168 Encyclopedia of the Solar System
of vast volcanic plains along with nearly ubiquitous tectonic
features. There are tens of thousands of volcanic features
from small-scale (hundreds of meters) flows, vents, and
shields, to hundreds of large-scale (>100 km) shield volca-
noes that blanket the surface. The pervasive volcanism may
have buried the earliest, heavily cratered surfaces, or they
may have been destroyed through tectonic processes. Tec-
tonic features range in scale from pervasive linear fractures
and polygons at the limit of resolution to highland plateaus
composed of tessera terrain 1000–2000 km in diameter.
Despite the similarities between Venus and Earth, Earth
is the only body in our solar system that developed the
system of plate tectonics that has so shaped the geologic
and environmental evolution our planet. The atmosphere
of Venus lost nearly all of its water early in its evolution. The
loss appears to have affected the interior as well, causing
the lithosphere to be too strong to break into the plates ob-
served on Earth, and the asthenosphere to be too strong
to facilitate rapid horizontal plate motion. This same loss
of water has contributed to the dominance of CO 2 in the
atmosphere and the resulting greenhouse effect that cre-
ated the scorching surface conditions. Why Venus lost its
water is not understood, but as with Mars, the absence of a
magnetic field exposes the atmosphere to erosion by solar
wind. In turn, a planet must be losing heat rapidly enough
to drive the formation of a magnetic dynamo. The interior
volatile content affects the processes through which planets
lose heat and appears to be the key to whether or not plate
tectonics develops. Was Venus originally on the same evolu-
tionary path as Earth? What was the pivotal event or process
that sent Venus down an alternate path to the hellish, unin-
habitable planet we observe today? We can begin to address
these questions, thus better understanding the evolution of
our own planet, through future missions to understand the
coupled evolution of the atmosphere, surface, and interior.Bibliography
Brougher, S. W., Hunten, D. M., and Phillips. R. J., eds. (1997).
“Venus II.” Univ. Arizona Press, Tucson.
Fegley, Jr., B., and Treiman, A. H. (1992). Chemistry of
atmosphere–surface interactions on Venus and Mars. In “Venus
and Mars: Atmospheres, Ionospheres, and Solar Wind Interac-
tions” (J. G. Luhmann, M. Tatrallyay, and R. G. Pepin, eds.),
pp. 7–71. Geophysical Monograph No. 66. American Geophys-
ical Union, Washington D.C.
Venus data are available through the Planetary Data System at
pds.nasa.gov.