaspects relevant to US Space Policy.

FIG. 6. Earthrise over the Moon. Courtesy of NASA.


Background. Humans are increasingly perturbing Earth’sbiogeochemical cycles. In addition to impacting the carboncycle, humans have doubled the natural global sulfur emis-sions to the atmosphere, doubled the global rate of nitrogenfixation, enhanced levels of phosphorus loading to the ocean,altered the silica cycle, and perhaps, most critically, alteredthe hydrological cycle. When compared to many natural per-turbations, the effects of human activities have been ex-tremely rapid. Understanding how these changes will affectplanetary climate, ecosystem structure, and human habitatsis an urgent research priority in which astrobiology can playan important role.

A conceptual continuum embraces the development ofbiogeochemical cycles, the evolution to the modern bio-sphere, and ongoing human effects. Studies of processes overlong time scales (millennia to millions of years) offer an ob-servational context that extends and strengthens the inter-pretation of shorter time scale (annual to century) phenom-ena. While longer-term changes in Earth’s ecosystems arestrongly affected by processes such as tectonics and evolu-tion, the relatively rapid rates of recent change, influencedby anthropogenic forcing, may only have analogues in pre-vious important events such as major extinctions.

A key objective for elucidating the sign of the feedbacksin biogeochemical cycles and for understanding how the cy-cles respond to perturbations is to develop quantitative mod-els that incorporate the interactions between metabolic andgeochemical processes. For example, how are the key bio-geochemical cycles of the light elements (e.g., C, N, O, S, P,etc.) related? What constrains these cycles on time scales ofyears to millions of years? How are these cycles altered byrapid changes in climate? Does functional redundancy, as in-dicated by a great diversity within microbial ecosystems, en-sure ecosystem resilience? Are specific metabolic pathwaysmore sensitive to perturbations than others? How have thebiogeochemical cycles co-evolved with terrestrial environ-ments on time scales of millions of years? Our vision of thefuture will be sharpened by a retrospective view offered bya biogeochemical model that is verified by preserved records.This effort is needed in order to expand the current focus onshort-term changes and “what is happening” in order to per-form more hypothesis testing and thus address “why is thishappening.”

Life forms that are transported beyond their planet of ori-gin will probably experience conditions more extreme thanthose in the most extreme habitats on Earth. Such conditionswill challenge their very existence in most, if not all, cases.Understanding survival and evolution beyond the planet oforigin is essential for evaluating the potential for the inter-planetary transfer of viable organisms and thus the poten-tial that life elsewhere in the Solar System might share a com-mon origin with life on Earth. Survivorship beyond Earth isan ultimate test of the resilience of terrestrial life and thus ofits potential to diversify far beyond the limits of our currentunderstanding. Microbes and other organisms are destinedultimately to play critical roles in life support during ex-tended human missions. Their study should be an integralcomponent of the expansion of mankind throughout the So-lar System as NASA carries out US Space Policy over thenext century.

• Objective 6.1—Effects of environmental changes on microbialecosystems. Conduct remote sensing, laboratory, and fieldstudies that relate the effects of present-day environmen-tal changes to the biogeochemical cycling of key elementsby microorganisms. Relate changes in elemental cyclingto effects on the structure and functioning of microorgan-isms, their ecosystems, and the surrounding environment.Develop predictive models that integrate biogeochemicalcycles with adaptation by microbial ecosystems and withenvironmental change (both those driven by anthro-pogenic and non-anthropogenic forces, including ex-traterrestrial events [see Objective 4.3]).


Document the ecological impact of changes in climate, habi-tat complexity, and nutrient availability upon the struc-ture and function of a selected ecosystem, as a guide tounderstanding changes that might occur over time scalesranging from abrupt events (a few years or less) to mil-lions of years.

Identify biosignatures associated with global change or mi-crobial stress.

Develop techniques for remote sensing of microbial biosig-natures at a variety of scales.

• Objective 6.2—Adaptation and evolution of life beyond Earth.Explore the adaptation, survival and evolution of micro-bial and other organisms under environmental conditionsthat simulate conditions in space or on other potentiallyhabitable planets. Identify survival strategies to evaluatethe potential for interplanetary transfer of viable organ-isms and to establish requirements for effective planetaryprotection. Identify and validate roles that microorgan-isms might play in life support and resource acquisitionduring human missions envisioned by US Space Policy.Develop tools to track the function and adaptation of mi-crobes and other organisms to extraterrestrial environ-ments during mankind’s exploration efforts.


Document the effects of the space environment upon micro-bial ecosystems.

Examine the survival, genomic alteration, and adaptation ofmicrobial ecosystems in a wide range of simulated martian environments. Interpret the significance of these experiments regarding the potential for the forward bio-logical contamination of Mars and for utilizing microor-ganisms to support the needs of human exploration.

Examine the effects of the space environment upon thebiosynthesis and utilization of biomolecules that play keyroles in biogeochemical processes and also upon the via-bility of microbes that might be transferred between plan-ets by natural processes (e.g., impact ejection).

Develop automated assay tools to monitor the adaptation oforganisms in lunar and martian environments, especiallythose areas most likely to be visited by human explorersover the next century.

DES MARAIS ET AL.14

Our concepts of life and biosignatures are inextricablylinked. To be useful for exploration, biosignatures must bedefined in terms that can be measured and quantified. Mea-surable attributes of life include its complex physical andchemical structures and also its utilization of free energyand the production of biomass and wastes, phenomena thatcan be sustained through self-replication and evolution.Habitable planets might create nonbiological features thatmimic biosignatures and therefore must be understood inorder to clarify our interpretations. We must create a libraryof biosignatures and their nonbiological mimics of life aswe know it. A strategy is needed for recognizing novelbiosignatures. This strategy ultimately should accommo-date a diversity of habitable conditions, biota, and tech-nologies in the universe that probably exceeds the diver-sity observed on Earth.

