Findings and Recommendations
As outlined in the Executive Summary, the committee was charged by NASA to develop an astrobiology strategy for the exploration of Mars which addresses the following topics:
The characteristics of potential targets for Mars exploration particularly suited for elucidating the prebiotic and possibly biotic history of Mars, and methods for identifying these targets;
A catalog of biosignatures that reflect fundamental and universal characteristics of life (i.e., not limited to an Earth-centric perspective);
Research activities that would improve exploration methodology and instrumentation capabilities to enhance the chances of astrobiological discovery; and
Approaches to the exploration of Mars that would maximize the astrobiological science return.
Before describing the committee’s findings and recommendations relating to these four specific areas, it is appropriate to make some general comments about the search for life on Mars
THE SEARCH FOR LIFE ON MARS
Researchers know that life took hold on Earth but do not know if life ever existed anywhere else in the solar system. Mars is the most Earth-like of the other planets and satellites in the solar system, in terms of its geological environment and the availability of liquid water at or near the surface throughout time. It is the most logical place to look for other life and, among places in the solar system that might have life, is the most accessible. The finding of evidence for past or present life would have profound philosophical and scientific repercussions, and a finding either that life was present or that it was not would have dramatic implications for the prospects for life elsewhere in the universe. Because of its compelling importance the search for life should be the main focus of the Mars Exploration Program.
The committee recognizes that life may never have started on Mars or gained a foothold there. However, the search will lead to a broad understanding of the planet as a whole. It requires investigation of the geological and geophysical evolution of Mars, the history of volatiles and climate, the nature of the surface and the subsurface, the history and geographical distribution of liquid water, and the availability of other resources that are necessary to support life, as well as of the processes that controlled each of these. The search also requires a detailed understanding of the nature of terran life and how it functions in different environments.
The Priority of the Search for Life on Mars
Although it is not the only possible emphasis for the Mars program, astrobiology provides a scientifically engaging and broad approach that brings together multiple disciplines to address an important set of scientific questions that also are of tremendous interest to the public. And, more than any other scientific focus, it integrates the disciplines together into a coherent approach to understanding Mars as a planet.
Finding. The search for evidence of past or present life, as well as determination of the planetary context that creates habitable environments, is a compelling primary focus for NASA’s Mars Exploration Program.
A Broad Approach to the Search for Life on Mars
Addressing astrobiology science goals requires a broad approach to understanding Mars. It involves not just searching for present and past life, but also understanding the geological and environmental context that determines planetary habitability. It will entail determination of the geological and geochemical evolution of the planet, its internal structure, and the nature of its interaction with the space environment. Such an approach will provide the information necessary to be able to apply the results to assessing the potential for life throughout the galaxy. In addition, because of the interconnected nature of the martian geological, geochemical, geophysical, and climatological systems, a broad approach will likely enable astrobiologists to determine which characteristics result from nonbiological processes and which result from biological processes and so could be used as biosignatures. While most aspects of Mars exploration have connections to astrobiology, the emphasis should be on those areas that are most directly related to habitabilitythe potential for life and the presence of its building blocks, and the possible occurrence of life.
NASA’s 1995 report An Exobiological Strategy for Mars Exploration took the approach of starting from the global perspective and focusing increasingly on the local perspective. This approach involved a series of steps:
Global reconnaissance that focused on the history of water and the identification of sites for detailed in situ analysis;
In situ analysis at sites that hold promise for understanding the history of water;
Deployment of experiments that address astrobiology science questions, including the nature of martian organic molecules and the presence of features indicative of present-day or prior life;
Return of martian samples to Earth for detailed study; and
Human missions that would provide the detailed geological context for astrobiology measurements and the detection of modern-day “oases” for life.
This reasoned and measured approach provides the best opportunity for determining the geological and geochemical context in which the most useful and appropriate astrobiological measurements can be determined, implemented, and then properly interpreted and understood. It combines a broad, interdisciplinary approach to understanding Mars as a whole with the detailed, focused investigations that allow researchers to understand the astrobiology of Mars.
