Astronomers using ESO’s Very Large Telescope have detected neutral atoms of two heavy metals — iron (Fe I) and nickel (Ni I) — in the atmospheres of several Solar System comets, even far from the Sun. The findings appear in the journal Nature.
The detection of iron (Fe) and nickel (Ni) in the fuzzy atmosphere of a comet are illustrated in this image, which features the spectrum of light of C/2016 R2 (PANSTARRS) on the top left superimposed to a real image of the comet taken with the SPECULOOS telescope at ESO’s Paranal Observatory; each white peak in the spectrum represents a different element, with those for iron and nickel indicated by blue and orange dashes, respectively. Image credit: ESO / L. Calçada / SPECULOOS Team / E. Jehin / Manfroid et al.
Astronomers know that heavy metals exist in comets’ dusty and rocky interiors.
But, because solid metals don’t usually sublimate at low temperatures, they did not expect to find them in the atmospheres of cold comets that travel far from the Sun.
“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,” said Dr. Jean Manfroid, an astronomer in the STAR Institute at the University of Liège.
“Comets formed around 4.6 billion years ago, in the very young Solar System, and haven’t changed since that time,” added Dr. Emmanuel Jehin, also from the STAR Institute at the University of Liège.
“In that sense, they’re like fossils for astronomers.”
The astronomers analyzed the high-resolution ultraviolet-optical spectra of about 20 different comets located between 0.68 and 3.25 AU from the Sun.
The spectral data were collected by the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on the 8-m UT2 telescope of ESO’s Very Large Telescope.
The researchers spotted weak, unidentified spectral lines in their UVES data and on closer inspection noticed that they were signaling the presence of neutral atoms of iron and nickel.
They estimate that for each 100 kg of water in the comets’ atmospheres there is only 1 g of iron, and about the same amount of nickel.
“Usually there is 10 times more iron than nickel, and in those comet atmospheres we found about the same quantity for both elements,” said Dr. Damien Hutsemékers, also from the STAR Institute at the University of Liège.
“We came to the conclusion they might come from a special kind of material on the surface of the comet nucleus, sublimating at a rather low temperature and releasing iron and nickel in about the same proportions.”
J. Manfroid et al. 2021. Iron and nickel atoms in cometary atmospheres even far from the Sun. Nature 593, 372-374; doi: 10.1038/s41586-021-03435-0