Comets contain the least altered remnant material dating to the birth of our solar system. Near-IR (~ 2.8 - 5.2 microns) spectrographs having spectral resolving power &gt; 20,000 on ground-based telescopes (Keck and the NASA-IRTF) measure the composition of ices in cometary nuclei and provide a viable means of assessing ices in the proto-solar disk mid-plane over a range of distances, for comparison with disk chemical models. Our sample is comprised of comets from the two primary dynamical reservoirs: long-period and non-returning comets perturbed from the Oort cloud, and short-period (ecliptic) comets fed from the Kuiper belt, which represents the residual disk material from the epoch of solar system formation. In their dynamical end state, ecliptic comets evolve into short-period orbits, becoming “Jupiter-family comets” that can be observed from the Earth’s surface and/or targeted by space missions. As a comet enters the inner solar system its ices sublime, releasing “parent volatiles” (molecules) into its coma. Production rates are measured for up to 10 parent molecules: H2O (the dominant ice in most comets), CO, H2CO, CH3OH, CH4, C2H2, C2H6, HCN, NH3, OCS. A taxonomy of their relative abundances has emerged over the past 20+ years that now includes in excess of 40 comets from both dynamical classes, however, it remains dominated by comets from the Oort cloud. Significant compositional diversity is revealed, both among comets and in some cases even within an individual comet. Recent work has both augmented this compositional taxonomy and opened important new questions for better understanding the comet-early solar system connection, for two principal reasons: (1) The new IRTF spectrograph (iSHELL) was commissioned in late 2016, and this together with the IRTF’s daytime capability (unique among near-IR platforms) has greatly expanded the number of accessible comets. (2) The period from early 2017 through early 2019 featured 12 comets having favorable circumstances for observation, six from the Oort cloud and six from the still-underrepresented ecliptic dynamical class. In addition to molecular abundances, modern IR spectrographs also provide long-slit coverage that records the spatial distribution of emissions in the coma. Comparing spatial profiles of molecular emitters can provide information regarding active source region(s) on the nucleus. A summary will be presented with emphasis on recent observations of both short- and long-period comets.

Building a taxonomy of comats based on native ice compositions

ACS Spring Meeting 2021

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