WHERE DO WE DRAW THE LINE? EVALUATING TECHNIQUES FOR MAPPING IMPACT CRATER DEPOSITS ON PLANETARY BODIES

Impact craters are pervasive on most bodies in the solar system, and they come in a wide range of sizes, shapes and ages. Craters are beneficial for studying the geologic history of a planetary surface because they provide windows into the subsurface allowing stratigraphy to be observed (or sampled), and they can provide a relative age for the surface, and thereby the geologic materials on which they superpose.

In general, impact craters can consist of several structures – a raised rim, terraces along the inner wall, and a central peak and/or peak ring (dependent on crater size) – and geologic units that may be emplaced along their wall as talus, on their floor as impact melt, and surrounding their rim as ejecta. Due to the pervasiveness of impact craters, we expect that mapping them across the solar system should be a straightforward process; but this is not necessarily so. A number of variables can complicate mapping craters including impactor type, target properties, surface properties, the presence (or lack) of an atmosphere, data resolution, and image lighting. Complications in mapping arise especially with ejecta deposits. In some cases, clear distinctions can be made between impact ejecta and the surrounding terrain. For example, Venusian impact craters stand out as radar bright (high back-scatter) features against darker (low back-scatter) terrain, and many Martian crater ejecta deposits have distinct topographic relief above their surroundings, thus providing clear boundaries along which contacts can be mapped and the units characterized. However, ejecta deposits on many bodies, especially airless bodies, are difficult to map because the ejecta lacks significant albedo or textural differences with its surroundings, or the ejecta forms a thin or diffuse mantle over the surface, or the deposit is spectrally indistinct from the surroundings. In these cases it is incumbent upon the mapper to identify the “deposit” as either a geologic unit or a surface, which can result in inconsistent mapping within a map, or among maps and mappers.

This abstract is intended to present this issue, as well as mapping strategies, to the community of planetary geologic mappers. We do not intend to provide answers, rather to elicit discussion amongst the community in order to refine the methodologies of how mappers define these features.