Quantifying the Hypervolatile Abundances in Jupiter-family Comet 46P/Wirtanen

Abstract

Abstract 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 CH 4 , 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 CO 2 /H 2 O ratio, as CO 2 cannot be observed directly from the ground, and space-based assets sensitive to CO 2 were not able to observe 46P during this apparition. We focused our analysis on H 2 O, CO, CH 4 , C 2 H 6 , CH 3 OH, and CO 2 (via [O i ] emission). We detected strong emissions from H 2 O, C 2 H 6 , and CH 3 OH. Over the 3 nights, we found evidence for changing mixing ratios, mostly due to a variable H 2 O production rate. In 46P, C 2 H 6 and CH 3 OH are enriched compared to cometary averages, with mixing ratios relative to H 2 O of ∼1% and ∼3%, respectively. Measurements of CH 4 and CO have been especially rare in JFCs. We report significant 3σ upper limits on CH 4 /H 2 O < 0.97% and CO/H 2 O < 0.54%. They place CH 4 being near-average or depleted, and CO being strongly depleted in 46P compared with Oort cloud comets. 46P has comparable CO/H 2 O to the few other measurements in JFCs, but enriched in C 2 H 6 and CH 3 OH. Our inferred CO 2 /H 2 O 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 CO 2 -driven water-rich ice grain production proposed for 103P/Hartley 2 may be operating on 46P as well.

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