Vision and Voyages for Planetary Science in the - Solar System ...

Important Questions
Some important questions for using the bulk composition of the terrestrial planets to understand
their formation 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 improved understanding of the various types of rock and
regolith making up the crusts and mantles of the inner planets, via 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 characterization of the Moon’s lower mantle and core, and developing 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 noble gas molecular and
isotopic composition of the Venus atmosphere. Improved modeling of solar system formation and
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.
Characterize Planetary Interiors to Understand How They Differentiate
and 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 generation, distribution of heat-producing radioactive elements, styles and
extent of volcanism, and the role of giant impacts. Analysis of lunar samples implies 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 rocky planets, or may have been stochastically
distributed based on the occurrences of giant impact processes. All the large terrestrial planets
differentiated into rocky crusts and mantles, and metallic cores, and variously continued to dissipate
internal energy via mantle convection, magmatism, magnetic dynamos and faulting, though 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. 5 The dynamic nature of
Mercury’s interior has been supported by MESSENGER flyby on the internal origin of the planet’s
magnetic field 6 and its discovery of extensive volcanic deposits. 7 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 last decade, recent results
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