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

Defining criteria for mapping material units on airless, rocky bodies is challenging. Where the primary geologic process for the bulk of a small body’s history is impact cratering, traditional mapping approaches can be problematic, because differences in morphological characteristics among the various cratered surfaces can be subtle to absent, and surface morphology is muted by the regolith’s physical and mechanical properties. In constructing a global geologic map of Vesta at 1:300,000-scale using the Dawn Framing Camera (FC), DTM-derived slope and contour, and color (visible wavelength) and spectroscopic data, we have been evaluating how much weight each dataset should be given in defining criteria for unit boundaries, and what the consequences of those choices are.

Our initial approach was to characterize units based on morphology, surface textures, and albedo, as well as crater size-frequency distribution and superposition relations. Color data from the FC (and VIR) were examined as an overlay on the first draft of units, to refine unit boundaries where morphologic characteristics provided more than one possible border, or the interpretation of the unit type was ambiguous. However, we found that unique information provided by color data was being lost in the mapping process and as a result, not being incorporated synergistically into interpretations.

To counter this problem, we are using a hybrid method that requires creating two maps: one based on morphology/topography, and another based primarily on color/spectral data. The unique results of each are then combined, with the ultimate objective being to integrate color data into meaningful map units, presented in a new visual scheme.

We note that color/multispectral boundaries are often gradational, so it can be problematic to tie a combination of colors to a rock body. It is possible to interpret the composition of the sampled layer, but because that layer is only a few µm thick, in the absence of other information it is not always clear how or whether that color data correlate with a surficial unit. However, on an airless body, the ejecta of an impact event can persist relatively unchanged, potentially over geologic timescales. Thus, even the upper microns of the surface can contain records of the vertical composition of the rock body, as observed on Vesta.