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Quantifying the hypervolatile abundances in Jupiter-family comet 46P/Wirtanen

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journal contribution
posted on 2023-08-05, 13:15 authored by Adam McKay, Michael A. DiSanti, Anita L. Cochran, Boncho BonevBoncho Bonev, Neil Dello Russo, Ronald J. Vervack, Jr., Erika Gibb, Nathan X. Roth, Mohammad Saki, Younas Khan, Hideyo Kawakita

We present analysis of IR and optical spectroscopy of Jupiter-family comet (JFC) 46P/Wirtanen obtained in 2019 January, when the comet had sufficient geocentric velocity to enable studies of the hypervolatiles CO and CH4, as well as [O I] emission. These species could not be studied near closest approach in mid-December because there was insufficient Doppler shift to separate the cometary emission from their corresponding telluric absorption lines. We employed the [O I] observations as a proxy for the CO2/H2O ratio, as CO2 cannot be observed directly from the ground, and space-based assets sensitive to CO2 were not able to observe 46P during this apparition. We focused our analysis on H2O, CO, CH4, C2H6, CH3OH, and CO2 (via [O I] emission). We detected strong emissions from H2O, C2H6, and CH3OH. Over the 3 nights, we found evidence for changing mixing ratios, mostly due to a variable H2O production rate. In 46P, C2H6 and CH3OH are enriched compared to cometary averages, with mixing ratios relative to H2O of ∼1% and ∼3%, respectively. Measurements of CH4 and CO have been especially rare in JFCs. We report significant 3σ upper limits on CH4/H2O < 0.97% and CO/H2O < 0.54%. They place CH4 being near-average or depleted, and CO being strongly depleted in 46P compared with Oort cloud comets. 46P has comparable CO/H2O to the few other measurements in JFCs, but enriched in C2H6 and CH3OH. Our inferred CO2/H2O mixing ratio is ∼15%, though accounting for systematic uncertainties from the lack of knowledge of [O I] photochemistry means a value between 10% and 20% is likely. The compositional profile of 46P is similar to another small, hyperactive comet: 103P/Hartley 2. The mechanism of CO2-driven water-rich ice grain production proposed for 103P/Hartley 2 may be operating on 46P as well.



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Planetary Science Journal, Volume 2, Issue 1, February 2021, Article number abd71d.


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