MAPPING VESTA USING A HYBRID METHOD FOR INCORPORATING SPECTROSCOPIC AND MORPHOLOGIC DATA

In constructing a global geologic map of Vesta at 1:300,000-scale, we have utilized a hybrid method of mapping that first requires creating two maps independently. The first map uses morphology and topography to define map units; the second uses spectral data to define units. The unique results of each map are then combined into map units that contain key information from both datasets.

Multispectral data provide unique insight into stratigraphy (material brought up through cratering processes) that can be lost when using an albedo mosaic as the basemap. However, solely using a “color” ratio mosaic as a basemap can magnify potentially misleading data, because spectroscopy in the UV-VIS-near IR samples only the upper few µm of the surface. Thus, clear criteria should be established to define hybrid map units. The crucial exercise in retaining unique data when combining the two maps was to create a decision tree for determining which data would be primary in drawing unit boundaries:

1. If saturated colors (meaning the color signal in color-ratio spectral data was strong and the color itself was easy to describe) matched unit boundaries derived from morphology, there was no conflict. For example, saturated colors on Vesta tend to be associated with fresher exposures of regolith, which are more likely found at the youngest, freshest craters, easily demarcated morphologically.
2. If muted colors exist, where the morphology is relatively clear, the morphology is the primary guide for unit definition, as it retains the least altered record of geologic processes and the most reliable record of the nature of the rock bodies. Colors provide additional characteristics of such units.
3. If saturated colors are not associated with morphologic boundaries, color boundaries are interpreted to record the most recent impact evidence. In such cases we mapped saturated color data as impact material. This preserves the underlying morphology information while supporting stratigraphic interpretations based on excavated subsurface layers revealed by crater ejecta.
4. In the case of muted colors where the morphology is unclear, decisions were made case-by-case, using all available data to make a reasonable determination of where to mark unit boundaries.