Parent Volatiles in Jupiter-family Comet 103P/Hartley 2 with NIRSPEC and Comparisons with Previous Apparitions

Comets contain the best-preserved known remnant material of solar system formation. Their volatile composition is thought to provide clues as to extant conditions in the protosolar disk at the time and place of their formation. Subsequent gravitational interactions with the young giant planets scattered comet nuclei into the Kuiper belt and Oort cloud reservoirs, representing the primary source regions for Jupiter-family comets (JFCs) and long-period/non-returning comets (Oort cloud comets, OCCs), respectively. Of these two populations of comets, JFCs remain under-represented in the overall compositional taxonomy, owing largely to their relatively small sizes and smaller gas production rates, making their characterization more challenging compared with OCCs. However, in recent years this disparity has been addressed thanks to favorable JFC apparitions as well as improved instrumentation on ground-based telescopes. Owing to their short orbital periods, JFCs can permit compositional comparisons among parent volatiles across perihelion passages (i.e., across apparitions), an emerging field in cometary science.

We performed high-resolution (λ/Δλ ~ 25,000), near-infrared (λ ~ 2.8 – 3.6 µm) spectroscopic observations of JFC 103P/Hartley 2 (hereafter 103P) with NIRSPEC at Keck 2, on three UT dates: 2023 October 14 (near perihelion), 26, and 27. We report values (or meaningful upper limits) for production rates (molecules s^-1) and abundance ratios for H₂O, C₂H₆, CH₃OH, HCN, C₂H₂, NH₃, and H₂CO, and compare these both short term (day-to-day) and longer term (over approximately a two week period). We will compare our results from the 2023 apparition of 103P with previous measurements, to test whether its observed decrease in water production from 1997 to 2010 [1] continued in 2023. Our study will also test potential changes in parent volatile composition from 2010 [2,3], which could indicate heterogeneity with depth in the nucleus.

This work was supported by the NASA Solar System Observations program under award 22-SSO22_0013, and NSF award 2009398. We also thank the Keck Observatory staff for their assistance in making these challenging non-sidereal observations successful.

References: [1] Combi et al. 2011 ApJ 734:L6, [2] Mumma et al. 2011 ApJ 734:L7, [3] Dello Russo et al. 2011 ApJ 734:L8