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2014 GSA PRESIDENTIAL ADDRESS

                          Mapping the Planets—Geology Stakes Its Claim

                          Harry Y. McSween Jr., Dept. of Earth and Planetary Sciences,                      besides the Moon with features that were resolved through tele-
                          University of Tennessee, Knoxville, Tennessee 37996-1410, USA;                    scopes was Mars, and that planet famously was argued to have
                          mcsween@utk.edu                                                                   canals built by sentient beings. In fact, the better part of a century
                                                                                                            of GSA history would elapse before the Planetary Geology
                          ABSTRACT                                                                          Division was established in 1981.

                            Planetary geoscience had very little presence in GSA’s first                      Planetary geoscience, though, has had a surprisingly long pres-
                          century, but it has a long history in GSA publications. Beginning                 ence in GSA publications (Fig. 1). GSA Bulletin featured what
                          with the Moon, the transformation of the planets and their                        I consider to be its first planetary paper in 1921. Other GSA
                          satellites from astronomical objects into geological worlds has                   publications have followed suit: The very first issue of Geology
                          taken place largely by geologic mapping using telescope and                       contained two planetary papers; GSA Today published its first
                          spacecraft imagery and by the application of stratigraphic prin-                  planetary article during its first year, Geosphere during its second
                          ciples to these new data sets. Compositional data from orbital                    year, and Lithosphere during its third year.
                          remote sensing, chronological information from crater densities,
                          and the added dimension of petrology and geochemistry from                        BEGINNING WITH THE MOON
                          surface rovers and laboratory analyses of samples, where avail-
                          able, have cemented geology’s central place in planetary explora-                   Planetary geology began, appropriately enough, with the
                          tion. The present focus on characterizing planetary                               geologic mapping of our nearest neighbor. Although cartography
                          paleoenvironments and the search for life further buttresses                      from telescopic observations of the Moon had been conducted for
                          geology’s role in planetary exploration and serves as the next                    more than three centuries, the first lunar geologic map of the
                          step in the expansion of our discipline beyond Earth.                             region surrounding Copernicus crater (Fig. 2), based on the strati-
                                                                                                            graphic principles so useful in terrestrial geology, appeared in a
                          PLANETARY GEOLOGY AND GSA                                                         landmark study by Shoemaker in 1962. Later that same year,
                                                                                                            Shoemaker and Hackman (1962) divided the lunar timescale into
                            The inaugural GSA Presidential Address (Stevenson, 1899)                        periods delineated by cataclysmic impacts, with major formations
                          ended this way: “The world must advance or retrograde; it cannot                  defined as the ejecta blankets of these impact basins (Fig. 3). That
                          stand still.” J.J. Stevenson was referring to the world of science,               was a new twist on time and rock units, but it was respectful of
                          and more specifically to geology. As prescient as he was, the                     the principle of linking rocks and time and has worked well for
                          Society’s first President might not have imagined that geology                    heavily cratered planets. Lunar geologic units, as in terrestrial
                          would advance to other worlds. At that time, the only body                        maps, were integrated into a stratigraphic column, and were dated
                                                                                                            first with relative ages determined from crater-density measure-
                                                                                                            ments. Shoemaker recognized the value that geologic maps would
                                                                                                            have in selecting landing sites for the Apollo program and in
                                                                                                            extrapolating data from these sites to the rest of the Moon. By
                                                                                                            1966, 28 lunar quadrangle maps had been produced from tele-
                                                                                                            scopic imagery; subsequent lunar geologic maps and cross
                                                                                                            sections have been based on observations at higher spatial
                                                                                                            resolution from orbiting spacecraft. Similar to stratigraphic
                                                                                                            columns on Earth, which initially had only relative ages until
                                                                                                            radioactive isotope dating techniques were developed, lunar stra-
                                                                                                            tigraphy was relative until crater densities could be calibrated with
                                                                                                            radiometric ages from volcanic or shock-melted rocks returned by
                                                                                                            the Apollo astronauts.

GSA TODAY | JANUARY 2015  Figure 1. A timeline of GSA publications, comparing dates of establishment        EXPLORING PLANETS AND SMALL BODIES
                          and publication of first planetary papers (red arrows), as well as establishment
                          of GSA’s Planetary Geology Division.                                                From that beginning, geoscientists have moved forward with
                                                                                                            the audacious goal of mapping the entire solar system.
                                                                                                            Interestingly, geologic mapping of the planets has moved in an
                                                                                                            opposite direction from mapping on Earth. Local maps of our
                                                                                                            own planet are pieced together to produce regional and eventually
                                                                                                            global maps. On the other hand, planetary explorers have had a

     GSA Today, v. 25, no. 1, doi: 10.1130/GSAT-14PresAdrs.1

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