Methanol in Comet 46P/Wirtanen: Rapidly Varying and Anisotropic Outgassing Measured with ALMA

The study of comets affords a unique window into the birth, infancy, and subsequent history of the solar system through study of their coma volatiles. Pure rotational transitions of methanol (CH₃OH) have been observed in comets at millimeter wavelengths since the 1990s (e.g., Bockelée-Morvan et al. 1994, A&A, 287, 647) and are often used to probe coma rotational and kinetic temperature. More recently, the high spatial and spectral resolution provided by the Atacama Large Millimeter/Submillimeter Array (ALMA) has enabled the coma to be probed in unprecedented detail. These studies have revealed insights into the production mechanisms and thermal chemistry of CH₃OH in comets, and time-resolved measurements have revealed rapid variations in gas and dust production (e.g., Cordiner et al. 2017, ApJ, 837, 166; Cordiner et al. 2017, ApJ, 838, 147). The historic 2018 perihelion passage of comet 46P/Wirtanen afforded an opportunity to couple the capabilities of ALMA with the most favorable apparition of a Jupiter-family comet in the previous decade. We targeted multiple CH₃OH transitions in 46P/Wirtanen using five executions of the ALMA Atacama Compact Array (ACA) spread across UT 2018 December 8 and 9, with a spectral resolution of 0.15 km s^-1 and spatial resolution (synthesized central beam) of 279 km × 502 km. Our time-resolved measurements revealed that CH₃OH production in Wirtanen was anisotropic, and that the spectral line asymmetry varied on timescales of minutes. Variations in line asymmetry and CH₃OH production rates were consistent with the reported nucleus rotation period of 9 hours. We will present spectrally integrated flux maps of CH₃OH emission on each date, report CH₃OH production rates and rotational temperatures, and place our results into the context of comets characterized to date.