QUANTITATIVE MORPHOLOGY AS A PROBE TO TRANSPORT MECHANISMS AND COMPOSITION: OLD TECHNIQUE, NEW PLANET

Assessment of clast morphology (primarily size, shape and roundness) is a standard analysis technique employed at many terrestrial sites. We have utilized this technique in combination with multispectral data to assist in deconvolving rock lithology and sedimentary transport history for rocks in the Rock Garden at the Mars Pathfinder landing site.

Two-dimensional images of rocks were used to yield quantitative shape and roundness data. A total of 597 rocks greater than 10 mm diameter were assessed for size, sphericity, elongation and roundness values. A series of spectra were then taken for each rock, and these were used to categorize them in terms of potential composition. Rocks were classified as “grey” (relatively flat spectrum between 671-801 nm with a weak kink at 968 nm), “red” (maximum at ~752 nm and a steeper slope from 480-670 nm) or “pink” (maximum at 801 nm and a steep slope from 480-670nm).

Rocks were mostly pebble-to-cobble sized (0.11 m) and had an average sphericity of 0.75, elongation of 0.63 and relative roundness of 0.083 (sub-angular to sub-rounded). Rocks of each spectral class had nearly identical shape and roundness averages (0.74 and 0.094 respectively). There is thus essentially a single average morphology within the study area. The implication is that all rocks in the scene within the size limits of this study are of the same general composition and were transported to the site in a similar fashion. Rock shape and roundness values were not associated with rock size, distribution or distance from the lander, as would be expected if these morphologic indices were a function of resolution. Our conclusion is that all rocks have a similar composition, regardless of the variant spectral signatures displayed at their surfaces. We preliminarily identify all rocks in the study as gray rocks; red and pink spectra are interpreted to be due to chemical rock coatings, mantling dust or surrounding soil.