Europa 443
FIGURE 17 (Right) The Tyre impact structure has numerous
rings around a smoother central region. Outside the rings are
many small craters, which are secondary craters caused by ejecta
from the impact. This false-color image highlights the reddish
material associated with the Tyre rings. (Left) Model for
formation of multiring basins on icy satellites. This model may be
applicable to structures like Tyre, which are thought to have
formed in a relatively thin, brittle layer (white) over a fluid layer
(gray). (Image: NASA/JPL.)
overlie everything in their path, and their brightness sug-
gests they are so young that they have not been darkened
or significantly eroded by charged particle irradiation or
micrometeorite bombardment. Pwyll’s distinctive topogra-
phy and central peak imply that it formed in relatively solid
ice, rather than a thin layer of ice overlying liquid water or
slush.
One of Europa’s largest impact structures is Tyre
(Fig. 17), with a diameter of∼44 km. Tyre, along with
one other known feature named Callanish, are multiringed
structures, somewhat analogous to impact basins on the
terrestrial worlds. Tyre’s rim crest is difficult to identify and
it exhibits a complex interior with a smooth, bright cen-
tral patch interpreted to be impact melt, or frozen rem-
nants of fluid material that may have been emplaced from
below during the impact event. Tyre’s most striking char-
acteristic is its concentric troughs and fractures, tectonic
features resulting from the impact process. These struc-
tures are thought to originate when an impact occurs into
relatively fluid material, allowing for rapid collapse and infill
of thetransient crater, and dragging the cold and brittle
overlying crust inward to break along concentric faults (Fig.
17).GalileoNear-Infrared Mapping Spectrometer (NIMS)
observations of Tyre show that dark material associated with
the troughs is similar to the reddish material seen elsewhere
on the surface.
Europa’s simple and complex craters have morphologies
consistent with impact into a solid (though warm and weak)
ice target. In contrast, the larger (∼40 km) impacts inferred
to have formed Tyre and Callanish, with their distinctive
rough topography and concentric ring systems, imply pen-
etration of the transient crater to a fluid layer at a depth of
∼20 km. These observations are consistent with Europa’s
solid ice shell being∼20 km thick, overlying a fluid layer
that is probably Europa’s liquid water ocean.5. Surface Composition and Thermal State
It has long been known from Earth-based telescopic ob-
servations that Europa’s surface is predominantly covered
with water ice, as amply confirmed byGalileo’s NIMS in-
strument. However, the composition is distinctly different
in Europa’s darker regions, which are associated with many
landforms such as ridges and chaos (Fig. 18). This material
is thought to contain impurities such ashydratedsalts,
along with a reddish component.
Spectra from the NIMS instrument show highly dis-
torted water bands in the dark regions, indicative of one
or more hydrated minerals (Fig. 19). These deposits have
been interpreted to indicate the presence of hydrated salt
minerals, sulfates, and possibly carbonates. Some thermal
evolution models of Europa predict large quantities of mag-
nesium sulfate hydrate within the ocean, and mixtures of
this and sodium hydrate are predicted on Europa. Another
candidate for the surface material is sulfuric acid (H 2 SO 4 )
hydrate (more commonly known to us as battery acid). Al-
though all of these candidate compounds are colorless, ir-
radiation of the surface by charged particles may be the
reason why Europa’s dark areas appear reddish. Because
of its proximity to Jupiter, Europa’s surface is constantlyFIGURE 18 Composite image of a false-color NIMS infrared
image overlain on a monochrome camera image. Blue areas
represent relatively clean, icy surfaces, while redder areas have
high concentrations of dark, non-ice materials, which may be
from a subsurface ocean. The infrared image is about 400 km
across. (NASA/JPL.)