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5 The Inner Planets: The Key to Understanding Earth-Like Worlds Earth’s inner solar system companions, Mercury, Venus, the Moon, and Mars, are diverse bodies, each of which provides data critical for understanding the formation and evolution of habitable worlds like our own. These ter- restrial (or rocky) planetary bodies have a range of compositions and geologic histories—each is a unique world that reveals information crucial for understanding the past, present, and future of Earth. This chapter focuses on three particular inner bodies, Mercury, Venus, and the Moon (Figure 5.1). All are essential to understanding how terrestrial planets form and change with time.1 Current knowledge of these bodies differs, with exploration challenges and major accomplishments (Table 5.1) at each. Within the past decade, initial results from the MESSENGER spacecraft have revealed aspects of the complex early history of Mercury. Venus, with its greenhouse atmosphere, Earth-like size, and volcanic surface, has been a focus of recent international missions but remains a challenge for in situ exploration. Recent exploration of the Moon has revealed a geochemically complex surface and polar volatiles (e.g., hydrogen or ice), leading to significant unanswered questions about the Earth-Moon system. The detailed study of Mars 2 over the past 15 years has greatly increased our understanding of its history, which in turn has allowed us to formulate specific questions to constrain terrestrial planet origin, evolution, and habitability. Thus, the initial reconnaissance of the terrestrial planets is transitioning to more in-depth, in situ study. In this new phase, specific observations can be made to allow the testing of hypotheses and significant progress in finding answers to basic questions that can lead us to an improved understanding of the origin and evolution of all of the terrestrial planets, including Earth. All three of the crosscutting science themes for the exploration of the solar system include the inner planets, and studying the inner planets is vital to answering several of the priority questions in each of the three themes. The building new worlds theme includes the question, What governed the accretion, supply of water, chemistry, and internal differentiation of the inner planets and the evolution of their atmospheres, and what roles did bom- bardment by large projectiles play? The planetary habitats theme includes the question, Did Mars or Venus host ancient aqueous environments conducive to early life, and is there evidence that life emerged? The workings of the solar systems theme includes two questions that can be answered by the study of the inner planets. First, the lunar impact record holds key information of relevance to the question, What solar system bodies endanger Earth’s biosphere, and what mechanisms shield it? Second, studies of Venus and Mars relate directly to the question, Can understanding the roles of physics, chemistry, geology, and dynamics in driving planetary atmospheres and climates lead to a better understanding of climate change on Earth? Questions about how the inner planets formed, about 111
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112 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 5.1 Mercury (left), Venus (middle), and the Moon (right) are essential to understanding how terrestrial planets form and change with time. SOURCE: Mercury, NASA/JPL; Venus, NASA/JPL/USGS; Moon, NASA/JPL. their composition, and about the processes by which they have evolved are a major part of the question, How have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time? SCIENCE GOALS FOR THE STUDY OF MERCURY, VENUS, AND THE MOON The overarching concept that drives the study and exploration of Mercury, Venus, and the Moon is comparative planetology—the idea that learning about the processes and history of one planet (including Earth) is enabled by an understanding of and comparison to other planets. An understanding of any individual planet relies on knowledge of the whole solar system, which in turn relies on an in-depth exploration of every component of the system: from dust to planets, from Mercury to the outermost comets, from the Sun’s deep interior to the far reaches of the interstellar medium. Comets and asteroids (and meteorites and dust from them) preserve clues to the formation of the solar system and its planets; now-quiescent bodies like the Moon and Mercury preserve evidence of the early histories of the ter- restrial planets; large, active planets like Venus and Mars show some of the variety of geologic and climatic processes; all help in understanding Earth’s past, present, and possible futures. And, as the number of known extrasolar planets continues to grow, the goal of understanding Earth and its life takes on the broader dimension of the search for habit- able bodies around other stars. The goals for research concerning the inner planets for the next decade are threefold: • Understand the origin and diversity of terrestrial planets. How are Earth and its sister terrestrial planets unique in the solar system, and how common are Earth-like planets around other stars? Addressing this goal will require constraining the range of terrestrial planet characteristics, from their compositions to their internal structure to their atmospheres, to refine ideas of planet origin and evolution. • Understand how the evolution of terrestrial planets enables and limits the origin and evolution of life. What conditions enabled life to evolve and thrive on early Earth? The Moon and Mercury preserve early solar system his- tory that is a prelude to life. Venus is a planet that was probably much like Earth but is now not habitable. Together, the inner planets frame the question, Why is Earth habitable, and what is required of a habitable planet? • Understand the processes that control climate on Earth-like planets. What determines the climate balance and climate change on Earth-like planets? Earth’s climate system is extraordinarily complex, with many interrelated feedback loops. To refine concepts of climate and its change, it is important to study other climate systems, like those of Venus, Mars, and Titan, which permit us to isolate some climate processes and quantify their importance. Subsequent sections examine each of these goals in turn.
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113 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS TABLE 5.1 Major Accomplishments of Studies of Mercury, Venus, and the Moon in the Past Decade Major Accomplishment Mission and/or Technique Demonstrated from measurement of Mercury’s forced libration that the planet has a Earth-based radar studies liquid core Found evidence that volcanism has been widespread throughout Mercury’s geologic MESSENGER history, with compelling evidence for pyroclastic volcanism, which requires interior volatiles at higher abundances than were previously believed to exist Identified zones of locally higher emissivity associated with volcanic centers on Venus Express Venus, suggestive of geologically recent volcanic activity Measured lower atmospheric loss rates for hydrogen and higher rates for oxygen, Venus Express suggesting that Venus may be more hydrated and less oxidized than previously believed Discovered higher quantities of water on the Moon than were previously believed Lunar Prospector, Cassini, LRO/LCROSS, to exist, including interior endogenous water and exogenic water generated by solar Deep Impact, and Chandrayaan-1 wind interactions with silicates and cometary deposits in the extremely cold regions at the lunar poles Concluded that a potential lunar impact cataclysm also affected all planets in the Theory and modeling of orbital dynamics inner solar system and may have resulted from changes in the orbital dynamics of correlated with the history of impact the gas giants fluxes throughout the solar system UNDERSTAND THE ORIGIN AND DIVERSITY OF TERRESTRIAL PLANETS The solar system includes a diversity of rocky planetary bodies, including the terrestrial planets (Mercury, Venus, the Moon, Earth, and Mars), the asteroids, and many outer solar system satellites. Despite their differ- ences, common physical processes guided the formation and evolution of all these bodies. The inner planets are the most accessible natural laboratories for exploring the processes that form and govern the evolution of planets such as Earth. Understanding the origin and diversity of terrestrial planets encompasses the broad base of research through which scientists compare these terrestrial bodies and learn how they form and evolve. This knowledge is the foundation for understanding how rocky planets work: how they formed early in solar system history; how they acquired their compositions, internal structures, surfaces, and atmospheric dynamics; and what processes have been important throughout their histories. Key questions, such as those concerning the development and evolu- tion of life and the intricacies of planetary climate change, can only be formulated and addressed by building this base of knowledge. Fundamental objectives associated with the goal of understanding the origin and diversity of terrestrial planets include the following: • Constrain the bulk composition of the terrestrial planets to understand their formation from the solar nebula and controls on their subsequent evolution; • Characterize planetary interiors to understand how they differentiate and dynamically evolve from their initial state; and • Characterize planetary surfaces to understand how they are modified by geologic processes. Subsequent sections examine each of these objectives in turn, identify critical questions to be addressed, and suggest future investigations and measurements that could provide answers.
