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Introduction to Planetary Science

How do planets form? What combination of initial conditions and subsequent geologic, chemical, and biological processes led to at least one planet becoming the abode for innumerable life forms? What determines the fate of life on a planet? Such scientific enquiries reflect a basic human need to understand who we are, where we came from, and what the future has in store for humanity. “Planetary science” is shorthand for the broad array of scientific disciplines that collectively seek answers to these and related questions.

THE MOTIVATIONS FOR PLANETARY SCIENCE

In the past, scientists had only one planet to study in detail. Our Earth, however, the only place where life demonstrably exists and thrives, is a complex interwoven system of atmosphere, hydrosphere, lithosphere, and biosphere. Today, planetary scientists can apply their knowledge to the whole solar system, and to hundreds of worlds around other stars. By investigating planetary properties and processes in different settings, some of them far simpler than Earth, we gain substantial advances in understanding exactly how planets form, how the complex interplay of diverse physical and chemical processes creates the diversity of planetary environments seen in the solar system today, and how interactions between the physical and chemical processes on at least one of those planets led to the creation of conditions favoring the origin and evolution of multifarious forms of life. These basic motivational threads are built on and developed into the three principal science themes of this report—building new worlds, workings of solar systems, and planetary habitats—discussed in Chapter 3.

Current understanding of Earth’s surface and climate are constrained by studies of the physical processes operating on other worlds. The destructive role of chlorofluorocarbons in Earth’s atmosphere was recognized by a scientist studying the chemistry of Venus’s atmosphere. Knowledge of the “greenhouse” effect, a mechanism in the ongoing global warming on Earth, likewise came from studies of Venus. Comparative studies of the atmospheres of Mars, Venus, and Earth yield critical insights into the evolutionary histories of terrestrial planet atmospheres. Similarly, studies of the crater-pocked surface of the Moon led to current understanding of the key role played by impacts in shaping planetary environments. The insights derived from studies of lunar craters led to the realization that destructive impacts have wreaked havoc on Earth in the distant past, and as recently as 100 years ago a devastating blast in Siberia leveled trees over an area the size of metropolitan Washington, D.C. Three recent impacts on Jupiter provide our best laboratory for studying the mechanics of such biosphere-disrupting events. Wind-driven processes that shape Earth’s desert dunes operate on Mars and even on Saturn’s moon Titan.



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1 Introduction to Planetary Science How do planets form? What combination of initial conditions and subsequent geologic, chemical, and bio- logical processes led to at least one planet becoming the abode for innumerable life forms? What determines the fate of life on a planet? Such scientific enquiries reflect a basic human need to understand who we are, where we came from, and what the future has in store for humanity. “Planetary science” is shorthand for the broad array of scientific disciplines that collectively seek answers to these and related questions. THE MOTIVATIONS FOR PLANETARY SCIENCE In the past, scientists had only one planet to study in detail. Our Earth, however, the only place where life demonstrably exists and thrives, is a complex interwoven system of atmosphere, hydrosphere, lithosphere, and biosphere. Today, planetary scientists can apply their knowledge to the whole solar system, and to hundreds of worlds around other stars. By investigating planetary properties and processes in different settings, some of them far simpler than Earth, we gain substantial advances in understanding exactly how planets form, how the complex interplay of diverse physical and chemical processes creates the diversity of planetary environments seen in the solar system today, and how interactions between the physical and chemical processes on at least one of those planets led to the creation of conditions favoring the origin and evolution of multifarious forms of life. These basic motivational threads are built on and developed into the three principal science themes of this report—building new worlds, workings of solar systems, and planetary habitats—discussed in Chapter 3. Current understanding of Earth’s surface and climate are constrained by studies of the physical processes operating on other worlds. The destructive role of chlorofluorocarbons in Earth’s atmosphere was recognized by a scientist studying the chemistry of Venus’s atmosphere. Knowledge of the “greenhouse” effect, a mechanism in the ongoing global warming on Earth, likewise came from studies of Venus. Comparative studies of the atmospheres of Mars, Venus, and Earth yield critical insights into the evolutionary histories of terrestrial planet atmospheres. Similarly, studies of the crater-pocked surface of the Moon led to current understanding of the key role played by impacts in shaping planetary environments. The insights derived from studies of lunar craters led to the realization that destructive impacts have wreaked havoc on Earth in the distant past, and as recently as 100 years ago a devas- tating blast in Siberia leveled trees over an area the size of metropolitan Washington, D.C. Three recent impacts on Jupiter provide our best laboratory for studying the mechanics of such biosphere-disrupting events. Wind-driven processes that shape Earth’s desert dunes operate on Mars and even on Saturn’s moon Titan. 31

