Mapping the Planets—Geology Stakes Its Claim

Planetary geoscience had very little presence in GSA’s first century, but it has a long history in GSA publications. Beginning with the Moon, the transformation of the planets and their satellites from astronomical objects into geological worlds has taken place largely by geologic mapping using telescope and spacecraft imagery and by the application of stratigraphic principles to these new data sets. Compositional data from orbital remote sensing, chronological information from crater densities, and the added dimension of petrology and geochemistry from surface rovers and laboratory analyses of samples, where available, have cemented geology’s central place in planetary exploration. The present focus on characterizing planetary paleoenvironments and the search for life further buttresses geology’s role in planetary exploration and serves as the next step in the expansion of our discipline beyond Earth. PLANETARY GEOLOGY AND GSA The inaugural GSA Presidential Address (Stevenson, 1899) ended this way: “The world must advance or retrograde; it cannot stand still.” J.J. Stevenson was referring to the world of science, and more specifically to geology. As prescient as he was, the Society’s first President might not have imagined that geology would advance to other worlds. At that time, the only body besides the Moon with features that were resolved through telescopes was Mars, and that planet famously was argued to have canals built by sentient beings. In fact, the better part of a century of GSA history would elapse before the Planetary Geology Division was established in 1981. Planetary geoscience, though, has had a surprisingly long presence in GSA publications (Fig. 1). GSA Bulletin featured what I consider to be its first planetary paper in 1921. Other GSA publications have followed suit: The very first issue of Geology contained two planetary papers; GSA Today published its first planetary article during its first year, Geosphere during its second year, and Lithosphere during its third year. BEGINNING WITH THE MOON Planetary geology began, appropriately enough, with the geologic mapping of our nearest neighbor. Although cartography from telescopic observations of the Moon had been conducted for more than three centuries, the first lunar geologic map of the region surrounding Copernicus crater (Fig. 2), based on the stratigraphic principles so useful in terrestrial geology, appeared in a landmark study by Shoemaker in 1962. Later that same year, Shoemaker and Hackman (1962) divided the lunar timescale into periods delineated by cataclysmic impacts, with major formations defined as the ejecta blankets of these impact basins (Fig. 3). That was a new twist on time and rock units, but it was respectful of the principle of linking rocks and time and has worked well for heavily cratered planets. Lunar geologic units, as in terrestrial maps, were integrated into a stratigraphic column, and were dated first with relative ages determined from crater-density measurements. 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 telescopic 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 stratigraphy was relative until crater densities could be calibrated with radiometric ages from volcanic or shock-melted rocks returned by the Apollo astronauts. EXPLORING PLANETS AND SMALL BODIES 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 2014 GSA PRESIDENTIAL ADDRESS Figure 1. A timeline of GSA publications, comparing dates of establishment and publication of first planetary papers (red arrows), as well as establishment of GSA’s Planetary Geology Division.