A Report from the NASA Astrobiology Institute to the National Research Council Solar System Exploration Decadal Strategy Working Group
Prepared by the NASA Astrobiology Institute Executive Council:
Bruce Jakosky (1), David Des Marais (2), Baruch Blumberg (2), Steven D1Hondt (3), Jack Farmer (4), M. Reza Ghadiri (5), Rosalind Grymes (2), Andrew Knoll (6), David McKay (7), Victoria Meadows (8), Kenneth Nealson (8), Hiroshi Ohmoto (9), Bruce Runnegar (10), Mitchell Sogin (11), Sean Solomon (12), Michael Thomashaw (13), Peter Ward (14)
(1) University of Colorado; (2) NASA/Ames Research Center; (3) University of Rhode Island; (4) Arizona State University; (5) Scripps Research Institute; (6) Harvard University; (7) NASA/Johnson Space Center; (8) NASA/Jet Propulsion Laboratory; (9) Pennsylvania State University; (10) University of California at Los Angeles; (11) Marine Biological Laboratory; (12) Carnegie Institute of Washington; (13) Michigan State University; (14) University of Washington
I. Executive summary
The NASA Astrobiology Institute prepared this report in order to articulate the relationship between astrobiology and solar-system exploration, and the role of astrobiology in planetary science. This report provides input into the National Research Council's task of constructing of a decadal strategy for solar system exploration.
Astrobiology as related to solar-system exploration addresses far more than just the search for life in our solar system. It is about understanding the planets in our solar system as representing different outcomes in the formation of planets, the nature of processes that affected those outcomes, and how those same processes might have operated elsewhere. It is about understanding planetary habitability as well as the actual distribution of life (in our solar system and as applied to elsewhere in the galaxy). In this context, for example, finding no life on Mars or Europa is not a failure but is a scientific result that is just as important as finding life; it allows us to better understand the conditions required for a planet to support life and the relationship between biology and planetary processes.
With this broad perspective on the connections between astrobiology and planetary science, it is clear that many aspects of planetary science are closely linked to astrobiology. While we do note that some of these connections are stronger than others, all of them are substantive nonetheless. The overall strength of these connections is clear if one examines a list of spacecraft missions that are either currently operating or under development; most, if not all, are addressing questions and themes that are linked strongly to astrobiology.
Astrobiology is an integrating theme that brings together a substantial fraction of the issues in solar-system exploration under a common thread of understanding planetary habitability. This theme allows us to explain to the non-expert the connections between different component disciplines within planetary science, and to do so in a way that most people will appreciate as addressing core issues in human thought. Astrobiology and its connections to space science (and solar-system exploration in particular) are the primary means by which NASA tries to implement one of its prime objectives (going back to the highest-level defining documents of NASA and of the NRC) of understanding life's origins and its distribution in the universe.
Astrobiology is certainly one of the several highest-level themes that unites and integrates solar-system exploration and, as such, should be strongly integrated into the solar system strategy.
The NASA Astrobiology Institute (NAI) was asked by the National Research Council (NRC) to provide formal input on issues of relevance to astrobiology into the development of a decadal strategy for solar system exploration. The request came from the NRC Committee on the Origin and Evolution of Life (COEL), acting as a subcommittee of the Decadal Strategy Steering Group. Because the request came relatively late in the current phase of the NRC process, and given the desire to provide timely input that might impact the direction of the report, the NAI responded on an accelerated schedule to articulate the major issues in a brief report. The basic outline of the report was developed in consultation with one of the co-chairs of COEL in early August, the NAI Executive Council (EC) discussed the process and outline at its monthly videocon held on 24 August, the EC met by telecon on 31 August for a detailed discussion of the issues, a draft report was discussed at a previously scheduled meeting of the EC on 11 September, and the report was finalized in time for distribution to the NRC DSWG at its 19 September meeting.
Given the rapid timescale required for development of the report, no effort has been made yet either to be all-inclusive and encompassing or to solicit or include comments from the general astrobiology community. Such inputs indeed can be provided during the coming weeks and months.
