The enchanting honeycomb patterns found in salt deserts such as Badwater Basin in California’s Death Valley and Salar de Uyuni in Bolivia have baffled tourists and inspired sci-fi filmmakers for decades. Scientists have also struggled to elucidate the mechanism behind the iconic shapes.
Now physicists believe they have finally solved this natural puzzle.
“The fantastic scenery requires an explanation,” Lucas Goehring (opens in new tab)an associate professor of physics at Nottingham Trent University in England, said in a announcement (opens in new tab). “What we’ve shown is that a simple, plausible explanation is there, but hidden underground.”
The answer lies in the groundwater beneath the salt crust, according to a study published Feb. 24 in the journal Physical examination X (opens in new tab). In the study, researchers describe how layers of salt and less salty water circulate up and down in donut-shaped currents, which are compressed horizontally to form the regular pattern.
Earlier, suggested researchers (opens in new tab) that the cracks and ridges form as the salt crust expands and dries out, bending and fragmenting beneath the trunk.
Now the researchers note that previous attempts to understand the iconic landscape did not take into account the consistent size of the hexagons, which are always 3 to 6 feet (1 to 2 meters) across, no matter where in the world they are found.
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The new study confirms the widely accepted idea that the geometric patterns are formed by a mechanism rooted in fundamentals thermodynamics, similar to the movement of hot and cold water in a radiator or in a pot of boiling water. “The surface patterns reflect the slow overturning of saline water in the soil, a phenomenon somewhat like the convection cells that form in a thin layer of simmering water,” Goehring said.
Salt deserts are not as dry as they seem. Beneath the salt crust is a layer of extremely salty water, which can be reached by digging with the hands. The water evaporates in the hot summer months, leaving only a blanket of salt, some of which dissolves in the next layer of water. This layer is then denser than the one below it, and the salty water sinks in a ring surrounding fresher, less dense water that rises to replace it. The water evaporates and leaves a salt residue, which again dissolves in the upper water layer. The cycle repeats itself to form what scientists call a convection roll.
Research into salt deserts has focused either on these underground flows or on the crust. The new study posits that the two features interact and mirror each other to form the tessellations. Where the dense, salty surface water sinks, salt collects on the crust and forms ridges. The salt crust grows faster around the edges of each hexagon because it is in contact with saltier water than the center.
Normally, a convection roller would assume a circular donut shape. However, because there are so many of them packed closely together on a salt surface, the rolls are pressed against each other to form hexagons, the researchers said.
The authors present a convincing explanation for the striking pattern, Stuart King (opens in new tab), a researcher at the University of Edinburgh in Scotland who was not involved in the study, told LiveScience in an email. “It is well known that hexagonal patterns arise from convection and evaporation processes, (but) this paper links it to the penetrating convection of the underlying porous layer, which seems very plausible as a broader mechanism driving the whole salt formation.”
The researchers say their work was driven purely by curiosity. “Nature presents us with an obvious and fascinating puzzle that stimulates our curiosity and thereby encourages us to solve it – even without any direct further possibility of application in mind,” the study’s first author Jana Lasser (opens in new tab)a postdoctoral researcher at the Graz University of Technology in Austria, said in the statement.