MORPHOLOGICAL ATTRIBUTES OF MARTIAN STEPPED FANS

Stepped fans—conical forms with stair-step topography—are relatively rare on both Earth and Mars. Multiple formation scenarios for stepped fans have been proposed, including erosive wave action, repeated alluvial fan deposition with periodic sheetwash flows to create the benches, or punctuated lake level rise. We characterize the morphological attributes of martian stepped fans to ascertain variability among this class of landform and assess the plausibility of formation hypotheses. Stepped fans and their associated catchment were examined in CTX and HiRISE image data to document their size, feeder channel system, planimetric form and step spacing. In addition, the topographic form (step heights and inter-step slope) were documented for select stepped fans where elevation data could be derived from HiRISE stereo image pairs.

Thirteen martian fans are concentrated in the equatorial region, typically located in impact craters, with a few present in canyons. Half of the martian stepped fans have a low-order, branched feeder network, while the other half are fed by a single channel with no discernible tributaries in sub-meter-scale images. Surprisingly, initial longitudinal profiles reveal some steps identified in image data have minimal relief and are merely albedo differences.

We have developed a preliminary classification scheme based on our image observations. ‘Classic’ stepped fans have a triangular planview shape and equally-spaced, concentric steps. ‘Complex’ stepped fans are characterized by multiple lobes and greater variability in step curvature and spacing. We will continue to refine the attributes of fans within each category through further interrogation of fan topography, and evaluate whether these differences reflect distinct processes. This study could yield critical information regarding Mars’ complex history of standing water on its surface.