Below, we discuss the relationship between astrobiology and the ongoing exploration of our solar system and the role of astrobiology in solar-system research. We include a specific discussion of the relevance of astrobiology to each of the fields represented by the discipline subcommittees within the NRC process, and we summarize the relevance of astrobiology to each of the spacecraft missions that currently are active or in development. We also address specific concerns that have been raised in the past to using astrobiology as a central theme in the solar-system-exploration program.
III. The relationship between astrobiology and solar system exploration
A. The nature of astrobiology
The intellectual goals of astrobiology are often described as being embraced by three questions: How does life begin and develop? Does life exist elsewhere in the universe? What is the future of life on Earth and beyond? In this discussion, we focus on those aspects of astrobiology that are most closely related to exploration of our solar system.
From the breadth of these questions, we see that the relationship between astrobiology and our solar system must encompass much more than just determining where in our solar system life exists today or has existed in the past. It is about understanding what conditions are required for a planet to be able to sustain life, and how the processes involved in the formation of our solar system and in the subsequent geological history of the planets led to such habitable conditions; and it is about using the multiple planets in our solar system to understand both what determines habitability and what determines when a planet is not habitable. It is about determining from in-depth analysis of our own solar system what processes affect the architecture of planetary systems in general, and whether the characteristics of our system that allow life to exist here are likely to be common or rare in the galaxy. It is about understanding the different planetary outcomes in our own solar system?the different climates on Earth, Venus, and Mars, for example?and how the large range in outcomes affects the distribution of life. It is about understanding why life is absent from some planets and satellites as well as why life may be or is present on others.
In other words, astrobiology is about understanding solar system formation and evolution, what makes a planet habitable and what makes it uninhabitable, and the coupled evolution of life and planets. In this context, we easily see the relevance to astrobiology of studies of the history and present-day state of our solar system and of the individual objects within it. These connections are elucidated in more detail below.
The terrestrial planets, as an ensemble, tell us about the ranges in planetary characteristics that can result as different outcomes from the processes of planet formation in an inner solar system. The planets embody a remarkable diversity in size, composition, internal structure, role and nature of tectonic processes, abundance and distribution of volatiles, and climate history, for example, with only one of them known today to support life. What processes led to such fundamentally different planets? How did the initial boundary conditions (such as size, distance from the Sun, etc.) contribute to such different end results? Which properties are connected in a fundamental way to the presence of habitable conditions and life, and which are coincidental?
Even when examining a single terrestrial planet, such as Mars, it is clear that all of the disciplines within planetary science contribute to understanding the potential for life and determining whether life is present or not. This is especially the case in that we want to understand not just whether Mars has life, but, if it does, which properties allowed life to exist, and, if it does not, why not. We need to understand the history of water and climate, the role of geochemical and geological processes, the role of Sun-planet interactions and the nature of the magnetic field in affecting the history of CO2, the geological and geophysical history and the nature of martian tectonics, and so on. The history of the detailed analysis of ALH84001, with the profound ambiguities associated with trying to infer the actions of geological versus biological processes, demonstrates that an understanding of biology also requires detailed contributions from several key disciplines.
The outer solar system has had a dramatic effect on the potential for life in our solar system. The gas giant planets must have had a large impact, so to speak, on the nature of the inner solar system, through their role in scattering planetesimals, comets, and Kuiper-belt objects into the inner solar system and thereby affecting the habitability of the terrestrial planets. Thus, understanding the formation and history of the outer planets is central to understanding the potential for life. For example, almost none of the other ~75 known planetary systems have a structure similar to ours, with gas giants in the outer regions of their systems; although this may be an observational selection effect, this cannot be demonstrated with certainty today. What processes led to the Jovian planets in our solar system being in their present location and having their present masses and compositions? Given how these processes might have combined in different ways elsewhere and resulted in other outcomes, is the architecture of our solar system likely to be representative of planetary systems in general?