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114 VISION AND VOYAGES FOR PLANETARY SCIENCE Constrain the Bulk Composition of the Terrestrial Planets to Understand Their Formation from the Solar Nebula and Controls on Their Subsequent Evolution Understanding the bulk composition of a planet is key to constraining its origin and subsequent evolution. A planet’s bulk composition reflects the interplay and convolution of many processes in the early solar system: the transport of dust and gas in the early solar nebula, compositional gradients in the early nebula imposed by time or distance from the Sun, the accretion of solids to form self-gravitating bodies, the gravitational scattering of those bodies, impacts among those bodies (possibly with chemical fractionation), and the redistribution of volatile ele- ments in response to thermal gradients and impact events. After formation, a planet’s bulk chemical composition is key to its subsequent evolution; for example, the abundance and distribution of heat-producing elements underlie planetary differentiation, magmatism, and interior dynamical and tectonic processes. Basic information on surface composition, internal structure, and volatile inventories provides important con- straints on the bulk major-element composition of the terrestrial planets. Although little progress has been made in the past decade to help determine Venus’s bulk composition, major strides have been made in understanding the bulk compositions of Mercury and the Moon. Mercury’s high bulk density implies that it is rich in metallic iron. Reflectance spectra from Earth and initial observations from the MESSENGER spacecraft are ambiguous with regard to the composition of Mercury’s crust. These spectra suggest that Mercury’s surface materials contain little ferrous iron,3,4 whereas preliminary results by MESSENGER’s neutron spectrometer suggest abundant iron or titanium (Figure 5.2).5 Substantial research efforts in the past decade using Lunar Prospector and Clementine data, plus new basaltic lunar meteorites, have provided refined estimates of the compositions of the lunar crust and mantle. New observations from Apollo samples have been interpreted as indicating that the bulk volatile content of the Moon is more water-rich than had been thought; if true, this has profound implications for the origin of the Earth-Moon system. Important Questions Some important questions for using the bulk compositions of the terrestrial planets to understand their forma- tion from the solar nebula and controls on their subsequent evolution include the following: • What are the proportions and compositions of the major components (e.g., crust, mantle, core, atmosphere/ exosphere) of the inner planets? • What are the volatile budgets in the interiors, surfaces, and atmospheres of the inner planets? • How did nebular and accretionary processes affect the bulk compositions of the inner planets? Future Directions for Investigations and Measurements Significant progress in understanding the bulk compositions of the inner planets can be made through in situ and orbital investigations of planetary surfaces, atmospheres, and interiors. Future investigations and measurements should include the development of improved understanding of the various types of rock and regolith making up the crusts and mantles of the inner planets, through remote sensing of Mercury’s crust, in situ investigation of Venus’s crust, and sample return of crust and mantle materials from the Moon. Key geophysical objectives include the characterization of the Moon’s lower mantle and core and the development of an improved understanding of the origin and character of Mercury’s magnetic field. Understanding Venus’s bulk composition and interior evolution awaits the critical characterization of the noble gas molecular and isotopic composition of the Venus atmosphere. Improved modeling of solar system formation and the facilitation of searches for and analyses of extrasolar planetary systems hold great promise for understanding the composition and evolution of the terrestrial planets in general.
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115 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS FIGURE 5.2 Rembrandt impact basin on Mercury photographed by MESSENGER. Rembrandt spans more than 700 km and at 4 billion years old is possibly the youngest large impact basin on the planet. Geologic analysis indicates that the basin expe- rienced multiple stages of volcanic infilling and tectonic deformation. SOURCE: Courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, from the cover of Science, Vol. 324, No. 5927, May 1, 2009; reprinted with permission from AAAS. Characterize Planetary Interiors to Understand How They Differentiate and Dynamically Evolve from Their Initial State Knowledge of the internal structure of the terrestrial planets is key to understanding their histories after accretion. Differentiation is a fundamental planetary process that has occurred in numerous solar system bodies. Important aspects of differentiation include heat-loss mechanisms, core-formation processes, magnetic-field gen- eration, distribution of heat-producing radioactive elements, styles and extent of volcanism, and the role of giant impacts. The analysis of lunar samples implies that the Moon formed hot, with a magma ocean more than 400 km deep. The heat of accretion that led to magma oceans on Earth and the Moon may have been common to all large
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116 VISION AND VOYAGES FOR PLANETARY SCIENCE rocky planets, or it may have been stochastically distributed based on the occurrences of giant impact processes. All of the large terrestrial planets differentiated into rocky crusts, rocky mantles, and metallic cores, and variously continued to dissipate internal energy through mantle convection, magmatism, magnetic dynamos, and faulting, although only Earth appears to have sustained global plate tectonics. Radar observations of Mercury’s rotational state from Earth and improved knowledge of Mercury’s gravity field by MESSENGER have led to the detection of a liquid outer core on Mercury, advancing our understanding of the internal structure and thermal state.6,7 The dynamic nature of Mercury’s interior has been supported by MESSENGER flyby on the internal origin of the planet’s magnetic field 8 and its discovery of extensive volcanic deposits.9,10 The discovery of new lunar rock types from both meteorites and remote sensing data has provided insight into the differentiation of the Moon and the composition and evolution of its crust and mantle. Studies of lunar meteorites as well as improved knowledge of the ages, compositions, and spatial distribution of volcanics have offered new insights into the thermal and magmatic history of the Moon. Although there has been limited progress on understanding the internal structure, evolution, and dynamics of Venus over the past decade, recent results from Venus Express and Galileo may suggest a dynamic history with potentially evolved igneous rock compositions in some tessera areas, as well as very young volcanism. 11,12 Important Questions Some important questions concerning characterizing planetary interiors to understand how they differentiate and evolve from their initial state include the following: • How do the structure and composition of each planetary body vary with respect to location, depth, and time? • What are the major heat-loss mechanisms and associated dynamics of their cores and mantles? • How does differentiation occur (initiation and mechanisms) and over what timescales? Future Directions for Investigations and Measurements Advancing the understanding of the internal evolution of the inner planets can be achieved through research and analysis activities as well as by data from new missions at the Moon, Mercury, and Venus. Obtaining higher- resolution topography of Venus would provide new insights into the emplacement mechanisms of features such as mountains and lava flows. Key lunar investigations include determining the locations and mechanisms of seismicity and characterizing the lunar lower mantle and core. New analysis of the ages, isotopic composition, and petrology (including mineralogy) of existing lunar samples, of new samples from known locations, and of remotely sensed rock and regolith types, and the continued development of new techniques to glean more information from samples will form the basis of knowledge regarding the detailed magmatic evolution of the Moon. Experimental petrology, fluid, and mineral physics and the numerical modeling of planetary interiors are crucial to understanding processes that cannot be directly observed