Strange ingredient in interstellar Comet Borisov offers a clue to its origins

Astronomers have revealed the unusual chemical composition inside 2I/Borisov (opens in new tab), the interstellar comet that visited our solar system last year. A strange ingredient has provided new clues about where this traveling space rock originated.

2I/Borisov was discovered on Aug. 30, 2019, by amateur astronomer Gennady Borisov. Following the appearance of the interstellar object 'Oumuamua (opens in new tab) in 2017, this was the second object from another solar system ever discovered wandering through our cosmic neighborhood. On Dec. 15 and 16 in 2019, astronomers took a closer look (opens in new tab) at 2I/Borisov using the Atacama Large Millimeter/submillimeter Array (ALMA), a giant radio telescope in Chile Atacama Desert.

In the new study, an international team of researchers, led by planetary scientists Martin Cordiner and Stefanie Milam from NASA's Goddard Space Flight Center (opens in new tab), analyzed 2I/Borisov's chemical makeup.

Related: Interstellar comet: Here's why it's got scientists so pumped up (opens in new tab)

A visualization of the interstellar comet 2I/Borisov. — (Image credit: NRAO/AUI/NSF, S. Dagnello)

The researchers found that the gas coming from the comet contained more carbon monoxide (CO) than has been detected in any other comet this close to the sun (less than 186 million miles, or 300 million kilometers). In fact, the concentration of CO in the gas coming from this comet was between nine and 26 times higher than in the average comet in our solar system, according to a statement from the National Radio Astronomy Observatory (opens in new tab) (NRAO), which oversees ALMA.

Using ALMA, the team detected both CO and hydrogen cyanide (HCN). However, they found a similar amount of HCN in 2I/Borisov that's found in other comets in our solar system, so that discovery wasn't much of a surprise. But the unexpectedly high quantities of CO offered a major clue as to where this comet came from. "The comet must have formed from material very rich in CO ice, which is only present at the lowest temperatures found in space, below minus 420 degrees Fahrenheit (minus 250 degrees Celsius)," Milam said in the same statement.

"If the gases we observed reflect the composition of 2I/Borisov's birthplace, then it shows that it may have formed in a different way than our own solar system comets, in an extremely cold, outer region of a distant planetary system," Cordiner said in the same statement

Astronomers do not yet know what kind of star 2I/Borisov formed around, but these researchers suspect that 2I/Borisov came from a cold region in a larger protoplanetary disk (opens in new tab), or rotating disk of dust and gas around a young star from which planets and planetary objects form, Cordiner said. "Many of these disks extend well beyond the region where our own comets are believed to have formed, and contain large amounts of extremely cold gas and dust. It is possible that 2I/Borisov came from one of these larger disks," Cordiner said.

These images of the interstellar Comet 2I/Borisov show an unusually high amount of carbon monoxide gas (right) as well as hydrogen cyanide gas (left). These images were taken by the ALMA radio telescope in Chile. (Image credit: Credit: ALMA (ESO/NAOJ/NRAO), M. Cordiner & S. Milam; NRAO/AUI/NSF, S. Dagnello)

2I/Borisov was traveling pretty quickly (about 21 miles per second (33 km/s)) when it zoomed through our solar system. Researchers think that, whichever solar system it came from, it was likely torn from that system by the gravity of a passing star or large planet, according to the statement. After a long trip through space, it has made history as one of only two interstellar visitors ever identified in our solar system.

With this new study, not only are researchers discovering more about the possible origins of 2I/Borisov, they are also learning more about interstellar objects and other planetary systems in general. "2I/Borisov gave us the first glimpse into the chemistry that shaped another planetary system," Milam said.

Further studies and possible future observations of other interstellar comets will add to this understanding and also show whether 2I/Borisov and its unusual makeup is typical for such an object.

"Only when we can compare the object to other interstellar comets will we learn whether 2I/Borisov is a special case, or if every interstellar object has unusually high levels of CO," Milam said.

This study was published today (opens in new tab) (April 20) in the journal Nature Astronomy.