How did the formation and history of the Jovian planets affect the number, composition, size, and distribution of planetary satellites in the outer solar system? Are conditions on Europa conducive to an origin of life and its possible continued existence? Do Ganymede and Callisto have sub-ice liquid-water oceans, and are they conducive to the presence of life? More broadly, what processes determined the different outcomes in the history of outer-solar-system satellites, with various satellites showing evidence for a sub-ice liquid-water ocean, a thick atmosphere, the presence of organic compounds, possible organic-rich lakes or "groundwater", active sulfur-rich volcanism, and so on? Are the processes that led to this tremendous diversity of objects likely to lead to as much diversity in planetary systems elsewhere, and are conditions conducive to the presence of life likely to exist elsewhere?
The Kuiper belt and Oort cloud are important for understanding the planetary outcomes in the rest of the solar system. The distribution of objects in the Kuiper belt provides a "witness plate" to understanding the dynamical history of the outer planets and their original locations following formation. The composition of both Kuiper-belt Objects (KBOs) and Oort-cloud comets tell us about the composition of the protoplanetary disk out of which the planets formed. They also likely played a role in supplying water and other volatiles to the inner solar system, and thus would have been instrumental in the origin and continued existence of life.
Looking now at the integrative perspective, in order to chart the potential distribution of habitable conditions and life in our solar system and beyond, we must characterize those processes that together have shaped planetary environments. To the extent that solar system objects differ in bulk composition, size, mass, solar influx, and orbital dynamics, each object contributes to our overall understanding about the origin and evolution of habitable conditions and life. Planetary processes such as tectonics and the geochemical cycling of biologically relevant elements are not only essential for sustaining habitable conditions, they also are responsible for having extensively modified planetary crusts and atmospheres. Astrobiology represents an inclusive approach to planetary science, in that it requires that interactions between these key factors be evaluated, in this case for their potential to support life.
Water is a prime example of an agent whose history in our solar system is centrally important to planetary science in general, and to astrobiology in particular. Water is a key agent in the transformation of primitive materials into the various products of planetary processing that, on Earth, for example, we recognize as igneous, sedimentary and metamorphic rocks. Water-related processes have amplified and stabilized the dichotomy between oceanic (basaltic) and continental (granitic) crust. Comparative studies of comets, asteroids and larger, more-evolved bodies are necessary in order to document the full diversity of such transformations.
The processing of crustal materials affects other biologically important elements and thus forms the basis for shaping planetary atmospheres and bodies of water and, therefore, the habitability of planetary environments. Liquid water is considered essential for life, and a search for life elsewhere often starts with a search for liquid water. Indeed, a program that charts the origins and evolution of planetary crusts, atmospheres, and environments is almost identical to one that effectively and substantively addresses habitable conditions and life.
Mars offers a specific example of the importance of water. The number and diversity of features that appear to require aqueous processes in their formation or evolution is continually increasing, and it is clear that water has played a major role in martian history. Water might even have contributed to the formation of the dichotomy between the martian highlands and lowlands. Still, we remain remarkably unfamiliar with the chemical and mineralogical composition of the martian crust, even though such properties record the entirety of crustal history and not just those processes that created the morphology still visible today. For example, the recently discovered deposits of crystalline hematite seem out of context within the modern environment, yet they might indicate processes that extensively modified the crust long ago. Perhaps the hematite deposits are a beacon that draws us to sites that will clarify the roles of water and the history of the martian crust. As significant as aqueous activity might have been for climate and life, the chemical and mineralogical record of such activity might be rare or at least heterogeneously distributed among potential landing sites. Accordingly, we need to understand the full three-dimensional distribution of surface properties and composition in order to decipher the evolution of the crust. While these issues are widely recognized within the planetary community today, they arose most emphatically out of a need to understand the processes relevant to the potential for life.
More broadly, what role does the presence or absence of water play in the very different tectonic styles of Venus, Earth, and Mars? How did water affect the chemical history of surface materials via chemical weathering processes? What is the nature of water- and ice-driven tectonic processes in outer-solar-system satellites? How would these same processes have acted on objects such as Titan or Pluto/Charon, which formed and evolved under very different conditions? What processes determined the composition of KBOs and comets, given the apparent very different D/H ratio in comets from that of the Earth, the asteroids, or Jupiter? What role did these objects play in supplying volatiles to the inner planets or to the outer-planet satellites?
