4
Stratigraphy and Chronology

PRESENT STATE OF KNOWLEDGE

Relevance of Stratigraphy and Chronology to Understanding Mars

The long and complex geological history of Mars, notably the history of its water, can be unraveled by understanding the relative and absolute ages of the planet’s geological units, which have been produced or deposited by the various geological processes that have operated throughout the planet’s history. Moreover, understanding the geology of a landing site, and therefore, that of samples examined in situ or returned to Earth, requires placing such samples in their appropriate time-stratigraphic geological contexts. Thus, understanding the stratigraphy of Mars is of high priority, as is the ability to date surface units. The absolute ages of surface units will remain necessarily uncertain until samples from known surface locations have been dated in situ and/or on Earth.

Geologic Units and the Stratigraphic Column

The Mariner 9, Viking orbiter, and Mars Global Surveyor spacecraft have provided a wealth of data from which the types and abundances of Mars’s geological units can be surveyed, their relative stratigraphic ages derived, and their absolute ages estimated. The planet’s various geological units are distinguished and characterized on the basis of their morphologic, topographic, and spectral properties. Their relative ages are determined through examination of their crosscutting and superpositional relationships and the number of superposed impact craters.1,2 Analyses of this sort have allowed confident derivation of a stratigraphic column for Mars and corresponding chronological ordering of the major geological events in the planet’s history (see Figure 4.1).

The major geological units of Mars, their stratigraphic positions, and their locations and extents are reviewed by Tanaka and colleagues.3 The major divisions thus recognized are the following:

  1. Highland rocks, notably those of the Hellas, Argyre, and Isidis impact basins and the southern high plains;

  2. Lowland rocks, notably the northern plains and degraded rocks along the highland/lowland boundary;

  3. Volcanic and tectonic regions, including highland volcanic paterae, the Tharsis and Elysium volcanic regions, and Valles Marineris;

  4. Channel systems, including those originating near Valles Marineris; and

  5. Polar regions, including ice and associated layered deposits.



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4 Stratigraphy and Chronology PRESENT STATE OF KNOWLEDGE Relevance of Stratigraphy and Chronology to Understanding Mars The long and complex geological history of Mars, notably the history of its water, can be unraveled by understanding the relative and absolute ages of the planet’s geological units, which have been produced or deposited by the various geological processes that have operated throughout the planet’s history. Moreover, understanding the geology of a landing site, and therefore, that of samples examined in situ or returned to Earth, requires placing such samples in their appropriate time-stratigraphic geological contexts. Thus, understanding the stratigraphy of Mars is of high priority, as is the ability to date surface units. The absolute ages of surface units will remain necessarily uncertain until samples from known surface locations have been dated in situ and/or on Earth. Geologic Units and the Stratigraphic Column The Mariner 9, Viking orbiter, and Mars Global Surveyor spacecraft have provided a wealth of data from which the types and abundances of Mars’s geological units can be surveyed, their relative stratigraphic ages derived, and their absolute ages estimated. The planet’s various geological units are distinguished and characterized on the basis of their morphologic, topographic, and spectral properties. Their relative ages are determined through examination of their crosscutting and superpositional relationships and the number of superposed impact craters.1,2 Analyses of this sort have allowed confident derivation of a stratigraphic column for Mars and corresponding chronological ordering of the major geological events in the planet’s history (see Figure 4.1). The major geological units of Mars, their stratigraphic positions, and their locations and extents are reviewed by Tanaka and colleagues.3 The major divisions thus recognized are the following: Highland rocks, notably those of the Hellas, Argyre, and Isidis impact basins and the southern high plains; Lowland rocks, notably the northern plains and degraded rocks along the highland/lowland boundary; Volcanic and tectonic regions, including highland volcanic paterae, the Tharsis and Elysium volcanic regions, and Valles Marineris; Channel systems, including those originating near Valles Marineris; and Polar regions, including ice and associated layered deposits.