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Chelsea “Foxanne” Gohd joined Space.com in 2018 and is now a Senior Writer, writing about everything from climate change to planetary science and human spaceflight in both articles and on-camera in videos. With a degree in Public Health and biological sciences, Chelsea has written and worked for institutions including the American Museum of Natural History, Scientific American, Discover Magazine Blog, Astronomy Magazine and Live Science. When not writing, editing or filming something space-y, Chelsea "Foxanne" Gohd is writing music and performing as Foxanne, even launching a song to space in 2021 with Inspiration4. You can follow her on Twitter @chelsea_gohd and @foxannemusic.

9 Comments Comment from the forums

dfjchem721

Well, this chemistry pretty much blows away the alien space ship theory. Either that, or it indicates that other life forms could arise with vastly different chemistries. I am going with the "not alien" conclusion, and will take on all who challenge it!

Carbon monoxide (CO) is a pretty reactive molecule, which is one reason you only see it in the deep freeze in objects from space. CO as a solid needs to stay super-cold as it is also extremely volatile. The boiling point of CO is −313 °F (ca. 80K). That is pretty chilly.

The CO observation that it is "between nine and 26 times higher than in the average comet" has me thinking about outliers. Using the term "average comet" makes me wonder if there are some comets known to occur in our solar system that approach the lower limit noted. If so, then it is not overly unusual, but interesting to be sure.

I know one person here who might be able to tell us if this level of CO out-gassing could be matched by a small subset of comets we already know about.

After all, if our own comet population has some that are even nine times above "the average comet", it would be interesting to note. More to ponder.
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dfjchem721

Here is an interesting abstract to a research article written on carbon monoxide from comets*:

"The composition of ices in comets may reflect that of the molecular cloud in which the Sun formed, or it may show evidence of chemical processing in the pre-planetary accretion disk around the proto-Sun. As carbon monoxide (CO) is ubiquitous in molecular clouds, its abundance with respect to water could help to determine the degree to which pre-cometary material was processed, although variations in CO abundance may also be influenced by the distance from the Sun at which comets formed. Observations have not hitherto provided an unambiguous measure of CO in the cometary ice (native CO). Evidence for an extended source of CO associated with comet Halley was provided by the Giotto spacecraft, but alternative interpretations exist. Here we report observations of comet Hale-Bopp which show that about half of the CO in the comet comes directly from ice stored in the nucleus. The abundance of this CO with respect to water (12 per cent) is smaller than in quiescent regions of molecular clouds, but is consistent with that measured in proto-stellar envelopes, suggesting that the ices underwent some processing before their inclusion into Hale-Bopp. The remaining CO arises in the coma, probably through thermal destruction of more complex molecules."

end quote

After reading up on the issue, it appears that much of the CO in many comets is actually in their interior. It seems possible that the analysis of the gases seen with I2/Borisov was a result of its breaking up, and therefore releasing more CO than it would have otherwise. After all, I2/Borisov was showing an unstable nucleus for some time, having already let off a chunk prior to it breaking up completely just a few weeks ago.

One wonders if data exists prior to the observed high levels of CO, which might have been lower at earlier times prior to its breaking up. Of course distance from the sun might tend to cloud estimates. It would be nice to get as much data out of this unique object as possible.

* Nature. 1999 Jun 17;399(6737):662-5.
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Catastrophe

dfjchem721 said:
Here is an interesting abstract to a research article written on carbon monoxide from comets*:

"The composition of ices in comets may reflect that of the molecular cloud in which the Sun formed, or it may show evidence of chemical processing in the pre-planetary accretion disk around the proto-Sun. As carbon monoxide (CO) is ubiquitous in molecular clouds, its abundance with respect to water could help to determine the degree to which pre-cometary material was processed, although variations in CO abundance may also be influenced by the distance from the Sun at which comets formed. Observations have not hitherto provided an unambiguous measure of CO in the cometary ice (native CO). Evidence for an extended source of CO associated with comet Halley was provided by the Giotto spacecraft, but alternative interpretations exist. Here we report observations of comet Hale-Bopp which show that about half of the CO in the comet comes directly from ice stored in the nucleus. The abundance of this CO with respect to water (12 per cent) is smaller than in quiescent regions of molecular clouds, but is consistent with that measured in proto-stellar envelopes, suggesting that the ices underwent some processing before their inclusion into Hale-Bopp. The remaining CO arises in the coma, probably through thermal destruction of more complex molecules."

