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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).
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 imagers, of the bedded rocks encountered provide sufficient infor-
basins. Adapted from Wilhelms (1987). 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.
GSA TODAY | JANUARY 2015 SURFACE GEOLOGY AT HUMAN SCALE PLANETARY SAMPLES
Once a planetary body has been mapped from orbit, the next The return of lunar samples to Earth and the identification of
logical step is landing on its surface. The recent operation of meteorites from the Moon, Mars, and asteroid Vesta have also
mechanical rovers on Mars has allowed high-resolution geologic provided valuable ground truth for spacecraft remote sensing and
mapping at scales with which field geologists can readily identify. better geologic interpretations of these data. For example, litho-
The traverse maps made by Mars rovers resemble those compiled logic interpretation of lunar compositional terranes from their
from observations of the Apollo astronauts on the Moon, but thorium and iron abundances (shown previously in Fig. 4)
rovers have extended their traverses much farther. Images and required comparison with laboratory measurements of those
remote sensing data from Spirit, Opportunity, and Curiosity elements in Apollo rocks (Jolliff et al., 2000). Interpretation of the
provide the basis for surface outcrop maps. An example is Spirit’s unexpected discovery of hydrogen in Vesta’s regolith (Fig. 8) using
7.7-km, 6-year traverse map though the Columbia Hills in Gusev neutron absorption measurements by the Dawn spacecraft
crater (Crumpler et al., 2011), reproduced in part in Figure 6. (Prettyman et al., 2012) was made possible because some mete-
Identifications of rock types analyzed by the rover have been orite breccias from Vesta contain water-bearing chondrite clasts.
extended farther afield using spectrometers that can “see” for tens Comparisons of laboratory geochemical analyses of geologically
of meters, making the traverse map more representative. Mars young martian basaltic meteorites with rover and orbiter analyses
surface mapping has also been supplemented with detailed strati- of older volcanic rocks on the ground (Fig. 9) have provided new
graphic context from the mapped and analyzed walls of impact insights into the evolution of martian magmatism through time
craters, such as the Burns Formation section in Endurance crater (McSween et al., 2009). Although the specific locations from
analyzed by Opportunity (Fig. 7) (Grotzinger et al., 2005). which meteorites were extracted from their parent bodies is not
Spectroscopic analyses of chemistry and mineralogy, and spatial known, the ability to perform petrologic and geochemical
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