444 Encyclopedia of the Solar System
FIGURE 19 Infrared spectrum (top after Pappalardo, 1999,
Scientific American) of Castalia Macula (bottom), one of the
reddest, darkest spots on Europa. The spectrum of the icy plains
material is distinct from the red material of the spot, which is
more similar to something like epsom salt or hydrated sulfuric
acid. (Image: NASA/JPL.)
irradiated by ions and electrons. This radiation is sufficient
to rip apart molecules of water ice and other compounds,
allowing them to recombine in a process known asradioly-
sis. This could allow sulfur ions (at least some of which likely
originate on Io) and sulfur-containing compounds such as
sulfuric acid to synthesize long molecular chains that are
ochre in color. These sulfur chains may be responsible for
the reddish color of material that has been emplaced on the
surface relatively recently. Sulfuric acid itself could result
from the breakdown and recombination of ice and sulfur
dioxide frost, which has also been detected on Europa.
Generally the stratigraphically youngest features on Eu-
ropa are the darkest, implying that the darkening and red-
dening process is rapid relative to the age of observable
surface features. (Also, the fact that Europa’s older features
are relatively bright implies that some other process bright-
ens features over time, as discussed below.) Whatever their
specific origin, the close association of these hydrated min-
erals with areas of presumed surface disruption suggests
they are related to endogenic processes and may have orig-
inated in the subsurface ocean.
Strong absorptions in the infrared region of the spec-
trum by Europa’s H 2 O-bearing minerals easily mask the
signatures of minor constituents; however, hydrogen perox-
ide (H 2 O 2 ) is observed and is probably a radiolysis product
of water ice. An ultraviolet absorber identified on the trail-
ing side of Europa has been attributed to sulfur from Io,
delivered to Europa’s surface via the jovian magnetosphere.
TheGalileospacecraft carried a Photopolarimeter Ra-
diometer (PPR) instrument that showed that temperatures
at low latitudes are in the range 86–132 K, with higher tem-
peratures where the surface is dark, and colder tempera-
tures where it is bright. This inverse correlation between
brightness and temperature holds on a global scale, but sig-
nificant local temperature variations are inferred below the
spatial resolution of the PPR instrument. These may be due
to local-scale variations in surface physical properties, and a
distinct anomaly around the crater Pwyll may imply a rela-
tively warm ejecta blanket. Other thermal variations such as
lower than expected temperatures on the equator at dusk
are harder to explain. These may be due to variations in
grain sizes and structures of ice, but endogenic heat fluxes
indicative of interior activity cannot be ruled out.6. Surface Physical Processes
Processes affecting Europa’s surface materials are domi-
nated by thermal processing and radiation bombardment,
with meteorite bombardment playing a lesser role.
Jupiter’s magnetosphere sweeps up and traps particles
including electrons, protons, and heavy ions such as S and
O. Because of its close proximity to Jupiter, these particles
result in a high-energy (<10 MeV) radiation flux at the sur-
face of Europa. The heavy ions in particular are responsi-
ble forsputtering, where molecules are physically blasted
from the surface, creating anexosphereof sputtered prod-
ucts, including sodium and low-energy electrons.There is
much still to be learned about the effects of irradiation of
ices and the stability of hydrated salt minerals at Europa’s
surface temperatures.
Europa’s water ice exhibits a variety of grain sizes and is
particularly abundant and fine-grained (< 100 μm diameter)
between± 60 ◦latitude on the leading side, but it is less
abundant with coarser grains (> 400 μm) on the trailing
hemisphere. The polar regions have a mixture of particles
with a range of grain sizes. Bright regions of Europa’s surface
are topped with a 1μm layer ofamorphousice, which is