Geologic mapping of tectonic planets

Geological analysis of planets typically begins with the construction of a geologic map of the planets’ surfaces using remote data sets. Geologic maps provide the basis for interpretations of geologic histories, which in turn provide critical relations for understanding the range of processes that contributed to the evolution. Because geologic mapping should ultimately lead to the discovery of the types of operative processes that have shaped a planet surface, geologic mapping must be undertaken in such a way as to allow such discovery. I argue for modifications in current planetary geologic mapping methodology that admit that tectonic processes may have contributed to the formation of a planet surface, and I emphasize a goal of constraining geologic history rather than determining global stratigraphy. To this end, it is imperative that secondary (tectonic) structures be clearly delineated from material units; each record different aspects of planet surface evolution. Neglecting such delineation can result in geologic maps and interpreted geohistories in which both the spatial limits and the relative ages of material units and suites of secondary structures are incorrect. Determination of absolute time is fundamentally difficult to constrain in planetary studies. In planetary geology the only means to estimate absolute age is the density of impact craters on a surface. Crater density surface ages are more akin to terrestrial εNd average mantle model ages, which reflect the average time at which all of a rock’s components were extracted from the Earth’s mantle. Rocks, or planetary surfaces, with very different geohistories could yield the same average model age. Dating tectonic events or determining rates of tectonic events is even more difficult.