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FIGURE 4.1 Relative ages of major geological events in Mars’s history, derived from stratigraphic analyses of geological units and corresponding crater densities (right: expressed as number of craters greater than the indicated diameter per 106 km2 of surface area), grouped here by geological process (SOURCE: J.W. Head III, R. Greeley, M.P. Golombek, W.K. Hartmann, E. Hauber, R. Jaumann, P. Masson, G. Neukum, L.E. Nyquist, and M.H. Carr, “Geological Processes and Evolution,” Space Science Reviews 96:263–292, 2001). A correspondence between absolute ages and crater densities has not been established with confidence. The figure is subject to amendment; it shows Tharsis volcanism to have occurred in the Hesperian epoch, but recent Mars Global Surveyor data have indicated that the Tharsis complex of volcanoes was formed in the upper Noachian epoch.

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The planet’s geological units are assigned to three major time-stratigraphic systems.4 The oldest is the Noachian System, named for the ancient rugged materials of Noachis Terra in the southern highlands. Most of the southern highland terrain consists of Noachian-age materials. Rocks of the Hesperian System overlie Noachian units and are characterized by the ridged plains materials of the northern lowlands. Much of the northern lowlands consists of Hesperian age units. The most recent system is the Amazonian, represented by the plains and volcanic materials of Amazonis Planitia. Volcanic materials of the Elysium and Tharsis Montes volcanic regions are Amazonian in age. These three time-stratigraphic systems correspond to three epochs, major time periods having durations and absolute ages that have been estimated from model crater production curves (see the next subsection, “Cratering Chronology”). Data from the Mars Global Surveyor (MGS) spacecraft provide rich insight into martian stratigraphic relationships and the timing of major geological events. Many specific stratigraphic issues have been addressed and new questions have arisen, notably regarding the presence, timing, and extent of liquid water on the surface.5,6 Moreover, MGS images have revealed thick layered sequences, within Valles Marineris and surrounding troughs and elsewhere across the cratered highlands, which may be volcanic and/or sedimentary in origin.7,8 These recent observations reveal the rich, active, and wet geological history of early Mars. Cratering Chronology The absolute ages of Mars’s geological events, and thus the time history of the planet’s evolution, will be fully understood only when the relative chronology derived from stratigraphy is tied to an absolute chronology. The density of superposed craters provides a means of estimating absolute chronology, but this technique is dependent on imperfect models of the cratering rate at Mars through time. Estimates of the impactor flux on Mars through time are based on extrapolation from known lunar fluxes. On the Moon, radiometric dating of samples collected by the Apollo astronauts allows correlation of rocks of known age to impact crater densities observed in images of the sample-collection sites, allowing an absolute chronology to be established for the Moon and extrapolated across the lunar surface.9 The size-frequency distribution of lunar craters permits derivation of a model curve, known as the “lunar production function,” that describes the cratering rate as a function of time.10 This model curve then must be scaled to the cratering rate as a function of time on Mars. Adapting the lunar production curve to Mars requires knowledge of the difference in impact rates between the planetary bodies; the nature of crater-forming projectiles; and the effects of differing gravity, impact velocity, and target properties on impact crater formation.11,12 Modeling of these parameters permits an estimate of the ratio of the lunar to martian production functions, allowing estimation of martian crater ages to within about a factor of two.13,14 The factor-of-two uncertainty in age has relatively little effect on interpretation of the absolute age of Noachian terrains, expected to have been originally nearly saturated with craters. Similarly, the factor of two has relatively little effect on interpretation of the age of very young terrains, where a surface with a nominal age of ~10 million years (Myr) is young in any case. However, the factor-of-two uncertainty means that ages of terrains which fall in middle martian history are very poorly constrained. Model crater ages can constrain the boundaries between Mars’s major epochs as defined by its three time-stratigraphic systems. Compilation of various crater age estimates from the literature places the Noachian/Hesperian boundary in the age range of 3.5 billion to 3.8 billion years (Gyr), and the Hesperian/Amazonian boundary within the broad age range of 1.5 to 3.5 Gyr.15 A recent assessment by Hartmann and Neukum places the Noachian/ Hesperian boundary in the nominal age range of 3.5 to 3.7 Gyr, and the Hesperian/Amazonian boundary within the nominal range of 2.9 to 3.3 Gyr.16 The uncertainty in the martian crater production function implies that late Hesperian through mid-Amazonian ages are the most poorly characterized. The number of observable martian craters is affected by geological processes of erosion and deposition, with the preservation time of a given crater being dependent on its size and the geological processes that have acted to modify it. Notably, lava flows and eolian activity can partially fill or completely obliterate craters. Eolian activity can exhume a cratered surface that has been protected from cratering for an uncertain amount of time, implying that its crater density would indicate a somewhat younger age than its actual age.17 While clear evidence for