and to providing frameworks for future observations. Characterize Planetary Surfaces to Understand How They Are Modified by Geologic Processes The distinctive face of each terrestrial planet results from dynamic geologic forces linked to interactions among the crust, lithosphere, and interior (e.g., tectonism and volcanism); between the atmosphere and hydrosphere (e.g., erosion and mass wasting, volatile transport); and with the external environment (e.g., weathering and erosion, impact cratering, solar wind interactions). The stratigraphic record of a planet records these geologic processes and their sequence. The geologic history of a planet can be reconstructed from an understanding of these geologic processes and the details of that planet’s stratigraphic record. New data from Clementine, Lunar Prospector, LRO, and various international missions (Smart-1, Kaguya, Chang’e-1, and Chandrayaan-1) illustrate a diversity of surface features on the Moon, including fault scarps, lava tubes, impact melt pools, polygonal contraction features, and possible outgassing scars. The timing and extent of lunar magmatism have been extended by means of crater counting and new meteorite samples. The understanding of impact processes has been enhanced by models of crater formation and ejecta distribution, and knowledge of
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117 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS the lunar impact flux has been improved using dynamical modeling and new ages for lunar samples. Although the nature of lunar polar volatile deposits was probed by the LCROSS impactor mission and by instruments aboard LRO and Chandrayaan-1, the form, extent, and origin of such deposits are not fully understood. Continued analysis of Magellan measurements has revealed extensive tectonism and volcanism on Venus, with great debate over the rates of resurfacing; recent infrared emissivity results from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the Venus Express spacecraft show that resurfacing processes have continued as recently as 2 million years ago.13 MESSENGER flybys of Mercury have provided views of the regions unseen by Mariner 10 and indicate a surface history that is more dynamic than previously thought. The diversity of terrains observed by MESSENGER suggests a complex evolution, including extensive tectonism and young volcanism and pyroclastic activity.14,15,16 Important Questions Some important questions concerning the characterization of planetary surfaces to understand how they are modified by dynamic geologic processes include the following: • What are the major surface features and modification processes on each of the inner planets? • What were the sources and timing of the early and recent impact flux of the inner solar system? • What are the distribution and timescale of volcanism on the inner planets? • What are the compositions, distributions, and sources of planetary polar deposits? Future Directions for Investigations and Measurements Major advances in our understanding of the geologic history of the inner planets will be achieved in the coming decade through the orbital remote sensing of Venus, the Moon, and Mercury, as well as from in situ data from Venus and the Moon. Key among these achievements will be the global characterization of planetary morphology, stratigraphy, composition, and topography; the modeling of the time variability and sources of impacts on the inner planets; and the continued analysis of sample geochronology to help provide constraints on the models. Also crucial will be developing an inventory and isotopic composition of lunar polar volatile deposits to understand their emplacement and origin, modeling conditions and processes occurring in permanently shadowed areas of the Moon and Mercury, and the continued observation of Mercury’s volatile deposits to understand their origin. UNDERSTAND HOW THE EVOLUTION OF TERRESTRIAL PLANETS ENABLES AND LIMITS THE ORIGIN AND EVOLUTION OF LIFE Is Earth the only planet that has (or had) life? Understanding how the evolution of the terrestrial planets enables and limits the origin and evolution of life is closely aligned with other NASA efforts, including astrobiology and Mars exploration. This goal is also is relevant to the study of Mars; moons like Europa, Enceladus, and Titan; and terrestrial planets orbiting stars other than the Sun. The existence of life, present or past, depends on planetary context and the availability of energy, nutrients, and clement environments. Thus, it is crucial to explore the inner solar system in great detail in order to understand the constraints on and possible timing of habitable conditions. The Moon and Mercury are unlikely to harbor life, but they provide critical records of processes and information about the early solar system when life emerged on Earth. Earth is the single known planet that provided all of the necessities for the origin and persistence of life, but Venus may have once supported oceans of liquid water and so, possibly, life. Similarly, Mars’s surface shows signs of abundant water in its distant past and may likewise have supported life. Finally, learning about the cir- cumstances that limit or promote the origin and evolution of life will inform current understanding of extrasolar planets and the search for life in the universe. Fundamental objectives that will help in understanding how the evolution of terrestrial planets enables and limits the origin and evolution of life are as follows:
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118 VISION AND VOYAGES FOR PLANETARY SCIENCE • Understand the composition and distribution of volatile chemical compounds; • Understand the effects of internal planetary processes on life and habitability; and • Understand the effects of processes external to a planet on life and habitability. Subsequent sections examine each of these objectives in turn, identify critical questions to be addressed, and suggest future investigations and measurements that could provide answers. Understand the Composition and Distribution of Volatile Chemical Compounds To address objectives relating to the composition and distribution of volatile chemical compounds, it is crucial to improve the understanding of the sources, sinks, and physical states of water and of chemical compounds con- taining hydrogen, carbon, oxygen, sulfur, phosphorus, and nitrogen on and in the inner planets (including Mars), as functions of time and position in the solar system. These compounds are the basis of life as we know it, as well as the prebiotic chemistry that can form under a limited known range of physical conditions (e.g., pressure, temperature, electromagnetic fields, and radiation environments). The understanding of the distribution of volatiles in the inner solar system has advanced significantly in the past decade, due in large part to ongoing NASA spacecraft missions and research programs. Remote sensing of the Moon has shown that broad areas near the poles contain significant hydrogen; recent radar data suggest that some of this hydrogen is present as water ice. The LCROSS impact experiment detected abundant volatiles at one shadowed polar region. Results from the Moon Mineralogy Mapper spectrometer on India’s Chandrayaan-1 spacecraft have detected widespread water (or hydroxyl) in the regolith at higher latitudes. In addition, sample analyses show that some beads of lunar volcanic glass and minerals from mare basalts contain concentrations of hydrogen high enough to suggest that their parent magma contained as much water as Earth’s mantle does. These results are new, and their interpretation is still in flux, but they may overturn the conventional wisdom that the Moon is “dry.” Regarding Mercury, Earth-based radars have located deposits in polar craters that are probably water ice. Among the MESSENGER spacecraft’s discoveries so far are young volcanic pyroclastic deposits, which suggest sufficient internal volatiles to nucleate and grow bubbles in ascending magmas. More evidence on the presence and perhaps distribution of hydrogen on the surface of Mercury can be anticipated from the spacecraft’s neutron spectrometer (which will map the abundance of hydrogen in the regolith) and its VNIR spectrometer (which may detect some hydrous minerals if they are present). The understanding of the volatile budget and history of Venus has also advanced, mostly through improved knowledge of its current atmosphere. Venus Express VIRTIS and Galileo NIMS infrared images of Venus’s surface suggest that tesserae may be composed of felsic rock (e.g., perhaps comparable to granites on Earth), a finding that would be consistent with the production of hydrous (and perhaps sodium- and/or potassium-rich) magmas in Venus’s early history. Important Questions Some important questions relating to the composition and distribution of volatile chemical compounds include the following: • How are volatile elements and compounds distributed, transported, and sequestered in near-surface environ- ments on the surfaces of the Moon and Mercury? What fractions of volatiles were outgassed from those planets’ interiors, and what fractions represent late meteoritic and cometary infall? • What are the chemical and isotopic compositions of hydrogen-rich (possibly water ice) deposits near the Moon’s surface? • What are the inventories and distributions of volatile elements and compounds (species abundances and isotopic compositions) in the mantles and crusts of the inner planets? • What are the elemental and isotopic compositions of species in Venus’s atmosphere, especially the noble gases and nitrogen-, hydrogen-, carbon-, and sulfur-bearing species? What was Venus’s original volatile inven-
