Iris, and the Dinosaurs' End

Tennant named iridium for Iris, the rainbow goddess who in Greek myth carried messages between the gods and mortals along the arc of the rainbow, choosing her for the varied colours of the metal's compounds, so that iridium joins chromium and rhodium among the elements christened for colour. The metal later acquired a startling scientific fame: iridium is rare in the Earth's crust but relatively abundant in asteroids, and in 1980 the physicist Luis Alvarez and his son Walter found a thin worldwide layer unusually rich in iridium at exactly the geological boundary marking the extinction of the dinosaurs. That iridium anomaly became the key evidence that a giant asteroid impact ended the age of the dinosaurs, giving the rainbow-named element an unexpected role in the story of life on Earth.

The Alvarez hypothesis, initially controversial, gained decisive physical support in the 1990s when geologists identified the Chicxulub crater buried beneath Mexico's Yucatan Peninsula, an impact structure of almost exactly the right size and age to match the iridium-rich boundary layer found worldwide, closing much of the debate over what had actually struck the Earth sixty-six million years ago. Iridium anomalies have since become a standard tool geologists check for whenever they suspect an impact event anywhere in the rock record.