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exhumation is found in MGS images, the effect on cratering chronology has not yet been studied in detail. Plainly, understanding the geological context of a region is critical to an accurate estimate of its age based on its crater density. Absolute Chronology The SNC meteorites (discussed in Chapters 1 and 3) have contributed useful constraints on surface ages, though their exact provenance on Mars is unknown. Their radiometric ages indicate that some near-surface rocks are as old as 4.5 Gyr and that martian volcanic activity occurred as recently as ~175 Myr ago.18 These ages are consistent with the interpretation that the heavily cratered highlands date from the earliest history of Mars, and with the interpretation from Mars Global Surveyor images that some very young lava flows have ages of ~10 Myr or less.19 Though little is currently known of the absolute chronology of Mars, fruitful future opportunities exist in this area. In situ analysis of rock samples offers a potential means of constraining the absolute chronology of the planet. Analyses of cosmic-ray exposure ages offer the potential for in situ dating of martian rocks in the age range of ~105 to 107 yr.20,21,22 This method takes advantage of the fact that a rock within ~1 m of the martian surface has been bombarded by cosmic rays, which through spallation produce nuclei including the noble gases 3He, 21Ne, 22Ne, and 38Ar. Measurement of rock elemental abundances along with these noble-gas abundances can allow an estimate of the production rate of the noble gases and, hence, the length of time the sample has been exposed near the surface. Approximate uniformity of the measured cosmic-ray exposure age of many samples in a region would provide confidence that the crystallization age of the local rock unit has been accurately measured. K-Ar dating may provide a viable means of dating martian samples with ages >106 yr, including those as old as the planet.23 Using this method in the laboratory to date martian meteorite samples of known radiogenic age, Swindle finds that K-Ar can be used to date samples in situ to an accuracy of ~20 percent.24 This method assumes that all the 40Ar in a rock sample has been produced by decay of K, and that no 40Ar has escaped the sample over its lifetime. Although Bogard and colleagues argue that K-Ar is not a reliable method for in situ dating on Mars— because atmospheric and cosmogenic 40Ar and 36Ar would confound accurate measurements and calibration,25 and because 40Ar may be lost from the sample over time—Swindle counters that measurement of 36Ar and 38Ar would allow correction for the atmospheric and cosmogenic contributions of Ar, and that sampling of unshocked and unweathered surface rocks would ensure accurate K-Ar dating for most rock types.26 In situ age-dating provides promise for more effectively constraining the present factor-of-two uncertainty in crater-based ages, especially for sites in middle Mars history (late Hesperian through mid-Amazonian). Site(s) dated in situ would better constrain the martian crater production curve, thereby improving estimates of the absolute ages of Mars’s other geological units. In order to reduce the uncertainty in rock types sampled and in the interpretation of sample ages, a sampled site or sites should be broad, homogeneous, and geologically well understood. Multiple sites that span middle Mars history would provide the best constraints on the planet’s absolute chronology. Sample return will ensure accurate and precise radiometric age determination, allowing the sample(s) to be subject to intensive laboratory examination. Effective radiometric dating methods could include K-Ar, 39Ar-40Ar, Rb-Sr, Sm-Nd, and U-Th-Pb; by analogy to dating of existing martian meteorites, laboratory precision in age determination would approach 107 yr.27 Accurate understanding of provenance and geological history must be ensured for any returned sample. Only by understanding the geological context can sample ages be generalized to cratering and stratigraphic chronologies and, thus, to the overall history of Mars. NEAR-TERM OPPORTUNITIES Orbiter missions provide the opportunity for imaging and spectroscopic studies that can aid stratigraphic and geological analyses and enable improved crater statistics and understanding of geological processes, including erosion and exhumation. The 2001 Mars Odyssey mission carries the Thermal Emission Imaging System (THEMIS) instrument, which will aid in unit characterization (both morphology and composition) through high-resolution imaging and imaging spectroscopy. The Mars Express orbiter’s High Resolution Stereo Camera (HRSC)

