Volatile Composition and Outgassing in C/2018 Y1 (Iwamoto): Extending Limits for High-resolution Infrared Cometary Spectroscopy between 2.8 and 5.0 μm

• Published in: The planetary science journal 2021-12, Vol.2 (6), p.225
• Main Authors: DiSanti, Michael A., Bonev, Boncho P., Dello Russo, Neil, McKay, Adam J., Roth, Nathan X., Saki, Mohammad, Gibb, Erika L., Vervack Jr, Ronald J., Khan, Younas, Kawakita, Hideyo
• Format: Article
• Language: English
• Subjects: Comae; Comet volatiles; Comets; Small Solar System bodies; Solar system
• ISSN: 2632-3338; 2632-3338
• DOI: 10.3847/PSJ/ac07ae

We used the powerful high spectral resolution cross-dispersed facility spectrograph, iSHELL, at the NASA Infrared Telescope Facility (IRTF) to observe C/2018 Y1 (Iwamoto), a long-period comet from the Oort cloud. We report production rates for water and eight other parent molecules (native ices), C 2 H 6 , CO, CH 4 , H 2 CO, CH 3 OH, HCN, NH 3 , and C 2 H 2 , on three preperihelion UT dates, 2019 January 13 and (near perihelion) February 4–5. We present abundance ratios relative to both C 2 H 6 (a nonpolar molecule) and H 2 O (a polar molecule), thereby providing a more complete picture of the parent volatile composition of C/2018 Y1 and potential associations of ices in its nucleus. Overall, the measured spatial distributions for polar molecules (in particular, H 2 O and CH 3 OH) were broader, exhibiting more complex structure compared with nonpolar or weakly polar species (CH 4 , C 2 H 6 , and CO). Our January 13 results permitted quantitatively assessing the significant improvement in sensitivity delivered by iSHELL compared with previous capabilities at the IRTF. The efficient spectral coverage of iSHELL plus the favorable geocentric Doppler shift of C/2018 Y1 allowed for measuring >50% of CH 4 ν 3 -band emission intensity on both January 13 and February 5. Compositionally, compared to their respective mean abundances among comets from the Oort cloud, C 2 H 6 and CH 3 OH were enriched, CH 4 and HCN were near normal, and all other species were depleted. The abundance ratio CH 3 OH/C 2 H 6 was higher by 45% ± 8% on January 13 versus February 5, whereas CH 4 /C 2 H 6 was unchanged within the uncertainty, suggesting nonhomogeneous composition among regions of the nucleus dominating activity on these dates.