Finding. The search for life and understanding the broad planetary context for martian habitability will require a broad, multidisciplinary approach to Mars exploration.
Finding. At the same time, the astrobiological science goals can best be addressed by an implementa-
tion that allows researchers to address increasingly focused questions that relate to astrobiology goals in particular.
Applying a broad approach to determining whether life ever existed on Mars will require focusing on the elements most relevant to life, especially carbon. It will be necessary to determine whether organic molecules are present on Mars and where, and which chemical characteristics will allow researchers to distinguish between meteoritic (nonbiological), prebiotic, and biological molecules. In addition, there is still much to be learned about the history and availability of water, and “following the water” is a strategy that researchers will also have to continue to apply.
Finding. The very successful intellectual approach of “follow the water” should be expanded to include “follow the carbon,” along with other key biologically relevant elements.
Planetary Protection Considerations
As researchers move toward increasingly detailed examination of the forms in which carbon exists within surface materials, characterization of the nature of possible organic molecules within soil or rock samples, and measurements to search for and identify possible biosignatures, it is clear that contamination by terrestrial materials is a significant concern. Obtaining the desired science results demands that issues related to terrestrial contamination be addressed by appropriate planetary protection approaches. It would be a mistake to have to avoid the sites identified as most promising for scientific discovery because of concerns over planetary protection. NASA must ensure that adequate measures are built into missions from the beginning, incorporating lessons learned from past analyses.
Finding. The desire to visit and sample the highest-priority astrobiological sites requires that future surface missions to Mars take the necessary and appropriate planetary protection measures.
The Importance of Sample Analysis
Useful science analysis of martian surface samples can be carried out either in situ on the martian surface or in terrestrial laboratories with samples returned from Mars to Earth. In situ missions have the advantage of being able to address questions appropriate to specific regions or to obtain measurements of characteristics that might be unstable (e.g., trace chemical species that might represent chemical disequilibrium). In addition, analysis in situ can be done more cheaply than sample-return missions. However, sample return offers the advantages of being able to carry out many more analyses on a sample than can be done in situ, of following up exciting measurements with additional measurements that had not previously been anticipated, and of being able to make measurements or observations using instruments that are not amenable to being accommodated on a lander or rover mission or that were not available at the time of mission development. Indeed, numerous previous studies have consistently pointed out the important contributions that sample-return missions from planetary bodies can make to virtually every area of solar system exploration in general, and to Mars exploration in particular.1–5 A Mars sample-return mission has thus been an essential component of the Mars exploration strategies advocated by the National Research Council for 30 years.6–11 Even from the narrower perspective of the search for life on Mars, “evidence” for martian life observed only in situ would create controversy, not conviction. That is, the discovery of past or present life on Mars would be of such importance that it is highly unlikely that the scientific community would be convinced by anything less than the power of laboratory analysis. Therefore, the anticipated astrobiology science results that would be obtained from a sample-return mission are much greater than those that would come from an in situ mission.
Finding. The greatest advance in understanding Mars, from both an astrobiology and a more general scientific perspective, will come about from laboratory studies conducted on samples of Mars returned to Earth.
The astrobiology science goals that have been put forward can be addressed appropriately via a series of robotic
spacecraft missions that could be carried out over the next one to two decades. It is critical that any astrobiological evidence that might be present on Mars not be compromised by robotic or human activities before definitive measurements or sample return occur.
Finding. The scientific study of Mars, including return to Earth of astrobiologically valuable samples that can be used to address the questions being asked today, can be done with robotic missions.
CHARACTERISTICS OF POTENTIAL TARGET SITES
What are the characteristics of potential targets for Mars exploration that are particularly suited for elucidating the prebiotic and possibly biotic (and postbiotic) history of Mars, and what methods can be used to identify these targets? Current understanding of the environmental requirements for supporting an origin or the continued evolution of life and the nature of the interplay between organisms and their geological and planetary environment is based on our terrestrial example. This understanding informs researchers’ views of the types of martian environments that might be fruitful for detailed investigation and the types of individual materials at those sites that might contain pertinent evidence.