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will provide decimeter-scale stereoscopic imaging resolution, greatly aiding stratigraphic studies and refinement of crater-based chronology. NASA’s Science Definition Team for the 2005 Mars Reconnaissance Orbiter imaging system has recommended decimeter-scale imaging resolution along with context imaging and spectroscopy, with sufficient swath width (at least 3 km, and preferably 4 to 6 km) to permit local stratigraphy and cratering chronology to be inferred. Landed missions provide the opportunity to constrain absolute chronology by dating rocks in situ or by collecting and returning samples to Earth for laboratory analyses. The goals of the two 2003 MER missions’ rovers do not explicitly address relative or absolute chronology. The specific goals of future lander missions (2007 or 2009 and beyond) have not yet been defined, but could include in situ age measurements and are expected to include sample return. NASA’s Mars Scout missions (as yet undefined) also provide potential opportunities for stratigraphic and chronological studies. RECOMMENDED SCIENTIFIC PRIORITIES Past recommendations by COMPLEX and other scientific advisory groups regarding strategies for the exploration of Mars have noted the importance of stratigraphy and absolute chronology—they are essential to an understanding of the history and evolution of the planet, notably its water and climate, and to an understanding of the geological context of martian samples. In its 1978 report Strategy for Exploration of the Inner Planets 1978–1987, COMPLEX emphasized that a basic understanding of the times scales of surface materials (Appendix B: [1.2]) and understanding of “the nature and chronology of the major surface forming processes” [1.3] are primary objectives in Mars exploration.28 Moreover, in its 1990 report The Search for Life’s Origins, the Committee on Planetary Biology and Chemical Evolution (CPBCE) recommended studies to “[r]econstruct the history of liquid water and its interactions with surface materials on Mars through photogeologic studies, space-based spectral reflectivity measurements, in situ measurements, and analysis of returned samples” [2.2].29 In discussing the most promising areas from which to return samples, COMPLEX recommended that sample sites be chosen with regard to “the geological context, age, and climatic environment in which the materials were formed” [7.8],30 and has pointed out the importance of understanding the geological history of martian samples and of the overall planet [9.2].31 In its 2001 report, NASA’s Mars Exploration Payload Assessment Group (MEPAG) also emphasizes the importance of stratigraphic and chronological studies to a broad range of issues relevant to martian geology, climate, and astrobiology [11].32 Past reports have recommended both in situ studies and sample return, though their relative emphasis has varied. COMPLEX’s 1978 report explicitly recommends that chronological determination should include (1) measurement of “cosmic-ray exposure ages of soil and rock materials for both long and short time scales”; and (2) determination of “crystallization ages of igneous rocks, recrystallization ages of metamorphic rocks, and depositional ages of sedimentary rocks” (Appendix B: [1.5]).33 The same report notes that sample return “will allow the full range of the most sophisticated analytical techniques to be applied for the study of chronology” [1.19]. The 1990 CPBCE report agrees on the importance of sample return, but emphasizes that even coarse in situ age-dating, which might be accomplished by landed science packages, “can be very valuable in some cases,” greatly enhancing our understanding of Mars [4.2].34 COMPLEX’s 1996 letter report “Scientific Assessment of NASA’s Mars Sample-Return Mission Options” recommends a focus on sample return to understand Mars as a possible abode of life [6.1].35 That report notes the overall relevance of in situ measurements for martian exploration and recommends their development [6.2], but does not explicitly refer to in situ age-dating. NASA’s 1996 Mars Expeditions Strategy Group report points out that in situ surface studies are essential,36 but in order to employ appropriately sophisticated and high-precision experiments, “the essential analyses of selected samples must be done in laboratories on Earth” [7.3]. ASSESSMENT OF PRIORITIES IN THE MARS EXPLORATION PROGRAM The Mars Exploration Program outlined in Appendix A includes the broad goals of understanding the climate and the geological history of the planet within the overarching theme “Follow the water.” These goals encompass

