Venus: Surface and Interior 167
FIGURE 18 Radar image of a
section of lineated plains
approximately 35 km across,
centered at 30◦N, 333.3◦E. These
gridded plains are located in
Guinevere Planitia and are
incredible uniform in orientation,
size, and space over nearly
1000 km.associated with a corona for example, but more commonly
they cover thousands of kilometers. These larger sets are
likely to be gravitational spreading of high topography into
lower regions and can be seen to form rings around some
large topographic features (see Fig. 4). Other sets cannot be
clearly associated with topographic highs. One hypothesis is
that these features result from thermal contraction due to
climate-change driven atmospheric temperature changes.
In some regions, there are two sets of wrinkle ridges, al-
though one set is usually better developed.
7.5 Plains Fractures, Grids, and Polygons
A wide range of long, narrow, approximately straight frac-
tures occur in the plains. Some fractures are wide enough
to be resolved as graben, but most are too narrow (less than
0.5 km) to be resolved as more than fractures. Most are in-
terpreted as extensional fractures because they parallel re-
solvable graben and because of their shape. Some are clearly
associated with local features such as volcanoes or corona
and are probably due to extension above dikes. In some lo-
cations, there are either single sets or intersecting grids of
fractures that cover hundreds of kilometers (Fig. 18). They
are very regularly spaced, with separations of 1–2.5 km.
The narrow spacing suggests that a thin layer is involved
in the deformation. It is not obvious how a uniform stress
can be transmitted to such a thin layer over such a broad
regions. Shear deformation be required to produce grids of
intersecting lineations.
Another type of extensional feature observed on Venus is
polygons, which are found in over 200 locations on Venus.
These features are analogous to mud cracks in that they
form in a uniform, extensional stress field. However, they
form not as water is lost but instead when rock cools and
contracts. The typical diameter is∼2 km, but some are up to
25 km across. Some areas have multiple scales of deforma-
tion. Again, some of these features can be associated with
local events such as volcanoes, but others cover very broad
regions and do not have an obvious origin. Polygons are most
commonly associated with small volcanic edifices, and fre-
quently appear to form synchronously (Fig. 9). Some may
form by actual cooling of lava flows. Such basaltic columns
are common on Earth, but the scale of the features found on
Venus is orders of magnitude larger, implying that the flow
thickness on Venus would probably to too large to be plau-
sible. Another mechanism, as proposed for wrinkle ridges,
is the possible heating and cooling of the upper crust due
to climate change.8. Summary
Venus provides a unique window in to the evolution of ter-
restrial planets. It is essentially identical to Earth in size and
bulk composition, yet its geologic history is entirely differ-
ent. Venus’ level of geologic activity over the last billion years
is comparable to that of Earth and exhibits many of the same
geologic processes. The convecting interior drives geologic
activity at the surface, creating a dozen major highlands.
These highlands include hot spots, which form above man-
tle plumes, and the more enigmatic and intensely deformed
highland plateaus. The majority of the surface is composed