Behind the Cover: July 2021

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

The research: Plants known as hyperaccumulators thrive in soils that contain otherwise toxic levels of metals—such as nickel, cobalt, and rare-earth elements—by absorbing them into their biomass. Those elements are crucial for clean-energy applications but are not mined in significant amounts in the US. As Physics Today‘s David Kramer reported in the July cover story , the US Department of Energy’s Advanced Research Projects Agency–Energy (ARPA–E) is exploring ways to use the plants to “phytomine” the metals as part of the agency’s efforts to green up the production of steel and nonferrous metals, from the mine through to the finished products.

Cobalt is increasingly the focus of such efforts because of its scarcity, its price, and the rapidly rising demand for use in lithium-ion batteries. Most of the world’s cobalt is mined in the Democratic Republic of the Congo, where social and political instability is high. ARPA–E fellow Elizabeth Troein says that planting hyperaccumulators in a field the size of Rhode Island (2700 km²) could satisfy the current 10 000-ton-per-year US demand for cobalt for 10 years or so until the soil is eventually depleted. Troein says there are at least 16 000 km² of land in the US with the type of surface rock (ultramafic) that weathers to produce soils rich in cobalt and other valuable metals.

The cover: The x-ray fluorescence image shows chemical elements in live seedlings of the nickel hyperaccumulator plant Berkheya coddii. Calcium appears in red, nickel in green, and potassium in blue. The image was supplied by one of the experts whom Kramer interviewed, the University of Queensland’s Antony van der Ent.

The design: Contenders for the July cover included satellite images of the Tiuia-Muiun radium mine from Robynne Mellor’s article about uranium extraction in the Soviet Union and a CAD illustration of the instrumental equipment discussed in an article about neutron skins . But in the end, the art and editorial teams gravitated to nickel hyperaccumulators. Those plants offered various images, from the cheery flowers of Alyssum murale, on the left below, to the eerie blue-green sap of the Phyllanthus balgooyi tree, on the right. Van der Ent’s otherworldly fluorescence image offered a bold illustration of hyperaccumulators’ functionality.