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the objectives of understanding the location and nature of ancient warm and wet environments and of understanding how the planet’s climate operated in the more distant past. The Mars Exploration Program explicitly recognizes the importance of understanding the timing of events in Mars history, and emphasizes that the planet may be used as a location from which to provide absolute calibration of the timing of major solar system events. COMPLEX reiterates past committee recommendations that place high priority on understanding the stratigraphic history and absolute chronology of Mars. Priority should be given to those studies that relate to the evolutionary history of the planet’s heat and water and, thereby, to the planet’s astrobiological potential. The planned suite of NASA missions to Mars addresses these important issues in part. Very high resolution imaging, as planned for the Mars Reconnaissance Orbiter and Mars Express missions, could permit relative age-dating of Mars’s youngest terrains through analysis of crater distributions at the meter scale, evaluation of the nature of geological boundaries, and better understanding of the active small-scale geological processes that affect the surface. High-resolution imaging also plays a practical role in Mars exploration, being essential to considerations of safety, choice of landing sites, establishment of the context for in situ analyses and returned samples, navigational support for rovers, and so on. The Mars Reconnaissance Orbiter imager and the Mars Express HRSC are expected to accomplish these goals for geological units of interest, and also to provide context imaging and spectroscopy. In general, it is important that very high resolution observations obtain contiguous coverage sufficient to derive meaningful crater statistics, and/or that simultaneous context images be obtained. However, even following these planned missions, portions of the surface of Mars may remain unimaged at sufficient resolution to characterize geological units and determine local stratigraphy and crater-based chronology. Astronomical observations and dynamical studies of asteroidal and cometary bodies are quite pertinent to improved understanding of the martian crater production function and, thus, its cratering chronology; such studies should be pursued. Landed missions will allow samples to be dated in situ or to be collected and returned to Earth for much more precise analyses. In situ dating is a developing and as yet unproved technique, but one that shows promise and could be a cost-effective means of constraining absolute chronology. Moreover, analysis of many samples could reduce the error inherent in this measurement technique. Recommendation. COMPLEX recommends that studies of the feasibility of in situ determination of rock ages, by robotic spacecraft, be pursued. In situ dating could constrain the surface emplacement ages for multiple sites of igneous activity on Mars. This might be achieved through several surface lander and/or rover missions, each of which would examine multiple rock samples within a locale of well-understood geological context and crater density. By constraining the absolute age of several specific geological units, this method can tie the crater age of the planet’s geological units to an absolute chronology, revealing the absolute timing of events in martian history. For the purpose of establishing an absolute chronology, sites formed in middle Mars history should have initial priority for age-dating. While the 1978 COMPLEX report Strategy for Exploration of the Inner Planets explicitly recommends age determination for soil, igneous rock, and metamorphic rock samples (Appendix B: [1.2, 1.3, 1.5]),37 this committee concludes that age-dating of igneous rock samples should have clear priority in constraining the emplacement age of a given terrain. Rock ages determined in situ would be complementary to age-dates acquired by analysis of samples returned to Earth. Sample return will permit precise and accurate age-dating drawing on techniques that cannot practically be used in situ, and will provide significant additional benefits (see Chapter 11 of this report). Overall, the NASA strategy of “Seek, In Situ, Sample” is a sound one with respect to understanding the relative and absolute timing of events in Mars’s history. REFERENCES 1. K.L. Tanaka, D.H. Scott, and R. Greeley, “Global Stratigraphy,”pp. 354–382 in Mars, H.H. Kieffer, B.M. Jakosky, C.W. Synder, and M.S. Matthews (eds.), University of Arizona Press, Tucson, 1992. 2. J.W. Head, R. Greeley, M.P. Golombek, W.K. Hartmann, E. Hauber, R. Jaumann, P. Masson, G. Neukum, L.E. Nyquist, and M.H. Carr, “Geological Processes and Evolution,”Space Science Reviews96: 263–292, 2001.

