The Galileo mission has returned the first high-resolution (21 m/pixel
) images of the surface of Europa, These images reveal structures with
morphologies reminiscent of those seen on terrestrial sea ice. Althou
gh it is premature to make one-to-one analogies between sea ice and Eu
ropa's surface, a review of the types of surface features commonly for
med on Earth and of various sea-ice processes can provide insight into
the complex geology of Europa. For example, deformation of terrestria
l sea ice results from winds, tides, and currents and from thermally i
nduced stresses; the resulting features include fractures ranging in w
idth from millimeters to kilometers, pressure ridges, shear ridges, an
d rafted ice. Potential agents of deformation on Europa are more likel
y to be limited to tidal flexing and possibly convection, but could pr
oduce similar features and perhaps account for the ridges and fracture
s seen in many areas. Subtle differences in albedo and color in terres
trial sea ice result from differences in ice thickness and grain size,
attributed to factors such as the rate of ice-crystal growth, water t
urbulence, age of the ice, and deformation. Similar factors could acco
unt for differences observed in the bright icy plains of Europa, Moreo
ver, salts in both the solid form and as brine vary in concentration a
nd composition as a function of space and time on Earth, leading to di
fferences in density and the strength of ice sheets. Salts are also su
spected in the europan ice and could lead to similar differences, enha
ncing the creation of topographic relief from density contrasts and th
e formation of fractures from brittle failure of the ice. Differences
in the environments between Europa and terrestrial sea ice in terms of
parameters such as temperature, gravity, time, and ice compositions s
uggest caution in drawing direct analogies. Future work by the planeta
ry and sea-ice communities must include understanding the terrestrial
processes sufficiently for extrapolation in Europa. (C) 1998 Academic
Press.