Iron in the night sky
DOI: 10.1063/1.3582233
Iron in the night sky. Even in the absence of the Moon and stars, the night sky would not be black. Chemical reactions in the upper atmosphere emit faint light called airglow, whose spectrum has been studied for more than a century. Much of the emission comes from atomic and molecular oxygen, hydroxyl radicals, sodium, and, at higher latitudes, nitrogen dioxide. But after subtracting those species’ contributions from the airglow spectrum at lower latitudes, observers have noted a broad band of unaccounted-for emission in the orange part of the visible spectrum, between roughly 550 and 650 nm. Using measurements from the Optical Spectrograph and Infrared Imager System (OSIRIS) on Sweden’s Odin spacecraft, Dick Gattinger and Ted Llewellyn (University of Saskatchewan) and colleagues have now identified the source of that band: iron monoxide. Several factors supported the team’s conclusions. Nitrogen dioxide was ruled out since sunlight destroys it at lower latitudes and, moreover, it has a different spectral signature. Laboratory measurements of FeO, on the other hand, yield a similar emission spectrum. Iron’s atmospheric abundance is comparable to that of sodium, a known airglow contributor. And an orange emission band observed a decade ago originating from iron-containing meteors was also matched to the laboratory FeO spectrum. Subsequent ground-based measurements show that the FeO emission exhibits dramatic temporal fluctuations. The team postulates that FeO is generated through reactions between atmospheric iron and ozone. (
