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2014 GSA PRESIDENTIAL ADDRESS

Figure 2. The earliest lunar geologic map based on stratigraphic principles (Shoemaker, 1962), shown without the legend or cross sections. This map of the
Copernicus region was originally published in black and white; colorized unit contacts and labels have been added for legibility.

global perspective from the outset, and their maps progress down- on the Moon (Jolliff et al., 2000) based only on iron and thorium GSA TODAY | www.geosociety.org/gsatoday/
ward to regional and local scales as spatial resolution improves. abundances obtained by the orbiting Lunar Prospector (Fig. 4).
Gamma-ray spectra are especially sensitive to these two elements,
Geologic mapping of the planets (see Carr, 2013, for a recent and their concentrations vary greatly in different lunar lithologies.
historical review) nowadays still depends on imagery but has been
augmented by the application of remote-sensing tools. The identi- Other planetary bodies present different challenges. The
fication of minerals from their visible, near-infrared, and thermal surfaces of Venus and Titan (a moon of Saturn) are obscured by
infrared spectra provides a means of mapping compositional thick clouds. However, they have been imaged using radar,
units on the Moon, Mars, and Mercury. Spectroscopy can often allowing the mapping of geologic units based on their topography
identify only a few minerals with diagnostic absorption or emis- and radar reflectivity.
sion features, and then only if they are sufficiently abundant, but
adding any mineralogic information to maps allows much more Mapping is not restricted only to large planets. Geologic maps
rigorous interpretation. Even from orbital altitudes, the spatial have been compiled for all the satellites imaged by orbiting or
resolution of spectral maps can be as small as a few tens of meters, flyby spacecraft. Moons of the giant planets show remarkably
although coarser resolutions are more common. For example, the complex geologic units, comprised of jumbled blocks of icy crust
CRISM spectrometer on the Mars Reconnaissance Orbiter has (Europa), crosscutting tectonic features and superposed impact
distinguished and mapped concentrations of olivine and phyllo- ejecta (Ganymede), erupting volcanoes with associated pyro-
silicates. Orbital tools for geochemical analysis are also available. clastic deposits of compositionally exotic materials (Io), and lakes
Gamma-ray and neutron spectroscopy measures only a handful of of liquid methane (Titan). Even smaller bodies—asteroids and
elements at fairly coarse spatial resolution, but any chemical abun- comet nuclei—have been mapped where spacecraft imagery is
dances are useful in distinguishing and interpreting geologic available. The most recent example is a geologic map of asteroid
units. A prime example is a global map of compositional terranes 4 Vesta (Williams et al., 2014), assembled from images and spectra
obtained by the Dawn orbiter (Fig. 5).

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