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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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