evolution of the planet as a whole in order to understand discoveries relating to distributions of water and conditions that might have allowed life to develop.

Great improvement in our knowledge of the martian interior could come from a passive seismic experiment. There are good reasons, based on terrestrial and lunar experience, to believe that the rate of detectable seismicity on Mars would provide adequate sources for imaging the interior. The impacts of meteoroids would also provide useful seismic signals. Determining the rate, mechanism, and location of seismicity and the rate of meteoroid impacts are high-priority objectives in themselves. Crustal structure in a single area could potentially be constrained with a local network of three stations. Seismic attenuation properties should depend on whether the shallow crust is wet (as on Earth) or dry (as on the Moon). More stations would be needed to locate Mars quakes accurately and to determine the velocity and attenuation structure of the mantle and the size and state of the core. For example, accurate constraints on the size and state of the core would require stations at a wide range of distances from seismic sources. Thus, a phased approach is suggested, with a deployment of four stations to determine the locations of seismic sources and constrain crustal properties, while later experiments would add more stations to focus on the deep interior.

The seismic experiment part of the NetLander mission would take the first step in a phased seismic study of the martian interior. It must, however, be emphasized that seismometers need to operate simultaneously over a period of years to provide constraints on the interior structure. Given the high priority that COMPLEX places on the range of objectives that seismic experiments could accomplish, the committee strongly recommends that NASA support passive seismic experiments.

1. W.M. Folkner, C.F. Yoder, D.N. Yuan, E.M. Standish, and R.A. Preston, “Interior Structure and Seasonal Mass Redistribution of Mars from Radio Tracking of Mars Pathfinder,”Science278: 1749–1752, 1997.

2. M.H. Acuña, J.E.P. Connerney, N.F. Ness, R.P. Lin, D. Mitchell, C.W. Carlson, J. McFadden, K.A. Anderson, H. Reme, C. Mazelle, D. Vignes, P. Wasilewski, and P. Cloutier, “Global Distribution of Crustal Magnetization Discov-ered by the Mars Global Surveyor MAG/ER Experiment,”Science284: 790–793, 1999.

3. M.T. Zuber, S.C. Solomon, R.J. Phillips, D.E. Smith, G.L. Tyler, O. Aharonson, G. Balmino, W.B. Banerdt, J.W. Head, C.L. Johnson, F.G. Lemoine, P.J. McGovern, G.A. Neumann, D.D. Rowlands, and S.J. Zhong, “Internal Structure and Early Thermal Evolution of Mars from Mars Global Surveyor Topography and Gravity,”Science287: 1788–1793, 2000.

4. R.J. Phillips, M.T. Zuber, S.C. Solomon, M.P. Golombek, B.M. Jakosky, W.B. Banerdt, D.E. Smith, R.M.E. Will-iams, B.M. Hynek, O. Aharonson, and S.A. Hauck, “Ancient Geodynamics and Global-Scale Hydrology on Mars,” Science291: 2587–2591, 2001.

5. See, for example, H. Wänke and G. Dreibus,“Chemical Composition and Accretion History of Terrestrial Planets,”Philosophical Transactions of the Royal Society of LondonA235: 545–557, 1988.

6. W.M. Folkner, C.F. Yoder, D.N. Yuan, E.M. Standish, and R.A. Preston, “Interior Structure and Seasonal Mass Redistribution of Mars from Radio Tracking of Mars Pathfinder,”Science278: 1749–1752, 1997.

7. See, for example, C.M. Bertka, and Y. Fei, “Implications of Mars Pathfinder Data for the Accretion History of the Terrestrial Planets,”Science281: 1838–1840, 1998; C.M. Bertka and Y. Fei,“Density Profile of an SNC Model Martian Interior and the Moment-of-Inertia Factor of Mars,”Earth and Planetary Science Letters157: 79–88, 1998; F. Shol and T. Spohn,“The Interior Structure of Mars: Implications from SNC Meteorites,”Journal of GeophysicalResearch102: 1613–1635, 1997; and D.H. Johnston and M.N. Toksöz, “Internal Structure and Properties of Mars,” Icarus32: 73–84, 1977.

8. H. Moritz, “Fundamental Geodetic Constraints,”Travaux de L’Association Internationale de Géodésie25: 411–418, 1976.

9. C.M. Bertka and Y. Fei,“Implications of Mars Pathfinder Data for the Accretion History of the Terrestrial Planets,”Science281: 1838–1840, 1998.

10. D.H. Johnston and M.N. Toksöz,“Internal Structure and Properties of Mars,”Icarus32: 73–84, 1977.

11. T. McGetchin andJ.R. Smythe, “The Mantle of Mars: Some Possible Geological Implications of its High Density,”Icarus34: 512–536, 1978.

12. C.M. Bertka and Y. Fei,“Density Profile of an SNC Model Martian Interior and the Moment-of-Inertia Factor of Mars,”Earth and Planetary Science Letters157: 79–88, 1998.

13. A.E. Ringwood, Composition and Petrology of the Earth’s Mantle, McGraw-Hill, New York, 1975.

Front Matter (R1-R12)

Executive Summary (1-4)

1. Introduction (5-8)

2. Interior and Crustal Structure and Activity (9-17)

3. Geochemistry and Petrology (18-25)

4. Stratigraphy and Chronology (26-33)

5. Surface Processes and Geomorphology (34-47)

6. Ground Ice, Groundwater, and Hydrology (48-53)

7. Life, Fossils, and Reduced Carbon (54-63)

8. Lower Atmosphere and Meteorology (64-70)

9. Climate Change (71-76)

10. Upper Atmosphere, Ionosphere, and Solar Wind Interactions (77-82)

11. Rationale for Sample Return (83-88)

12. Assessment of the Mars Exploration Program (89-104)

13. Conclusions (105-106)

Appendix A: The NASA Mars Exploration Program (107-117)

Appendix B: Compilation of Recommendations Concerning Mars Exploration Made by COMPLEX and Other Advisory Groups (118-130)

Appendix C: Acronyms (131-132)