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Assessment of Solar System Exploration Programs 1991 (Chapter 2) Assessment of Solar System Exploration Programs 1991 2 Exploration of the Inner Planets The committee's Strategy far the Exploration of the Inner Planets: 1977- 1987 (SSB, 1978) established a comprehensive set of scientific goals and objectives for the exploration of Mercury, Venus, Earth, the Moon, and Mars, together with a number of related policy and program recommendations. In that report, COMPLEX concluded that observation and measurement of the morphologic, physical, and chemical character of Mars, Venus, Mercury, and the Moon on a global scale have high general scientific importance and are basic to all planetological studies. The committee recommended that the triad of terrestrial planets—Venus, Earth, and Mars—should be the major focus in exploration of the inner solar system for the succeeding decade. REPORT MENU NOTICE SCIENCE OBJECTIVES MEMBERSHIP FOREWORD Specifically, the committee established the following scientific objectives SUMMARY in the 1978 report for the inner planets: CHAPTER 1 CHAPTER 2 CHAPTER 3 CHAPTER 4 CHAPTER 5 Mercury CHAPTER 6 CHAPTER 7 Determine the chemical composition of the planet's surface on both a CHAPTER 8 global and regional scale. CHAPTER 9 REFERENCES Determine the structure and state of the planet's interior. Extend the coverage [to the entire planet] and improve the resolution of orbital imaging. file:///C|/SSB_old_web/ssep91ch2.htm (1 of 8) [6/18/2004 1:57:33 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 2) Venus Obtain a global map of the topography and morphology of [the planet's] surface at sufficient resolution to allow identification of the gross processes that have shaped it. [Obtain some images] of a limited number of selected regions at a substantially higher resolution. Determine the major chemical and mineralogical composition of the surface material. Determine the concentrations of photochemically active gases in the 65-135 km altitude region. Investigate the physical and chemical interactions of the surface with the atmosphere and study the composition and formation of atmospheric aerosols. Earth Resolve the following fundamental questions: 1. Why the Earth alone possesses vast water oceans, and why the Earth's atmosphere has a markedly different mass composition and evolutionary sequence than either Venus or Mars; 2. Why the Earth and Venus, with nearly identical mass and diameter, and why Mars with smaller diameter and bulk density, each had markedly different thermal and tectonic histories; 3. What are the rates and mechanism of transport of materials from the deep interior of the planets to their exteriors, and what are the chemical and physical processes of exchange of matter between the interior and the crusts of the planets and their atmospheres; 4. Why the Earth, Venus, and Mars possess very different internal magnetic fields; 5. Why particular conditions on Earth have led to the evolution of living organisms but apparently not on Venus or Mars; 6. How and why the atmospheric circulation and the long-term climatic variations on Earth differ from those of her nearest neighbors; file:///C|/SSB_old_web/ssep91ch2.htm (2 of 8) [6/18/2004 1:57:33 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 2) 7. How man and his culture will influence and modify the biosphere and the physical and chemical composition of the Earth on a global scale. Moon Determine the chemistry of the lunar surface on both a global and regional scale. Determine the surface heat flow on both a global and a regional scale. Determine the nature of any central metallic core in the Moon. Mars [Study local areas intensively] (a) to establish the chemical, mineralogical, and petrological character of different components of the surface material, representative of the known diversity of the planet; (b) to establish the nature and chronology of the major surface forming processes; (c) to determine the distribution, abundance, and sources and sinks of volatile materials, including an assessment of the biological potential of the Martian environment, now and during past epochs; (d) to establish the interaction of the surface material with the atmosphere and its radiation environment. Explore the structure and general circulation of the Martian atmosphere. Explore the structure and dynamics of Mars's interior. Establish the nature of the Martian magnetic field and the character of the upper atmosphere and its interaction with the solar wind. Establish the global chemical and physical characteristics of the Martian surface. Fields and Particles [Determine] the strength and character of the internal magnetic fields (both global fields and, where possible, small-scale remanent fields). file:///C|/SSB_old_web/ssep91ch2.htm (3 of 8) [6/18/2004 1:57:33 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 2) Both cruise and orbiting phases of planetary missions should be utilized to conduct appropriate interplanetary and solar measurements. [Characterize] each planet's interaction with the solar wind. CURRENT STATUS OF NASA'S EXPLORATION OF THE INNER PLANETS The committee recently completed a review of the 1978 document, which was published as the 1990 Update to the Strategy for Exploration of the Inner Planets (SSB, 1990b). Most of the scientific goals and objectives established in the original strategy were found to be still relevant and unfulfilled. Rather than establishing an entirely new science strategy, therefore, the committee left the 1978 strategy intact with the following modifications: Mercury The report (SSB, 1978) concluded that insertion of an appropriately instrumented planetological payload into a relatively low-altitude, circular orbit around Mercury required development of a low-thrust propulsion system. Such a mission is now deemed feasible with conventional rocket launches and gravity assists at Venus and Mercury. Therefore, a Mercury mission is a possible near- term activity, and justification of such a mission should rest on the important role of Mercury in understanding the origin and evolution of all the terrestrial planets. In the 1978 report, exploration of Mercury's magnetosphere was relegated to a secondary objective. One of the major unsolved problems in geophysics is understanding how the Earth's geodynamo works; characterization of Mercury's magnetic field should provide crucial insights and constraints on dynamo theories. Because of the direct connections between Mercury's magnetic field and the size and physical state of its core, determination of the multipole structure of the planet's magnetic field should be a primary science objective along with surface chemistry, internal structure, and imaging. Venus Essentially all of the strategy developed for Venus in the 1978 report