Non-detection of water-ice grains in the coma of comet 46P/Wirtanen and implications for hyperactivity

Hyperactive comets are unusual in that their water gas production rates require an active fraction of ∼100%, while for most comets the active fraction is typically between a few percent to 30% (A’Hearn et al. 1995). Deep Impact’s observations at comet 103P/Hartley 2 (hereafter, 103P) laid out our current understanding of comet hyperactivity: sublimating water-ice in the coma, whether in the form of small grains and/or in large chunks of nucleus, increases the available surface area for water sublimation, which in turn contributes to the high observed water production rate (A’Hearn et al. 2011, Kelley et al. 2013, Protopapa et al. 2014). To verify whether other small hyperactive comets display a water-ice grain halo, we took advantage of the historic close approach between Earth and comet 46P/Wirtanen (hereafter, 46P) in 2018. Comet 46P represents an ideal candidate to test the present model for hyperactivity since it resembles 103P in dynamical class, level of activity and nucleus size. We report a detailed characterization of the inner coma of comet 46P through high spatial resolution, near-infrared spectroscopic measurements acquired with IRTF/SpeX. No evidence for the 1.5- or 2.0-μm water-ice absorption bands is found in six 0.8–2.5 μm spectra taken around perihelion and closest approach to Earth, which span a range of heliocentric distance from 1.1 au inbound, to 1.05 au near perihelion, and back to 1.1 au outbound. In addition, the strong 3.0-μm water-ice absorption band is absent in a 2.0–5.3 μm spectrum taken near perihelion. Radiative transfer modeling of the spectroscopic data combined with calculations of water-ice grain sublimation lifetimes rule out 103P-like small water-ice grains (Protopapa et al. 2014) in the coma of 46P. Pure water ice grains on the order of 1-μm in diameter are cool and long lived due to low sublimation temperatures. With the given upper-limit abundance, <1.4% by area, it is unlikely that such grains contribute to the hyperactivity of 46P. We consider instead dirty small ice grains (1.2-µm in diameter) with carbon fractions ≥0.5% as possible candidates to account for the comet water production rates. The small amount of low albedo dust limits the observability of the water-ice grains at heliocentric distances of 1.0–1.1 au, accounting for our observations. Moreover, it quickly converts the ice into water vapor, which is needed to account for the hyperactivity (58% active fraction). This model is based, in part, on the ice grains observed in the coma of comet C/2013 US10, which has been suggested to be a hyperactive comet by Protopapa et al. (2018).