Life has the potential to exist in a broad range of environments. Although any place on the surface might provide pertinent information, the most likely places should be explored first. These include places where liquid water might be present today or might have been present for extended periods at some time in the past.
Such sites are pertinent both for searching for evidence of present or past life and for understanding the nature of martian habitability. For present-day or geologically recent water, these sites include the surface, interior, and margins of the polar caps; cold, warm, or hot springs or underground hydrothermal systems; and the source region or outflow region associated with near-surface aquifers that might be responsible for the “gullies” that have been observed. For geologically ancient water, pertinent sites include the source or outflow regions for the catastrophic flood channels, the ancient highlands that formed at a time when surface water might have been widespread (e.g., in the Noachian), and deposits of minerals that are associated with surface or subsurface water or with ancient hydrothermal systems or cold, warm, or hot springs.
Recommendation. Sites that NASA should target with highest priority to advance astrobiology science objectives are those places where liquid water might exist today or might have existed in the past and where organic carbon might be present or might have been preserved.
Although the committee anticipates that new measurements are likely to include major discoveries relevant to astrobiology that will point researchers to specific sites, a foundation of data is already available to identify exciting and appropriate sites for either in situ analysis or sample return.
There is no single site that will provide all of the answers pertinent to the astrobiology science goals for Mars. Further, the Mars Reconnaissance Orbiter (MRO) spacecraft is just beginning its mission at this writing, and it will be several years before those results are fully integrated into the (anticipated) revision of our understanding of Mars. Thus, it is premature either to identify a specific landing site for future detailed investigation (either in situ on the surface or as a source of samples for return to Earth) or to limit the range of places that can be visited by spacecraft.
An argument has been made that if only a single sample-return mission is programmatically feasible in the foreseeable future, then it must return the “right” sample from the “right” site, the “right” sample being one that has the best chance for uniquely providing astrobiologically significant information (such as information bearing on the detection of life). The committee disagrees with this viewpoint and argues that there is no such thing as the “right” sample. Any well-selected samples—i.e., samples selected carefully from a thoughtfully chosen site—would provide information that would be incredibly valuable for addressing the scientific goals for the exploration of Mars, in general, and astrobiological goals, in particular. Furthermore, the information necessary to choose an appropriate sampling site is available within existing data sets or data sets whose acquisition is imminent (e.g., those from MRO). Although they cannot be known to identify specific sites as having life, the relevant data do
identify sites that have had liquid water or chemical alteration typically associated with liquid water and that have morphologies indicative of the long-term presence of liquid water.
Finding. Identification of appropriate landing sites for detailed analysis (whether in situ or by sample return) can be done with the data sets now available or imminently available from currently active missions.
The potential target sites listed above that would important for in situ investigation pertinent to ancient or recent life are at high elevations or at polar latitudes. These include most of the ancient Noachian terrain that would tell researchers about the potential earliest life and polar regions where melting of ice (e.g., at relatively recent epochs of high obliquity) could provide liquid water to sustain life. Accessing these sites will require an increased capability to land at a wider range of latitudes and elevations than are accessible by the Mars Science Laboratory (MSL), for example. Among the requirements are advances in landing site selection and in entry, descent, and landing technologies, and the provision of more capable power systems to ensure spacecraft survival during extended missions in the polar regions. In addition, given the importance of mobility as demonstrated by the Mars Exploration Rover (MER) mission, future rovers should have adequate capability to visit a wide range of geographical and geological terrains on a single mission.
Recommendation. Future surface missions must have the capability to visit most of the martian surface, including Noachian terrains and polar and high-latitude areas, and to access the subsurface.
