Metals like iron and nickel usually require high temperatures to evaporate. But now astronomers have detected gaseous iron and nickel in the shell of several far-off comets. They are emitted from the comet’s core in small quantities, although the temperatures there are only between minus 90 and plus 60 degrees. A second team has also discovered these gaseous metals in the shell of the interstellar comet 2l / Borisov. These surprising finds now raise the question of how this iron and nickel can sublime despite the cold.
Iron and nickel are among the most common metals in our solar system. They are not only found in the core of the earth and other planets, but also in meteorites, meteors and interplanetary dust. Most of these metals are in the form of solid compounds. They only become gaseous through sublimation when they are very hot, such as in the atmospheres of some very hot exoplanets or when comets fly very close to the sun. The first detection of neutral nickel vapor in the coma of a comet came in the 1970s when the comet Ikeya – Seki (C / 1965 S1) flew past our star at a distance of 13 to 30 solar radii. The fact that both iron and nickel compounds are present in cometary nuclei is also confirmed by the results of the Stardust space probe, which brought the first dust samples from comet 81P / Wild back to Earth in 2006.
Gaseous iron and nickel around freezing comets
In order to find out more about the composition of comets and their gas envelope, a team of astronomers headed by Jean Manfroid from the University of Liège in Belgium has been evaluating observation data on comets for almost 20 years that were collected at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). collected in Chile. Above all, they analyze the spectrum of light that emanates from the comet. Because every chemical element leaves a unique signature in it in the form of spectral lines. However, lines of gaseous iron or nickel atoms were never noticed in the spectrum of the comet’s envelopes – until now. When Manfroid and his team took another closer look at the spectra of 20 distant comets, they discovered several faint spectral lines in the blue region of the spectrum, the characteristics of which corresponded to those of neutral iron and nickel.
“We were very surprised to find iron and nickel atoms in the atmosphere of all 20 or so comets that we have observed in the past two decades, and even in those that are far from the Sun in the cold environment of space.” says Manfroid. The density of metal atoms was highest near the comet’s nucleus and then decreased with increasing distance. Overall, however, the number of atoms detected was very small: for every 100 kilograms of outgassing water vapor, there is only one gram of iron and about the same amount of nickel. But even in such tiny amounts, the discovery of the gaseous metals is surprising, as the researchers explain. Because the temperatures on the surfaces of the observed comets were between an icy minus 93 degrees Celsius and around 60 degrees plus. At these temperatures, iron and nickel should normally not sublime and become gaseous.
Nickel vapor also around 2l / Borisov
A second study suggests that the comets in our solar system are obviously not an isolated case: Piotr Guzik and Michal Drahus from the Jagiellonian University in Krakow have also demonstrated the spectral signature of nickel vapor in the coma of the interstellar comet 2l / Borisov. They had also observed the comet, discovered on August 31, 2019, for several nights at the end of January 2020 with a spectrograph at ESO’s Very Large Telescope. At this point in time, 2l / Borisov was already 2.3 times as far from the sun as our earth and the temperature on its surface was around minus 93 degrees. “At first we found it difficult to believe that really atomic nickel could be present in 2I / Borisov so far from the sun,” reports Guzik. “It took numerous tests and checks before we could finally convince ourselves.” This suggests that comets from different planetary systems have even more in common than assumed. “Now imagine that the comets in our solar system have real relatives in other planetary systems – how cool is that?” Says Drahus.
These discoveries also raise the question of where the gaseous iron and nickel in the comet’s gas envelopes come from – and how they can outgas at such low temperatures. The ratio of iron and nickel in the coma of the icy lumps provides a possible clue: While meteorites typically contain ten times more iron than nickel, Manfroid and his team found both metals in almost equal proportions. From this they conclude that the two metals on the comet’s surface were probably not present as a metal alloy or sulfide, but rather possibly in an organic complex compound such as a metal carbonyl. This could also be indicated by the relatively high proportion of carbon monoxide and carbon dioxide radicals in the spectrum of some comets. “If these compounds are present in comets, then, unlike silicates and sulfides, these carbonyls could sublime even at low temperatures and far from the sun,” the researchers explain.
Whether there are actually carbonyls on the comets and whether they are the source of the gaseous metals has yet to be clarified. The astronomers hope that new telescopes and instruments such as ESO’s Extremely Large Telescope, which is currently under construction, will help.
Source: Nature, Jean Manfroid (Université Liège, Liège) et al., doi: 10.1038 / s41586-021-03435-0; Guzik and Drahus (Jagiellonian University, Krakow), doi: 10.1038 / s41586-021-03485-4