Physics and Geomorphology of Sand Ripples on Earth and in the Solar System

Aeolian impact ripples are abundant in arid regions on Earth, on the surface of Mars and probably on the surface of Venus and Titan. Aeolian impact ripples develop from the instability of an initially flat bed of cohesionless sand that is mobilized into saltation by wind shear stress. Impact ripples in relatively fine desert sands typically have unimodal grain size distributions, with coarser-than-average grains concentrated at ripple crests. Megaripples are more extreme forms that develop in poorly sorted sands with bimodal grain size distributions in which the coarsest grains covering crests are limited to short creep movements only, driven by impacts from the finer saltating grains. Observations of normal sand ripples indicate that they are effectively two-dimensional bedforms, displaying only small modulations in the direction transverse to the wind. In contrast, most megaripples have greater sinuosity due to the presence of a transverse instability, causing small undulations along the ripple crest to grow over time. Recently, it was suggested that the megaripple can mechanistically be understood as an unconventional type of small dune, created by coarse grains that are too heavy to participate in saltation but move in tiny steps (so-called ‘reptation’) when kicked by finer saltating grains.

On Earth, ordinary impact ripple wavelengths typically are < 30 cm and less than 1 cm high. However, on Mars, ripple-like bedforms with crests lacking very coarse grains can be much larger both in wavelength and height. Two size modes of these ripples were observed: small (decimeter scale) ripples similar to impact ripples that commonly cover dune surfaces on Earth and large, meter scale ripples that have no corresponding terrestrial analog. It’s important to note that these very large Martian ripples do not have crests covered with very coarse grains, so are not like terrestrial or Martian megaripples. These large ripples on Mars were detected in orbital images and first visited in situ by the NASA Mars Exploration Rover (MER) Spirit at the El Dorado ripple field in Gusev Crater. Recently, a continuous transition between large ripples and megaripples has been observed on Mars by analyzing HiRISE images (High Resolution Imaging Science Experiment). Based on data sent by the NASA Mars Science Laboratory (MSL) rover in Gale Crater an alternative hypothesis for the origin of the large (2.1 meter in wavelength) ripples that superimpose dune surfaces at the MSL landing site was suggested. According to their theory, the large ripples are fluid drag ripples which are similar in their morphology to subaqueous ripples.

Despite the fact that the study of sand ripples is almost 100 years old, many fundamental questions are still not fully understood. This Research Topic aims to collect the latest research progress and achievements on sand ripples on Earth and in the Solar System, contributions could include, but are not limited to:

• Morphology of normal ripples and megaripples;
• The debate on the formation of large ripples on Mars;
• The physics of the sorting process;
• Wind tunnel studies of ripples formation;
• Mathematical models of sand ripples formation;
• The mechanism of ripple flattening;
• CFD (Computational Fluid Dynamics) simulations of the flow above ripples and megaripples;
• The formation of TARs (Transverse Aeolian Ridges);
• Using remote sensing methods in the study of ripples dynamics;
• Sand ripples on other bodies in the solar systems like Venus and Titan;
• The formation of fluid drag ripples.