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119 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS tory, and how has this inventory been modified during Venus’s evolution? How and to what degree are volatiles exchanged between Venus’s atmosphere and its solid surface? • Are Venus’s highlands and tesserae made of materials suggestive of abundant magmatic water (and possibly liquid water on the surface)? Future Directions for Investigations and Measurements Key to constraining the character of volatile chemical compounds on Venus, the Moon, and Mercury is deter- mining (1) the state, extent, and chemical and isotopic compositions of surface volatiles, particularly in the polar regions on the Moon and Mercury; (2) the inventories and isotopic compositions of volatiles in the mantle and crust of all of the terrestrial planets; and (3) the fluxes of volatiles to the terrestrial planets (e.g., by impact) over time. Of high importance for Venus is to obtain high-precision analyses of the light stable isotopes (especially carbon, hydrogen, oxygen, nitrogen, and sulfur) in the lower atmosphere and noble gas concentrations and isotopic ratios throughout its atmosphere. Also key is the continued evaluation of the effects of meteoroid impact fluxes and intensities on the development and evolution of life on the inner planets through an analysis of the impact record on the Moon and Mercury. Understand the Effects of Internal Planetary Processes on Life and Habitability It is crucial to understand how planetary environments can enable or inhibit the development and sustain- ment of prebiotic chemistry and life. This objective focuses on the availability of accessible energy and nutrients (chemicals and compounds) and on the establishment and maintenance of clement, stable environments in which life could have arisen and flourished. Also important are the initiation and termination of planetary magnetic fields, which can enable the shielding of a planet’s surface from external radiation. Despite the dearth of spacecraft missions to explore the inner planets in the past decade, there have been several important discoveries about internal processes. Recent flybys of Mercury by MESSENGER have confirmed the dipole field measured by Mariner 10. Flyby data also confirm that Mercury’s plains are volcanic and show that some are far younger than previously had been proposed. Further improvements in our knowledge of Mercury’s internal structure and geologic history are expected after MESSENGER enters its mapping orbit in 2011. Constraints on Venus’s current tectonic style and extensive volcanism are based mostly on radar imagery and altimetry from the Magellan mission. Recent results from VIRTIS on the Venus Express spacecraft provide evidence that Venus’s tesserae are more felsic than mafic, and that Venus’s volcanoes have been active in the geologically recent past (consistent with models of gradual rather than catastrophic resurfacing). For the Moon, although much of what was learned about its interior in the Apollo era remains intact, new evidence of volatiles in lunar magmas is altering that view. Important Questions Some important questions concerning the effects of internal planetary processes on life and habitability include the following: • What are the timescales of volcanism and tectonism on the inner planets? • Is there evidence of environments that once were habitable on Venus? • How are planetary magnetic fields initiated and maintained? Future Directions for Investigations and Measurements Progress can be made in understanding how internal processes affect planetary habitability through focused measurements and research that “follow the volatiles” from the interiors, to the surfaces, to escape from the atmo- spheres of the inner planets. Future investigations should include determining the transport rates and fluxes of
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120 VISION AND VOYAGES FOR PLANETARY SCIENCE volatile compounds between the interiors and atmospheres of the inner planets, specifically Venus; determining the composition of the Venus highlands; constraining the styles, timescales, and rates of volcanism and tectonism on Venus, the Moon, and Mercury through orbital and in situ investigations; and measuring and modeling the characteristics and timescales of planetary magnetic fields and their influence on planetary volatile losses and radiation environments. Understand the Effects of Processes External to a Planet on Life and Habitability External processes can be crucial enablers or inhibitors of the origin and evolution of life. Understanding these external processes overlaps partially with the objective of understanding the composition and distribution of volatile chemical compounds. In other words, volatiles can be brought to a planet or leave by means of external processes (e.g., comet impacts delivering volatiles, or solar wind removing them). The origin and evolution of life can be influenced by other external processes, such as stellar evolution, atmospheric losses to space, effects of impacts, orbital interactions of planetary bodies, cosmic-ray fluxes, supernovae, and interstellar dust clouds. The previous decade saw progress in many aspects of external influences on planets. There has been significant progress in understanding impact processes and the delivery of volatiles and in finding potential mechanisms for impact “swarms” like the putative late heavy bombardment (e.g., the “Nice model” of orbital evolution in the outer solar system).17 Additionally, the sample returns from comets and of the solar wind and the continued analyses of meteorite samples have increased our understanding of the distribution and compositions of volatiles in the solar system. Astronomical observations of star-forming regions and of supernovae provide important constraints on the origins of solar systems (and potential early processes), the effects of supernovae, and the nature and potential effects of interstellar dust clouds. Important Questions Some important questions concerning how processes external to a planet can affect life and habitability include the following: • What are the mechanisms by which volatile species are lost from terrestrial planets, with and without sub- stantial atmospheres (i.e., Venus versus the Moon), and with and without significant magnetic fields (i.e., Mercury versus the Moon)? Do other mechanisms of loss or physics become important in periods of high solar activity? • What are the proportions of impactors of different chemical compositions (including volatile contents) as functions of time and place in the solar system? • What causes changes in the flux and intensities of meteoroid impacts onto terrestrial planets, and how do these changes affect the origin and evolution of life? What are the environmental effects of large impacts onto terrestrial planets? Future Directions for Investigations and Measurements Fundamental models of delivery and loss of volatiles relevant for understanding how processes external to a planet can enable or thwart life and prebiotic chemistry can be constrained by investigation of the rates of loss of volatiles from planets to interplanetary space, in terms of solar intensity, gravity, magnetic-field environment, and atmospheric composition. Also key are the characterization of reservoirs of volatiles that feed volatiles onto terrestrial planets after the main phases of planetary accretion (e.g., a late chondritic veneer, heavy bombardment) and an evaluation of impact intensity and meteoritic and cometary fluxes to the terrestrial planets through time, including calibration of the lunar impact record.
