440 Encyclopedia of the Solar System
extension has been accommodated. Other features (such as
ridges), along with net global expansion (from freezing of its
ice shell), may play important roles, but the mystery of how
Europa’s surface extension is balanced is yet to be solved.
4.2 Lenticulae and Chaos
Much of Europa’s surface is covered with dark terrain
with a mottled appearance, termed “mottled terrain” from
Voyagerimages. High-resolutionGalileoimages show that
in these areas the surface has been endogenically disrupted
at small and large scales.
4.2.1 LENTICULAE
Many areas of Europa’s surface are disrupted by subcircular
to elliptical pits, spots, and domes, and microchaos regions
(collectively termed “lenticulae”), which are∼10–15 km in
diameter, with a variety of morphologies (Fig. 11). Domes
can be convex with upwarped but unbroken margins where
they meet the plains. Pits are topographically low areas
where the surface has downwarped while preserving the
preexisting terrain. Many of these features are associated
with dark plains material thatembayssurrounding valleys
in the ridged terrain, so it was probably relatively fluid when
emplaced. Spots were apparently flooded with dark plains
material. Lenticulae known as “microchaos” typically con-
sist of a fine-scale hummocky material, including embedded
small plates of preexisting material, commonly with some
associated dark plains material. These microchaos regions
resemble the larger chaos terrains described later.
Although a range of dome, pit, and spot sizes exists, there
is a strong preferred diameter of∼10 km. This consistency
in size and the range in their morphologies suggests that
FIGURE 11 Lenticulae are found in a range of morphologies,
including domes (a, b), microchaos (c), pits (d), and
combinations of these morphologies (e), which may or may not
have dark plains material associated with them (f). (After
Pappalardo et al., 1998.)
FIGURE 12 Model for the formation of lenticulae through
diapiric upwelling of buoyant warm ice.they are genetically related; the size and range are consis-
tent with an origin from convective upwelling of buoyant
icediapirswithin Europa’s icy shell (Fig. 12). Convection
is predicted within a tidally heated ice shell greater than
about 20 km thick overlying a liquid water ocean. The ice
may be either thermally buoyant (commonly referred to by
the counterintuitive term “warm” ice) or compositionally
buoyant, where the rising diapiric ice is “clean” relative to
its surroundings. Compositional buoyancy of diapirs is pos-
sible if they are cleaned out of low-melting-temperature
substances (e.g., salts, see Section 5), allowing the clean ice
to be more buoyant than the surrounding salty ice. In this
model, domes form by buoyant diapirs that would reach
and break through the surface, and pits may form when
a diapir does not quite make it to the surface but softens
and/or melts out impurities from the ice above it, allowing
the surface to sag downward.
The range of morphologies and levels of degradation that
are observed in microchaos regions supports the suggestion
that upwelling diapirs may partially melt pockets of briny ice