The imperative to understand water in our solar system demonstrates that astrobiology and planetary science share many of the goals in the exploration of our solar system, as well as the tools needed to achieve those goals. It is neither prudent nor, in fact, possible to separate the scientific goals and objectives of solar-system exploration from those of astrobiology. While some differences might exist in the priorities for how these goals might be achieved, which missions to fly, or what measurements to make for each object, they reflect only slightly different approaches to examining the same concepts, objects, and histories. To focus on astrobiology as a cross-cutting theme is not to exclude components of our solar system. Rather, it allows us to see the connections between objects and the processes that formed them, and between present-day states and their ancestry.
B. Other issues in solar-system exploration
Some aspects of solar-system exploration are so intimately connected to issues in astrobiology as to be inextricably intertwined. These issues, related to site selection for upcoming Mars (or Europa) missions, life detection in returned samples, and planetary protection, serve to point out some of the innate connections.
Astrobiology has played an important role in site selection for the upcoming Mars rovers and sample-return missions. The common goals for astrobiology and Mars exploration of "follow the water" and the search for potential habitats have dominated the site-selection process. Together, they demonstrate that astrobiology and detailed planetary exploration share common measurement objectives as well as common science goals.
The question of life detection is one not just of determining whether returned samples are safe to distribute to other scientists. The recent experience with ALH84001 demonstrates that biological and geochemical processes each could leave behind measurable characteristics that are indistinguishable from the other. This means that, in order to understand the geochemical implications of a sample, for example, one must understand the biological implications at the same time. This also means that, at present, we do not know how to "do" life detection, and that any attempts to do so will have uncertain results.
This last issue is important for planetary protection issues with regard to sample return. If we cannot verify that a sample is not a biohazard, for example by demonstrating that there are no organisms present, then we cannot confirm that it is safe to distribute.
C. Astrobiology as a central theme of solar-system exploration
Astrobiology has three characteristics that would qualify it as one of the appropriate overarching themes for the Solar System Exploration program.
First, as discussed above, an astrobiology program, when viewed from the proper broad perspective, takes much of our understanding of solar system formation, evolution, and present-day processes and integrates it together under a common umbrella or theme. Without such an integrating theme, the various subdisciplines within planetary science often are seen as distinct and unconnected.
Second, astrobiology allows the major goals of planetary exploration to be articulated to the non-expert in a way that is readily understood and appreciated as an integrated theme. This is particularly valuable in getting the public and Congress to support the solar-system exploration program.
Third, astrobiology achieves these first two goals while at the same time being recognized as addressing and answering questions that have a deep-seated and intrinsic meaning and value to the public?at is the nature of life, is life likely to be widespread, and, in essence, what does it mean to be alive and to be human?
While essentially all aspects of solar-system exploration are indeed relevant to astrobiological issues, they all are not equal in this quality. Some issues touch much more closely on astrobiology than do others.
For example, understanding the geochemical evolution of the martian surface and subsurface is central to correctly interpreting measurements that might tell us about the presence of life at some time in martian history (as we learned from the ALH84001 experience). Or, learning about the dynamical history of the Galilean satellites would be important for understanding the nature and history of possible sub-ice oceans on each of the objects at different times in their history. As such, these issues touch very closely on astrobiological themes.
On the other hand, exploration of Pluto or Mercury, while still having astrobiology relevance, is less strongly connected. These objects certainly would tell us about the outcomes in planetary architecture that are possible and the composition of different regions in the protoplanetary disk. As the smallest and innermost of the terrestrial planets, though, Mercury may not have as much to tell us about planetary habitability or about conditions that favored formation of planets farther out in the inner solar system where the Earth resides. Pluto may be the largest of the KBOs, and as such is of fundamental importance for understanding our solar system; but, it might not tell us the composition of average or representative materials or of primitive materials in the Kuiper belt, and it may not tell us much about the dynamical history of the disk itself. Despite this, these objects are of clear relevance to understanding our solar system. Dynamics of the Jupiter atmosphere, as a process occurring late in the history of the solar system, may not tell us much about the dynamical importance of Jupiter in the earliest history of the solar system. Although a connection to astrobiology is present, it is weaker than in other examples.