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32 VISION AND VOYAGES FOR PLANETARY SCIENCE Planetary science transcends national boundaries. Even during the depths of the Cold War, planetary scientists from both East and West frequently cooperated by exchanging samples from their respective lunar missions or by coordinating their independent missions to Halley’s Comet. Now, decades later, planetary science is a truly global endeavor. Spacecraft that explore the planets come not only from the United States, but also from China, India, Japan, and the nations of Western Europe. If the list is expanded to include nations with some space-based capacity—those that use spacecraft data, build spacecraft instruments, operate relevant ground-based facilities, or contribute in some other way to the advancement of planetary science—planetary science encompasses the globe. This chapter reviews the recommendations of the 2003 planetary science decadal survey and summarizes some of the most exciting recent scientific achievements. The chapter concludes with a discussion of the organization of this report, articulating how this and subsequent chapters relate to the the statement of task (Appendix A) for the Committee on the Planetary Science Decadal Survey. THE 2003 SOLAR SYSTEM EXPLORATION DECADAL SURVEY In the 1970s and 1980s, science strategies for exploring the solar system were drafted by the National Research Council’s (NRC’s) Committee on Planetary and Lunar Exploration (COMPLEX), which addressed separately the inner planets, the outer planets, and primitive bodies. Early in the 1990s, COMPLEX crafted a single solar system strategy that united and updated the several preexisting documents. The resulting report, An Integrated Strategy for the Planetary Sciences: 1995-2010,1 showed that it was both feasible and appropriate to establish a set of self- consistent, solar-system-wide priorities for planetary science. The Integrated Strategy provided the foundation upon which the planetary community’s first decadal survey was built, with the process starting in 2001. Unlike the precursor reports from COMPLEX, which only considered science goals, the 2003 decadal survey— New Frontiers in the Solar System: An Integrated Exploration Strategy—both outlined science priorities and identified new ini- tiatives needed to address those priorities.2 The study also advocated the creation of a new class of medium-size missions, named New Frontiers. The 2003 decadal survey’s statement of task from NASA called for prioritized missions binned in small, medium, and large categories with respective costs of less than $325 million, less than $650 million, and more than $650 million in then-year dollars. That survey prioritized Mars missions separately from missions to other solar system destinations. The present report provides status updates for the missions recommended in the 2003 survey. Non-Mars Mission Priorities in 2003 Large In the 2003 planetary science survey the only large mission identified was Europa Geophysical Explorer: a spacecraft to orbit Europa and determine the nature and depth of the subsurface ocean postulated to exist beneath Europa’s ice shell. Although much planning has occurred, the mission has not been initiated. Current efforts focus on implementing this mission in the context of a joint NASA-ESA Europa Jupiter System Mission (Chapters 8 and 9). Medium The 2003 planetary science decadal survey identified five medium-class initiatives to collectively initiate the competitively selected line of New Frontiers missions. These initiatives were, in priority order: 1. Kuiper Belt-Pluto Explorer—a mission to perform the initial spacecraft reconnaissance of the Pluto/Charon system as well as one or more other Kuiper belt objects. This mission is currently being implemented as the New Horizons mission launched in 2006 and scheduled to reach Pluto in 2015 (Figure 1.1). Subsequently, the spacecraft will be redirected so that it passes near to at least one additional Kuiper belt object, as was recommended in the 2003 planetary science decadal survey.

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33 INTRODUCTION TO PLANETARY SCIENCE FIGURE 1.1 Launch of the New Horizons mission in January 2006. New Horizons is the first of the New Frontiers missions and was a top priority in the 2003 decadal survey. New Horizons explores a completely new region of the solar system, the Kuiper belt, a region discovered by ground-based observers. SOURCE: NASA.