end quote

After reading up on the issue, it appears that much of the CO in many comets is actually in their interior. It seems possible that the analysis of the gases seen with I2/Borisov was a result of its breaking up, and therefore releasing more CO than it would have otherwise. After all, I2/Borisov was showing an unstable nucleus for some time, having already let off a chunk prior to it breaking up completely just a few weeks ago.

One wonders if data exists prior to the observed high levels of CO, which might have been lower at earlier times prior to its breaking up. Of course distance from the sun might tend to cloud estimates. It would be nice to get as much data out of this unique object as possible.

* Nature. 1999 Jun 17;399(6737):662-5.
That is quite an impressive comment.
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dfjchem721

Cat, what surprised me most was the notion of the source of CO in the eject, resulting from incomplete conversion of its out-gassing hydrocarbon content rather than an internal CO source. Clearly any synthesis of CO is likely coming from the incomplete oxidation of hydrocarbons, with reactions powered by UV and high energy solar particles, I suspect. As in many cases of the incomplete oxidation of hydrocarbons, particularly methane, CO is a primary product. I would think that there might even be an optimal distance from the sun that could produce large amounts of CO since such reactions will be highly dependent on that distance, and the rate of out-gassing, assuming substantial quantities of hydrocarbons are present.
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rod

Looks like dfjchem721, the biochemist is catching up on comets :) Here is a note form another report, ALMA reveals unusual composition of interstellar comet 2I/Borisov "The ALMA observations from a team of international scientists led by Martin Cordiner and Stefanie Milam at NASA's Goddard Space Flight Center in Greenbelt, Maryland, revealed that the gas coming out of the comet contained unusually high amounts of carbon monoxide (CO). The concentration of CO is higher than anyone has detected in any comet within 2 au from the Sun (within less than 186 million miles, or 300 million kilometers)..."Carbon monoxide is one of the most common molecules in space and is found inside most comets. Yet, there's a huge variation in the concentration of CO in comets and no one quite knows why. Some of this might be related to where in the solar system a comet was formed; some has to do with how often a comet's orbit brings it closer to the Sun and leads it to release its more easily evaporated ices. "If the gases we observed reflect the composition of 2I/Borisov's birthplace, then it shows that it may have formed in a different way than our own solar system comets, in an extremely cold, outer region of a distant planetary system," added Cordiner."

Perhaps the H2O issue in the protoplanetary disk for the Sun or an exoplanet protoplanetary disk, could be some issues here too for H2O and quantity. Perhaps 2I/Borisov points to exoplanets with plenty of CO and little H2O too :)--Rod
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Catastrophe

" Yet, there's a huge variation in the concentration of CO in comets and no one quite knows why."

Is it too simplistic to ask whether it depends on the oxygen available?
I am not being facetious but asking if factors governing oxygen availability might be linked. Comes of being a chemical engineer rather than a chemist - mass transfer and all that :)
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dfjchem721

There is plenty of oxygen in the water and CO2 present, Cat. And you bring up a good question. What is the nature of the reactions that form the CO noted in that Nature abstract? Since O2 is not likely present, the hydrocarbons are probably reacting with water and/or CO2 as they are the primary source for oxygen. Since CO is being formed during out-gassing, it must be due to high energy UV etc. from the sun, and not by the standard reactions when "burning" hydrocarbons on earth.

From the Nature abstract:

"Here we report observations of comet Hale-Bopp which show that about half of the CO in the comet comes directly from ice stored in the nucleus. " and then "The remaining CO arises in the coma, probably through thermal destruction of more complex molecules."