Astrobiology as a theme is an excellent umbrella that integrates together a wider subset of solar-system issues and questions than perhaps any other single theme. It has to be considered as one of the highest-level themes that are central to the exploration of our solar system.
IV. Relevance of astrobiology to each of the discipline subcommittees
Although the connections between astrobiology and planetary science are described above, we now go through the individual disciplines, as represented by the subcommittees of the NRC Decadal Strategy Working Group, and discuss the relevance of astrobiology to each and of each to astrobiology. We've chosen to present this in bullet format in order to avoid repeating too much of the above discussion. (Note that we treat Mars separately from the inner planets as a whole because the NRC discipline subcommittees are formed this way and because of the emphasis on Mars in the current program.)
A. Inner planets
B. Giant planets
C. Large satellites
D. Primitive bodies
V. Relevance of astrobiology to flight missions currently operating or under development
A substantial number of the ongoing flight missions currently operating or under development have strong astrobiology themes. Thus, astrobiology already is a central component of the solar-system exploration program. These missions are listed below, along with a brief enumeration of their astrobiological connections. The list of missions was obtained directly from the NASA/OSS web page, is limited to those operated out of the Solar System Exploration theme within OSS, and includes foreign missions on which there is NASA participation.
A. Operating missions
Cassini: Explore composition and organic and isotope geochemistry of Titan's atmosphere and surface; infer history of Saturn.
Deep Space 1: Define composition, nature, and volatile-related processes of comets.
Galileo: Understand composition and infer history of Jupiter; explore nature of Galilean satellites and determine their radiation environment and potential for sub-ice oceans.
Genesis: Determine elemental and isotopic composition of Sun from the solar wind as a constraint on composition of the protoplanetary disk out of which the planets formed.
Mars Global Surveyor: Determine nature and distribution of water- and possibly other volatile-related processes via geomorphology, geophysics, surface mineralogy, and atmospheric behavior.
Mars Odyssey: Map mineral composition at high spatial resolution, in order to identify sites of potential relevance to biology and volatile history and to understand potential landing sites for future missions; map surface thermophysical properties for landing site certification; map global elemental abundances to determine effects of crustal processing, including effects of water.
Nozomi: Understand nature of solar-wind interactions with Mars, as relevant to determining atmospheric history.
Stardust: Determine composition of grains that come from comets, as constraints on history of early solar system.
B. Missions in development
Contour: Understand the physical properties, composition, and evolutionary processes on comets.
Deep Impact: Understand composition of pristine material in the interior of a comet, as clues to understanding the early history of volatiles in our solar system.
Mars Exploration Rovers: Explore surface sites of aqueous or biological relevance on Mars. Understand full range of effects of volatiles upon chemical and mineralogical composition of crustal materials.
Mars Express: Map subsurface structure and search for subsurface liquid water and aqueous environments (using radar system)
MESSENGER: Explore Mercury and understand the processes that resulted in its planetary outcome being different from that of the other terrestrial planets
[Note that missions such as Europa orbiter, Mars '05 orbiter, Mars '07 lander, and Pluto/Kuiper are listed on the NASA/OSS web site as "Under study", so we do not discuss them despite their obvious astrobiological connections. Similarly, SOFIA and SIRTF have clear connections to astrobiology, but are operated out of the Astronomy and Physics theme.]
VI. Examples of astrobiological investigations that would contribute to solar system exploration
Finally, we list some missions, instruments, and observations that are of astrobiological relevance that might not have as high a priority within the general solar-system exploration program. Our purposes are to demonstrate that astrobiology as a central theme has priorities that might differ from those connected solely to planetary exploration without astrobiology and to demonstrate that astrobiology alone has objectives that are of high scientific value. We have made no effort to be thorough or exhaustive, and these are for example only. We also recognize that some of these do overlap with objectives within solar system exploration in the absence of astrobiology, but include them here as they are likely to be of higher priority within astrobiology.
