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

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: transport of dust and gas in the early solar nebula, compositional gradients in the early
nebula imposed by time or distance from the Sun, accretion of solids to form self-gravitating bodies,
gravitational scattering of those bodies, impacts among those bodies (possibly with chemical
fractionation); and redistribution of volatile elements 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 constraints on the bulk major-element composition of the terrestrial planets. While little
progress has been made in the last 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 regards to the composition of Mercury’s crust.
These spectra suggest that Mercury’s surface materials contain little ferrous iron, 3 whereas preliminary
results by MESSENGER’s neutron spectrometer suggest abundant iron or titanium (Figure 5.2). 4
Substantial research efforts in the last 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.
FIGURE 5.2 Rembrandt impact basin on Mercury photographed by MESSENGER. Rembrandt spans
over 700 kilometers and at 4 billion years old is possibly the youngest large impact basin on the planet.
Geological analysis indicates the basin experienced 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.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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