So you can see how a comet breaking up could release a large plume of CO, making increased release subject to various interpretations. Since I have read that CO from comets is released directly from the nucleus (but not limited to it), one is inclined to believe that a comet could release varying amounts of CO for various reasons. Based on all observations, this could depend on its structural integrity more than anything else. Clearly an object breaking up will expose much larger surface areas for out-gassing.

Adding to the uncertainty in all this is the synthesis issue. The closer to the sun, likely the greater the rate of synthesis, and the greater the overall value one would measure from the comet's entire CO output (regardless of source). Even this should vary depending on the amounts of hydrocarbon in an individual comet, and as noted before, a possible optimal distance from the sun for maximum CO production.

Remember that the Hale-Bopp observations appear rather precise in that they were able to detect two different sources, presumably by resolving the CO coming from the comet's nucleus, and the CO being synthesized in the coma. Curiously, the ALMA report did not mention this possibility at all :

"Carbon monoxide is one of the most common molecules in space and is found inside most comets. Yet, there's a huge variation in the concentration of CO in comets and no one quite knows why. Some of this might be related to where in the solar system a comet was formed; some has to do with how often a comet's orbit brings it closer to the Sun and leads it to release its more easily evaporated ices."

Since I2/Borisov was breaking up when this data was obtained, I suspect this significantly increased CO output as a result. It would then seem unlikely from all these observations that one can draw any conclusions about the origin of this object within a protoplanetary disc, especially based on the limited data obtained from this object.

There is little doubt about one thing at least. There is much to learned about comets, and that simple observations like the amount of CO out-gassing (or synthesis) can have various explanations. We wouldn't want it any other way!
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Catastrophe

Just found the Elsevier book.
When I have a month to spare I'll read this:
Chapter X: The Oxidation of Carbon Monoxide
"Yet, there's a huge variation in the concentration of CO in comets and no one quite knows why."To be simplistic, CO is partially oxidised with respect to CO2. Conditions will be variable all over a galaxy, hence variable degree of oxidation.
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dfjchem721

Here is another abstract from a Nature Astronomy article with substantially different values for the CO output from 2I/Borisov (cut down to provide the most relevant aspects)*:

".....knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars, with only one detection of carbon monoxide. Here we report that the coma of 2I/Borisov contains substantially more CO than H2O gas, with abundances of at least 173%, more than three times higher than previously measured for any comet in the inner (<2.5 au) Solar System. Our ultraviolet Hubble Space Telescope observations of 2I/Borisov provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star that is substantially different from our own."

It never ceases to amaze me what they have found in exowhatevers. It appears that CO was actually detected from an exocomet!!!

To summarize the above abstract quote, their data is reducing the CO from 2I/Borisov to "at least 173%, more than three times higher than previously measured for any comet in the inner (<2.5 au) Solar System." This was based on data from the UV telescope on Hubble.

The data of the space.com article was based on the "Atacama Large Millimeter/submillimeter Array (ALMA), a giant radio telescope in Chile Atacama Desert". The later suggests a CO level "between nine and 26 times higher than in the average comet in our solar system". That indicates a lower limit about 3 times higher than the minimal amount "detected" by Hubble's UV telescope, a non-trivial difference. And I do appreciate that the abstract suggests "at least 173%, more than three times higher..." In science, that is significantly different from a minimum estimate of 'nine times'.

Not surprisingly I suppose, there is a significant difference between these analyses based on the instruments used, how the CO values were deduced, and even possibly viewing times. The ALMA data was collected on December 15-16, 2019, while it is not clear when the Nature Astronomy data was obtained as it is not in the abstract, and I am not paying for the whole article! Anybody who cares to pay for that is more than welcome to provide their viewing times,

However, it seems likely the difference could be due to the times of observation. If the Nature Astronomy article's data is significantly different than December 15-16, that might explain the variation, assuming it is real.

Anyone with knowledge of the two techniques used to make these determinations and how they may differ is welcome to explain it all.

* https://www.nature.com/articles/s41550-020-1095-2
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