ALMA observations reveal unusual composition of intestellar comet 2I/Borisov

An artistic rendering shows carbon monoxide sublimating from the interstellar comet 2I/Borisov. Photo by NRAO/AUI/NSF, S. Dagnello<br>
An artistic rendering shows carbon monoxide sublimating from the interstellar comet 2I/Borisov. Photo by NRAO/AUI/NSF, S. Dagnello

April 20 (UPI) -- Late last summer, astronomers spotted the first-ever interstellar comet to visit our solar system. A few months later, scientists pointed one of the world's most powerful telescopes at 2I/Borisov.

The data collected by the Atacama Large Millimeter/submillimeter Array -- detailed Monday in the journal Nature Astronomy -- showed the icy interloper's interior features surprisingly large concentrations of carbon monoxide.


'Oumuamua was the first interstellar visitor spotted flying through our solar system, but by the time researchers saw it, the object was moving away from the sun. Luckily, astronomers and ALMA caught 2I/Borisov on its way toward the sun.

As comets get closer to the sun, volatile materials like trapped ice and gas begin to heat up and sublimate, or escape as a gas. By studying sublimated materials, scientists can gain insights into the interior composition of a comet.


"At submillimeter wavelengths, we are looking at gases that are considered native to the comet -- in other words the actual composition of the nucleus," study co-author Stefanie Milam, NASA researcher and deputy project scientist for planetary science for James Webb Space Telescope, told UPI in an email. "We have done this for a number of comets in the solar system and try to do so at almost all favorable apparitions."

Milam and her research partners were surprised when the ALMA data revealed elevated levels of carbon monoxide, or CO. The discovery helped scientists estimate the comet's likely origin.

"In order to get CO to freeze into a body, the environment needs to be really cold. This means that the material in this object is from somewhere cold in its protoplanetary disk," Milam said.

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Scientists were excited when they first successfully detected hydrogen cyanide gas sublimating from 2I/Borisov, but they couldn't be certain ALMA would be able to detect other gases.

"When we found the CO detection, we were simply blown away to see it, let alone how prominent it was in our data!" Milam said. "That alone gave us the clue that this CO abundance was well beyond what we normally detect in solar system comets."


The interstellar comet's unusual fast speed suggests it was ejected from its host system by a passing star or giant planet. And the unusually large concentrations of CO suggest it was born somewhere rather cold. But until astronomers can study other interstellar comets, it will be hard to know much else about 2I/Borisov -- whether its composition is normal or atypical of interstellar travelers.

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Astronomers estimate hundreds, possibly thousands, of interstellar objects pass through the solar system every year, but so far, astronomers have only spotted two. Milam is hopeful that the number grows in the near future.

"New survey facilities covering the full sky are coming on line that will enable the detection of these objects sooner and when they are further away!" she said. "We hope this will allow for more detection of such unique and mysterious objects. However, as vast as space is, and considering how far these tiny objects must travel from their own planetary system to find our solar system is quite spectacular."

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