PHYSICAL WEATHERING FEATURES OF BARBERTON GROUP COBBLES AT MERIDIANI PLANUM, MARS: IMPLICATIONS FOR SURFACE RESIDENCE TIMES

Barberton Group (BG) cobbles investigated by the Opportunity rover at Meridiani Planum are considered stony/stony-iron meteorite candidates, containing either one or both of
 kamacite or troilite, and other
 ferrous and ferric iron phases [1, 2]. The ferric
 iron is likely a product of aqueous 
alteration of these minerals, lending importance to this group of rocks for helping to constrain Late Amazonian chemical weathering rates in the region [3]. BG group members Barberton, Santa Catarina, Santorini, and Kasos were examined at locations along the Opportunity traverse separated by nearly 10 kilometers. The cobbles represent a separate category from the iron meteorites discovered at Meridiani Planum, Gusev crater, and Gale crater.

Pancam and MI images show that BG cobble surfaces have been physically modified while on the martian surface. Santorini surfaces are scalloped/faceted and differentially eroded. Weathering on Santa Caterina appears to attack structurally or mineralogically weak zones, producing differential mass removal resulting in overhanging lips of rock with serrated margins. Sculpted features do not appear to show signs of preferred orientation, and none of the BG cobbles examined have obvious regmaglypts or fusions crusts. The occurrence of significant, post-fall surface modifications demonstrate that BG rocks did not arrive recently, and likely have extended residence times on the surface. This erosion could occur as the result of aeolian abrasion alone but likely works in concert with aqueous alteration as indicated by the ferric iron, which would weaken rocks structurally where alteration occurs. Further, evidence from oxide coatings on the surfaces of several iron meteorites at Meridiani Planum show the current epoch to be a destruction epoch not a production epoch for iron oxide [4]. A plausible explanation is that iron oxides would form during exposures to equatorial water ice during periods of high obliquity, with cumulative effects from contributions of many cycles over hundreds of thousands of years.

[1] Fleischer, I. et al., (2010) J. Geophys. Res., 115, E00F05. [2] Schröder, C. et al., (2010) J.Geophys. Res., 115, E00F09. [3] Schröder, C. et al., (2015) 46^th LPSC; abs. #2354. [4] Ashley, J. W. (2015) Elements 11, No. 1.