  The release of volatiles from comets is usually from direct sublimation of
ices on the nucleus, but for very or hyper-active comets other sources have to
be considered to account for the total production rates. In this work, we
present new near-infrared imaging and spectroscopic observations of
46P/Wirtanen taken during its close approach to the Earth on 2018 December 19
with the MMIRS instrument at the MMT Observatory to search for signatures of
icy or ice-rich grains in its inner coma that might explain its previously
reported excess water production. The morphology of the images does not suggest
any change in grain properties within the field of view, and the NIR spectra do
not show the characteristic absorption features of water ice. Using a new
MCMC-based implementation of the spectral modeling approach of Protopapa et al.
(2018), we estimate the areal water ice fraction of the coma to be less than
0.6%. When combined with slit-corrected Afrho values for the J, H, and K bands
and previously measured dust velocities for this comet, we estimate an icy
grain production rate of less than 4.6 kg/s. This places a strict constraint on
the water production rate from pure icy grains in the coma, and in turn we find
that for the 2018-2019 apparition approximately 64% of 46P's surface was
sublimating water near perihelion. WE then discuss 46P's modern properties
within the context of other (formerly) hyper-active comets to understand how
these complex objects evolve.
