164 Encyclopedia of the Solar System
FIGURE 14 This false color,
perspective radar image of Alpha
Regio is approximately 2000 km
across and is centered at 25◦S, 5◦E.
The blank strip is a data gap. The
texture of the deformed regions is
similar to that of Tellus Regio (Fig.
13). A corona is located at the
southwest edge of Alpha. To the
west are several small pancake
domes. An impact crater is seen on
the western margin of Alpha.7.2 Chasmata and Fracture Belts
Chasmata(chasmameans canyon) are regions of exten-
sional deformation, as indicated by their locally low topog-
raphy andgrabenor graben-like morphology. There are
5 major chasmata on Venus that extend for thousands of
kilometers and are several kilometers deep: Parga, Hecate
(see Fig. 15), Dali/Diana, Devana, and Ganis Chasmata.
The fracture zones in these regions are typically∼200 km
wide, with topographic troughs that are generally narrower,
with widths of∼50–80 km. There are 7 smaller chasmata,
with lengths of hundreds of kilometers and proportionately
narrower fracture belts and troughs. Several of the chasmata
occur on the flanks of hot spot rises and may be a result of
topographic uplift above a plume. The majority of other
chasmata form synchronously with coronae, as discussed
earlier. Although chasmata are not required for coronae to
form, nor vise versa, it is clear that the presence of one
increases the likelihood of the other. Both extension and
upwelling plumes can thin the lithosphere, which may fo-
cus additional extension and upwelling in an area.
Fracture belts appear similar to minor chasmata but
are less intensely fractured, implying lesser amounts of
extension. A curious feature of fracture belts is that they
are topographically broad swells rather than topographic
lows. The positive relief suggests that they went through a
compressional stage, and that the fractures may be due to
topographic uplift rather than regional extension.
7.3 Coronae
Coronae are large (>100 km across) circular features sur-
rounded by concentric ridges and fractures (Fig. 16). Over
500 coronae have been identified on Venus; the largest one
is Artemis Corona at 2500 km across. Coronae often have
volcanoes in their interiors, and many are surrounded by
extensive lava flows. Coronae tend to be raised at least 1 km
above the surrounding plains, but others are depressions,
rimmed depressions or rimmed plateaus. Most coronae are
located along rift or chasmata systems, although some are at
topographic rises, and others occur in the plains away from
other features. Coronae are thought to form over thermal
plumes or rising hot blobs, smaller in scale and probably
rising from shallower depths than the plumes that form
topographic rises. The wide range in corona topographic
shapes indicates that coronae evolution also involves de-
lamination or sinking of lithospheric material in its later
stages. Studies of the gravity signatures of some large coro-
nae indicate that many coronae are likely to be isostatically
compensated, and thus probably inactive. The fact that we
do not see coronae on Earth may be due to the lack of an
asthenosphere on Venus.7.4 Ridge Belts and Wrinkle Ridges
Ridge belts occur in a variety of morphologies and are dis-
tributed around the planet. Based on the morphology of