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34 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 1.2 The Juno mission to Jupiter under construction. SOURCE: NASA/JPL-Caltech/Lockheed Martin Space Systems. 2. South Pole-Aitken Basin Sample Return—a mission to return a sample from the oldest and deepest impact basin on the Moon. An implementation of this mission called MoonRise was a runner-up for the second New Frontiers selection and is currently a finalist for the third. Selection of the third New Frontiers mission is scheduled for 2011.3 3. Jupiter Polar Orbiter with Probes—a mission to determine the internal structure of Jupiter. An implementa- tion of this priority without probes called Juno was selected as the second New Frontiers mission. Juno is scheduled for launch in 2011 (Figure 1.2). 4. Venus In Situ Explorer—A mission to determine the geochemical characteristics of the surface of Venus and to study its atmosphere. An implementation of this mission was a runner-up for the second New Frontiers selection, and a new concept called the Surface and Atmosphere Geochemical Explorer (SAGE) is currently a finalist for the third selection.4

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35 INTRODUCTION TO PLANETARY SCIENCE 5. Comet Surface Sample Return—a mission to collect and return surface samples of a comet to Earth. This mission has not yet been attempted. The selection of New Horizons and Juno as the first two New Frontiers missions prompted NASA in 2007 to request a new NRC study to suggest additional candidate missions to supplement the remaining three. The subse- quent report, Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity ,5 identified five additional candidates. They were, in alphabetical order: • Asteroid Rover/Sample Return—a mission to rendezvous with an asteroid, land, collect surface samples, and return them to Earth for analysis. An implementation of this mission called the Origins Spectral Interpretation Resource Identification Security-Regolith Explorer (OSIRIS-REx) is currently a finalist for the third New Frontiers launch opportunity.6 • Ganymede Observer—a mission to perform detailed studies of the third of Jupiter’s Galilean satellites, the largest satellite in the solar system. • Io Observer—a mission to study the innermost of Jupiter’s Galilean satellites, the most volcanically active body in the solar system. • Network Science—a mission to deploy an array of small landers on the Moon or one of the terrestrial planets to perform coordinated geophysical and/or meteorological observations. • Trojan/Centaur/Reconnaissance—a mission to perform the initial characterization of one or more Trojan asteroids and a Centaur. Small The 2003 decadal survey identified two small-class initiatives. They were, in priority order: 1. Discovery program. The 2003 survey recommended that the Discovery line of innovative, principal- investigator-led missions should continue and that a new one should be launched approximately every 18 months (Figure 1.3). This mission line has continued, but the flight rate has not matched the 2003 decadal survey’s expectations. 2. Cassini extended mission. The 2003 decadal survey recommended that the Cassini Saturn orbiter mission be extended beyond its 4-year nominal lifetime. Operation of this highly successful and scientifically productive spacecraft (Figures 1.4 and 1.5) now extends through 2017. Mars Mission Priorities in 2003 Large Mars Sample Return. Although no large Mars mission was recommended, the 2003 survey called for initia- tion of the technology development necessary to enable a mission to collect and return martian samples to Earth in subsequent decades. Much programmatic planning and scientific groundwork have been performed to deter- mine how such a mission might be undertaken (Figures 1.6, 1.7, and 1.8), but not all of the necessary technology development has taken place. Medium The 2003 survey identified two medium-class Mars initiatives. They were, in priority order: 1. Mars Science Laboratory—At the time the 2003 survey was conducted, this mission was understood to be a lander capable of carrying out sophisticated surface observations and validating some of the technologies for a sample return mission. Since then, the concept has evolved into a large, highly capable rover mission with

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36 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 1.3 The nucleus of comet Tempel 1 at the moment it was struck by the impactor from the Deep Impact spacecraft on July 4, 2005. This was a Discovery mission. SOURCE: NASA/JPL-Caltech/University of Maryland. a comprehensive payload of remote and in situ instruments (Figure 1.9). In the process, the cost of the mission grew substantially, to more than $2 billion. Launch is scheduled for late 2011. 2. Mars Long-Lived Lander Network—This globally distributed array of small landers would be equipped to make comprehensive measurements concerning Mars’s interior, surface, and atmosphere. It has not yet been implemented.