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121 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS UNDERSTAND THE PROCESSES THAT CONTROL CLIMATE ON EARTH-LIKE PLANETS Terrestrial life and human civilizations have been profoundly affected by climate and climate change. To understand and predict climate variations, one must understand many aspects of planetary evolution on different timescales. Critical issues include the variation of terrestrial climate over geologic timescales, the causes of extreme climate excursions (e.g., snowball Earths and the Paleocene/Eocene Thermal Maximum approximately 55 million years ago), the development of an understanding of the stability of our current climate, and clarification of the effects of anthropogenic perturbations. This goal is closely aligned with other NASA efforts, especially in Earth science. A key tenet is that detailed exploration and intercomparisons of the inner planets contribute significantly to understanding the factors that affect Earth’s climate—past, present, and future. Fundamental objectives on the path to understanding the processes that control climate on Earth-like planets include the following: • Determine how solar energy drives atmospheric circulation, cloud formation, and chemical cycles that define the current climate on terrestrial planets; • Characterize the record of and mechanisms for climate evolution on Venus, with the goal of understanding climate change on terrestrial planets, including anthropogenic forcings on Earth; and • Constrain ancient climates on Venus and search for clues into early terrestrial planet environments so as to understand the initial conditions and long-term fate of Earth’s climate. Subsequent sections examine each of these objectives in turn, identify critical questions to be addressed, and suggest future investigations and measurements that could provide answers. Determine How Solar Energy Drives Atmospheric Circulation, Cloud Formation, and Chemical Cycles That Define the Current Climate on Terrestrial Planets Results from Venus Express show that Venus’s atmosphere is highly dynamic, with abundant lightning, unexpected atmospheric waves, and auroras and nightglows that respond to high-altitude global winds. Venus Express has also found evidence of relatively recent volcanism, in a geographic correlation of low near-infrared emissivity with geologic hot-spot volcanoes.18 These observations support the model which holds that Venus’s current climate is maintained, at least in part, by the volcanic emission of sulfur dioxide that feeds the global clouds of sulfuric acid. These inferences confirm that some climate processes on Venus are similar to those on Earth and that a better understanding of Venus’s climate system will improve our understanding of Earth’s and provide real-world tests of computer codes—general circulation models (GCMs)—that attempt to replicate climate systems. Important Questions Some important questions concerning how solar energy drives atmospheric circulation, cloud formation, and chemical cycles that define the current climate on terrestrial planets include the following: • What are the influences of clouds on radiative balances of planetary atmospheres, including cloud proper- ties: microphysics, morphology, dynamics, and coverage? • How does the current rate of volcanic outgassing affect climate? • How do the global atmospheric circulation patterns of Venus differ from those of Earth and Mars? • What are the key processes, reactions, and chemical cycles controlling the chemistry of the middle, upper, and lower atmosphere of Venus? • How does the atmosphere of Venus respond to solar-cycle variations?
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126 VISION AND VOYAGES FOR PLANETARY SCIENCE SUPPORTING RESEARCH AND RELATED ACTIVITIES Research and Analysis For stability and scientific productivity, long-term core NASA research and analysis (R&A) programs are needed that sustain the science community and train the next generations of scientists. For flexibility, these core programs are complemented by R&A programs that target strategic needs (e.g., planetary cartography, compara- tive planetary climatology, and planetary major equipment) and shorter-term specific needs (e.g., data-analysis programs and participating-scientist programs). R&A programs like planetary cartography are also critical for mission planning, ensuring that (for instance) cartographic and geodetic reference systems are consistent across missions to enable proper analysis of returned data. Comparative Climatology To complement existing R&A programs, the committee recognizes a current need for a new focus on com- parative climatology. There is a pressing need for more data and better models of climate evolution, prompted in part by the recognition of possible anthropogenic effects on Earth’s climate and the need to understand the robustness of current climate trends, and a need for determination of whether apparent cause-and-effect relation- ships are accurate. Climate research cuts across the standard disciplines. Climate and its change on a single planet cannot be understood without in-depth knowledge of geology, hydrology, and meteorology. And each terrestrial planet (and satellite) with a “thick” atmosphere provides a different mix of processes and forcings that can inform and constrain models for the other planets. NASA’s R&A programs support portions of this research (e.g., Titan hydrology in Outer Planets Research, Mars meteorology in Mars Fundamental Research), but there is no program in which cross-disciplinary, multi-planet climate research can be realized and funded. TECHNOLOGY DEVELOPMENT Although the inner solar system is Earth’s immediate neighborhood, the exploration of Mercury, Venus, and the Moon presents unique challenges that require strategic investments in new technology and new spacecraft capabili- ties. Orbital missions to all of these bodies have been conducted or are underway now; however, in situ exploration requires that spacecraft be able to survive harsh chemical and physical environments. The lack of an atmosphere at Mercury and the Moon, for example, coupled with their relatively large masses, means that landed missions incur either a substantial propulsion burden for soft landing or large landing shocks at impact. The development of a robust, airless-body lander system incorporating high-impulse chemical propulsion, impact attenuation, and low-mass subsystems will enable extensive surface exploration in the coming decades. Venus and Mercury, and to a lesser extent the Moon, also represent extreme thermal environments that will stress spacecraft capabilities. High-temperature survivability technologies such as new materials, batteries, elec- tronics, and possibly cooled chambers will enable long-term in situ missions. The development of robust scientific instruments and sampling systems, including age-dating systems, spectrometers, seismometers, and subsurface drilling and related technologies, is also critical in addressing the science objectives for the coming decades. New capabilities for in situ age dating are of particular importance, as they can help to provide constraints on models of the surface and interior evolution of all the terrestrial planets. ADVANCING STUDIES OF THE INNER PLANETS Previously Recommended Missions A series of National Research Council (NRC) reports, culminating in the 2003 planetary science decadal survey,26 affirm that the exploration of Mercury is central to the scientific understanding of the solar system. The successful achievement of science objectives of the NASA MESSENGER and the European Space Agency-
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127 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS Japan Aerospace Exploration Agency (ESA-JAXA) BepiColombo missions remains a high priority. Given all the advances that will likely come from MESSENGER and BepiColombo, as well as ongoing technology and capability enhancement work, the high priority of Mercury landed science could be revisited at the earliest opportunity in the mid to late years of this decade. Previously Recommended New Frontiers Missions The 2003 planetary decadal survey included recommendations for New Frontiers missions to Venus and the Moon.27 They are as follows: • Venus In Situ Explorer (VISE) and • South Pole-Aitken Basin Sample Return. Venus In Situ Explorer VISE’s importance was reaffirmed in the NRC’s 2008 report Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity.28 The rationale for VISE is that many crucial analyses of Venus cannot be obtained from orbit and instead require in situ investigations. Sample return appears beyond current technology, and Venus’s thick atmosphere limits the primary tools for surface investigations from orbit to radar, radio science, gravity, and a few windows in near-infrared wavelengths. The science mission objectives for VISE from the 2003 and 2008 reports are as follows: • Understand the physics and chemistry of Venus’s atmosphere, especially the abundances of its trace gases, sulfur, light stable isotopes, and noble gas isotopes; • Constrain the coupling of thermochemical, photochemical, and dynamical processes in Venus’s atmosphere and between the surface and atmosphere to understand radiative balance, climate, dynamics, and chemical cycles; • Understand the physics and chemistry of Venus’s crust; • Understand the properties of Venus’s atmosphere down to the surface and improve our understanding of Venus’s zonal cloud-level winds; • Understand the weathering environment of the crust of Venus in the context of the dynamics of the atmo- sphere and the composition and texture of its surface materials; and • Look for planetary-scale evidence of past hydrological cycles, oceans, and life and for constraints on the evolution of the atmosphere of Venus. Achieving all of these objectives represents a flagship-class investment, 29 but achieving a majority is consid- ered feasible in the New Frontiers program.30 In the 2003 planetary science decadal survey, the long-term goal was extraction and return to Earth of samples (solid and gas) from the Venus surface, clearly a flagship-class mission, and VISE was considered in terms of its contribution to this sample return. The 2008 NRC report Opening New Frontiers in Space suggested that VISE not be tied to Venus sample return, given the huge (and so-far-unanswered) technical challenges posed by the latter. VISE-like missions do, however, provide the rare opportunities for technical demonstrations in the Venus near-surface environment, and inclusion of demonstration technologies on a VISE mission would be justified (on a non-interference, non-critical-path basis). South Pole-Aitken Basin Sample Return The exploration and sample return from the Moon’s South Pole-Aitken Basin are among the highest-priority activities for solar system science. The mission’s high priority stems from its role in addressing multiple science objectives outlined in this report, including understanding the interior of the Moon and the impact history of the solar system. Although recent remote sensing missions provide much valuable new data from orbit about the diver- sity of materials and the geophysical context of this important basin, achieving the highest-priority science objec-
