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Review of NASA's Planned Mars Program 2 Scientific Goals for the Exploration of Mars The Space Studies Board's key report outlining the principal scientific issues in the planetary sciences is An Integrated Strategy for the Planetary Sciences: 1995- 2010. 1 After summarizing current knowledge about the solar system and the key remaining scientific questions, the Integrated Strategy lists Mars among its four highest-priority objects for further study, the other targets being comets, Jupiter, and planetary systems around other stars. The Integrated Strategy describes a series of important measurements to be made at Mars. Strategies for the scientific exploration of Mars have also been written by other panels, including the Mars Science Working Group 2 and the International Mars Science Working Group. 3 All these reports identify the same three science themes for Mars exploration, namely: The search for indigenous life or evidence of past life, q Atmospheric dynamics and climate change, and q The evolution of the surface and interior. q COMPLEX now briefly elaborates on these topics and describes observations that, according to the Integrated Strategy, will best elucidate the primary scientific questions. Previous reports have defined specific measurement requirements arising from the scientific objectives (see below) relating to these themes.4-6 It is not necessary to repeat here all those requirements; clearly, without capable instruments, Mars Surveyor will not accomplish the scientific goals that are the program's rationale. LIFE There is increasingly compelling evidence that Mars was, and may still be, water- rich and that it has undergone major changes in its climate. This evidence, coupled with indications from molecular phylogeny of the conditions under which primitive life may have existed on Earth, has heightened interest that life may also have started on Mars. Nevertheless, survival of living forms on the present surface of Mars is considered highly improbable. 7,8 Accordingly, much of the emphasis in Mars exploration is on a search for evidence of past life. If some form of life did

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start on Mars in the distant past, it may have survived in protected niches such as long-lived volcanic vents or deep aquifers. 9 In order to better judge whether living forms might have arisen on Mars and where to search for evidence of them, we need to better understand climatic history; the inventory and distribution of volatiles and biogenic elements; and the locations and characteristics of potential past habitats such as hydrothermal systems and lakes. Even the likely case that life never developed on Mars is interesting by comparison with the terrestrial example: How did conditions differ for the two planets? The optimum strategy for the biologic exploration of Mars, endorsed by COMPLEX and other groups, is to focus first on global reconnaissance to better assess past surface conditions and planet-wide inventories of water as well as other volatiles and to identify promising sites where such materials might be available. Emphasis would then shift to surface exploration of these favorable locations to seek more clear-cut evidence of past conditions, and to search for more direct confirmation of past life such as the presence of biogenic elements and compounds, and anomalous isotopic fractionations. Ultimately, returned samples will be needed for definitive analysis in terrestrial laboratories. ATMOSPHERIC DYNAMICS AND CLIMATE CHANGE COMPLEX's goals for atmospheric science at Mars include two different but connected themes. The first concerns the dynamics and chemistry of the present martian atmosphere and how it compares with Earth's gaseous envelope; the second is to understand the evolution of the atmosphere and, in particular, past climatic conditions. In order to characterize the atmosphere's general circulation, systematic high-resolution soundings of the atmosphere for temperature, dust, water vapor, and aerosols must be made over at least 1 martian year. In addition, measurements of pressure, wind, humidity, and opacity need to be made at a number of widely distributed surface stations for at least 1 martian year, during the same period when vertical temperature profiles are being monitored from orbit. Between 15 and 20 stations, distributed over the surface and at various elevations sufficient to ensure that the characteristic separation of any two stations is no more than a planetary radius, appear to be the minimum necessary to acquire interpretable meteorological data. Mars may have undergone modest climatic changes in the recent geologic past, but larger variations in the more distant past. Evidence for the more recent changes are probably best preserved in the polar layered deposits, and so an exploration program should include a means of characterizing these deposits. Information on more ancient climates may be derived in various ways: from geomorphic evidence of past fluvial action and erosion rates, from the composition of gases trapped in ancient rocks, from the characteristics of sediments deposited in climate-sensitive environments such as lakes, from the mineralogy of weathering products, and so forth. An exploration strategy should, therefore, include schemes for identifying and locating climate-sensitive features as well as the means for characterizing and possibly sampling the relevant deposits. The atmosphere's

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composition-in particular the isotopic ratios for H, C, N, O, and the noble gases- also provides clues about its evolution. In addition, measurements of the escape rates of upper-atmospheric species are useful in constraining models of past climates. EVOLUTION OF SURFACE AND INTERIOR The decipherment of the origin and evolution of the solid planet, and its comparison with Earth's evolution, are both important goals in COMPLEX's strategy for Mars exploration. The surface's chemistry, lithology, and morphology result from a variety of internal (volcanism and tectonism) or external (impacts) processes, or interactions with the atmosphere (erosion and sedimentation). Any interpretation of the record will require global surveys of the chemistry, mineralogy, and morphology from orbit, followed by detailed surface measurements at locations of special interest (e.g., lake beds or hydrothermal deposits) that have been identified from the orbital data. Many of the critical measurements needed to unravel the geologic history-such as ages, determination of stable isotopes, and measurement of trace elements-at present seem to require sample return. The planet's interior provides information about how the solid body accumulated, differentiated, and evolved. Orbital measurements of the gravitational, topographic, and magnetic fields provide valuable constraints on interior properties, primarily about shallow structures. To specify core and mantle properties, as well as to locate present tectonic and volcanic activity, a seismic network will need to be emplaced. REFERENCES 1. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994. 2. NASA, Mars Science Working Group, A Strategy for the Scientifc Exploration of Mars, PL D-8211, Jet Propulsion Laboratory, Pasadena, Calif., February 1991. 3. International Mars Science Working Group, "An International Strategy for the Exploration of Mars," Planetary and Space Science, in press. 4. Space Science Board, National Research Council, Strategy for Exploration of the Inner Planets: 1977-1987, National Academy of Sciences, Washington, D.C., 1978. 5. Space Studies Board, National Research Council, 1990 Update of Strategy for Exploration of the Inner Planets, National Academy Press, Washington, D.C., 1990. 6. Space Studies Board, National Research Council, An Integrated Strategy for the

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Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994. 7. Space Studies Board, National Research Council, The Search for Life's Origins: Progress and Future Directions in Planetary Biology and Chemical Evolution, National Academy Press, Washington, D.C., 1990. 8. Space Studies Board, National Research Council, Biological Contamination of Mars: Issues and Recommendations, National Academy Press, Washington, D.C., 1992. 9. Exobiology Program Office, Exobiological Strategy for Mars Exploration, NASA, Washington, D.C., 1995.