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37 INTRODUCTION TO PLANETARY SCIENCE FIGURE 1.4 Saturn as imaged by the Cassini spacecraft. SOURCE: NASA/JPL. Small Two small-class Mars missions were identified in the 2003 survey. In priority order they were as follows: 1. Mars Scout Program—This line of competitively selected missions is similar in concept to the Discovery program. The 2003 survey envisaged one such mission every other Mars launch opportunity. Two Scout missions have been selected: Phoenix was selected in 2003 and launched in 2007, and the Mars Atmosphere and Volatile Evolution (MAVEN) mission was selected in 2008 for launch in 2013. Subsequently, the program was combined with Discovery. 2. Mars Upper Atmosphere Orbiter—This is an orbiter dedicated to studies of Mars’s upper atmosphere and plasma environment. The MAVEN mission, selected for the second and final Mars Scout launch opportunity, addresses the goals of this concept. Research Infrastructure In addition to identifying high-priority spacecraft missions, the 2003 decadal survey singled out two important new pieces of ground-based research infrastructure. They were, in alphabetical order: • Giant Segmented Mirror Telescope—This 30-meter-class general-purpose, optical telescope would be equipped with adaptive optics. The construction of such a facility has been a high priority in the last two (2001 and 2010) NRC astronomy and astrophysics decadal surveys.7,8 At least three consortia—one in Europe and two in the United States—have been developing plans and raising the funds necessary to begin construction of such a telescope.

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38 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 1.5 Titan beyond Saturn and its rings, as viewed by Cassini on May 10, 2006. SOURCE: NASA/JPL/Space Science Institute. • Large Synoptic Survey Telescope—This 8-meter-class special-purpose, wide-field telescope will survey the entire visible sky every three nights. This facility was the highest-priority ground-based initiative identified in the 2010 astronomy and astrophysics decadal survey and was also ranked highly in the 2001 survey. It is envisaged as being constructed and operated via a public-private consortium (Figure 1.10). RECENT ACHIEVEMENTS IN PLANETARY SCIENCE Twelve discoveries made since the publication of the 2003 planetary science decadal survey illustrate the vitality and diversity of planetary science. Listed below, these discoveries represent just a small fraction of the advances in planetary sciences over the past decade (see Chapters 4 through 8 for additional achievements). • An explosion in the number of known exoplanets. Confirmed examples have grown from a few dozen at the beginning of this decade to many hundreds, including numerous multi-planet systems. Multiple lines of evidence suggest that the majority are Uranus- and Neptune-size bodies, including microlensing surveys that seem to account for selection effects inherent in other detection techniques.

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39 INTRODUCTION TO PLANETARY SCIENCE FIGURE 1.6 A rock outcrop on Mars named Clovis, drilled and brushed by the Mars Exploration Rover (MER) Spirit in August 2004. In situ investigations, such as those performed by the MERs, have laid the scientific groundwork for future studies of Mars. SOURCE: JPL/NASA/Cornell University. • Evidence that the Moon is less dry than once thought. Evidence is mounting that the lunar surface and interior is not completely dry as previously believed. Apollo samples now show the Moon’s interior as holding more water than thought. Observations from Lunar Prospector, Lunar Reconnaissance Orbiter, LCROSS, Cassini, and Chandrayaan-1 also suggest small, but significant, quantities of water on the Moon, including exospheric and exogenic water generated by solar wind proton reduction and cometary deposits in the extremely cold regions of the lunar poles.

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40 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 1.7 Victoria crater as imaged by the Mars Reconnaissance Orbiter. The Opportunity rover traversed the edge of this crater before venturing into it. The synergistic combination of data from landers and orbiters has been a key aspect of the Mars science activities conducted in the past decade. SOURCE: NASA/JPL-Caltech/University of Arizona.