Exposure to strongly oxidizing environments or to high fluxes of radiation is not conducive to the preservation of biologically diagnostic carbon compounds; this knowledge should be factored into decisions about where to collect samples. Terrestrial-based knowledge suggests that fine-grained sedimentary rocks, evaporites, and hydrothermal deposits are examples of rock types that can preserve biosignatures. Inadvertent processing of samples by heating or shock during sampling, or processing prior to in situ analysis or return to Earth, should be avoided.
Recommendation. Selection of samples for analysis (either in situ or of samples returned from Mars to Earth) should emphasize those having the best chance of retaining biosignatures.
What biosignatures reflect fundamental and universal characteristics of life? Unfortunately, there is no single comprehensive or unique biosignature whose presence would indicate life and whose absence would uniquely indicate the absence of life. Experience from examination of the ALH 84001 meteorite and analysis of evidence relating to claims of the earliest life on Earth have demonstrated that the potential interplay between putative organisms and their geological environment is so complex that researchers may never be able to identify a unique biosignature that would work in all environments and at all times. Rather, the sum total of all measurements on a sample, in the context of understanding of the origin and evolution of the martian environment, will be required. What has been learned from the study of Earth’s earliest life and of the interplay between organisms and their planet, as well as from modern biology, provides the most appropriate guide to selecting targets on Mars and searching for biosignatures.
Of all the various life-detection techniques available, analysis of carbon chemistry is the first among equals. Organic analysis is likely to provide a more robust way to detect life than imaging technologies, mineral assemblages, isotopic measurements, or any one other single technique. This is the case because, on Earth, the patterns of biogenic carbon compounds reflect organized polymerization of smaller subunits, or precursors, and comprise mixtures with a limited range of atomic spatial arrangements very different from those made by abiological processes. However, organic analysis alone is insufficient to detect life. An ensemble of all of the relevant methodologies, combined with analysis of geological and environmental plausibility, will likely provide the best evidence for the presence or absence of life in a sample; there is no single, unique characteristic that would allow researchers to identify a region that might now have, or might once have had, life, or to determine whether life is, indeed, indicated.
Recommendation. The lack of a comprehensive understanding of all of the potential biosignatures for Mars exploration means that NASA should employ a combination of techniques that utilize both Earth-centric and non-Earth-centric approaches that focus on the basic concepts in carbon chemistry, imaging, mineral assemblages, and isotopic measurements.
Specific aspects of carbon chemistry that should be investigated include the following:
The presence of polymers based on repeating universal subunits;
Patterns in the carbon isotopic compositions of organic molecules that reflect organized polymerization of smaller subunits or precursors;
Patterns in the carbon numbers of organic compounds; and
The presence of carbon compounds that have only a subset of the possible connectivities or atomic spatial arrangements (i.e., just a few structural isomers or stereoisomers and/or strong chiral preferences).
EXPLORATION METHODOLOGIES AND INSTRUMENTATION
What research activities would improve exploration methodology and instrumentation capabilities to enhance the chances of astrobiological discovery? The vitality of Mars astrobiology science goals and investigations has not diminished with the delays in a Mars sample-return mission or the initiation of other activities. The ongoing missions (e.g., Mars Odyssey, Mars Express, Mars Exploration Rovers, and Mars Reconnaissance Orbiter) and the missions in development (Phoenix and Mars Science Laboratory) all are, or will be, returning scientific data that directly address astrobiology goals in substantive ways. Missions that are being planned for the next decade also have strong astrobiology components, and the Mars program is intimately intertwined with astrobiology science objectives. Thus, if astrobiologists are to advance their science goals for the exploration of Mars, they must work with NASA to ensure that the upcoming missions proceed as scheduled and, then, take advantage of the scientific data these spacecraft will collect.
Ensuring the success of future missions will require attention to the following activities:
Research and analysis activities, and
Supporting activities such as studies of martian meteorites and Mars-analog environments on Earth.
