• Physics 16, 31
The geometric patterns on dry, salty lake beds are generated by convection of high- and low-salinity water underground, in line with simulations and observations.
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Salt flats, deserts, and dry lakes host mysterious and nonetheless unexplained patterns of interlocked polygons shaped from slender ridges that develop on the salt-encrusted floor. Now researchers consider they’ll clarify how these patterns type via the interplay between fluid flowing within the soil and fluid evaporating on the floor [1]. The brand new work seems to supply a unifying rationalization for the expansion of such polygons in a variety of various bodily situations.
Salt polygons type in areas the place the speed of evaporation of water from the bottom considerably exceeds the speed of precipitation. In such locations, evaporation on the floor is balanced by a steady stream of groundwater from extra distant sources. For instance, Badwater Basin in Loss of life Valley, California, is provided by water from the close by mountains. This water rises to the floor and results in steady evaporation, even within the absence of rainfall, and it leaves a salt residue behind. However why these residues type into ridges that hint out polygonal patterns has remained mysterious. Additionally puzzling is why the patterns develop into so comparable wherever they happen, regardless of native variations in geology, salt chemistry, and environmental situations.
“Salt deserts are a few of the most lovely landscapes on the planet,” says physicist Jana Lasser of the Graz College of Know-how in Austria. The polygons are “all the time just a few meters throughout, however nobody has been capable of clarify the origins of those patterns with their distinctive dimension and form.”
To seek out a proof, Lasser and her colleagues developed a mathematical mannequin of the processes occurring at and simply beneath the floor. This mannequin, based mostly on fluid dynamics and different geophysical rules, predicts {that a} typical dry lake mattress will develop a heavy salt-rich layer of fluid close to its floor because of the salty residue above.
This dense, salty layer rests unstably on prime of less-dense fluid beneath. If the driving drive of evaporation is sufficiently robust, a convective stream develops within the fluid: saltier water sinks and less-salty water rises, which generates upward- and downward-flowing columns of fluid, like people who seem in boiling water. Utilizing fluid parameters attribute of actual salt lake beds, the researchers predict these so-called convection rolls to be roughly 1–2 m throughout—suspiciously just like the sizes of noticed polygons.
In simulations, the crew went additional to check the long-term evolution of this fluid stream. The simulations present that the convection rolls finally generate a set of slender plumes of high-salinity water descending from the floor. These plumes then type right into a closed polygonal community of downwelling vertical sheets that encompass patches of upwelling fluid.
This community of convection rolls, Lasser says, drives the formation of the salt ridges. Within the inside of every convection cell, the place low-salinity water flows upward, the salt crust grows extra slowly as a result of it’s involved with less-salty water. In distinction, on the cell boundaries situated immediately above the downwelling sheets, a better flux of salt flows upward into the crust. Because of this, slender ridges develop quickly and construct the polygonal community sample.
To check this mannequin, Lasser and her colleagues used detailed simulations of the fluid dynamics mannequin and in contrast these with observations from area websites, together with Loss of life Valley. Excavating beneath the ridge sample, they discovered proof of salty, down-flowing plumes, precisely as predicted, with brisker water flowing upward within the flat facilities of the polygons. General, the researchers discovered that their mannequin may clarify not solely the essential emergent patterns, but additionally the important thing size scales—the widths of each the convective cells and the salt ridges.
“This analysis presents compelling proof that salt polygons are the floor signature of this convection mechanism,” says mathematician Duncan Hewitt of College School London. “It’s a moderately lovely consequence, which supplies a possible rationalization for a broadly noticed geological sample.”
–Mark Buchanan
Mark Buchanan is a contract science author who splits his time between Abergavenny, UK, and Notre Dame de Courson, France.
References
- J. Lasser et al., “Salt polygons and porous media convection,” Phys. Rev. X 13, 011025 (2023).
