Abstract
1 The water ice on the three outermost Galilean satellites of Jupiter
has a lattice structure that can vary from crystalline to amorphous.
Amorphous ice is crystallized by heating, while crystalline ice is
amorphized through disruption by particle radiation. We determine
ice lattice order using infrared spectra from the Near Infrared Mapping
Spectrometer on the Galileo Jupiter orbiter. The shape of the reflectance
peak near 3.1 mum is diagnostic of the lattice order in the top micrometers
of the surface. A narrow, temperature-sensitive band near 1.65 mum,
from similar to1 mm depth, is missing for amorphous ice. Spectral
averages of > 100 pixels were used for Europa and Callisto, because
of high radiation noise and small ice amounts, respectively. Model
comparisons show that the surface ice is predominantly amorphous
on Europa and predominantly crystalline on Callisto, while both types
of ice are found on Ganymede. The distribution of Ganymede ice properties
shows a broad global pattern of more amorphous ice in the high-latitude
Jovian-facing hemisphere and in the low-latitude trailing hemisphere.
The ice at similar to1 mm depth on all three satellites is predominantly
crystalline. The radiation flux increases by similar to 300 times
between Callisto and Europa, while the thermal crystallization rate
may vary over five orders of magnitude among the three satellites
( being the fastest at Callisto). The occurrence of crystalline and
amorphous ice suggests a balance between the disruption and crystallization,
with Callisto dominated by thermal crystallization and Europa by
radiative disruption, and with nearly equal rates between the two
processes at Ganymede.
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