100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64

CHAPTER 8

Venus: Surface and

Interior

Suzanne E. Smrekar

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

Ellen R. Stofan

Proxemy Research

Rectortown, Virginia

1. History of Venus Exploration 4. Interior Processes 7. Tectonics


2. General Characteristics 5. Composition 8. Summary


3. Impact Craters and Resurfacing History 6. Volcanism Bibliography

V

enus plays a pivotal role in understanding the evolution
of the terrestrial planets, the four rocky bodies closest
to the Sun. Venus is the planet most similar to the Earth
in terms of radius and density, implying a very similar bulk
composition. Because terrestrial planets have all formed via
the same process, condensing out of the solar nebula, the
primary factor that distinguishes them is their size, and to a
lesser extent, distance from the Sun. The energy available to
drive geologic evolution comes from the heat of accretion
and from decay of radiogenic isotopes. Over time, radio-
genic decay becomes more dominant. Thus, larger planets
have a greater abundance of radiogenic elements and can be
expected to be geologically active longer. Earth has abun-
dant geologic activity today. We are uncertain about the
present-day level of activity on Venus, but it has clearly been
extremely active within the last billion years. The majority
of geologic activity on Mars occurred over 3 Ga (billion
years) ago. Mercury has not been active since the earli-
est part of solar system evolution, the heavy bombardment
era. [SeePlanetary Impacts.] Most importantly, Venus
has evolved without the system of plate tectonics that gov-
erns the pattern of geologic activity on the Earth. Clearly,
size is important in determining the duration of geologic
activity, but other factors must affect the overall style of
geologic evolution. The atmospheric conditions on Venus
are also wildly different from those on Earth. The green-
house effect, in which abundant carbon dioxide causes the

atmosphere to heat up, was discovered on Venus. Its thick,

dense atmosphere gives Venus a surface temperature of

about 468◦C (874◦F), and a pressure 90 times greater than

Earth’s. For this reason, Venus has been called Earth’s “evil

twin.”

Volatiles on a planet are essentially the link between the

atmosphere, the surface, and the interior, as well as an es-

sential element in the habitability. A planet’s atmosphere

forms primarily through the outgassing of volatiles from the

interior. Outgassing results from the eruption and degassing

of lava onto the surface. The rate of resurfacing is a function

of the broad-scale geologic processes operating on a planet.

These processes are driven by heat loss from the interior,

which is primarily fueled by decay of radioactive elements.

The interiors of the larger terrestrial planets are hot enough

to convect, allowing hot material to rise and cold material to

sink on timescales of millions of years. On Earth, convec-

tion is linked to surface processes via the process ofplate

tectonics. The presence of water in the interior of Earth

acts to reduce the strength of the rock, which in turn allows

the exterior shell of the Earth to be broken up into plates.

As plates are pushed back into the interior, water is recycled

back into the interior. Volatiles on Earth are also strongly

affected by both the hydrosphere and the biosphere, both

lacking on Venus.

Although plate tectonics has controlled the evolution of

Earth for at least 3 Ga, Venus has no trace of such a process.

Encyclopedia of the Solar System 2e©C2007 by Academic Press. All rights of reproduction in any form reserved. 149