remains valid. The highest priority objective to provide a global map of the surface of Venus at high spatial resolution is now being accomplished by the Magellan mission. Results from missions by NASA's Pioneer Venus and the Soviet Union's Venera spacecraft series raised some significant questions pertaining to middle and lower atmospheric composition, including time variability file:///C|/SSB_old_web/ssep91ch2.htm (4 of 8) [6/18/2004 1:57:33 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 2) and the combined effects of dynamics and chemistry. Therefore, COMPLEX in its 1990 update recommended that "characterization of the basic structure, composition, and dynamics of the Venus atmosphere be a primary objective." The committee also elevated studies of the Venus interior to primary status, together with studies of the surface and the atmosphere, so that acquisition of seismic data is now a primary objective. The committee therefore restated the recommendation of the 1978 report to emphasize that "acquisition of seismic data from Venus should be maintained as a highly desirable goal," and that "serious study of instruments operating at Venusian surface temperatures should be undertaken and preliminary studies should be conducted to determine the technical feasibility of sample return from Venus." In addition, the committee noted that a more sensitive search for any intrinsic magnetic field of Venus is of primary significance for determining the nature of the planet's interior. The Moon In its 1990 update, the committee again endorsed the recommendations in the 1978 report, and stated: "Measurement of the Moon's global chemical composition remains a high priority, but the committee recommends that global mineralogical measurements at high spatial and spectral resolution also be given a high priority." Mars As described in the 1978 report, the detailed analysis of surface materials involved samples to be collected from a region within easy reach of a landed vehicle. The current scientific consensus is that such samples are no longer considered adequate to address this objective. Rather, samples need to be obtained from a variety of locations spread out over perhaps hundreds to thousands of kilometers, and they need to be identified with sufficient information to provide the geologic context for each sample. Only determination of the whole-planet and major-unit chemical composition at low spatial resolution from orbit is called out specifically in the 1978 statement of measurement requirements. Remote sensing instruments available now or under development, however, permitted the committee to extend this objective to include regional- and intermediate-scale surveys of surface mineralogy and physical properties (e.g., density and grain size)-a particularly important consideration for Mars, with its chemical and physical diversity at various scales. file:///C|/SSB_old_web/ssep91ch2.htm (5 of 8) [6/18/2004 1:57:33 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 2) In 1989 the Soviet Phobos probe provided data on the Martian atmosphere and surface, primarily in the equatorial regions, and on the Martian charged-particle environment and its interaction with the solar wind. Because of the short mission lifetime and the limited geographic extent of observations, however, none of the major goals outlined in the 1978 report was addressed completely. The U.S. Mars Observer mission is a near-polar orbiting spacecraft scheduled for launch in 1992, with global mapping in late 1993. Its goal is to map the surface and atmosphere for an entire Martian year. If Mars Observer is successful, the original COMPLEX objectives of establishing the nature of the magnetic field and characterizing the global distribution of chemical and physical characteristics. of the surface should be partly accomplished. In addition, contributions will be made to the objectives of exploring atmospheric structure and circulation, establishing the distribution of volatiles, and constraining the planet's interior structure by means of topography and gravity data. Mars Observer will also provide a base of data to guide the selection of sampling sites for a Mars sample return mission and to identify resources to be used in potential future human exploration of the planet. The principal components of the 1978 report's objectives that will remain largely unaddressed after a successful Mars Observer mission are the prime objective of in situ (or remotely sampled) elemental, mineralogical, and petrological studies of selected areas; seismological and precessional studies of the interior structure of the planet; direct measurements of winds; and the dynamical and chemical properties of the upper atmosphere and its interactions with the solar wind. Also, the question of past life on Mars will remain open. If life developed in the more clement ages on Mars, it may have left chemical and fossil evidence. COMPLEX therefore recommended in its 1990 update report "that the geochemical, isotopic, and paleontological study of Martian surface material for evidence of previous living material be a prime objective of future in situ and sample return missions." Space Exploration Initiative The administration and NASA are planning a major program called the Space Exploration Initiative (SEI), which involves human exploration and permanent habitation of the Moon and then Mars. COMPLEX is currently contributing to a Space Studies Board report on the interaction of science programs with the human exploration of space. That study addresses the scientific information required to enable safe and effective human activities on the Moon and Mars, and the opportunities that arise for planetary science as a part of any program that prepares for, and carries out, prolonged human space missions. These opportunities include, of course, those enhanced by the presence of humans. Many of the same objectives for understanding the environment of Mars and the Moon are shared by the scientific strategy for planetary exploration and the concern for astronaut safety. The design and file:///C|/SSB_old_web/ssep91ch2.htm (6 of 8) [6/18/2004 1:57:33 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 2) implementation of future spacecraft missions to meet these combined objectives should be a major goal as NASA carries out the Space Exploration Initiative. In its 1990 update, however, the committee also made a recommendation with regard to program balance. It urged that "exploration of the inner planets in the next two decades should include further exploration of Mercury and Venus because a program of planetary exploration that includes only Mars and the Moon is scientifically inadequate." Last update 12/12/00 at 3:34 pm Site managed by Anne Simmons, Space Studies Board file:///C|/SSB_old_web/ssep91ch2.htm (7 of 8) [6/18/2004 1:57:33 PM]