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3. K.L. Tanaka, D.H. Scott, and R. Greeley,“Global Stratigraphy,”pp. 354–382 in Mars, H.H. Kieffer, B.M. Jakosky, C.W. Synder, and M.S. Matthews (eds.), University of Arizona Press, Tucson, 1992. 4. K.L. Tanaka, D.H. Scott, and R. Greeley,“Global Stratigraphy,”pp. 354–382 in Mars, H.H. Kieffer, B.M. Jakosky, C.W. Synder, and M.S. Matthews (eds.), University of Arizona Press, Tucson, 1992. 5. J.W. Head, R. Greeley, M.P. Golombek, W.K. Hartmann, E. Hauber, R. Jaumann, P. Masson, G. Neukum, L.E. Nyquist , and M.H. Carr, “Geological Processes and Evolution,”Space Science Reviews96: 263–292, 2001. 6. S.C. Solomon, C.L. Johnson, J.W. Head, M.T. Zuber, G.A. Neumann, O. Aharonson, R.J. Phillips, D.E. Smith, H.V. Frey, M.P. Golombek, W.B. Banerdt, M.H. Carr, and B.M. Jakosky,“What Happened When on Mars?: Some Insights into the Timing of Major Events from Mars Global Surveyor Data,”Abst. P31A-06, Eos82(20), Spring meeting suppl., 2001. 7. A.S. McEwen, M.C. Malin, M.H. Carr, and W.K. Hartmann, “Voluminous Volcanism on Early Mars Revealed in Valles Marineris,”Nature397: 584–586, 1999. 8. M.C. Malin and K.S. Edgett,“Sedimentary Rocks of Early Mars,”Science290: 1927–1937, 2001. 9. D. Stöffler and G. Ryder,“Stratigraphy and Isotope Ages of Lunar Geologic Units: Chronological Standard for the Inner Solar System,”Space Science Reviews96: 9–54, 2001. 10. G. Neukum, B. Ivanov, and W.K. Hartmann, “Cratering Records in the Inner Solar System in Relation to the Lunar Reference System,”Space Science Reviews96: 55–86, 2001. 11. R.G. Strom, S.K. Croft, and N.G. Barlow,“The Martian Impact Cratering Record,”pp. 383–423 in Mars, H.H. Kieffer, B.M. Jakosky, C.W. Synder, and M.S. Matthews (eds.), University of Arizona Press, Tucson, 1992. 12. B. Ivanov, “Mars/Moon Cratering Rate Ratio Estimate,”Space Science Reviews96: 87–104, 2001. 13. B. Ivanov, “Mars/Moon Cratering Rate Ratio Estimate,”Space Science Reviews96: 87–104, 2001. 14. W.K. Hartmann and G. Neukum, “Cratering Chronology and the Evolution of Mars,”Space Science Reviews96: 165–194, 2001. 15. J.W. Head, R. Greeley, M.P. Golombek, W.K. Hartmann, E. Hauber, R. Jaumann, P. Masson, G. Neukum, L.E. Nyquist, and M.H. Carr, “Geological Processes and Evolution,”Space Science Reviews96: 263–292, 2001. 16. W.K. Hartmann and G. Neukum, “Cratering Chronology and the Evolution of Mars,”Space Science Reviews96: 165–194, 2001. 17. R. Greeley, R.O. Kuzmin, and R.M. Haberle, “Aeolian Processes and Their Effects on Understanding the Chronology of Mars,”Space Science Reviews96: 393–404, 2001. 18. L.E. Nyquist, D.D. Bogard, C.-Y. Shih, A. Greshake, D. Stöffler, and O. Eugster, “Ages and Geologic Histories of Martian Meteorites,”Space Science Reviews96: 105–164, 2001. 19. W.K Hartmann and D.C. Berman, “Elysium Planitia Lava Flows: Crater Count Chronology and Geological Implica-tions,”Journal of Geophysical Research105: 15011–15025, 2000. 20. See, for example, K. Marti, and T. Graf, “Cosmic-ray Exposure History of Ordinary Chondrites,”Annual Reviews ofEarth and Planetary Science20: 221–243, 1992. 21. T.D. Swindle, “In Situ Noble-gas Based Chronology on Mars,”pp. 294–295 in Concepts and Approaches for MarsExploration, LPI Contribution #1062, 2000. 22. T.D. Swindle, “Applying Noble-gas Geochronology Techniques In Situ on Planets and Asteroids,”Eleventh Annual Goldschmidt Conference, Abstract #3718 (CD-ROM), 2001. 23. T.D. Swindle, “In Situ Noble-gas Based Chronology on Mars,”pp. 294–295 in Concepts and Approaches for MarsExploration, LPI Contribution #1062, 2000. 24. T.D. Swindle, “Could In Situ Dating Work on Mars?”, 31st Lunar and Planetary Science Conference, Abstract #1492 (CD-ROM), 2001. 25. D.D. Bogard, J.L. Birck, O. Eugster, A. Greshake, W.K. Hartmann, G. Neukum, L. Nyquist, M. Ott, G. Ryder, D. Stöffler, and G. Turner, “Letter to the Mars Exploration Program Assessment Group (MEPAG),”Working Group on the Chronology of Mars and the Inner Solar System, International Space Science Institute, Bern, Switzerland, Apr. 15, 2000. 26. T.D. Swindle, “Could In Situ Dating Work on Mars?”, 31st Lunar and Planetary Science Conference, Abstract #1492 (CD-ROM), 2001. 27. L.E. Nyquist, D.D. Bogard, C.-Y. Shih, A. Greshake, D. Stöffler, and O. Eugster, “Ages and Geologic Histories of Martian Meteorites,”Space Science Reviews96: 105–164, 2001. 28. Space Science Board, National Research Council, Strategy for Exploration of the Inner Planets 1978–1987, National Academy of Sciences, Washington, D.C., 1978. 29. Space Studies Board, National Research Council, The Search for Life’s Origins: Progress and Future Directions inPlanetary Biology and Chemical Evolution, National Academy Press, Washington, D.C., 1990. 30. Space Science Board, National Research Council, “Scientific Assessment of NASA’s Mars Sample-Return Mission Options,”National Research Council, Washington, D.C., 1996. 31. Space Science Board, National Research Council, “Assessment of NASA’s Mars Exploration Architecture,”National Research Council, Washington, D.C., 1998.