(i) Determination of the composition, abundance, and distribution of organic materials in the solar system.
(ii) Exploration of the radiation environment at the surface and near-surface regions of Europa and the other Galilean satellites.
(iii) Detailed three-dimensional determination of the elemental, chemical, isotopic, and mineralogical composition of the surfaces of planets and satellites (including Mars, outer solar system satellites, and icy objects).
(iv) Exploring Titan to understand the nature of organic processes and chemisy on primitive bodies and the relationship between internal composition and structure and atmospheric composition (i.e., methane history).
(v) Further exploration of Mars, including detailed radar sounding to look for subsurface liquid water and possible ground-ice inventories, full determination of surface mineralogy, and spatial and temporal juxtaposition of liquid water and sources of energy that could support life.
VII. Discussion of commonly raised concerns to astrobiology as one of the central themes in solar-system exploration
Several concerns have been raised regarding using astrobiology as one of the central themes in solar-system exploration. These appear to be based on misunderstandings or misperceptions of what astrobiology is, and we wish to mention and discuss them explicitly.
There is the perception that the search for life elsewhere in our solar system is the primary goal in astrobiology but should not also be a central goal in planetary exploration. Should we explore Mars or Europa and not find life, then an exploration program that is based on such a search might cease to be of interest to the public and would consequently be scaled back dramatically. The history of Mars exploration is used as an example, with the lack of a discovery of life by Viking in the mid-1970s seeming to bring Mars exploration to a halt for two decades. There are several issues here that are fundamental to understanding what astrobiology is: First, as described above, astrobiology is about much more than just the search for life. While the explicit search for life is an important part of astrobiology, it is not necessarily the central component and certainly is not the sole component. Second, a finding of no life on Mars or Europa is just as important scientifically as a finding of life, both in understanding the distribution of life and in understanding what makes a planet habitable. We clearly need to be vigilant, however, that the Mars program is sold not as "finding life on Mars" but, instead, as "finding out if habitable conditions existed and whether there is life on Mars" and that the implications of either result are profound. Third, the reality is that the tremendous recent increase in Mars exploration not only coincides with the reappraisal of Mars as a potential abode for life but, in fact, is a direct result of that interest. Importantly, the current incarnation of the Mars program emphasizes the search for life as a central theme, but it emphasizes it as just one of several central themes. The implementation of the program addresses key issues in Mars' geological and geophysical history, climate, and atmosphere, as well as life.
Some object to astrobiology as a central theme in planetary exploration because they see astrobiology as a very narrow discipline that focuses primarily on issues that connect directly to biology: Much of planetary science does not have a direct connection to biology, such that astrobiology is not the truly integrating theme that it is presented as being. Again, as discussed above, many questions that are central to astrobiology do not relate directly to the interactions between biota and their environment. Astrobiology requires an understanding of planetary environments in general and how they formed and changed, both for environments in which life could possibly reside and for environments in which it does not or cannot reside. It requires an understanding of the relationship between stars and planets, between planets and satellites, and between planetary objects and environments, as well as between environments and life.
Finally, some object that that there is little scientific justification for thinking that life could be present elsewhere, and that the astrobiology community's use of this argument to justify an expanded exploration program is, at best, exaggerated and misleading to the public. In fact, astrobiology is an intellectually stimulating endeavor of enormous scientific value with direct relevance to how we address questions in solar system exploration. The intellectual arguments for the potential for life on other planets have emerged from an increased understanding of both terrestrial biology and planetary environments. The very early origin of life on Earth, the ability of life to thrive in what we consider to be extreme environments, the possibility that life originated in one of these extreme environments, and the fact that these environments are likely to exist on other planets, all point to the potential that life could have had an independent origin elsewhere or could have been transferred elsewhere from the Earth. The discipline of astrobiology is about exploring these logical inferences and trying to find out whether we have a full picture of the connections between stars, planets and biology.