Missions such as Mars Sample Return and the Astrobiology Field Laboratory will require significant technical advances if they are to be carried out successfully. Technology development must occur both in mission-related areas (e.g., entry, descent, and landing systems, including a precision landing capability; sample-return technology; in situ sample processing and handling; and planetary protection) and in astrobiological science instrumentation (so that the necessary next generation of instruments is ready to go). In particular, a means must be developed to take instruments from the low and middle technology readiness levelsa up to TRL 6 so that they are ready for flight when needed.
Recommendation. The Mars Exploration Program must make stronger investments in technology development than it does currently.
Research and Analysis Activities
It is through the basic research and analysis (R&A) programs that results are obtained from the data returned by missions and ideas are developed for future missions. In particular, R&A programs are the primary vehicle by which the Mars Exploration Program can maintain its vitality in response to new discoveries. R&A programs should include analysis of existing and about-to-be-obtained data from Mars missions, analysis of basic martian processes to help in developing ideas about Mars evolution and history, and comparative planetology that allows
a better understanding of environmental processes on each planet. In addition, R&A programs also must address the need for basic understanding of the interplay between organisms and their geological environments, the nature of biosignatures, and the astrobiology of Mars.
In addition to preparing for the next generation of missions and the science objectives to be addressed by them, ongoing analysis and investigation put the science community in a better position to understand the nature of the questions that are being asked or the results that are being obtained. Because the education and the training of scientists, engineers, and managers who will work on the next generation of missions are funded through the R&A and technology programs, the health of those programs is vital to the development of a future workforce.
Recommendation. Continued strong support of NASA’s basic research and analysis programs is an essential investment in the long-term health of the Mars Exploration Program.
Studies of Martian Meteorites and Mars-Analog Environments
Analysis of martian meteorites has been central to the development of the current understanding of Mars, its potential for life, and ideas about detection of present or fossil life. It continues to be essential to developing ideas and protocols relevant to analyzing samples that are pertinent to astrobiology and life-detection science goals. It is especially important to search for martian meteorites that formed during early periods of Mars’s history, as well as meteorites of sedimentary origin. This is an important area of collaboration between NASA and the National Science Foundation.
Recommendation. Collection and analysis of martian meteorites must continue, even though biases in the compositions and ages of these meteorites, their uncertain provenance, and the effects of impact-ejection and transfer to Earth mean that they cannot take the place of samples returned from Mars.
As with the analysis of martian meteorites, studies of Mars-analog sites are essential to mission development and execution and to the training of the scientists engaged in current and future missions. Such studies focus on the technological aspects of missions and not on the development of basic scientific concepts regarding how life behaves and leaves signatures. Studies of Mars-analog sites on Earth should continue to be a fundamental aspect of Mars astrobiological research because they provide critical understanding of Mars-like environments for the testing and development of instrumentation and sample-handling protocols; understanding that supports the development and validation of techniques for detecting and measuring biosignatures, including establishing baseline abiotic signatures; and discovery of novel organisms and metabolisms and the chemical/isotopic imprints of these metabolisms on Mars-like environments.
Recommendation. Terrestrial analog studies should include testing instrumentation, developing techniques for measuring biosignatures under Mars-like conditions, and conducting technological proof-of-concept studies.
MAXIMIZATION OF SCIENCE
What approaches to the exploration of Mars will maximize the astrobiological science return? The astrobiology science goals for Mars are extremely broad. They legitimately include determining the volatile inventory of the planet upon formation and the evolution of volatiles through time, the geological and geophysical evolution of the planet, the interplay between the geology and the atmosphere and the history of habitability, and, of course, the determination of whether there is life today or has been life at some time in the past. As researchers learn more about Mars, they are finding that there is an incredible diversity of local and regional environments—as exemplified by the landing site of the rover Opportunity—that have chemical properties and physical implications that are very different from what had been expected. There clearly is the potential for identifying an incredibly diverse range of properties as more is learned about more places in detail via remote sensing and in situ analysis. Thus, any astrobiological exploration of Mars has to take into account the incredible diversity of Mars, as well as the diversity of questions that bear on the issues.