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128 VISION AND VOYAGES FOR PLANETARY SCIENCE tives requires precision of age measurements to better than ±20 million years and accuracy of trace elemental compositions to the parts-per-billion level, which is only achievable through sample return. The principal scientific reasons for undertaking a South Pole-Aitken Basin Sample Return mission are as follows: • Determine the chronology of basin-forming impacts and constrain the period of late heavy bombardment in the inner solar system and thus address fundamental questions of inner solar system impact processes and chronology; • Elucidate the nature of the Moon’s lower crust and mantle by direct measurements of its composition and of sample ages; • Characterize a large lunar impact basin through “ground truth” validation of global, regional, and local remotely sensed data of the sampled site; • Elucidate the sources of thorium and other heat-producing elements in order to understand lunar differen- tiation and thermal evolution; and • Determine ages and compositions of farside basalts to determine how mantle source regions on the far side of the Moon differ from regions sampled by Apollo and Luna. Landing on the Moon, collecting appropriate samples, and returning them to Earth requires a New Frontiers- class mission, which has been demonstrated through the 2003 decadal survey and the New Frontiers proposal process. The committee places very high priority on the return of at least 1 kg of rock fragments from the South Pole-Aitken Basin region, selected to maximize the likelihood of achieving the above objectives. Such a mission is significantly enabled by recent orbital missions that have provided high-resolution surface images, allowing a reduction in the risk associated with appropriate site selection and hazard avoidance. Current technology for in situ instrumentation is not adequate for obtaining the required isotopic, geochemical, and mineral-chemical analyses on the Moon; terrestrial laboratories and instrumentation can do the requisite analyses, but expertise in the sample analysis must be sustained through core NASA R&A programs. A robotic lunar sample return mission has extensive “feed-forward” to future sample return missions from other locations on the Moon as well as Mars and other bodies in the solar system. New Missions: 2013-2022 Flagship Class The most recent report from the Venus Exploration Analysis Group (VEXAG) details the community-based consensus on scientific priorities for the exploration of Venus.31 Well over half of the science objectives and the suggested high-priority investigations to accomplish them target a deeper understanding of Venus’s complex climate system. Smaller Discovery and New Frontiers missions, while able to accomplish some of the highest- priority VEXAG science objectives, do not have the capability to address all of the interrelated aspects of climate (Figure 5.3). A flagship mission focused on studying the climate of Venus would answer many of the outstanding science questions that remain about the Venus climate system. In 2009, NASA tasked the Venus Science and Technology Definition Team to define the science objectives for a possible flagship-class mission to Venus with a nominal launch date in the mid-2020s. The resulting Venus Flagship Design Reference Mission (VFDRM)32 addresses three overarching science goals: 1. Understand what Venus’s greenhouse atmosphere can tell us about climate change; 2. Determine how active Venus is (including the interior, surface, and atmosphere); and 3. Determine where and when water, which appears to have been present in the past, has gone. The VFDRM comprises synergistic measurements from two landers, two balloons, and a highly capable orbiter. However, while there are synergisms that can be realized by conducting these investigations within the same mission, much can be accomplished with multiple smaller (Discovery, New Frontiers, or smaller flagship-class) missions that address subsets of the VFDRM objectives, such as the Venus Climate Mission (VCM) described below.
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129 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS FIGURE 5.3 Venus’s climate is controlled by interior processes (e.g., the rate of volcanism), processes within the atmosphere, and atmospheric escape processes. SOURCE: Courtesy of David Grinspoon and Carter Emmart. Venus Climate Mission The Venus Climate Mission will greatly improve our understanding of the current state and dynamics and evolution of the strong carbon dioxide greenhouse climate of Venus, providing fundamental advances in the understanding of and ability to model climate and global change on Earth-like planets. The VISE mission focuses on the detailed characterization of the surface and deep atmosphere and their interaction, whereas VCM provides three-dimensional constraints on the chemistry and physics of the middle and upper atmosphere in order to iden-
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130 VISION AND VOYAGES FOR PLANETARY SCIENCE tify the fundamental climate drivers on Venus. The VCM is a mission that can only be accomplished through in situ, simultaneous measurements in Venus’s atmosphere. The principal science objectives of the Venus Climate Mission are as follows: • Characterize the strong carbon dioxide greenhouse atmosphere of Venus, including variability over longi- tude, solar zenith angle, altitude, and time of the radiative balance, cloud properties, dynamics, and chemistry of Venus’s atmosphere. • Characterize the nature and variability of Venus’s superrotating atmospheric dynamics, to improve the ability of terrestrial general circulation models to accurately predict climate change due to changing atmospheric composition and clouds. • Constrain surface/atmosphere chemical exchange in the lower atmosphere. • Determine the origin of Venus’s atmosphere. • Search for atmospheric evidence of recent climate change on Venus. • Understand implications of Venus’s climate evolution for the long-term fate of Earth’s climate, including if and why Venus went through radical climate change from a more Earth-like climate in the distant past, and when Earth might go through a similar transition. Synergistic observations from an orbiter, a balloon, a mini-probe, and two dropsondes will enable the first truly global three-dimensional (and to a large extent four-dimensional, including many measurements of temporal changes) characterization of Venus’s atmosphere. The mission will return a data set on Venus’s radiation balance, atmospheric motions, cloud physics, and atmospheric chemistry and composition. The relationships and feedbacks among these parameters, such as cloud properties and radiation balance, are among the most vexing problems limiting the forecasting capability of terrestrial GCMs. Evidence will also be gathered for the existence, nature, and timing of the suspected ancient radical global change from habitable, Earth-like conditions to the current, hostile, runaway greenhouse climate, with important implications for understanding the stability of climate and our ability to predict and model climate change on Earth and extrasolar terrestrial planets. This mission does not require extensive technology development and could be accomplished in the coming decade, providing extremely valuable data to improve our understanding of climate on the terrestrial planets. New Frontiers Class Important contributions can be made by a lunar geophysical network (LGN) to the goals for the study of the inner planets. Lunar Geophysical Network The 2003 NRC decadal survey identified geophysical network science as a potential high-yield mission concept. The importance of geophysical networks to both lunar and solar system science was strongly affirmed by subsequent reports.33,34,35 Deploying a global, long-lived network of geophysical instruments on the surface of the Moon to understand the nature and evolution of the lunar interior from the crust to the core will allow the examination of planetary differentiation that was essentially frozen in time some 3 billion to 3.5 billion years ago. Such data (e.g., seismic, heat flow, laser ranging, and magnetic-field/electromagnetic sounding) are critical to determining the initial composition of the Moon and the Earth-Moon system, understanding early differentia- tion processes that occurred in the planets of the inner solar system, elucidating the dynamical processes that are active during the early history of terrestrial planets, understanding the collision process that generated our unique Earth-Moon system, and exploring processes that are currently active at this stage of the Moon’s heat engine. Important science objectives that could be accomplished by an LGN mission are as follows: • Determine the lateral variations; the structure, mineralogy, composition, and temperature of the lunar crust and upper mantle; the nature of the lower mantle; and the size, state, and composition of a lunar core to understand the formation of both primary and secondary crusts on terrestrial planets (Figure 5.4).
