Honeycomb patterns form in salt deserts all over the world, for example in the Badwater Basin of Death Valley in California or in the Salar de Uyuni in Bolivia. These enigmatic salt structures attract tens of thousands of visitors every year. The alien-looking patterns of the Salar de Uyuni can also be recognised in film sets, for example in “Star Wars: The Last Jedi,” where they served as the backdrop for the desert planet Crait.
Until now, a mechanism that was able to explain the origin of these patterns and their iconic shape and size has been elusive. Previously, it was suspected that the salt crust of the desert dries out and cracks form around which the patterns grow. Another hypothesis explains the pattern formation by the fact that the salt crust grows continuously and bends due to lack of space, thus forming the patterns. Neither of these explanations, however, can account for the always consistent size – one to two metres – and the honeycomb shape of the patterns. A more plausible explanation has now been provided by Jana Lasser from the Institute of Interactive Systems and Data Science at TU Graz together with researchers from Germany and England. In the journal Physical Review X, they describe how convection of saline water in the subsurface is responsible for the honeycomb-like formation of the salt patterns. The constant size of the honeycombs of one to two metres and the speed with which the patterns grow can also be attributed to this.
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Driven by curiosity
First author Jana Lasser from TU Graz says: “This is a great example of curiosity-driven basic research. Nature presents us with an obvious and fascinating puzzle that stimulates our curiosity and thereby prompts us to solve it – even without any direct further possibility of application in mind.”
To get to the bottom of this mystery of nature, Lasser and her collaborators combined the research fields of fluid dynamics from physics and geomorphology from the geosciences and investigated the phenomenon from several directions. They observed in laboratory experiments how saline water moves in sandy soils, analysed in numerical simulations the length scale of the patterns under different conditions, and in two field studies in California observed the patterns in nature and collected samples to show that the currents in the subsurface are related to the patterns visible on the surface. They were the first research group to record in a video how the salt pattern grows: www.youtube.com/watch
Hexagonal honeycomb patterns thanks to convection
The evidence gathered by the researchers in experiments, simulations and field studies all points to a consistent picture. The driving mechanism for pattern formation is the convection or circulation of salty water in the soil beneath the salt crust. The salt deserts in which these patterns occur are by no means bone dry. On the contrary, the highly salty groundwater often reaches directly below the salt crust. When this brine then evaporates in the hot summer sun, the salt remains. This makes the groundwater directly below the surface saltier, and thus heavier than the fresher water below. If this difference in salinity is high enough, the saltier water near the surface begins to sink downwards, while fresher water rises from below. Similar to hot and cold water circulating by convection in radiators, convection rolls of salty and less salty water form in the subsurface.
A single convection roll would have a circular shape, which maximises the volume enclosed by the roll while minimising its circumference. However, when many convection rolls develop next to each other in the ground, they are “squeezed” together and produce hexagonal, honeycomb-like patterns, along the edges of which very salty water sinks down. In places with a particularly high salt content, salt also crystallises more on the surface. Over time, the resulting crust forms the raised humps and edges that create the honeycomb salt pattern.
In addition to satisfying curiosity, an understanding of the topography of salt deserts helps predict how much mineral-rich dust will be dislodged from the surfaces of salt deserts and swirled into the atmosphere. This dust in turn plays an important role in the formation of clouds and the transport of minerals into the oceans.
Jana Lasser worked on this topic as part of her doctoral thesis at the Max Planck Institute for Dynamics and Self-Organisation at Göttingen. She was supervised by Lucas Goehring, who is now researching at Nottingham Trent University. Since 2021, Lasser has been researching at the Institute of Interactive Systems and Data Science at TU Graz, where a substantial part of the writing work and the many revisions of the publication for Physical Review X (PRX) have been done. In addition to Lasser and Goehring, Marcel Ernst from the Max Planck Institute for Dynamics and Self-Organisation in Göttingen, Jo Nield from the University of Southampton, Cédric Beaume and Mathew Threadgold from the University of Leeds, Volker Karius from the University of Göttingen and Giles Wiggs from Oxford University were also involved in the publication in PRX.
