A systematic method for classifying and grouping late noachian and early hesperian rock targets analyzed by the mars exploration rover opportunity at endeavour crater, mars

The Mars rover Opportunity has collected in situ compositional data with the Alpha Particle X-ray Spectrometer at almost 500 sites. To analyze these data, hierarchical clustering analysis and an error-weighted similarity index are applied to a subset of 57 APXS target compositions and selected Martian meteorites. Hierarchical clustering provides a rapid first approximation of compositional relationships, whereas the error-weighted similarity index provides an in-depth and quantifiable comparison of individual composition pairs. These analyses are combined into a statistical grouping model that provides insight into lithologic relationships and is critically informed by examination of Panoramic Camera and Microscopic Imager images. Major lithologies are (1) the Burns formation sulfate sandstones; (2) Shoemaker impact breccias (Endeavour crater ejecta/rim deposits); (3) the morphologically distinct Grasberg formation, associated with Endeavour crater rim deposits; (4) the Matijevic formation, an exposure interpreted to be Endeavour crater target rocks; and (5) erratics or other rocks that do not cluster with groups 1–4. The Grasberg formation is more similar to the Shoemaker formation than any other formation, and thus likely incorporated eroded Shoemaker material. The lowest Shoemaker member (Copper Cliff breccia) may contain material from the preimpact Matijevic formation. The Matijevic formation is the most chemically distinct formation and is most similar to the volcanic erratic rock “Marquette Island.” Clustering and similarity index values also show that regolith breccia Martian meteorites (represented by the NWA 7475/7034 paired meteorites) are similar in bulk composition to Mars surface materials at Meridiani, especially the Matijevic formation.