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2014 GSA PRESIDENTIAL ADDRESS

                                                                                                        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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