  Physical processing of Europan surface water ice by thermal relaxation,
charged particle bombardment, and possible cryovolcanic activity can alter the
percentage of the crystalline form of water ice compared to that of the
amorphous form of water ice (the ''crystallinity'') on Europa's surface. The
timescales over which amorphous water ice is thermally transformed to
crystalline water ice at Europan surface temperatures suggests that the water
ice there should be primarily in the crystalline form, however, surface
bombardment by charged particles induced by Jupiter's magnetic field, and vapor
deposition of water ice from Europan plumes, can produce amorphous water ice
surface deposits on short timescales. The purpose of this investigation is to
determine whether the Europan surface water ice crystallinity derived from
ground-based spectroscopic measurements is in agreement with the crystallinity
expected based on temperature and radiation modeling. Using a 1D thermophysical
model of Europa's surface, we calculate a full-disk crystallinity of Europa's
leading hemisphere by incorporating the thermal relaxation of amorphous to
crystalline water ice and the degradation of crystalline to amorphous water ice
by irradiation. Concurrently, we derive the full-disk crystallinity of Europa's
leading hemisphere using a comparison of near-infrared ground-based spectral
observations from Grundy et al. (1999), Busarev et al. (2018), and the Apache
Point Observatory with laboratory spectra from Mastrapa et al. (2018) and the
Ice Spectroscopy Lab at the Jet Propulsion Laboratory. We calculate a modeled
crystallinity significantly higher than crystallinities derived from
ground-based observations and laboratory spectra. This discrepancy may be a
result of geophysical processes, such as by vapor-deposited plume material, or
it may arise from assumptions and uncertainties in the crystallinity
calculations.
