Scientists have solved the riddle of the mysterious honeycomb patterns found in salt deserts around the world.
These places are among some of the most extreme and inhospitable on Earth, their bizarre polygon-shaped structures attracting hundreds of thousands of tourists each year.
Salt deserts occur in areas including Badwater Basin in Death Valley, California, and Salar de Uyuni in Chile, which fans of Star Wars: The Last Jedi will recognize as the backdrop to the desert planet Crait.
The new study suggests that the shape and size of the honeycomb pattern may be due to the movement of salty water (with a high concentration of dissolved salt) below the surface.
Patterns on the surface reflect the slow turning of salt water into the soil, a phenomenon similar to convection cells that form in a thin layer of boiling water.
Dr Lucas Goehring, Nottingham Trent University
The constant size of the features and the speed with which the patterns grow can also be attributed to this, say the researchers from Nottingham Trent University and TU Graz in Austria.
It was previously thought that the desert salt crust dries out and cracks form, around which patterns grow.
Another suggestion was that the salt crust continually grows and folds due to lack of space, forming the patterns.
However, none of the theories explained the constant size, always one to two meters, and the honeycomb shape.
Dr Lucas Goehring, Associate Professor of Physics at Nottingham Trent University’s School of Science and Technology, said: “In salt deserts, the first thing you see, almost all you see, is an endless mosaic of hexagons. and other ordered forms.
“The patterns on the surface reflect the slow overturning of salt water into the soil, a phenomenon similar to convection cells that form in a thin layer of boiling water.
“Although beautiful, the wind blowing over salt deserts is a major source of atmospheric dust, and our results will help understand processes like this in desert environments.”
The scientists conducted laboratory experiments to see how salty water moves in sandy soils and analyzed the patterns under different conditions.
In two field studies in California, they observed patterns in nature and collected samples to show that currents in the ground reflect patterns visible on the surface.
The salt deserts in which these patterns occur are not dry, and highly salty groundwater often reaches directly beneath the salt crust.
While people would quickly reach this water by digging by hand, it would be too salty to drink.
When this brine evaporates in the hot summer sun, the salt remains, making the groundwater directly below the surface saltier and therefore heavier than the fresher water still lurking below.
If this salinity difference is high enough, the saltier water near the surface begins to sink downward, while the fresher water rises from below.
Research suggests that when many convective coils develop next to each other in the ground, they squeeze together and produce hexagonal, honeycomb-like patterns, along the edges of which highly salty water sinks.
Where there is a particularly high salt content, the salt also crystallizes more on the surface.
Over time, the resulting crust forms the bumps and raised edges that create the honeycomb pattern of salt.
The study, which involved the Max Planck Institute for Dynamics and Self-Organization, the University of Southampton, the University of Leeds, the University of Gottingen and the University of Oxford, is published in the journal Physical Review X.