Sample Return Is Essential
Although almost any measurements made at Mars provide information that is pertinent to astrobiology, the value of having an astrobiology science strategy is in being able to prioritize the possible measurements and the missions that can make them in order to provide useful scientific guidance.
The approach outlined in NASA’s 1995 report An Exobiological Strategy for Mars Exploration involved missions of increasing capability and focused on astrobiology-related issues. In the intervening decade, NASA has carried out a program of orbital reconnaissance and in situ analysis to understand the role of water. Similarly, missions are in development (e.g., Phoenix and Mars Science Laboratory) that will explore the chemistry and the biological potential of the surface in detail. With this information in hand, and following that approach, analysis of samples returned from Mars to Earth will yield the greatest increase in our understanding of Mars and thus support for addressing astrobiology science goals as well as science goals related to other aspects of Mars. A commitment to carrying out a Mars sample-return mission is necessary to ensure that such a mission does not continue to be pushed farther into the future.
Samples collected for return to Earth should include a well-chosen suite of materials collected from a diverse set of locations by a capable rover, and should include both weathered and unweathered materials with minimal thermal and shock histories. Given the MER experience and current understanding of the nature of materials on the martian surface, a “grab sample” obtained from a stationary lander is not likely to be sufficient to provide the necessary data.
Finding. Sample return should be seen as a program that NASA and the Mars science community have already embarked upon rather than as a single, highly complex, costly, and risky mission that is to occur at some future time.
As an example, the committee notes that technology development pertinent to sample return (e.g., sample handling and packaging, Mars ascent vehicle, precision landing, planetary protection, and so on) has been ongoing but needs to move forward in earnest immediately so that the necessary technology will be available in a timely manner. There is a significant heritage from ongoing Mars missions (such as MER and MSL) that will carry over to the implementation of Mars Sample Return; from sample-collection missions such as Stardust and Genesis; and from the heritage of sample planning and analysis conducted over the last two decades.
Programmatically, sample return should be phased over multiple launch opportunities. A first phase could involve caching samples on Mars; a second phase, putting samples into orbit; and a third phase, returning samples to Earth. This approach would also allow some independent science investigations at each phase that would continue to engage the science community and the public, and it would increase resilience of the program in the face of the failure or delay of individual spacecraft missions.
However, the program’s emphasis should be on sample return. Missions subsequent to MSL should emphasize science, technology, and programmatic issues that lead directly to sample return, and return of samples to Earth should be carried out at the earliest opportunity.
Recommendation. The highest-priority science objective for Mars exploration must be the analysis of a diverse suite of appropriate samples returned from carefully selected regions on Mars.
As part of an effective sample-return strategy, sample caching could be carried out by each surface mission, utilizing a minimalist approach so as not to make sample caching a cost- or technology-driver. That is, caching should not be made so complicated as to preclude actually carrying it out. Because researchers cannot predict with confidence which landing sites might provide astrobiologically interesting samples, samples should be cached from all visited sites. This strategy should allow collection of diverse samples and mitigate the costs of sample-return missions. It is, of course, dependent on the development of a precision landing capability.
Recommendation. If it is not feasible to proceed directly toward sample return, then a more gradual approach should be implemented that involves sample caching on all surface missions that follow the Mars Science Laboratory, in a way that would prepare for a relatively early return of samples to Earth.
Despite the compelling scientific arguments for the return of martian samples to Earth and irrespective of how such an endeavor is implemented, sample return will be a technically challenging, high-risk, high-cost endeavor. Although the committee was not composed appropriately to independently estimate the cost of such a mission, figures in the range of $3 billion to $5 billion are frequently quoted.12 Thus, the decision to implement such an undertaking would have an impact on NASA’s science program extending far beyond the Astrobiology Program or the Mars Exploration Program. As such, the decision to implement a Mars sample-return mission will hinge on factors such as the relative balance of NASA’s flight activities between the various scientific programs, the state of the agency’s budget, and opportunities presented by international cooperation—all factors whose examination is beyond the scope of this study. As such, it behooves the astrobiology community to plan for the possibility that a Mars sample-return mission is not an integral component of current mission plans.