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41 INTRODUCTION TO PLANETARY SCIENCE FIGURE 1.8 Sand cascades on Mars, photographed by the Mars Reconnaissance Orbiter (MRO). Images from MRO have played an important role both in advancing Mars science activities in the past decade and in setting the agenda for future studies. SOURCE: NASA/JPL/University of Arizona. • Minerals that must have formed in a diverse set of aqueous environments throughout martian history. Observations from multiple orbiters and rovers have identified a broad suite of water-related minerals including sulfates, phyllosilicates, iron oxides and oxyhydroxides, chlorides, iron and magnesium clays, carbonates, and hydrated amorphous silica. • Extensive deposits of near-surface ice on Mars. These deposits are a major reservoir of martian water, and because of oscillating climate conditions, potentially lead to geologically brief periods of locally available liquid water. • An active meteorological cycle involving liquid methane on Titan. Observations from Cassini and Huygens have confirmed the long-suspected presence of complex organic processes on Titan. Moreover, they have revealed that an active global methane cycle mimics Earth’s water cycle. • Dramatic changes in the atmospheres and rings of the giant planets. Notable examples include observations of three impacts on Jupiter in 2009-2010; striking atmospheric seasonal change on Saturn and Uranus; evidence for vigorous polar vortices on Saturn and Neptune; and the discovery of rapid changes in the ring systems of Jupiter, Saturn, Uranus, and Neptune.

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42 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 1.9 Mars Science Laboratory Curiosity rover undergoing a test of its sample arm while on a tilt table at the Jet Propulsion Laboratory in September 2010. SOURCE: NASA/JPL-Caltech. • Recent volcanic activity on Venus. The European Space Agency’s Venus Express spacecraft has found zones of higher emissivity associated with volcanic regions, suggestive of recent volcanic activity. If correct, this finding supports models postulating that ongoing volcanic emission of SO 2 feeds the global H2SO4 clouds. • Geothermal and plume activity at the south pole of Enceladus. Observations by the Cassini spacecraft have revealed anomalous sources of geothermal energy coincident with curious rifts in the south polar region of Ence- ladus. The energy source appears to be responsible for plumes of ice particles and organic materials that emanate from discrete locations along the rifts. • The anomalous isotopic composition of the planets. Analysis of data from the Genesis solar wind sample return mission has revealed that the Sun is highly enriched in oxygen-16. The long-standing belief was that, rela- tive to the planets, the Sun was depleted in this isotope. The only materials that seem to have the average solar system composition of oxygen, besides the Sun, are refractory inclusions in chondrites. Some unknown process must be depleting the protoplanetary nebula’s oxygen-16 prior to the formation of the planets. • The differentiated nature of comet dust. Analysis of samples returned by the Stardust mission revealed that cometary dust contains minerals that can form only at high temperatures, close to the Sun (Figure 1.11). This result has changed ideas concerning the physical processes within the protoplanetary disk. • Mercury’s liquid core. Radar signals transmitted from NASA’s Deep Space Network station in California and detected by NRAO’s Green Bank Telescope detected Mercury’s forced libration and demonstration that the planet has a liquid core.

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43 INTRODUCTION TO PLANETARY SCIENCE FIGURE 1.10 The mirror for the Large Synoptic Survey Telescope prior to polishing and grinding. SOURCE: Howard Lester/ LSST Corporation. • The richness and diversity of the Kuiper belt. A combination of ground- and space-based telescopic studies has revealed the diversity of the icy bodies forming the Kuiper belt. This diversity includes many objects as large as or larger than Pluto and, intriguingly, a large proportion of binary and multi-object systems (Figure 1.12). Each of these recent achievements is a response to one or more of the basic motivations introduced at the beginning of this chapter that make planetary science a compelling field of study. Some of these achievements provide information on how planets form. Others say something about the physical and chemical processes that create planetary environments. Still others reveal something about the processes creating conditions favorable to life. The hallmark of these recent discoveries is their variety. From Mercury to the Kuiper belt at the solar system’s edge, from huge Jupiter to minuscule comet dust, no one class of objects dominates. Discoveries such as the plumes on Enceladus and the methane cycle on Titan were made by a NASA-ESA flagship mission. Other discoveries, such as the realization that cometary dust contains minerals that must have formed at high temperatures close to the Sun, came from small spacecraft costing a fraction of the cost of a flagship mission. Additional discoveries were made with ground-based telescopes supported by NSF and other national science agencies. Some of these discoveries were made by space-based telescopes supported by NASA and international space agencies; others—such as the recent spate of impacts on Jupiter—were found by amateur astronomers using backyard telescopes. In short, there is no one best way to do planetary science. A program that advances on a broad front is most likely to yield success.