Background. Astrobiological exploration is foundedupon the premise that signatures of life (biosignatures) willbe recognizable in the context of their environments. Abiosignature is an object, substance and/or pattern whoseorigin specifically requires a biological agent. The usefulnessof a biosignature is determined, not only by the probabilityof life creating it, but also by the improbability of nonbio-logical processes producing it. An example of such a biosig-nature might be complex organic molecules and/or struc-tures whose formation is virtually unachievable in theabsence of life. A potential biosignature is a feature that isconsistent with biological processes and that, when it is en-countered, challenges the researcher to attribute it either toinanimate or to biological processes. Such detection mightcompel investigators to gather more data before reaching aconclusion as to the presence or absence of life.


GOAL 7—Determine how to recognize signatures of life on other worlds andon early Earth. Identify biosignatures that can reveal and characterize pastor present life in ancient samples from Earth, extraterrestrial samplesmeasured in situ or returned to Earth, and remotely measured planetaryatmospheres and surfaces. Identify biosignatures of distant technologies.

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


Suites of biosignatures must be identified that reflect fun-damental and universal characteristics of life and thus arenot restricted solely to those attributes that represent localsolutions to the challenges of survival. For example, certainexamples of our biosphere’s specific molecular machinery,e.g., DNA and proteins, might not necessarily be mimickedby other examples of life elsewhere in the cosmos. On theother hand, basic principles of biological evolution might in-deed be universal.

However, not all of the universal attributes of life will beexpressed in ancient planetary materials or detectable re-motely (e.g., by astronomical methods). For example, the pro-cesses of biological evolution are highly diagnostic for life,but evidence of biological evolution might not be readily de-tected as such in a sample returned from Mars. However,better-preserved evidence of life might include complexstructures that are often retained in aquatic sediments or canbe preserved in large quantities in the environment. Thus,for example, categories of biosignatures can include the fol-lowing: cellular and extracellular morphologies, biogenicfabrics in rocks, bio-organic molecular structures, chirality,biogenic minerals, biogenic stable isotope patterns in min-erals and organic compounds, atmospheric gases, remotelydetectable features on planetary surfaces (photosyntheticpigments, etc.), and characteristic temporal changes in globalplanetary properties (e.g., seasonally mediated respirationand biomass cycles). On Earth, biosignatures also includethose key minerals, atmospheric gases, and crustal reservoirsof carbon, sulfur, and other elements that collectively haverecorded the enduring global impact of the utilization of freeenergy and the production of biomass and wastes. Oxygen-producing photosynthesis has simultaneously created largereservoirs of atmospheric oxygen, marine sulfates, and sed-imentary ferric iron and sulfates (its oxidized products), aswell as large sedimentary reservoirs of biogenic organic mat-ter and sulfides (its corresponding reduced products). Again,in order to qualify as biosignatures, such features must besufficiently complex and/or abundant so that they retain adiagnostic expression of some of life’s universal attributes.Also, their formation by nonbiological processes should behighly improbable.

All biosignatures are characteristic of the modification ofa local or planetary environment by life. Human technologyand its resultant products and outputs can therefore also beconsidered as a biosignature, with the added benefit that itmight be detected remotely. Thus, although technology isprobably much less common than life in the universe, its as-sociated biosignatures perhaps enjoy much higher “signal-to-noise” ratios. Accordingly, current methods should be fur-

ther developed and novel methods should be identified fordetecting electromagnetic radiation or other diagnostic arti-facts that indicate remote technological civilizations.

• Objective 7.1—Biosignatures to be sought in Solar System ma-terials. Learn how to recognize and interpret any biosig-natures either in ancient rocks on Earth or in the crustalmaterials and atmospheres of other Solar System bodiesin order to characterize any ancient and/or present-daylife.

Example Investigations

Determine additional organic biomarkers that will help tochart the presence and development of photosynthetic mi-crobiota in Precambrian rocks.

Determine the features of sedimentary laminated texturesthat uniquely require biological processes.

Identify examples of chemical, mineralogical and stable iso-topic biosignatures that can indicate the presence of sub-surface biota (e.g., microbes living in water-saturatedrocks) and that can be preserved in crustal materials.

Characterize any potential gaseous biosignatures in the martian atmosphere via orbiting spacecraft and/orground-based observatories, and investigate their poten-tial sources.

• Objective 7.2—Biosignatures to be sought in nearby planetarysystems. Learn how to identify and measure biosignaturesthat can reveal the existence of life or technology throughremote observations.

Example Investigations

Determine the nature and fate of reduced gases that are pro-duced by specific microbial ecosystems in an anoxic (“pre-oxygenated”) biosphere.

Carry out laboratory, observational and modeling studies toseparate false from true biosignatures (e.g., atmospheric O2

in a range of planetary environments [see Objective 1.2]).Develop novel approaches to detect evidence of distant tech-

nologies.

Address reprint requests to:David J. Des Marais

Mail Stop 239-4NASA Ames Research Center

Moffett Field, California 94035

E-mail: [email protected]

DES MARAIS ET AL.16

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The NASA Astrobiology Roadmap · · 2008-09-25The NASA Astrobiology Roadmap provides guidance for research and technology development across the ... and what is the future of life