BRIGHT-TONED MEGARIPPLE MIGRATION ON MARS

Many aeolian bedforms on Mars are enigmatic as compared to classical terrestrial bedforms. For example, the origin of moderate-scale (10-100 m spacing and 1-14 m tall), light-toned Transverse Aeolian Ridges (TARs) has been long debated [1], as similar terrestrial features are rare. Similarly, smaller (1-5 m spacing and ~40 cm tall) dark-toned ripples (DTRs) generally do not form on Earth, implying an unique martian process is involved [2]. We view a continuum of aeolian bedforms on Mars, some of which are currently migrating today (DTRs) while others appear static (TARs). Here we describe the morphology and evolution of bedforms bridging these two seemingly different groups. For this task, long-term (>10 Earth years) HiRISE images (25 cm/pix) were utilized for change detection along with stereo images for topography.

Prior HiRISE surveys of dunes have shown variable bedform activity across Mars [3], but recently certain locations have been found with migrating ‘megaripples’, with spacing of ~8-18 m and heights of 0.8-2 m. Like DTRs, megaripples are superposed or flanking dunes, but are generally brighter in tone. Although dominantly transverse in morphology and motion, crescentic, oblique and star-like members can also occur. Migrating megaripples exist in a variety of regions and latitudes (N Polar erg, Hellaspontus, Nili Fossae), but all detections were surrounding high sand flux dunes. Megaripples are found in continuity with DTRs and with similar dynamics and orientations, which implies both formed by similar aeolian processes (saltation and creep). However, they cross over into the lower end of parameters previously described for TARs. This may suggest that these decameter-scale ripples exist on a continuum of Martian bedforms with even larger TARs [4]. Classic TARs may have been active under different climate regimes and obliquities, but since then were stabilized by induration and coarse grain armoring. Additionally, any aeolian model that attempts to explain the dynamics of DTRs or TARs should consider these transitional bedforms that we interpret as megaripples.

1. Geissler et al., Aeol. Res. 26, 63–71 (2017)
2. Lapotre et al., Science. 353, 55–58 (2016)
3. Chojnacki et al., Plan. Dunes Work V, #3033 (2017)
4. Silvestro et al., Geophys. Res. Lett. 38, L20201 (2011)