Not of this World


She may not know it, but the blushing bride with the sparkling new platinum wedding band is carrying clues about Earth’s past on her finger.

Gold, palladium, and other precious metals also carry such evidence, even as modern society taps them for everyday products such as mobile phones, computers, and catalytic converters in cars.

New research led by James Day of Scripps Institution of Oceanography at UC San Diego has revealed for the first time how and when such elements may have arrived on Earth. Geologists know that these metals are sealed away in the metallic core of Earth—as well as the cores of the Moon, Mars, and certain asteroids—but their abundance within reach of planetary surface-dwellers indicates metals were added some time after the planet’s formation. But how? And how long after?

Like detectives evaluating clues spanning millions of years, Day and his colleagues at the University of Maryland, Carnegie Institution of Washington, and University of Science and Technology of China studied the ages and composition of stony meteorites called diogenites, which are derived from asteroids and similar objects. They resemble small planets with features such as cores, mantles, and crusts like Earth and Mars, and also contain elements such as platinum and other precious metals.

The results of the scientific study, published in the July 22 issue of Nature Geoscience, show that asteroids and other planetary objects were responsible for delivering these precious elements and did so through massive collisions that occurred 2 to 3 million years after the formation of the solar system—a snap of the fingers in planetary history time scales. Day and his colleagues believe the results point to the same scenario carrying the metals to Earth, the Moon, and Mars, and in roughly the same time frame.

“This is the first time that a hard and fast date has been provided for this process on any planet,” said Day, an assistant professor in the Geosciences Research Division of Scripps. “People had been speculating anywhere from a billion years after the formation of the Solar System to tens of millions of years, so to define that it happened so early is quite a dramatic change in our understanding.”

The results are relevant not only for understanding the state of precious metals on Earth, but for determining the availability of such elements on asteroids, which some companies are considering for space mining projects.

Day’s next steps include seeking direct evidence for similar processes from Earth, Moon, and Mars samples. He would also like to investigate how differently elements such as platinum are distributed on other planetary bodies. Because massive impacts could result in disparate distributions, precious metals may be spread in varying proportions.

“What we would like to do is try and figure out exactly how late these metals came to the earth and Moon,” said Day. “All of these materials that we use in manufacturing and to make wedding rings are very early vestiges of the earth, Moon, and other planets. These elements have intrinsic value for understanding how planets form, so in a sense these brides are wearing around their fingers the evidence for how planets form.”

– Mario C. Aguilera

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