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44 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 1.11 From a comet, to the desert, to the laboratory, to the Smithsonian—the Stardust mission encountered the comet Wild 2 in January 2004 and returned samples to Earth in January 2006 (top). The samples are still being examined (bottom) but have provided new understanding of our solar system. The capsule now resides in the National Air and Space Museum ( right). SOURCE: Top, bottom: NASA/JPL. Right: Smithsonian Institution National Air and Space Museum. SCOPE OF THIS REPORT The scientific scope of this report spans two dimensions: first, the principal scientific disciplines that collec- tively encompass the ground- and space-based elements of planetary science: i.e., planetary astronomy, geology, geophysics, atmospheric science, magnetohydrodynamics, celestial mechanics, and astrobiology; and second, the physical territory within the committee’s purview, the solar system’s principal constituents. This territory includes the following:

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45 INTRODUCTION TO PLANETARY SCIENCE FIGURE 1.12 Pluto and its large moon Charon, and two smaller moons, Nix and Hydra, discovered in 2005. SOURCE: NASA, ESA, H. Weaver (Johns Hopkins University Applied Physics Laboratory), A. Stern (Southwest Research Institute), and the Hubble Space Telescope Pluto Companion Search Team. • The major rocky bodies in the inner solar system: Mercury, Venus, the Moon, and Mars; • The giant planets in the outer solar system—Jupiter, Saturn, Uranus, and Neptune—including their rings and magnetospheres; • The satellites of the giant planets; and • Primitive solar system bodies: the comets, asteroids, satellites of Mars, interplanetary dust, meteorites, Centaurs, Trojans, and Kuiper belt objects.

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46 VISION AND VOYAGES FOR PLANETARY SCIENCE The committee imposed programmatic boundary conditions, derived largely from its statement of task, to ensure that this report contains actionable advice: • The principal findings and recommendations contained in New Frontiers in the Solar System and more recent NRC reports relevant to planetary science activities were assessed, and incorporated where appropriate. Missions identified in those past reports were reprioritized if they had not yet been confirmed for implementation. • Priorities for spacecraft missions to the Moon, Mars, and other solar system bodies were treated in a unified manner with no predetermined “set-asides” for specific bodies. This approach differs distinctly from the ground rules for the 2003 planetary science decadal survey, in which missions to Mars were prioritized separately. • The committee’s programmatic recommendations were designed to be achievable within the boundaries of anticipated NASA and NSF funding. • The report is cognizant of the current statutory roles of the National Science Foundation (NSF) and NASA, and how these roles may or may not be consistent with current practices within the two agencies regarding sup- port for specific activities—for example, the funding mechanisms, construction, and operation of ground-based observatories. • The report reflects an awareness of the science and space mission plans and priorities of potential foreign and U.S. agency partners (such as the Department of Energy for plutonium-238 and the Department of Defense for launch vehicles). This report’s recommendations are, however, addressed to NASA and NSF. To maintain consistency with other advice developed by the NRC and to ensure that this report clearly addresses those topics identified in the committee’s statement of task, the following topics are not addressed in this report: • Issues relating to the hazards posed by near-Earth objects and approaches to hazard mitigation. Relevant material on the hazard issue is contained in Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies.9 However, scientific studies of near-Earth asteroids are discussed in this report. • Study of the Earth system, including its atmosphere, magnetosphere, surface, and interior. A relevant discussion of these topics and recommendations relating to them can be found in Earth Science and Applications from Space—National Imperatives for the Next Decade and Beyond.10 • Studies of solar and heliospheric phenomena, with the exception of interactions with the atmospheres, magnetospheres, and surfaces of solar system bodies; and magnetospheric effects of planets on their satellites and rings. A relevant discussion of solar and heliophysics phenomena can be found in The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics11 and in a new heliophysics decadal survey scheduled for publication in 2012. • Ground- and space-based studies to detect and characterize extrasolar planets. Details and recommenda- tions relating to the detection and characterization of extrasolar planets and other aspects of contemporary stellar, galactic, and extragalactic astronomy are given in New Worlds, New Horizons in Astronomy and Astrophysics.12 However, the present report does contain a discussion of the scientific issues concerning the comparative planetol- ogy of the solar system’s planets and extrasolar planets, together with issues related to the formation and evolution of planetary systems. ORGANIZATION OF THIS REPORT The committee’s statement of task (Appendix A) calls for this report to contain three principal elements: a survey of planetary science; an assessment of and recommendations relating to NASA activities; and an assessment of and recommendations relating to NSF activities. The following sections map its chapters onto the specific tasks the committee was asked to address.

