2I Borisov Is a Carbon Monoxide-Rich Comet from Another Star
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The carbon monoxide-rich interstellar comet 2I/Borisov D. Bodewits1, J. W. Noonan2, P. D. Feldman3, M. T. Bannister4, D. Farnocchia5, W. M. Harris2, J.-Y. Li (李荐扬)6, K. E. Mandt7, J. Wm. Parker8, Z. Xing (邢泽曦)1,9 1Physics Department, Auburn University, Leach Science Center, Auburn, AL 36849, USA; 2Lunar and Planetary Laboratory, University of Arizona, 1629 E University Boulevard, Tucson, AZ 85721, USA; R3Department of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA; 4School of Physical and Chemical Sciences — Te Kura Matū , University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand; 5Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA; 6Planetary Science Institute, 1700 E. Ft. Lowell Road, Suite 106, Tucson, AZ 85719, USA; 7Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA; 8Department of Space Studies, Southwest Research Institute, Suite 300, 1050 Walnut Street, Boulder, CO 80302, USA; 9Department of Physics and Laboratory for Space Research, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China. Interstellar comets offer direct samples of volatiles from distant protoplanetary disks. 2I/Borisov is the first notably active interstellar comet discovered in our solar system1. Comets are condensed samples of the gas, ice, and dust that were in a star’s protoplanetary disk during the formation of its planets and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration moves volatiles2, organic material, and prebiotic chemicals in their host system3. In our solar system, hundreds of comets have been observed remotely, and a few have been studied up close by space missions4. However, 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 monoxide5. Here we report that the coma of 2I/Borisov contains significantly more CO than H2O gas, with abundances of up to 150%, more than three times higher than previously measured for any comet in the inner (<2.5 au) solar system4. Our ultraviolet observations of 2I/Borisov provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star that is significantly different from our own. 2I/Borisov is the second interstellar object discovered in our solar system, and it showed comet-like activity since it was discovered, indicating sublimating ices. Its brightness and location in the sky made it observable for months. This situation is very different from that of the first discovered interstellar object, 1I/'Oumuamua, which was much fainter, was visible for only a few weeks for most observers, and showed no detectable levels of gas or dust6. 2I/Borisov’s outgassing makes it possible to probe the chemical composition of volatiles stored within the nucleus. We report on its observation by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) during four epochs between December 11, 2019, and January 13, 2020 (Extended Data Table 1). An example of the HST/COS spectra is shown in Figure 1. In each of the four epochs, we clearly detected the emissions of several bands of the CO Fourth Positive system, excited by solar fluorescence7. The derived CO production rates range between 6.4 to 10.7 x 1026 molecules/s, equivalent to 30 – 50 kg/s. The volatile content of cometary nuclei is Bodewits et al. – The carbon-rich interstellar comet 2I/Borisov 2 generally considered to be a mixture of mostly H2O, CO2, CO, and a small percentage of 4 other molecules . Chemical abundances in the coma are often expressed relative to H2O. In Figure 2, we compare our measurements of the CO production rates with H2O production rates8,9,10. Reported water production rates peak around perihelion on Dec. 8, 2019, when 2I/Borisov came within 2.006 au of the Sun and decreased rapidly afterwards9. In contrast, we observed that the CO production rates remained constant during our observations, with a possible maximum around December 30, 2019. Interpolating between the Swift/UVOT measurements indicates that even just one week after perihelion, the production rate of CO exceeded that of H2O. Swift/UVOT and HST/COS observed Borisov simultaneously on Dec. 19 – 22, 2019 and on Jan. 13, 2020, and found CO to H2O abundances of 130 – 155%. This is a much higher CO/H2O abundance than has been measured in any comet in the inner solar system (<2.5 au), which mostly ranges between 0.2 – 23% with typical values around 4%4. CO ice is highly volatile and sublimates around 20 K in protoplanetary disks and around 25 K in cometary nuclei11. Its abundance, therefore, is very sensitive to both the local temperature of the region where comets formed and their thermal history since then. 12 Outside 2.5 au, several comets show coma abundances