by The spectacular, hexagonal shapes that texture salt deserts such as the Badwater Basin in Death Valley, California and the Salar de Uyuni in Bolivia have long mystified travelers and scientists alike.
Considered by some to be the result of cracks forming as the brine surface dries out or pushes against itself as the salt precipitates and expands, no explanation satisfies the uniform size and arrangement of the grooved cells.
A new study provides the most plausible explanation yet. Based on a mix of field research and numerical simulations, an international team of researchers suggests that contrasting concentrations of saline solutions circulating in donut-shaped currents may be responsible.
“This is a great example of curiosity-driven basic research,” says physicist Jana Lasser from Graz University of Technology (TU Graz) in Austria.
“Nature presents us with an obvious and fascinating puzzle that stimulates our curiosity and thus prompts us to solve it – even without any direct further application possibilities in mind.”
Here’s what the researchers propose happens: brine sitting near the surface is heated by the sun, causing a fraction of the water to evaporate into the air. As the water is removed, the remaining solution becomes saltier, increasing its density.
This relatively dense solution of salt water is heavier than the fresher, less dense salt water below, causing it to sink as the low-salinity water rises to replace it.
In salt deserts, the researchers suggest, this occurs widely over a large area, with each convective roll being squeezed tightly against neighboring cells. Edges of crystallized salt then collect in places where the water is saltier and sink, creating the familiar honeycomb pattern, with fresher water evaporating from the flatter spaces.
The team saw this happen in soil in the laboratory, which was combined with analyzes of samples taken from salt deserts. The researchers were also able to capture a time lapse video (below) of the phenomenon occurring in Owens Lake in California.
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“Our simulations, together with the field studies, provide a consistent picture,” says physicist Marcel Ernst, from the Max Planck Institute for Dynamics and Self-Organization in Germany.
“The driving mechanism for pattern formation is the convection-triggered rise and fall of salt water in the soil beneath the salt crust.”
While previous studies had suggested that the repeating ridge patterns could be created as the salt crust dries out or stretches over time, these earlier hypotheses did not really explain the uniformity or appearance of the shapes in many different regions.
The new proposal stands up to better scrutiny, say the researchers. It is consistent with the consistent size of the shapes (1 to 2 meters or 3.3 to 6.6 feet), and the speed with which they are known to form.
Also, it can help scientists in their calculations regarding how much dust, mineral-packed dust these deserts emit: it is important in terms of the speed at which clouds form and how minerals are transported through the air to the oceans.
“What we have shown is that a simple, plausible explanation is there, but hidden beneath the surface,” says physicist Lucas Goehring, from Nottingham Trent University in the UK.
“Although beautiful, the wind blowing across salt deserts is an important source of atmospheric dust, and our results will help to understand processes like this in desert environments.”
The research is published in Physical examination X.