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47 INTRODUCTION TO PLANETARY SCIENCE Survey of Planetary Science • Overview of planetary science, what it is, why it is a compelling undertaking, and the relationship between space- and ground-based planetary science research—The scientific context is discussed in Chapter 1, and the relationship between space- and ground-based research and related programmatic issues relating to planetary sci- ence activities at NASA and NSF are considered in Chapter 2. • Survey of the current state of knowledge of the solar system—A high-level overview of current knowledge, together with a discussion of three crosscutting themes and 10 high-priority questions underlying most current activities in this field, is presented in Chapter 3. The priority questions introduced in Chapter 3 are then developed and refined for the primitive bodies, the inner planets, Mars, the giant planets, and the satellites of the giant planets in Chapters 4, 5, 6, 7, and 8, respectively. • Inventory of the top-level science questions that should guide NASA flight mission investigations and supporting research programs and NSF’s activities—presented in Chapters 4, 5, 6, 7, and 8 and summarized in Chapter 3. NASA Activities • Optimum balance across the solar system and among small, medium, and large missions and supporting activities—Chapter 9. • Individual flight investigations for initiation between 2013 and 2022—Priority large, medium, and small spacecraft missions are discussed, and recommendations supported by decision rules are given, in Chapter 9. • Supporting research required to maximize the science return from the flight mission investigations—Chapter 10. • Strategic technology development needs and opportunities relevant to NASA planetary science programs— Chapter 11. • Discussion of potential opportunities for conducting planetary science investigations involving humans in situ and the value of human-tended investigations relative to those performed solely robotically —Chapter 2. • Opportunities for international cooperation—Chapter 2 and Chapter 9. NSF Activities • Assessment of NSF support for the planetary sciences—A detailed discussion of relevant NSF activities, including support for infrastructure and research programs, together with related recommendations, is given in Chapter 10. • Opportunities for joint ventures and other forms of international cooperation—discussed in Chapter 2 and Chapter 10. A Guide to Reading This Report There are many ways that individuals can and will read this report. Ideally, every reader will begin at the beginning and work his or her way through to the end. But this approach is not essential. Indeed, the desires of most readers will be satisfied by the selective reading of different parts of this report. The remainder of this section and Figure 1.13 serve as a reader’s guide. The fundamental principle used to frame this report derives from a hierarchy of science priorities. The core of the report (Chapters 4 through 9) is devoted to working from major, foundational topics that drive the overall planetary program—the origins of the solar system, the workings of planets, and the conditions that promote the emergence of life (the themes and priority questions discussed in Chapter 3)—to the science missions that the committee has identified as the top planetary science spacecraft activities for the coming decade (Chapter 9).

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48 VISION AND VOYAGES FOR PLANETARY SCIENCE Chapter  1   Introduc)on  to  Planetary  Science   Chapter  2   Na)onal  and  Interna)onal  Programs   Chapter  3   Priority  Ques)ons  for  the  Next  Decade   Chapter  4   Chapter  5   Chapter  6   Chapter  7   Chapter  8   Primi)ve  Bodies   The  Inner  Planets   Mars   The  Giant  Planets   Satellites             Science  Goals   Science  Goals   Science  Goals   Science  Goals   Science  Goals             Key  Ques)ons   Key  Ques)ons   Key  Ques)ons   Key  Ques)ons   Key  Ques)ons             Future   Future   Future   Future   Future   Inves)ga)ons   Inves)ga)ons   Inves)ga)ons   Inves)ga)ons   Inves)ga)ons             Priority  Missions   Priority  Missions   Priority  Missions   Priority  Missions   Priority  Missions   Chapter  9   Recommended  Flight  Inves)ga)ons   Chapter  10   Support  for  Research  and  Infrastructure   Chapter  11   Technology  Development   Chapter  12   A  Look  to  the  Future   FIGURE 1.13 Schematic showing the flow of discussion from one chapter to the next.

