Mars Pathfinder Near-field Rock Distribution Re-evaluation

Introduction: We have completed analysis of a new near-field rock count at the Mars Pathfinder landing site and determined that the previously published [1,2] rock count suggesting 16 % cumulative fractional area (CFA) covered by rocks is incorrect. The earlier value is not so much wrong (our new CFA is 20%), as right for the wrong reason: both the old and the new CFA’s are consistent with remote sensing data [1,2,3,4], however the earlier determination incorrectly calculated rock coverage using apparent width rather than average diameter. Here we present details of the new rock database and the new statistics, as well as the importance of using rock average diameter for rock population statistics. The changes to the near-field data do not affect the far-field rock statistics [5]. ShowstereoMap database: The first near-field rock data were limiting in that only two rock population parameters, apparent width and height, were evaluated in addition to rock position. In order to carry out more detailed studies of the rock population in the vicinity of the Pathfinder lander, we built a new rock database with more spatial information for each rock, including nine local-level frame coordinates to evaluate rock position and shape, as well as descriptive parameters like angularity, shape, burial, texture, and color. We used the JPL-developed ‘showstereo’ image analysis software to determine the spatial parameters for each rock. We thus call these data the ‘ShowstereoMap near-field rock database.’ The showstereo software performs user interactive stereo matching of individual image pairs. We chose images pairs from the entire post mast deployment IMP dataset that provided the best available resolution (lowest compression) for that part of the rock field. Generally these are red (670 nm) Superpan images [4], although some blue (440 nm) Superpan images, and even some Monsterpan images were used. Nine locallander frame (x,y, z) locations were measured for each rock with showstereo: (1) rock position is the leftmost point where the rock touches the ground, (2) leftmost point on the rock, (3) rightmost point on the rock, (4) left end of rock long axis, (5) right end of rock long axis; (6) left end of rock short axis, (7) right end of rock short axis, (8) topmost point on rock, and (9) lowest point on rock. Five shape categories (discoidal, sub-discoidal, spherical, sub-prismoidal, and prismoidal) and the six angularity categories (very angular, angular, sub-angular, sub-rounded, rounded, and well-rounded) were measured for each rock following accepted sedimentological grain shape studies. Burial was a qualitative assessment, assigning a value from 0 (perched) to 3 (nearcomplete burial) to each rock. Texture followed Parker et al.’s[6] criteria for bumpy, pitted, knobby, smooth, and lineated textures [7] in images. Rocks were allowed more than one texture type. All data were hand-entered via a web-interface into a mySQL database [8].