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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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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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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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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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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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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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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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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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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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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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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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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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OCR for page 136
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