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131 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS FIGURE 5.4 Understanding the interior of the Moon provides both a snapshot for the earliest stages of the interior evolu - tion of a terrestrial planet and an end member for understanding evolutionary pathways taken by planetary heat engines. SOURCE: J.W. Head III, Surfaces and interiors of the terrestrial planets, pp. 157-173 in The New Solar System (J.K. Beatty, C.C. Petersen, and A. Chaikin, eds.). Sky Publishing, Cambridge, Mass., Copyright 1999. Reprinted with permission of the Cambridge University Press. • Determine the distribution and origin of lunar seismic activity. Understanding the distribution and origin of both shallow and deep moonquakes will provide insights into the current dynamics of the lunar interior and their interplay with external phenomena (e.g., tidal interactions with Earth). • Determine the global heat-flow budget for the Moon and the distribution of heat-producing elements in the crust and mantle in order to better constrain the thermal evolution of Earth’s only natural satellite. • Determine the size of structural components (e.g., crust, mantle, and core) making up the interior of the Moon, including their composition and compositional variations, to estimate bulk lunar composition and how it relates to that of Earth and other terrestrial planets, how the Earth-Moon system was formed, and how planetary compositions are related to nebular condensation and accretion processes. • Determine the nature and the origin of the lunar crustal magnetic field to probe the thermal evolution of the lunar crust, mantle, and core, as well as the physics of magnetization and demagnetization processes in large basin-forming impacts. The overarching goal of the LGN is to enhance knowledge of the lunar interior. The technology developed for this mission also feeds forward to the design and installation of robotically emplaced geophysical networks on other planetary surfaces. A four-node network would accomplish much of the science outlined above. Such a network could be emplaced or enhanced with international contributions of nodes, as with the International Lunar Network (ILN) concept, providing opportunities for exploration synergies as well as cost savings among nations.
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132 VISION AND VOYAGES FOR PLANETARY SCIENCE Summary A combination of mission, research, and technology activities will advance the scientific study of the inner planets during the next decade and can guide future exploration (Box 5.1). Such activities include the following: • Flagship missions—The top and only priority for a flagship mission is the Venus Climate Mission, which would dramatically improve our understanding of climate on the terrestrial planets and provide an important context for comparison with the climate of Earth. This mission requires no new technology, can be accomplished in the next decade, and would serve as a key step toward more intensive exploration of Venus in the future. • New Frontiers missions—New Frontiers missions remain critical to a healthy program of mission activity throughout the inner solar system, providing opportunities for critical science in more challenging environments and for more comprehensive studies than can be supported under Discovery. A regular cadence of such missions is highly desirable. The committee points to three missions as being particularly important. They are, in priority order: 1. Venus In Situ Explorer, 2. South Pole-Aitken Basin Sample Return, and 3. Lunar Geophysical Network. • Discovery missions—Small missions remain an integral part of the exploration strategy for the inner solar system, with major opportunities for significant science return. A regular cadence of such missions is needed. Such missions may include orbital, landed, or mobile platforms that provide significant science return in addressing one or more of the fundamental science questions laid out earlier in this chapter. (See Box 5.2.) • Technology development—The development of technology is critical for future studies of the inner planets. Robust technology development efforts are required to bring mission-enabling technologies to technology readiness level (TRL) 6. The continuation of current initiatives is encouraged to infuse new technologies into Discovery and New Frontiers missions through the establishment of cost incentives. These could be expanded to include capabili- ties for surface access and survivability, particularly for challenging environments such as the surface of Venus and the frigid polar craters on the Moon. These initiatives offer the potential to dramatically enhance the scope of scientific exploration that will be possible in the next decade. In the long term, the infusion of new technologies will also reduce mission cost, leading to an increased flight rate for competed missions and laying the groundwork for future flagship missions. • Research support—A strong R&A program is critical to the health of the planetary sciences. Activities that facilitate missions and provide additional insight into the solar system are an essential component of a healthy planetary science program. An important opportunity for cross-disciplinary research exists concerning the climates of Venus, Mars, and Earth. • Observing facilities—Earth- and space-based telescopes remain highly valuable tools for the study of inner solar system bodies, often providing data to enable and/or complement spacecraft observations. Support for the building and maintenance of Earth-based telescopes is an integral part of solar system exploration. Chapter 10 contains a more complete discussion of observing facilities. • Data archiving—Data management programs such as the Planetary Data System must evolve in innova- tive ways as the data needs of the planetary community grow. Chapter 10 contains a more complete discussion of archiving issues. • Deep-space communication—Systems must be maintained at the highest technical level to provide the appropriate pipeline of mission data as bandwidth demands increase with improved technology, as well as S-band capability to communicate from the surface of Venus. Chapter 10 contains a more complete discussion of com- munications issues. • International cooperation—The development of international teams to address fundamental planetary sci- ence issues, such as the ILN and the NASA Lunar Science Institute (NLSI), is valuable. Continuing support by NASA for U.S. scientists to participate in foreign missions through participating scientist programs and Mission of Opportunity calls enables broader U.S. participation in the growing international space community.
