Three-Dimensional Documentation of the Transition from Sand Ripples to
Megaripples
Abstract
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.