Honeycombs in the desert

Convection of salty water creates hexagonal patterns

The honeycomb-like patterns that often occur in salt deserts, including Death Valley and Chile, look like something from another world. A team that included researchers from the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, explain the origin of the mysterious patterns for the first time.

Around the globe, honeycomb-like patterns form in salt deserts, for example in the Badwater Basin of Death Valley in California or in the Salar de Uyuni in Bolivia. These salt structures attract tens of thousands of people from all over the world every year, and they have also served as movie sets. For example, the alien-looking patterns of the Salar de Uyuni in Chile formed the backdrop for the desert planet Crait in "Star Wars: The Last Jedi."

Why these structures form was previously unknown. It was assumed that the salt crust of the desert dries out and cracks form from which the hexagons 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 meters - and honeycomb-like structure. A more plausible explanation has now been provided by a team led by Jana Lasser, who conducts research at Graz University of Technology and the Max Planck Institute for Dynamics and Self-Organization. Together with scientists from Germany and England, she concluded that convection, i.e. the circulation of salty water in the subsurface, leads to the honeycomb salt patterns. This also explains the constant size of the honeycombs of one to two meters and the speed at which the patterns grow.

Experiments, simulations and field studies

"This is a great example of basic research driven by curiosity," says Jana Lasser. "Nature gives us 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 solve nature's riddle, the researchers combined the fields of fluid dynamics from physics and geomorphology from the earth sciences to study the phenomenon from multiple directions: they observed how salty water moves in sandy soils in laboratory experiments and observed the patterns in nature in two field studies in California. In the process, they also collected samples and showed that subsurface currents are related to patterns visible at the surface. In addition, the researchers analyzed the size of the patterns under different conditions using numerical simulations. "Our simulations, together with the field studies, give a consistent picture," reports Marcel Ernst, doctoral student at the Max Planck Institute for Dynamics and Self-Organization. "The driving mechanism for pattern formation is the convection-triggered rise and fall of salty water in the soil beneath the salt crust," he explains.

Indeed, the salt deserts in which the honeycombs occur are by no means completely dry. On the contrary, the highly saline groundwater often reaches directly below the salt crust. When the water evaporates in the hot summer sun, the salt is left behind. As a result, the groundwater directly below the surface becomes more saline and thus heavier than the fresher water below. If this difference in salinity exceeds a certain threshold, the saltier water near the surface begins to sink downward, while fresher water rises from below. Similar to hot and cold water circulating by convection in radiators, convection rolls of saltier and less saltier water form in the subsurface. A single convection roll would form in a circular pattern as the roll then encloses as much volume as possible while minimizing its circumference. However, several convection rolls next to each other in the bottom are pressed into a hexagonal honeycomb pattern. At the edges of the hexagons, very salty water sinks to the bottom as a result. In places with particularly high salt content, more salt crystallizes on the surface. Over time, the resulting crust forms the raised humps and edges which result in the honeycomb salt pattern.

A better understanding of the topography of salt deserts also helps predict how much mineral-rich dust will break loose from the surfaces of salt deserts and swirl into the atmosphere. This dust plays a role in cloud formation and mineral transport to the oceans.

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