of CO that exceed that of H2O . H2O ice starts to sublimate efficiently from comet nuclei within 2.5 au from the Sun, while CO2 and CO ice have lower sublimation temperatures and can effectively sublimate within 13 au and 120 au of the Sun, respectively13. The activity of distant comets and Centaurs has therefore been attributed to these hypervolatiles13. Inside 2.5 au from the Sun, water sublimation from the nucleus scales approximately with the inverse square of the heliocentric distance and is a good indicator of underlying water abundance. A notable CO- rich comet is C/2016 R2 (PanSTARRS), though with a perihelion of 2.6 au, it never reached the inner solar system where water sublimation is fully effective, and while high CO+ and + N2 production levels were detected, no evidence of H2O or its fragments was observed in its coma14,15, which complicates the comparison with 2I/Borisov. Further complicating the comparison, in C/2016 R2 (PanSTARRS) the dominant volatile is most likely CO2, not CO 16 . For these reasons and uncertainties, we emphasize that the extremely high CO/H2O abundance measured in 2I/Borisov is significant in comparison to the large number of comets that reach the inner (< 2.5 au) solar system and have measured CO and H2O values. Another example of a CO rich comet was Comet C/2009 P1 (Garradd), a highly active, dynamically young comet that was well-observable over an extended time throughout its orbit. When its water production rates decreased after perihelion, its CO production rates increased continuously, leading to a CO abundance of 60% with respect to water at 2 au from the Sun, the highest abundance ratio ever measured17. This was interpreted as the result of surface erosion or seasonal illumination effects exposing more layers that reflected the comet’s primitive ice composition. Comets C/2009 P1 and 2I/Borisov show similar trends in the CO/H2O abundance ratio, increasing after perihelion (where mass loss rates and thus surface erosion peak). However, at similar distances from the Sun, 2I/Borisov has a CO/H2O abundance that is at least three times higher than that of C/2009 P1 (Garradd). Water production rates of 2I/Borisov dropped rapidly after perihelion9, but our results suggest that CO production rates remained constant or even increased during that period (Figure 2). This could be interpreted as illumination of parts of the nucleus that were obscured during approach, as a chemical heterogeneity of the nucleus, or as the exposure of deeper layers by surface erosion. For comet Bodewits et al. – The carbon-rich interstellar comet 2I/Borisov 3 67P/Churyumov-Gerasimenko, seasonal and evolutionary effects result in different erosion rates and chemical abundances for its southern and northern hemispheres18. As such, the composition of 67P’s more eroded southern hemisphere is likely more representative the primitive chemical abundance of the volatiles19. Pre-discovery observations of the dust surrounding 2I/Borisov indicated that its activity between 8 and 5 au was likely driven by an ice more volatile than H2O, such as CO2 20 or CO . However, the observed production rate trends imply that near perihelion H2O was driving the activity. Borisov likely has a small nucleus with a radius between 0.2 and 0.5 km21, and it has been estimated that the comet lost between 0.2 – 1.1 m of its surface, depending on the size of the nucleus9. Results from the Rosetta mission suggest an orbital thermal skin depth of a few tens of centimetres to 1 meter22, thus the activity around perihelion for 2I/Borisov would have exposed layers that had not been thermally altered. In this scenario above, the high CO to H2O abundance of 2I/Borisov is a primordial property that reflects the composition of the ices in the nucleus23. This requires that the CO originate from a pristine portion of the object that has remained below the 25 K sublimation point of CO since formation. Dynamical models of our solar system predicted that large amounts of the comets in our Oort Cloud could be interstellar interlopers24, but it was unclear what properties would set such objects aside from comets from our solar system. Borisov’s orbit firmly places its origins outside our solar system. Until now, most of its properties, including the colour of its dust, its brightness trend with respect to the distance to the Sun, and the presence of fragment species have been surprisingly similar to those of comets from our solar system1,9,20,23,25, which could partially be explained by the exposure to radiation during the time the comet spent in interstellar space. Two major differences between typical solar system comets and 2I/Borisov become apparent with our HST/COS observations.