PLANETARY GEOLOGIC MAPPING: PIONEERED BY EARLY AUTHORS OF LUNAR GEOLOGIC MAPS AND ITS ROLE IN EXPLORING PLANETS BEYOND OUR EARTH

The strategy of resolving geologic units on worlds beyond our own was described by USGS geologist and lunar mapping pioneer Don Wilhelms as “to portray ... not just similar-appearing surfaces or collections of similar topographic forms but, rather, three-dimensional bodies of finite horizontal and vertical extent which are in effect the building blocks of the ... crust.” The novel application and expansion of this strategy to identify discrete bodies of rock and sediment on the lunar surface by scientists in the 1950’s and 1960’s was no less pioneering than Smith’s geologic map of Great Britain in 1815. Planetary (non-terrestrial) geologic maps have since played an outsized, though largely unhailed, role in space exploration by helping downselect sites for spacecraft landing, plan traverses, and contextualize geologic observations across multiple scales. A second renaissance of planetary geologic mapping is now underway due to (1) the increasing fleet of spacecraft exploring the Solar System, (2) the analytical capabilities that enable rapid analysis of the data beamed back by these instruments, and (most urgently) (3) the reality of preparing for a long term, sustained human presence on the lunar surface with the Artemis missions. As such, we have an increasingly clear understanding that the value of geologic maps is not solely that they report investigative results regarding the three-dimensional characteristics and distributions of rock, sediment, and soil (which they do). Rather, they are most valuable when recognized as tools that encourage reasoned discussion and decisions regarding exploration, land use, and planning. The expansion of frontier boundaries, wherein landscapes simultaneously provide strategic positioning for civil infrastructure as well as the materials with which to build that infrastructure, will rely on the thoughtful diversification and targeted application of planetary geologic maps. The role of geologic maps in planetary exploration cannot be understated nor can the heritage of the people and programs that established maps as essential tools of exploration. We use lessons of the past to prepare for the future as we recognize that the that the absence of such tools decreases our understanding of geologic phenomena and increases our risk of exploration.