Enriched CO and CH4 abundances in Halley-Type Comet 13P/Olbers through high-resolution near-IR spectroscopy

Comets contain the best-preserved known remnant material of solar system formation. Their volatile compositions provide clues to extant conditions in the protosolar disk at the time and place of their formation [[i],[ii]]. Subsequent gravitational interactions with the young giant planets scattered comet nuclei into the Kuiper belt and Oort cloud reservoirs, representing the primary dynamical source regions for ecliptic comets and nearly isotropic comets, respectively. Ecliptic comets have low orbital inclinations and typically become observable as Jupiter-family comets (JFCs) at smaller heliocentric distances (R_h). Nearly isotropic comets have random inclinations and manifest as longer period or non-returning comets, a dynamical class often termed Oort cloud comets (OCCs). Through gravitational interaction with the giant planets, a subset of OCCs transition to Halley-type comets (HTCs), having semi-major axes < 40 au [[iii]].

 

Spectroscopic studies at IR and millimeter / submillimeter wavelengths permit quantifying constituent ices housed in cometary nuclei (referred to as “native” ices). When sublimed through solar heating, these ices release parent volatiles (molecules) into the coma. Measuring abundances for 8 – 10 distinct parent volatiles (at times more) has become common in the IR (from l ~ 2.8 – 5.0 µm). The number of comets so characterized to date is ~ 50, and these comprise a continually evolving parent volatile compositional taxonomy [[iv]]. Characterizing the parent volatile compositions of HTCs provides an important bridge between JFCs, which experience frequent solar heating due to their short orbital periods, and dynamically new or very long-period comets that have experienced relatively little heating. However, to date only two HTCs have been characterized in detail, 1P/Halley and 8P/Tuttle, with only 8P having been available for study using high-resolution IR spectroscopy. Therefore, more HTCs are needed to draw meaningful comparisons to the overall population of comets.

 

Fortuitously, 2024 provided relatively rare opportunities to measure the compositions of two HTCs, 12P/Pons-Brooks and 13P/Olbers. For results from our observational campaign on 12P, see the Gibb et al. presentation (this meeting). Here, we report production rates and abundance ratios for six parent volatiles in 13P from daytime observations using iSHELL [[v]] at the NASA-IRTF on UT 2024 August 16. H₂O and CO were measured together in one iSHELL setting, and CH₄, C₂H₆, CH₃OH, and H₂CO were measured together in a second setting.  Our study is coordinated with a dedicated 13P iSHELL run in early August that characterized the parent volatile composition in greater detail (see Saki et al. presentation, this meeting), yet lacked sufficient geocentric Doppler shift (Δ_dot) to test CO or CH₄. By mid-August, Δ_dot had increased sufficiently to test these two critical “hypervolatile” species.

 

Our measurements revealed significantly enriched abundances (relative to H₂O) of CO and CH₄ (the two most volatile ices/parent molecules measured in the IR), compared with their mean abundances among OCCs, whereas C₂H₆ and CH₃OH were close to their mean values [4]. Possible interpretations regarding the processing history of the ices incorporated into the nucleus of 13P will be discussed.

 

We acknowledge support through NASA SSO Program awards 22-SSO22_0013 (MD, SF, NXR) and 80NSSC22K1401 (NDR, BPB, RJV), and through NSF award AST-2009398 (NDR, BPB). We also thank the IRTF staff for their help in making these challenging daytime observations successful.

 

 

[i] Bockelée-Morvan, D., et al. 2004, in Comets II, ed. M. C. Festou, H. U. Keller, & H. A. Weaver (Tucson, AZ: Univ. Arizona Press), 391

[ii] Mumma, M.J. & Charnley, S.B. 2011 ARA&A 49, 471

[iii] Levison, H.F. 1996, Comet taxonomy, ASP Conference Series, 107, 173

[iv] Dello Russo, N., et al. 2016 Icarus 278, 301

[v] Rayner, J., et al. 2016 Proc SPIE 9908, 990884