Venus: Surface and Interior 161
FIGURE 10 Lava flows extend for
hundreds of kilometers across the
fractured plains shown in the
foreground to the base of Maat
Mons, which is located at about
0.9◦N latitude, 194.5◦E longitude.
Magellan data was combined with
radar altimetry to develop a
3-dimensional map of the surface.
The vertical scale in this perspective
has been exaggerated 22.5 times.
The simulated red color is based on
images recorded by the Soviet
Venera 13and 14 spacecraft that
indicate the atmosphere on Venus
would make the surface appear red
to our eyes. (The image was
produced at the JPL Multimission
Image Processing Laboratory.)as the surface expression of subsurface dikes. These dike
sets provide evidence that many large volcanoes have un-
dergone multiple episodes of intrusion and extrusion.
At the smaller end of the scale, volcanoes 5–50 km across
are also abundant on the surface of Venus (Fig. 11). Many of
the volcanoes resemble their terrestrial counterparts, with
summit calderas and radiating digitate flows. Venus also has
several types of volcanic features that differ from those on
Earth and other planets. The steep-sided or pancake domes
are flat-topped, with steep sides (Fig. 12), similar to the flat-
topped Inyo domes in California that are formed by silicic
lavas. The Venus domes may have a different composition
though, as they are much larger and have smooth rather
than blocky surfaces in comparison to the terrestrial domes.
Other unusual volcanoes on Venus resemble ticks, or bottle
caps. These small domes have scalloped margins and are
interpreted to be steep-sided domes whose margins have
collapsed.
TheMagellanradar also imaged channels, a few kilome-
ters wide and hundreds of kilometers long. The channels
are found many places within the plains, tend to be very
sinuous, and in places show evidence of levees and flow
breakouts. The channels have formed by lava of some un-
usual composition, so fluid that it behaved like water and
able to flow long distances without cooling. A number of
compositions have been proposed, including carbonate or
sulfur-rich lavas and ultramafic silicate melts. Others have
suggested that the channels were formed by erosion of the
surface by lava, similar to lunar rilles on the Moon. [See
The Moon.] Some of the channels extend for long dis-
tances, allowing them to be used as a time marker, as it
can be assumed that the channel formed over a relatively
short period of time. For example, the channel may super-
pose one feature, but be overlain or cut by another. Also, a
few channels now trend uphill, indicating that the surface
deformed after they formed.
Are volcanoes on Venus still active? There are over 1500
active volcanoes on Earth, but Venus probably has fewer
active volcanoes. Gravity studies indicate that a number
of volcanoes may be dynamically supported, and thus still
active. A decline in SO 2 over time observed by thePioneer
Venusspacecraft has been interpreted to possibly indicate
a relatively recent eruption. In addition, volcanism within
the last 10–50 Ma is supported by climate models. Future
missions to Venus monitoring the atmosphere may be able
to detect a future venusian eruption.7. Tectonics
For the larger terrestrial planets, Venus, Earth, and Mars,
mantle convection is the primary driving force for tectonic
processes. On Mars, most tectonic structures are associ-
ated with either the gigantic Tharsis rise or the global di-
chotomy. [SeeMars Surface and Interior.] The global
dichotomy divides the smoother northern lowlands from