In space, the location of an object has a major impact on its temperature. The closer the object is to its star, the hotter it is, most likely. Heat then plays an important role in the materials that are present in the atmosphere of the object, if any. Lighter elements like hydrogen and helium and much easier to take on a gaseous state and create an atmosphere. So it was a surprise when two different teams found much heavier elements in the atmosphere of comets that were relatively far from the Sun. And one of these comets happened to come from another solar system.
Many comets are metallic, so it was not surprising that elemental metals such as iron and nickel are present on their surface. And when metallic comets approach the sun, these elements can sometimes sublimate into the cloud of dust that surrounds the comet. Astronomers have been observing this phenomenon for years and it makes sense given the temperatures.
UT video explaining the origin of comets.
When a team of astronomers at the University of Liege studied the atmosphere of a number of comets relatively far from the solar system, they found some strange spectral lines that they couldn’t explain. Using the European Southern Observatory’s Very Large Telescope (VLT), they studied comets three times farther from the Sun than Earth, making them cold and stable compared to nearby comets.
After some more research, the team found that the spectral lines they were looking at were representative of very weak signals for iron and nickel. Oddly enough, the two elements had almost the same concentration with about 1 g iron and 1 g nickel per 100 kg of water in the comet’s atmosphere. Closer to the Sun, where it is more common to find these heavier elements in the comet’s atmosphere, there is usually about ten times the amount of iron than nickel.
Image of the VLT, in which the data of the comets were collected during a lunar eclipse.
Photo credit: ESO / Yuri Beletsky
This could mean that there is a new material that contains iron and nickel and has a much lower sublimation temperature. Or there could still be unexplained reasons for the occurrence of these metals. However, the team has already made some progress, including re-analyzing other older data to check for faint spectral lines as well. Soon they will also have a new tool for the extremely large telescope, the mid-infrared ELT imager and spectrograph (METIS), which will allow them to determine the exact composition of the material that led to atmospheric iron and nickel.
Unfortunately, the team investigating the other object found with heavy metals in its atmosphere cannot use METIS to observe it. This is because Object 2I / Borisov is an interstellar comet that flowed through our solar system a few years ago, splitting in two, and flying out of range of our instruments.
This image was captured with the FORS2 instrument on ESO’s Very Large Telescope in late 2019 when Comet 2I / Borisov passed near the Sun. With the comet moving at breakneck speeds of about 175,000 kilometers per hour, the background stars appeared as streaks of light as the telescope followed the comet’s trajectory. The colors in these stripes add a certain disco flair to the image and are the result of the combination of observations in different wavelength bands highlighted by the different colors in this composite image.
Credit – ESO
However, the team used a tool they had on hand at the time – the X-Shooter Spectrograph as the VLT. While they did not find iron in Borisov’s atmosphere, they found atomic nickel. Given that the comet was 300 million kilometers from the Sun when it was observed, this is again surprising, as previous estimates would have expected the comet to be too cold to have atmospheric nickel.
While Borisov himself may be out of sight, we are likely to find other interstellar invaders in the near future as we know what to look for. With better observation tools and new theories to understand why these unexpected elements appear in cometary atmospheres, the future looks bright for cometary science.
Learn more:
ESO – heavy metal vapors unexpectedly found in comets across our solar system – and beyond
Nature – iron and nickel atoms in cometary atmospheres also far away from the sun
Nature – Gaseous atomic nickel in the coma of the interstellar comet 2I / Borisov
Sky and Telescope – INTERETELLAR AND SOLAR SYSTEM COMETS SHARE A SURPRISING COMPONENT: NICKEL
Mission statement:
Representation of elements in comets with the VLT.
Credit – ESO
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