Free Astronomy Magazine September-October 2021

29 SEPTEMBER-OCTOBER 2021 first time heavy metals, usually asso- ciated with hot environments, have been found in the cold atmospheres of distant comets. “It was a big surprise to detect iron and nickel atoms in the atmosphere of all the comets we have observed in the last two decades, about 20 of them, and even in ones far from the Sun in the cold space environment,” says Jean Manfroid from the Univer- sity of Liège, Belgium, who lead the T his video starts by showing an animation of comet C/2016 R2 (PANSTARRS), which was done using real images taken by the SPECULOOS telescope at ESO’s Paranal Observatory. The video then zooms in on a blue comet. In a new study done with the UVES instrument on ESO’s Very Large Telescope, a team has spotted heavy metal atoms in the inner atmosphere of the comet, a discovery il- lustrated at the end of the video. There we see the spectrum of the comet and in particular the iron (Fe, blue) and nickel (Ni, orange) lines, marking the presence of the two elements in the atmosphere of the comet. [ESO/L. Calçada/M. Korn- messer, SPECULOOS Team/E. Jehin, Manfroid et al.] new study on Solar System comets published in Nature . Astronomers know that heavy met- als exist in comets’ dusty and rocky interiors. But, because solid metals don’t usually “sublimate” (become gaseous) at low temperatures, they did not expect to find them in the at- mospheres of cold comets that travel far from the Sun. Nickel and iron vapours have now even been de- tected in comets observed at more than 480 million kilometres from the Sun, more than three times the Earth-Sun distance. The Belgian team found iron and nickel in comets’ atmospheres in ap- proximately equal amounts. Material in our Solar System, for example that found in the Sun and in meteorites, usually contains about ten times more iron than nickel. This new re- sult therefore has implications for as- tronomers’ understanding of the early Solar System, though the team is still decoding what these are. “Comets formed around 4.6 billion years ago, in the very young Solar System, and haven’t changed since that time. In that sense, they’re like fossils for astronomers,” says study co-author Emmanuel Jehin, also from the University of Liège. While the Belgian team has been studying these “fossil” objects with ESO’s VLT for nearly 20 years, they had not spotted the presence of nickel and iron in their atmospheres until now. “This discovery went un- der the radar for many years,” Jehin says. The team used data from the Ultra- violet and Visual Echelle Spectro- graph (UVES) instrument on ESO’s VLT, which uses a technique called spectroscopy, to analyse the atmos- pheres of comets at different dis- tances from the Sun. This technique allows astronomers to reveal the chemical makeup of cosmic objects: each chemical element leaves a unique signature — a set of lines —