Dark-toned dunes in the western Medusae Fossae Formation: Characteristics, distribution, and source

Aeolian bedforms are nearly ubiquitous on Mars but the origin of the sediments remains unidentified. Dark-toned Martian sand may originate as volcaniclastic sediment (Edgett and Lancaster 1993). The Medusae Fossae Formation (MFF) has been hypothesized to be a volcaniclastic deposit. The two lobes of the western-most MFF (westMFF) host dark -toned sediments (Fig. 1) categorized here as aeolian based on morphologies, surface textures, and locations within lows. These sediments are bright in both day and night infrared (IR) images, indicating a large grain size and low albedo, and are concentrated along the westMFF southern margin, below the highland-lowland boundary (HLB) scarp. Indications of an MFF origin for this dark-toned dune sediment include: 1) gradation of tone: the dark sediments frequently grade into lighter toned MFF slope materials. 2) morphology and location: The dark dune morphologies indicate emplacement by a northerly (toward the south) wind regime (Fig. 1), for which the westMFF immediately to the north provides a sediment origin. 3) composition: Limited spectral data of the dark dunes indicate an olivine-poor composition, in contrast to the olivine-rich spectra of dunes in southern highland (SH) and Cerberus plains (Cp) craters, indicating a different source for those SH or Cp dunes than for the westMFF dunes. Thus, while minor amounts of sediment have likely been contributed from elsewhwere, we hypothesize that the dark-toned dunes in the westMFF originate(d) from the breakdown of MFF sediments, winnowing of bright fines, and concentration of dark, coarse sand into dunes. Given the putative origin of the MFF as volcaniclastic, this identification of the origin of the westMFF dark-toned dunes supports the paradigm of dark aeolian sediments on Mars originating as volcaniclastic material. Portion of P07_003769_1742_XN_05S209W, showing gradation between lighter- and dark-toned sediments (upper portion of image), and echo dune morphology (white oval) indicative of transport from the north (top).