New theory explains sand ripples on Mars and on Earth

Sand ripples are fascinating. They are symmetrical yet wind, which causes them, is very much not. Furthermore, they can be found on Mars and on Earth. They would be even more fascinating if the same effect found on Mars could be found here on Earth as well. What if one unified theory could explain their formation on two different planets of our solar system?

That is what Ben-Gurion University of the Negev physicist Prof. Hezi Yizhaq and Prof. Itzhak Katra and their colleagues from Denmark, Germany, Italy, China, and the US contend in a cover article published inNature Geoscience.

Sand ripples photographed on Mars by NASA's Curiosity rover in 2015 showed two distinct patterns – large ripples (meter scale) and a shorter "impact" ripples pattern (decimeter scale). The prevailing theory proposed since then argues that the smaller scale ripples are produced by the impact mechanism of the particles transported by the wind like normal ripples on Earth and the larger ripples form due to hydrodynamic instability like subaqueous ripples. Furthermore, it was believed that the physical conditions that produced them on Mars could not produce them on Earth.

However, Prof. Yizhaq and Prof. Katra have proven experimentally using Ben-Gurion University's wind tunnel and Aarhus University's Mars tunnel that such a phenomenon could exist on Earth – we just haven't noticed it yet because we didn't know we should be looking for it.

Imitating Martian sand was not easy because it's finer than sand here on Earth, explains Prof. Yizhaq, but the breakthrough occurred when they decided to try tiny glass balls to represent fine grains of sand.

Furthermore, the international research team has proposed a unified theoretical framework that would explain sand ripples on Mars and on Earth. At its most basic level, sand ripples on Mars caused by wind look like sand ripples on Earth caused by water.

"There is much more research, both fieldwork and experimentally, needed to prove our theory, but it is amazing to propose something so radically new in a field I have been studying for over 20 years. It is exciting to go out and try to find on Earth what can clearly be seen on Mars," says Prof. Yizhaq.

Prof. Yizhaq is a member of the Department of Solar Energy and Environmental Physics. Prof. Itzhak Katra is a member of the Department of Environmental, Geoinformatics and Urban Planning Sciences.

The research was supported by the Israel Science Foundation (Grant no. 1270/20), the German-Israel Foundation for Scientific Research and Development (GIF) (Grant no. 155-301.10/2018), the National Natural Science Foundation of China, Texas A&M Engineering Experiment Station, Europlanet grant no. 871149, and the Horizon 2020 Research and Innovation Program.

Photo caption: The wind tunnel at Aarhus University in Denmark allows the pressure to be reduced to that on Mars, which is 200 times smaller than atmospheric pressure on Earth. The tunnel is inside the large red tank.| Photo: Hezi Yizhaq

Photo caption: Waves received in the wind tunnel of Ben Gurion University of the Negev with glass balls with a diameter of 90 microns. Two scales of waves can be seen in the image.
Small waves with a wavelength of centimeters and large waves with a wavelength of about 10 centimeters resemble waves due to the flow of water. The existence of two scales of waves on Mars was discovered by the Mars Curiosity Rover.| Photo: Hezi Yizhaq

Photo caption: Several sizes of waves in the sand created by the wind in the Namib Desert. You can see down the high dune small, straight waves formed in fine-grained sand and larger mega waves covered with grainy coarse sand. The coarse sand is grayish while the fine sand that builds the dunes is reddish.| Photo: Hezi Yizhaq