Venus: Surface and Interior 157
FIGURE 7 Radar image of W. Eistla Regio centered at 22◦N, 354.5◦E with dimensions of
approximately 1725×1260 km. The western volcano is Sif Mons, 350 km in diameter, and the
eastern volcano is Gula Mons, 450 km in diameter. Radial radar-bright and dark flows surround
both volcanoes; radar-bright linear fractures of Guor Linea are seen in the southeastern corner.
lithosphere becomes too dense and breaks off into the man-
tle. A new estimate of lithospheric thickness variations also
suggests that the lower lithosphere may thicken and be-
come unstable locally. Although possibly important, such
a process is not going to be nearly as efficient a cooling
mechanism as subduction.
Volcanism, resulting from melting of the mantle and/or
lithosphere and the rise of hot magma, can contribute to
heat loss. As discussed earlier, Venus was completely resur-
faced, most likely by volcanism within the last billion years.
The cratering record shows that there has been less volcan-
ism, on average, since resurfacing than the terrestrial aver-
age. Thus, volcanism cannot be a dominant mechanism of
heat loss currently.
Another constraint on interior processes is the absence
(or extremely low level) of a magnetic field. TheMariner
flyby missions measured no magnetic field, indicating that,
if present, the field must be<0.005 Gauss at the surface.
Most models of interior dynamos indicate that a planet must
be losing large amounts of heat from the planet’s metal core
to provide enough energy for a dynamo. Some models have
suggested that relatively rapid heat loss through plate tec-
tonics is a good method of driving a dynamo. Thus, one
possible scenario is that Venus had early plate tectonics and
an active dynamo but eventually lost much of its water from
the crust through volcanism to the atmosphere, where it was
subsequently lost to space. This decrease in water increased
the strength of the lithosphere to the point that tectonics
ceased and the dynamo shut down. Heat is then lost pri-
marily by conduction through the lithosphere, causing the
mantle to heat up and increase the rate of volcanism, caus-
ing the planet to resurface. This idea is speculative, as there
is no direct evidence for an early plate tectonic period.
The unusual cratering record on Venus indicates that the
first 3.5 Ga of geologic history has been somehow erased,
with a lower rate of resurfacing occurring subsequently.