Interstellar visitor reveals its icy secrets

The first confirmed comet from another planetary system arrived last year. Astronomers scrambled to take advantage of this fleeting opportunity. Using the ALMA radio telescope, we captured molecular emission from 2I/Borisov, revealing an unusual chemical composition for this interstellar object.
Interstellar visitor reveals its icy secrets

The surprise apparition of two interstellar objects (ISOs) in the last three years is transforming the science of small Solar System bodies. 1I/'Oumuamua and 2I/Borisov arrived in our inner Solar System in September 2017 and December 2019, respectively,  delivering to our doorstep an unprecedented opportunity to investigate the physical and chemical properties of a new and truly alien class of object, leading to a paradigm-shift in our ability to study in detail the properties of distant planetary systems.

Interstellar comet 2I/Borisov was first discovered by amateur astronomer G. Borisov on August 30th 2019, more than three months before perihelion. Subsequent observations over the following days confirmed a strongly hyperbolic orbit, and the presence of a clear, extended gas/dust coma, allowing 2I/Borisov to be unequivocally categorized as an interstellar comet (originating in the direction of Cassiopeia, with an inbound heliocentric velocity of 33 km/s).

But 2I/Borisov was faint. It reached a visual magnitude of only 16, with a relatively large perihelion distance of 2 au, which made it a very challenging target for spectroscopic observations. The most powerful telescopes in the world (and in orbit) were called into action. Initial reports from optical observations of CN and atomic oxygen fluorescence showed close similarities with normal Solar System comets, but the information that could be gleaned from those gases, regarding the interstellar comet's intrinsic composition, was limited.

Our team (led by myself and Stefanie Milam at NASA Goddard Space Flight Center) obtained two sets of observations of 2I/Borisov in 2019 using the Atacama Large Millimeter/submillimeter Array (ALMA): the first set was in late September using the (7 m) Atacama Compact Array, and the second was in mid-December using the main (12 m) ALMA array. Reliable detections of molecular emission were only obtained from the (more sensitive), second set of observations, which also benefitted from the comet being closer to the Sun, and thus, more active. Our ALMA images of HCN and CO emission in the 0.9 mm wave band are shown in the following figure:

ALMA images (in false color) of molecular emission from 2I/Borisov, based on Figure 1 of our published article. The comet lies at the center of the field of view. Image credit: ALMA (ESO/NAOJ/NRAO), NRAO/AUI/NSF, S. Dagnello.

Full details of these observations are given in our published Nature Astronomy article. While exhilarating to detect any "native volatiles" (gases stored as ices inside the nucleus) of the first interstellar comet, it was the detection of carbon monoxide (CO) that really got us excited. We have looked for CO in seven of the previous Solar System comets targeted with ALMA, but never once detected it until now. It seemed impossible that CO would be found in such a faint comet as 2I/Borisov, but its signal was clear and unambiguous, alerting us to the idea that this interstellar voyager might have a distinctly different chemical composition to that of our own Solar System comets.

Our derived molecular production rates confirmed an unusually large CO abundance with respect to both HCN and H2O - higher than observed before in any comet within 2 au of the Sun. The heliocentric distance is important since beyond about 2.5 au, comets can be significantly less active, with reduced H2O sublimation, and their comae tend to be more enriched in the most volatile gases such as CO. Among all comets previously observed in our Solar System, only one - the chemically peculiar comet C/2017 R2 (PAnSTARRS) - had a higher CO/HCN mixing ratio. 

2I/Borisov's extremely high CO/HCN ratio with respect to Solar System comets is highlighted in the following figure from our published article:

2I/Borisov has many characteristics in common with our own Solar System comets, suggesting a commonality in the basic physical and chemical conditions occurring during planet formation around other stars in our Galaxy.  However, the remarkable, elevated CO abundance shows that key details regarding its formation must have been different. Given the importance of CO as the basis for the formation of more complex organic molecules in the solid phase in protoplanetary disks and star-forming regions, we may therefore expect differences is the inventory of possible prebiotic molecules, between different planetary systems. 

Based on a single data point, we cannot know the extent to which 2I/Borisov may be representative of the broader population of interstellar comets, but statistically, it is probable that the first confirmed interstellar comet should provide a template for future ISOs. Although the frequency of ISO apparitions bright enough for compositional studies is not currently well-constrained, more interstellar comets are expected to pass through the inner Solar System in the coming years. We should be prepared for surprises when the next opportunity to observe one arises.

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