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133 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS BOX 5.1 Planetary Roadmaps Roadmaps are important tools for laying out the exploration strategies for future exploration of the solar system, as has been demonstrated for Mars by the Mars Exploration Program Analysis Group. Such roadmaps include concepts for all mission classes and also identify supporting research, technology, and infrastructure. Elements of an inner planets roadmap are outlined below. For Mercury, the current MESSENGER mission will provide a wealth of new information that could further redefine our understanding of the planet and modify priorities for future missions. The planned European Space Agency (ESA) BepiColombo mission will augment those data and fill important data gaps. Given these missions, the next logical step for the exploration of Mercury would be a landed mission to perform in situ investigations, such as those delineated in the committee’s study of a Mercury lander concept (Appendixes D and G). Additional Discovery missions and ground-based observations (e.g., at the Arecibo Observatory in Puerto Rico and the National Radio Astronomy Observatory in Green Bank, West Virginia) will be important in addressing data gaps not filled by current and planned missions. Later Mercury missions would likely include the establishment of a geophysical network and sample return. The Venus Exploration Analysis Group has identified goals and objectives for the exploration of Venus, which will be met by future measurements from Earth and by orbital, landed, and mobile platforms. Currently ESA’s Venus Express continues to focus on measurements of the atmosphere. These measurements were to have been augmented by the Japan Aerospace Exploration Agency’s (JAXA’s) Akatsuki. Unfortunately, this spacecraft failed in its attempt to enter orbit around Venus, and its current status is unclear. Venus Express and Akatsuki (if it can be salvaged) will add significantly to the understanding of the structure, chemistry, and dynamics of the atmosphere. However, important gaps in atmospheric science key to understanding climate evolution will remain, requiring in situ measurements such as can be performed during atmospheric transit by landers like Venus In Situ Explorer (VISE), using balloons and/or dropsondes and probes. Significant new understanding of surface and interior processes on Venus will result from a landed geochemical mission such as VISE, as well as from orbital high-resolution imagery, topographic, polarimetric, and interferometric measurements, which will also enable future landed missions. There is a critical future role for additional VISE-like missions to a variety of important sites, such as tessera terrain (e.g., the Venus Intrepid Tessera Lander concept described in Appendixes D and G) that may represent early geochemically distinct crust. Later Venus missions would include the establishment of a geophysical network, mobile explorers (e.g., the Venus Mobile Explorer concept described in Appendixes D and G), and sample return, although these missions require technology development. There remains significant scope for Discovery-class missions to Venus, but more comprehensive, flagship-class missions will be needed to address the long-term goals for Venus exploration. The Lunar Exploration Analysis Group has developed a comprehensive series of goals and objectives for the exploration of the Moon involving both robotic and human missions. In addition, recent and ongoing orbital missions have shaped a new view of the Moon and have identified many opportunities for future exploration on Discovery and New Frontiers missions. The GRAIL mission, a recent Discovery selection, will soon launch to provide high-precision gravity data for the Moon that will generate significant new insight into lunar structure and history. Launching on a similar time frame, the LADEE will determine the global density, composition, and time variability of the fragile lunar atmosphere before it is perturbed by further human activity, implementing a priority enunciated by the National Research Council report The Scientific Context for Exploration of the Moon.1 Priority mission goals include sample return from the South Pole-Aitken Basin region and a lunar geophysical network, as identified in this chapter. Other important science to be addressed by future missions include the nature of polar volatiles (e.g., the Lunar Polar Volatiles Explorer concept described in Appendixes D and G), the significance of recent lunar activity at potential surface vent sites, and the reconstruction of both the thermal-tectonic-magmatic evolution of the Moon and the impact history of the inner solar system through the exploration of better characterized and newly revealed lunar terrains. Such missions may include orbiters, landers, and sample return. 1 National Research Council. 2007. The Scientific Context for Exploration of the Moon. The National Academies Press, Washington, D.C.
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134 VISION AND VOYAGES FOR PLANETARY SCIENCE BOX 5.2 The Discovery Program’s Value to Exploring the Inner Planets The Discovery program continues to be an essential part of the exploration and scientific study of the inner planets, Mercury, Venus, and the Moon. Their proximity to Earth and the Sun enables easy access by spacecraft in the Discovery class. During the past decade inner planets science has benefited greatly from the Discovery program. Past and ongoing missions include the following: • MESSENGER—The first mission to orbit Mercury, and • GRAIL—An effort to use high-quality gravity-field mapping of the Moon to determine the Moon’s interior structure (scheduled for launch in 2011). In addition, recent and planned missions to the Moon, although not Discovery missions, are generally equivalent to other missions in that program. The orbital LRO and impactor LCROSS missions address both exploration and science goals for characterizing the lunar surface and identifying potential resources, while LADEE will characterize the lunar atmosphere and dust environment. The proximity and ready accessibility of the inner planets provide opportunities to benefit from the frequent launch schedule envisioned by this program. Although Discovery missions are competitively and not strategically selected, Mercury, Venus, and the Moon offer many science opportunities for Discovery teams to seek to address. The most recent Discovery Announcement of Opportunity attracted more than two dozen proposals, including a number of inner planets proposals. At Mercury, orbital missions that build on the results from MESSENGER could characterize high- latitude, radar-reflective volatile deposits, map the chemistry and mineralogy of the surface, measure the composition of the atmosphere, characterize the stability and morphology of the magnetosphere, and precisely determine the long-term planetary rotational state. At Venus, platforms including orbiters, bal- loons, and probes could be used to study atmospheric chemistry and dynamics, surface geochemistry and topography, and current and past surface and interior processes. The proximity of the Moon makes it an ideal target for future orbital or landed Discovery missions, building on the rich scientific findings of recent lunar missions and the planned GRAIL and LADEE missions. The variety of tectonic, volcanic and impact structures, as well as chemical and mineralogical diversity, offer significant opportunity for future missions. • Education and outreach—It is important that NASA strengthen both its efforts to archive past education and public outreach efforts and its evaluations and lessons-learned activities. Through such an archive, future education and public outreach projects can work forward from tested, evaluated curricula and exercises. These mission priorities, research activities, and technology development initiatives are assessed and priori- tized in Chapters 9, 10, and 11, respectively. NOTES AND REFERENCES 1 . The term inner planets is used here to refer to Mercury, Venus, and the Moon, whereas the term terrestrial planets is used to refer to Earth, Mercury, Venus, Mars, and the Moon. 2 . Although scientific and programmatic issues relating to Mars are described in Chapter 6, it is not always possible to entirely divorce martian studies from studies of the other terrestrial planets. Therefore, when issues concerning Mercury, Venus, or the Moon naturally touch upon corresponding issues relevant to Mars they are mentioned in the spirit of com- parative planetology.
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135 THE INNER PLANETS: THE KEY TO UNDERSTANDING EARTH-LIKE WORLDS 3 . R. Jeanloz, D.L. Mitchell, A.L. Sprague, and I. de Pater. 1995. Evidence for a basalt-free surface on Mercury and impli - cations for internal heat. Science 268(5216):1455-1457, doi: 10.1126/science.7770770. 4 . D.T. Blewett, M.S. Robinson, B.W. Denevi, J.J. Gillis-Davis, J.W. Head, S.C. Solomon, G.M. Holsclaw, and W.E. McClintock. 2009. Multispectral images of Mercury from the first MESSENGER flyby: Analysis of global and regional color trends. Earth and Planetary Science Letters 285:272-282, doi: 10.1016/j.epsl.2009.02.021. 5 . D.J. Lawrence, W.C. Feldman, J.O. Goldsten, T.J. McCoy, D.T. Blewett, W.V. Boynton, L.G. Evans, L.R. Nittler, E.G. Rhodes, and S.C. Solomon. 2010. Identification and measurement of neutron-absorbing elements on Mercury’s surface, Icarus 209(1):195-209. 6 . J.L. Margot, S.J. Peale, R.F. Jurgens, M.A. Slade, and I.V. Holin. 2007. Large longitude libration of Mercury reveals a molten core. 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