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32. NASA, Mars Exploration Payload Assessment Group (MEPAG), “Mars Exploration Program: Scientific Goals, Objectives, Investigations, and Priorities, in Science Planning for Exploring Mars,”JPL Publication 01-7, Jet Propul-sion Laboratory, Pasadena, Calif., 2001. 33. Space Science Board, National Research Council, Strategy for Exploration of the Inner Planets 1978–1987, National Academy of Sciences, Washington, D.C., 1978. 34. Space Studies Board, National Research Council, The Search for Life’s Origins: Progress and Future Directions inPlanetary Biology and Chemical Evolution, National Academy Press, Washington, D.C., 1990. 35. Space Science Board, National Research Council, “Scientific Assessment of NASA’s Mars Sample-Return Mission Options,”National Research Council, Washington, D.C., 1996. 36. Mars Expeditions Strategy Group, National Aeronautics and Space Administration (NASA), “The Search for Evi-dence of Life on Mars,”1996, available online at <http://geology.asu.edu/~jfarmer/mccleese.htm>. Also available in National Aeronautics and Space Administration, Science Planning for Exploring Mars, JPL Publication 01-7, Jet Propulsion Laboratory, Pasadena, Calif., 2001. 37. Space Science Board, National Research Council, Strategy for Exploration of the Inner Planets 1978–1987,National Academy of Sciences, Washington, D.C., 1978.