GEOLOGIC MAPPING METHODS FOR SMALL, ROCKY BODIES: THE VESTA EXAMPLE

Defining criteria for mapping the boundaries of rock units on a small, airless, rocky body has its own particular challenges. Where the primary geologic process for the bulk of a body’s history is impact cratering, traditional approaches to mapping can be inadequate because the differences in morphological characteristics among the various cratered surfaces can be subtle to absent. For Vesta, as for many such bodies, surface morphology is muted by the regolith’s physical and mechanical properties. We are constructing a global geologic map of Vesta at 1:300,000-scale using the Dawn Framing Camera (FC) images as a basemap, while DTM-derived slope and contour maps yield the shape of the surface. Our map also incorporates elemental and mineralogical data, whereas previous maps did not. As we map, we are evaluating how much weight each dataset should be given in defining criteria for unit boundaries, and what the consequences of those choices are.

At this stage we have taken a conservative approach in choosing “certain” versus “approximate” boundaries. If we detected a boundary by changes in structure, topography, or in the level of cratering, we considered this a potential boundary. If we did not have confidence in being able to clearly define the morphological distinction between one unit and another, an “approximate” boundary was used. Further, even if the topography indicated the presence of a boundary, but the morphology did not, an “approximate” boundary was utilized.

We continue to debate other mapping issues relevant to small, airless bodies. For example, in many cases ejecta mantles but does not bury ancient cratered terrain. It is not clear how this ejecta should be mapped – as a unique geologic unit or as a surficial one. As another example, there are a number of “colors” in FC false-color data that might or might not indicate unique compositions (e.g., light teal ejecta, darker mantling, orange material). While we do not use these data as a primary means to define units, the information must be taken into account in unit subdivision and interpretation. However, it is currently unclear at what point these “colors” should be used to refine contact boundaries that are not observed in the visible imagery or topography. The question is one of emphasis - on rock body boundaries, or on rock body boundaries as expressed on the surface.