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 theJ = 4 - 3 transition of HCN showed generally uniform levels of outgassing activity during the observing period, 2018 December 14-20. Gas expansion velocities were similar to 0.6 km s(-1), and the derived average HCN production rate was 7.4 x 10(24) 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 mu m continuum emissions from icy coma dust particles of size similar to 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 similar to 1000 km from the nucleus. The total mass of those particles was similar to 2 x 10(8) kg.