Behind the Cover: April 2024

Each month, Physics Today editors explore the research and design choices that inspired the latest cover of the magazine.

Badwater Basin, located 86 meters below sea level, marks the lowest elevation in North America. On what was once the site of an ancient lake, water now flows underground, some of it just under the surface. When that groundwater evaporates, it leaves behind salts and other minerals that slowly accumulate. Instead of producing a flat surface, the minerals develop a network of narrow ridges that form polygons a few meters in diameter. The surreal landscape is pictured on the cover of Physics Today‘s April 2024 issue .

The dry salt lake surface is hard and brittle, and the ridges bordering the polygons are often broken up by cracks. But when Jana Lasser and Lucas Goehring toured the desert landscape and snapped the cover photo in 2016, they realized that the polygons were not themselves cracks. The researchers began making predictions about the polygons’ size from fluid flow in convection cells.

As Lasser and Goehring explain with Cédric Beaume in their cover story , groundwater evaporates from salt lakes worldwide at about the same rate of 0.1 millimeters per day. Salt is carried upward by water at the same rate. But it also diffuses laterally. The researchers theorized that if convection balances diffusion and fluid transport, the ratio of the two would yield the natural scale of the convection—about 1 meter. The researchers’ subsequent numerical simulations of the convection cells bear out that prediction.

To complement the striking Badwater Basin photo, Physics Today‘s art designer, Jason Keisling, gave the cover text a grainy texture: “I subtly roughened the letters’ interiors to make them appear granular, so that the type feels more cohesive with the landscape.”