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2014 GSA PRESIDENTIAL ADDRESS
Figure 3. The lunar time-stratigraphic system, with major time units defined context and textural analyses from panoramic and microscopic
by widespread formations produced as ejecta blankets from large impact
basins. Adapted from Wilhelms (1987).
GSA TODAY | JANUARY 2015 SURFACE GEOLOGY AT HUMAN SCALE
Once a planetary body has been mapped from orbit, the next
logical step is landing on its surface. The recent operation of
mechanical rovers on Mars has allowed high-resolution geologic
mapping at scales with which field geologists can readily identify.
The traverse maps made by Mars rovers resemble those compiled
from observations of the Apollo astronauts on the Moon, but
rovers have extended their traverses much farther. Images and
remote sensing data from Spirit, Opportunity, and Curiosity
provide the basis for surface outcrop maps. An example is Spirit’s
7.7-km, 6-year traverse map though the Columbia Hills in Gusev
crater (Crumpler et al., 2011), reproduced in part in Figure 6.
Identifications of rock types analyzed by the rover have been
extended farther afield using spectrometers that can “see” for tens
of meters, making the traverse map more representative. Mars
surface mapping has also been supplemented with detailed strati-
graphic context from the mapped and analyzed walls of impact
craters, such as the Burns Formation section in Endurance crater
analyzed by Opportunity (Fig. 7) (Grotzinger et al., 2005).
Spectroscopic analyses of chemistry and mineralogy, and spatial
6
Figure 4. Global lunar maps of compositional terranes, based solely on orbital
measurements of iron and thorium by Lunar Prospector. Modified from Jolliff
et al. (2006).
imagers, of the bedded rocks encountered provide sufficient infor-
mation to make detailed interpretations of geologic processes and
histories. These rovers have become virtual field geologists,
allowing their science teams to project human observational and
mapping skills onto the surface of Mars. The rovers have become
so anthropomorphic that Sojourner, the first primitive rover on
the Mars Pathfinder mission, was named a GSA Honorary Fellow
in 1997. And Spirit, Opportunity, and Curiosity have refined the
melding of humans, machines, and instruments to the point
where planetary geologic mapping can arguably be done as well or
better (albeit more slowly) by rovers than by astronauts.
PLANETARY SAMPLES
The return of lunar samples to Earth and the identification of
meteorites from the Moon, Mars, and asteroid Vesta have also
provided valuable ground truth for spacecraft remote sensing and
better geologic interpretations of these data. For example, litho-
logic interpretation of lunar compositional terranes from their
thorium and iron abundances (shown previously in Fig. 4)
required comparison with laboratory measurements of those
elements in Apollo rocks (Jolliff et al., 2000). Interpretation of the
unexpected discovery of hydrogen in Vesta’s regolith (Fig. 8) using
neutron absorption measurements by the Dawn spacecraft
(Prettyman et al., 2012) was made possible because some mete-
orite breccias from Vesta contain water-bearing chondrite clasts.
Comparisons of laboratory geochemical analyses of geologically
young martian basaltic meteorites with rover and orbiter analyses
of older volcanic rocks on the ground (Fig. 9) have provided new
insights into the evolution of martian magmatism through time
(McSween et al., 2009). Although the specific locations from
which meteorites were extracted from their parent bodies is not
known, the ability to perform petrologic and geochemical