Post contributed by Dr. Ryan C. Ewing, Department of Geology and Geophysics, Texas A&M University
Ripples cover the surfaces of sand dunes on Earth and Mars. On Earth, ripples formed in typical aeolian sand (e.g., 0.1 and 0.3 mm) range in wavelength between 10 and 15 cm and display a highly straight, two-dimensional crestline geometry. Ripples are thought to develop through a process dominated by the ballistic impacts of saltating sand grains in which wavelength selection occurs through the interplay of grain size, wind speed, the saltation trajectories of the sand grains, and ripple topography.
High Resolution Imaging Science Experiment (HiRISE) images of the surface of Mars show ripples forming at much longer wavelengths ranging between 1 and 5 m and with a variable planform geometry ranging from two-dimensional to sinuous, three-dimensional. The formative mechanism of these larger martian ripples has been thought to be similar to that of wind ripples on Earth, but with longer saltation trajectories due to the lower atmospheric density and gravity. Although some debate exists about the nature of saltation trajectories on Mars and the expected impact ripple wavelength that might result, this hypothesis implies that at any given location on a dune only one wavelength of ripple would be selected (i.e., smaller cm-scale ripples would not exist superimposed on the larger ripples).
Until recently, this hypothesis could not be texted because the 1 to 5 m large martian ripples were at the limit of the spatial resolution of HiRiSE and no lander or rover had visited an active sand dune to determine if another scale of ripple coexisted with the larger ripples. During the winter of 2015-2016, the Mars Science Laboratory Curiosity rover navigated through the Bagnold Dune Field in Gale Crater, Mars. Images from the Mast Camera revealed the first detailed images of an active dune and the large martian ripples. The imagery showed that the large ripples are unlike Earth’s small wind ripples. The ripples have a strongly asymmetric profiles with angle-of-repose lee slopes and sinuous crestlines. The images also showed ~10 cm ripples superimposed upon the larger ripples. This new observation has led to a new hypotheses about the large ripples.
A recent study by Lapotre et al. (2016) suggests that the smallest ripples on Mars are impact ripples, similar to what is found on Earth, but that the large ripples may form differently than wind-blown ripples on Earth. Instead, the study suggests that the large ripples develop in a way most similar to underwater current ripples, which have similar angle-of-repose slopes and sinuous crestlines and are thought to form via a fluid-drag mechanism. An analysis using martian atmospheric boundary conditions in the salient equations describing current-ripple formation indicate that wind-drag ripples can exist on Mars because of the high kinematic viscosity of the martian low-density atmosphere. Because the so-called wind-drag ripple wavelength scales with atmospheric density, these ripples have emerged as a key climate indicator of past changes in Mars’ atmospheric density.
Lapotre, M. G. A., et al. “Large wind ripples on Mars: A record of atmospheric evolution.” Science 353.6294 (2016): 55-58.
Sharp, Robert P. “Wind ripples.” The Journal of Geology (1963): 617-636.
Andreotti, Bruno, Philippe Claudin, and Olivier Pouliquen. “Aeolian sand ripples: experimental study of fully developed states.” Physical review letters96.2 (2006): 028001.
Almeida, Murilo P., et al. “Giant saltation on Mars.” Proceedings of the National Academy of Sciences 105.17 (2008): 6222-6226. b
Durán, Orencio, Philippe Claudin, and Bruno Andreotti. “Direct numerical simulations of aeolian sand ripples.” Proceedings of the National Academy of Sciences 111.44 (2014): 15665-15668.
Yizhaq, H., J. F. Kok, and I. Katra. “Basaltic sand ripples at Eagle Crater as indirect evidence for the hysteresis effect in martian saltation.” Icarus 230 (2014): 143-150.
Schmerler, Erez, et al. “Experimental and numerical study of Sharp’s shadow zone hypothesis on sand ripple wavelength.” Aeolian Research 22 (2016): 37-46.