Scientists have solved the mystery of the mysterious honeycomb patterns found in salt flats around the world.
These places are some of the most extreme and inhospitable on earth, and their strange polygonal structures attract hundreds of thousands of tourists every year.
Salt flats feature in areas like Badwater Basin in Death Valley, California and Salar de Uyuni in Chile, which Star Wars: The Last Jedi fans will recognize as the backdrop to the desert planet of Crait.
The new study suggests that the shape and size of the honeycomb pattern may be due to the movement of salt water (with a high concentration of dissolved salt) beneath the surface.
The surface patterns reflect the slow tipping over of salt water in the ground, a phenomenon similar to the convection cells that form in a thin layer of boiling water
dr Lucas Goehring, University of Nottingham Trent
The constant size of the features and the speed at which the patterns grow are also due to this, say the researchers from Nottingham Trent University and TU Graz in Austria.
It used to be thought that the salt crust of the desert dries up and cracks form around which the patterns grow.
Another suggestion was that the salt crust is continuously growing and due to lack of space, it bends and forms the patterns.
However, both theories did not explain 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 flats, the first thing you see – almost all you see – is an endless patchwork of hexagons and other ordered shapes.
“The surface patterns reflect the slow tipping over of salt water in the ground, a phenomenon similar to the convection cells that form in a thin layer of boiling water.
“Although the beautiful wind blows over salt flats, it is a major source of atmospheric dust and our results will help to understand such processes in desert environments.”
The scientists conducted laboratory experiments to see how saline water moves in sandy soils and analyzed the patterns under different conditions.
In two field studies in California, they observed the patterns in nature and collected samples to show that underground currents mirror the patterns seen on the surface.
The salt flats in which these patterns occur are not dry and the highly saline groundwater often reaches just below the salt crust.
While people would be quick to dig this water by hand, it would be far too salty to drink.
When this brine then evaporates in the hot summer sun, the salt remains and makes the groundwater just below the surface saltier and therefore heavier than the fresh water lurking below.
If this difference in salinity is high enough, the more saline water near the surface begins to sink, while fresher water from below rises.
The research suggests that when many convection coils develop side by side in the ground, they compress, creating hexagonal, honeycombed patterns, with very saline water sinking along the edges.
If the salt content is particularly high, salt also crystallizes out more on the surface.
Over time, the resulting crust forms the raised humps and ridges that create the honeycomb salt pattern.
The study, which involved the Max Planck Institute for Dynamics and Self-Organization, the University of Southampton, the University of Leeds, the University of Göttingen and the University of Oxford, is published in the journal Physical Review X.