If a commitment is not made for sample return, then high-quality and high-priority science still can be done at the surface of Mars, for example from an astrobiology field laboratory. Such an advanced rover mission should have significant analytical capabilities beyond those of MSL and should address science questions complementary to those of MSL. Alternatively, two Mid Rovers could investigate the geological and geochemical diversity of carefully selected sites on Mars. Either approach would provide high science value and would be compelling in itself.
Although a compelling Astrobiology Field Laboratory (AFL) mission could be defined today that would complement MSL, the launch of an AFL mission should be phased to take into account the results obtained from MSL. Such a mission would be most effective as part of an overall program that also addressed the broad range of issues related to astrobiology, including planetary habitability. However, it must be recognized that, although they would address some astrobiology science goals, such missions would have a much more limited ability than sample return to make fundamental discoveries and to respond to them. They would be incapable of addressing the most fundamental astrobiology and other science goals in the same substantive ways as sample return. An AFL mission would be complementary to a sample-return mission in that it would allow some extension of the detailed results from sample return to other locations via in situ analyses.
International collaboration in Mars missions has the potential to make expensive missions such as Mars Sample Return affordable. The benefit, however, has to be balanced against the political difficulties of working with multiple countries and multiple space agencies. The European Space Agency (ESA), for example, already values the role that astrobiology plays in Mars science. One area of relatively straightforward collaboration would involve encouraging ESA to include a sample-caching capability on its rovers currently under development that would be analogous or equivalent to that being encouraged by this committee for NASA rovers.
Recommendation. International collaboration in Mars missions should be pursued in order to make expensive missions affordable, especially in the areas of sample caching and sample return.
1. See, for example, M.J. Drake, W.V. Boynton, and D.P. Blanchard, “The Case for Planetary Sample Return Missions: 1. Origin of the Solar System,” Eos 68:105, 111-113, 1987.
2. See, for example, J.L. Gooding, M.H. Carr, and C.P. McKay, “The Case for Planetary Sample Return Missions: 2. History of Mars,” Eos 70:745, 754-755, 1989.
3. G. Ryder, P.D. Spudis, and G.J. Taylor, “The Case for Planetary Sample Return Missions: 3. The Origin and Evolution of the Moon and Its Environment,” Eos 70:1495, 1505-1509, 1989.
4. T.D. Swindle, J.S. Lewis, and L.A. McFadden, “The Case for Planetary Sample Return Missions: 4. Near-Earth Asteroids and the History of Planetary Formation,” Eos 72:473, 479-480, 1991.
5. National Research Council, Assessment of Mars Science and Mission Priorities, The National Academies Press, Washington, D.C., 2001, pp. 83-88.
6. National Research Council, Strategy for the Exploration of the Inner Planets: 1977-1987, National Academy of Sciences, Washington, D.C., 1978.
7. National Research Council, Update to Strategy for Exploration of the Inner Planets, National Academy Press, Washington, D.C., 1990.
8. National Research Council, International Cooperation for Mars Exploration and Sample Return, National Academy Press, Washington, D.C., 1990, pp. 1, 3, and 25.
9. National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994.
10. National Research Council, Review of NASA’s Planned Mars Program, National Academy Press, Washington, D.C., 1996, pp. 3, 26, and 29.
11. National Research Council, Assessment of Mars Science and Mission Priorities, The National Academies Press, Washington, D.C., 2001, pp. 3, 83-88, and 99-102.
12. In March, 2007, J. Douglas McCuistion, the director of NASA’s Mars Exploration Program, informed the NRC’s Committee to Assess Solar System Exploration that cost estimates for a Mars sample-return mission run the gamut from $3 billion to $10 billion, with $5 billion being most likely. Also see, National Research Council, Assessment of NASA’s Mars Architecture 2007-2016, The National Academies Press, Washington, D.C., 2006, p. 19, note 26.