A. T. Basilevsky

Morphologies of rocks within and near the Rock Garden at the Mars Pathfinder landing site

This paper considers the morphology of several rocks and rock fragments within and near the Rock Garden at the Mars Pathfinder landing site. We have analyzed stereo images taken both by the lander IMP camera and by the rover forward cameras. The rocks were found to differ in their roundness/angularity and in their densities of millimeter-to-centimter-sized pits and flute-like features on the rock facets. No correlation between rock roundness and the degree of rock pitting was found, indicating that rounding and pitting formed by different processes. Pits are either gas bubbles (implying the rocks are volcanic), or resulted from eolian abrasion/deflation (putting no constraints on the nature of the rocks), or both. If pits are of eolian origin, variations in the density of pits on different rocks imply that either the rock surfaces have been exposed for different times or the susceptibility of different rocks to pitting varies significantly from rock to rock, implying differences in composition and/or lithology. The densities of pits and flutes on the rocks show anti-correlation. This may be connected to their formation but probably is mostly related to issues of observation and definition of what is a pit and what is a flute. Flute-like features may be a result of eolian abrasion, but in this case, the spatial closeness of fluted rocks and those having delicately pitted textures requires explanation. Other mechanisms producing flute-like features, such as collisions in the deposit-forming flows and the breaking of rock in impact cratering, should not be ignored. Our observations agree with the conclusion of Golombek et al. [this issue] that deflational exhumation in this area was minor. The observed, relatively large, rocks typically do not show a concentration of small rock fragments at their feet. This implies diurnal thermal cycles are very ineffective at destructing rocks. The rock, Half Dome, may be a conglomerate (as suggested by Smith et al. [1997a] for this and several other rocks), but this hypothesis remains to be proven. Impact craters of a few meters to a few tens meters in diameter may have played a role in the local geology.