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49 INTRODUCTION TO PLANETARY SCIENCE The major questions forming the foundations of planetary science deal with topics that will almost certainly not be fully addressed in a single decade. Rather, many generations of scientists have already labored over them, and additional generations will likely follow suit. The topics discussed in Chapter 3 are too broad and too funda- mental to be fully addressed in the period 2013-2022. However, a general reader interested in the current scope of, and key motivations for undertaking, activities in the planetary sciences need only read Chapters 1, 2, and 3. Those general readers interested in a preview of the spacecraft missions recommended for implementation in the decade to come should jump to Chapter 9. A decadal plan must be based on the identification and exploitation of those components or subcomponents of the big, foundational topics showing the most promise of resolution in the coming 10 years. Chapters 4 through 8 contain the most basic breakdown of these foundational topics, divided largely in terms of locations in the solar system—i.e., the inner planets (Chapter 5), Mars (Chapter 6), the giant planets (Chapter 7) and their satel- lites (Chapter 8), and the myriad small bodies that are scattered throughout the solar system (Chapter 4). Thus, Chapters 4 through 8 are devoted to the identification of the particular aspects of Chapter 3’s crosscutting themes and questions showing the greatest promise for resolution in the next 10 years. Chapters 4 through 8 all follow the same general outline, starting with a link to key science questions in Chapter 3, outlining the science goals, identifying important questions and future directions, addressing any necessary technology development, and, finally, discussing potential missions. Some of the big questions can be better addressed at some specific destinations in the solar system rather than others. Chapters 4 through 8 lay out questions best addressed by visits to the inner planets, to Mars, to the giant planets and their satellites, and to primitive bodies such as asteroids and comets, and begin to define the missions that can gather the data that can answer specific aspects of important questions. Thus, readers with a deeper inter- est in current planetary science research activities should concentrate on Chapters 4 through 8 and then move on to the discussion of high-priority spacecraft missions in Chapter 9. If readers require more details on the research, infrastructure, and technology required to support these missions, they can turn to Chapters 10 and 11. Readers who are most interested in near-term matters of public policy will naturally turn to Chapters 9, 10, and 11 to understand what programs the committee has recommended for initiation or for continuation of fund- ing, but they will gain a full understanding of why the committee has reached these conclusions by starting with the big questions. NOTES AND REFERENCES 1 . National Research Council. 1994. An Integrated Strategy for the Planetary Sciences: 1995-2010. National Academy Press, Washington, D.C. 2 . National Research Council. 2003. New Frontiers in the Solar System: An Integrated Exploration Strategy. The National Academies Press, Washington, D.C. 3 . On May 25, 2011, following the completion of this report, NASA selected the OSIRIS-REx asteroid sample-return spacecraft as the third New Frontiers mission. Launch is scheduled for 2016. 4 . On May 25, 2011, following the completion of this report, NASA selected the OSIRIS-REx asteroid sample-return spacecraft as the third New Frontiers mission. Launch is scheduled for 2016. 5 . National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. The National Academies Press, Washington, D.C. 6 . On May 25, 2011, following the completion of this report, NASA selected the OSIRIS-REx asteroid sample-return spacecraft as the third New Frontiers mission. Launch is scheduled for 2016. 7 . National Research Council. 2001. Astronomy and Astrophysics in the New Millennium. National Academy Press, Wash- ington, D.C. 8 . National Research Council. 2010. New Worlds, New Horizons in Astronomy and Astrophysics. The National Academies Press, Washington, D.C. 9 . National Research Council. 2010. Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies. The National Academies Press, Washington, D.C. 10 . National Research Council. 2007. Earth Science and Applications from Space—National Imperatives for the Next Decade and Beyond. The National Academies Press, Washington, D.C.

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11 . National Research Council. 2003. The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics. The National Academies Press, Washington, D.C. 12 . National Research Council. 2010. New Worlds, New Horizons in Astronomy and Astrophysics. The National Academies Press, Washington, D.C.