Windblown sand produces distinctive bedforms at scales ranging from normal sand ripples to large reversing sand dunes. We explore how aeolian bedforms evolve at both extremes of this range. An investigation of the transition from sand ripples (<1 cm height) to granule-coated megaripples (25 cm height) is underway at Great Sand Dunes National Park and Preserve (GSDNPP) in central Colorado. Sand-to-megaripple transitions at GSDNPP were documented in May and Sept of 2019 using stereophotogrammetry that produced digital terrain models that resolved granule (1-2 mm) particles as well as some sand grains; these data show the spatial distribution of particles across sand ripples whose crests merge directly into crests of megaripples. To date we have not observed that sand ripples are a necessary prerequisite for the initiation and growth of megaripples; the spatial density of granule particles appears to influence the evolution of megaripples. Reversing sand dunes are being monitored using differential navigation satellite system data at GSDNPP (up to 10 m height) and at Bruneau Dunes State Park (BDSP) in central Idaho (individual dunes >100 m height). Surveys of the crests of reversing dunes at GSDNPP reveal a northeastward migration of individual dunes along the southern margin of the main dune mass, consistent with dominant local winds, yet the symmetric reversing dune profile is maintained during the translation. Surveys of the crests of large reversing dunes at BDSP reveal variable adjustments of the crests that may be affected by wind flow altered by the bulk of the dunes themselves, sheltering the southern end of the dunes from one of the seasonal bimodal winds. Results to date indicate that the deformable shape of aeolian bedforms affect wind flow at all spatial scales, influencing the evolution of the features over diverse time scales.

The transition from sand ripples to megaripples encodes information about the physics of how both sand-sized (moved via saltation) and coarsegrained (moved via impact creep) particles interact under Martian conditions. Previous studies have focused on the aeolian mobility of sand on Mars; here we examine how mobile sand interacts with larger particles moved by creep. HiRISE images of small dunes (lacking well developed slip faces) on Mars reveal a transition of aeolian bedform scale with increasing distance from the dune. Here we document the particles in a similar transition on Earth. High Resolution Imaging Science Experiment (HiRISE) images have documented that sand is moving at many locations around Mars under current conditions. Unlike the active sand deposits, enigmatic “Transverse Aeolian Ridges” (TARs; the non-genetic term for linear to curvilinear aeolian bedforms resulting from either dune- or ripple-forming processes) are found at locations widely distributed across Mars. Recently bright TARs were documented to have moved in HiRISE images taken many Earth years apart at three widely separated locations. Great Sand Dunes National Park and Preserve (GSDNPP) in Colorado has a bimodal particle size distribution along with a seasonal bimodal wind regime, providing the setting to examine the transition from sand ripples (<1 cm in height) to megaripples (typically ~25 cm in height). A Smithsonian Scholarly Studies Award for FY19 funded trips to GSDNPP during May and September of 2019 to collect thousands of digital photographs of ripple-megaripple transitions that were later processed using Multiview Stereo Photogrammetry software to produce detailed Digital Terrain Models (DTMs). The digital images were obtained using a Nikon camera that was motor-driven along a track above the study area. The track was then manually advanced following each photo traverse. Photos were obtained using both a 35 mm lens and a 85 mm Macro lens. The DTMs clearly resolve individual coarse (1-2-mm diameter) particles on the bedforms, providing a detailed record of the surface distribution of coarse grains across both sand ripple and megaripple bedforms, including cases where the crests were continuous between sand ripples and megaripples.