Mars Exploration Rover Spirit Investigation of the “ El Dorado ” Sand Deposit

Introduction: Reddish, relatively bright air fall dust is widespread across the Spirit landing site within Gusev Crater. The transience of dust devil tracks observed in the multi-year record of MOC images indicates dusty air fall continually obscures somewhat darker materials across the site. A few enduring dark-toned areas within the Columbia Hills are exceptional. MOC images show these darker patches consistently remain significantly darker than surrounding terrain, indicating air fall dust is less stable on substrate at these locations. Spirit visited the largest, 170 m-wide dark patch, termed " El Dorado, " for in situ investigation. Dark patches similar in some respects to El Dorado have been observed from orbit at many locations on Mars in a wide range of settings; many of these features have been interpreted as aeolian deposits of one kind or another (e.g., [1-5]). Objectives of Spirit's visit to El Dorado were to characterize this unusual unit not previously encountered, investigate potentially active aeolian material, and obtain data to help calibrate and inform interpretations of orbital observations of other, similar features elsewhere on the martian surface. Spirit's activities at El Dorado: Spirit visited El Dorado during sols 706-711 of its mission, driving 8 m into the unit and executing a turn-in-place wheel scuff. Undisturbed material was sampled by the Mössbauer spectrometer and APXS. APXS data were collected also from subsurface material exposed in the 2 cm-deep cavity created by the wheel scuff. MI images were obtained at both locations, plus a third location where wheel motion created and preserved a very shallow " road-cut " exposure. Bedforms and Presence of Dust: Bedform relief and trough widths near the rover's position were variable, but representative values are ~0.25 m and ~3 m, respectively. Much smaller secondary ripples with wavelengths 0.1 m or less occur on the flanks of the larger bedforms and within the troughs between them. A narrow, bright, reddish band exposed on SW-facing ripple flanks indicates brighter material can be incorporated into the bedforms, then become exposed subsequently by erosion, e.g., by bedform migration. No angle-of-repose slopes have been recognized in the rover's vicinity. The record of MOC images shows that subtle brightness changes occur over time from place to