Europa 435
FIGURE 4 Galileoimage of a typical portion of Europa’s ridged
plains, showing ridges and troughs criss-crossing and
overprinting each other. Double ridges are most common, but
complex ridges with more than two crests also occur, along with
simple troughs. The background terrain is so heavily overprinted
that it is no longer possible to distinguish individual features.
(NASA/JPL.)
3.5 Nonsynchronous Rotation
If a satellite is in a perfectly circular orbit around its primary,
it keeps the same face toward its parent planet, and this tidal
bulge remains fixed relative to the planet. Moreover, most
of the solar system’s large satellites, including the Earth’s
moon, rotate synchronously, keeping one hemisphere al-
ways facing their parent planet. Because of Europa’s ec-
centricity, however, a net torque tends to cause Europa to
rotate slightly faster than synchronously. Europa’s massive
rocky interior is expected to maintain a permanent mass
asymmetry to counter this effect, so beneath the icy shell,
Europa is probably synchronously locked (as is the Earth’s
moon). However, since Europa’s icy shell is decoupled from
the rocky interior—likely by liquid water—the ice shell can
rotate independently, and slightly faster than the interior
(Fig. 2). The rate of this “nonsynchronous rotation” is not
known, and it might not be constant through time, but a
lower limit for one complete rotation of the shell is thought
to be in the region of 10,000 years, based on comparisons of
VoyagerandGalileoimages. Regardless of the actual rate
of rotation, the nonsynchronous stresses are expected to
be large—many times larger than the diurnal stresses and
potentially sufficient to open deep (kilometer-scale) cracks
in the ice shell. The orientations of Europa’s major linea-
ments do not correspond to the current patterns expected
from tidal stresses alone, but if the shell is backrotated by
moving it “back in time” by∼ 30 ◦westward in longitude,
there is an overall good fit of lineaments to the predicted
stresses. This implies that Europa’s observable global-scale
lineaments may have formed over about 60◦of nonsyn-
chronous rotation of the ice shell. Mapping of crosscutting
relationships among lineaments in some areas of Europa
suggest that the ice shell has completed at least one full
rotation, and may have also undergone a small amount of
polar wander(i.e., tilt relative to the spin axis).4. Landforms on Europa
Europa exhibits two primary types of terrain: the bright
ridged plains criss-crossed by bright and dark linear fea-
tures and mottled terrain, which shows evidence foren-
dogenicdisruption and modification of the surface. Each
type of terrain and themorphologiesof their constituent
landformsare discussed in detail next.4.1 Ridges, Troughs, and Bands
Europa’s linear features are ubiquitous, covering most of
the satellite’s surface. These landforms exist at a variety of
sizes and scales, and exhibit a number of different mor-
phologies, some of which have not been observed on any
other solar system body. Many of these linear features have
overprinted and offset one another, sometimes by several
kilometers, making it difficult to piece together the history
of the surface. Some ridges have shallow topographic de-
pressions and/or fine-scale fractures alongside them, which
are suggestive of loading of the lithosphere either by the
weight of the ridge material from above or from withdrawal
of material from below. Understanding how ridged plains
form and evolve is important to the question of where liq-
uid water exists within Europa, how it is involved in the
formation of surface landforms, and possible niches for life.4.1.1 INDIVIDUAL TROUGHS
The simplest of Europa’s landforms, troughs (commonly
called “cracks”) may be several hundred kilometers long
and less than a few hundred meters wide (Fig. 4). They
can have subtle rims or none, and are generally V-shaped,
suggesting an origin as tension fractures. Some have under-
gone mass wasting along their sides, and some troughs have
elevated flanks and appear to be transitional forms between
simple troughs and double ridges. As discussed previously,
Europa’s troughs probably originate by tensile cracking due
to diurnal and nonsynchronous stresses.4.1.2 DOUBLE AND COMPLEX RIDGES
The most ubiquitous landform on Europa, ridges are most
commonly found in a “double ridge” form, with two parallel
ridges separated by an axial V-shaped trough (Fig. 4). Dou-
ble ridges can be from∼0.5 to∼2 km wide, and some span
thousands of kilometers in length. Slopes of these ridges