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James Clerk Maxwell Telescope Spectral and Continuum Imaging of Hyperactive Comet 46P/Wirtanen

The Jupiter-family comet (JFC) 46P/Wirtanen passed the Earth at a distance of 0.077 au on 2018 December 16 UT, presenting a rare opportunity to study the chemical structure of its coma. With the James Clerk Maxwell Telescope we achieved a resolution of 800 km, which is smaller than the scale lengths of some distributed cometary molecules at the comet’s heliocentric distance of 1 au. Spectroscopic observations of the J = 4 − 3 transition of HCN showed generally uniform levels of outgassing activity during the observing period, 2018 December 14–20. Gas expansion velocities were ∼0.6 km s−1, and the derived average HCN production rate was 7.4 × 1024 mol s−1. HCN and CH3OH emissions were detected at least 30″ (1600 km) from the nucleus, and the abundances of these species were typical for a JFC. The radial distribution of CH3OH is consistent with an extended source of sublimation such as a population of icy grains—cometary halo ice primaries, or CHIPs—as has been invoked previously to explain hyperactivity in comets. The abundance of H2CO is normal if it is a daughter species. HNC and CO were not detected, but a sensitive nondetection of CS implies an unusually low CS:H2O ratio of <0.02%. The peak brightness of the 850 μm continuum emissions from icy coma dust particles of size ∼1 mm fell from (52 ± 6) to (40 ± 3) mJy beam−1 through the week, while the size of the dust coma remained essentially constant, with dust extending to ∼1000 km from the nucleus. The total mass of those particles was ∼2 × 108 kg.