150 Encyclopedia of the Solar System
Plate tectonics creates a bimodal topography distribution on
Earth, with high continents and low ocean basins, as well as
an interconnected system of ridges and mountain belts at
plate boundaries. No such features are seen on Venus. Most
explanations of why Venus never developed plate tectonics
point to the very low amounts of water currently present
on Venus. The water in the atmosphere is equivalent to a
surface layer less than 10 cm thick. The abundance of heavy
hydrogen, or deuterium, in the atmosphere relative to the
normal hydrogen population indicates that a huge amount
of water was lost from Venus atmosphere early in it history.
The dry atmosphere implies a dry interior for Venus, which
is believed to make the outer shell on Venus too stiff to
break into the plates observed on Earth.
Although plate tectonics does not operate on Venus, it is
clearly an active planet with a relatively young surface and
a wealth of volcanic and tectonic features. The majority of
the planet is covered with volcanic features such as shield
volcanoes and lava plains directly analogous to Earth’s vol-
canic features. Many of the highland areas appear to form
over mantle plumes, where hot material from the interior
rises to the surface creating “hot spots” on the surface simi-
lar to Hawaii. In contrast, many of the tectonic features are
unique to Venus. Examples includecoronae, which are
believed to result from small-scaleplumesdeforming the
surface, andtessera, which are intensely deformed regions
with multiple intersecting fracture sets.
1. History of Venus Exploration
Venus has long been observed as one of the brightest ob-
jects in the evening or morning sky. Transits of Venus had
been used to determine its orbital period and diameter,
and Lomonsov discovered that Venus had an atmosphere
during the transit of 1761. But it was not until the 1960s
that the modern exploration of Venus began, with obser-
vation by Earth-based radio telescopes. Radio telescopes
at Arecibo in Puerto Rico and at Goldstone in California
were used to accurately measure the rotation period and
diameter of Venus. They also produced images of the sur-
face that showed large, continent-sized regions. However,
Earth-based radio telescopes were hindered by only being
able to image the same side of Venus that faced Earth at
inferior conjunction. [SeeThe Solar System at Radio
Wavelengths.]
Spacecraft observation of Venus began in 1962 with a
flyby by theMariner 2spacecraft. It observed Venus from
34,833 km, determining a 468◦C (874◦F) surface temper-
ature and observing that Venus lacked a magnetic field. In
1967,Mariner 5flew by Venus at an altitude of 4023 km, re-
turning data on atmospheric composition and surface tem-
perature. Also in 1967, the first probe entered the Venus
atmosphere, when the Soviet Union’sVenera 4returned
data for 93 minutes. TheVenera 5and 6 probes followed in
1969, sending back more atmospheric measurements. Two
moreVeneraprobes followed in 1970 and 1972 making soft
landings on the surface, withVenera 8in 1972 transmitting
data on surface temperature, pressure, and composition.
TheVenera 8measurements were initially thought to be
consistent with a granitic composition (see Section 5 for
more discussion).
The next U.S. mission to observe Venus wasMariner 10
in 1973, which was on its way to Mercury.Mariner 10pro-
vided observations of the atmospheric circulation of Venus
with both visible and ultraviolet wavelengths. In 1975, the
Soviet Union landed two more probes on the surface of
Venus,Venera 9and 10 , sending back panoramas of the sur-
face for the first time (see Section 2) and making detailed
geochemical measurements. These landers measured sur-
face compositions similar to terrestrialbasalts.
The U.S.Pioneer Venusmission in 1978 consisted of an
orbiter plus four atmospheric probes. The probes returned
data on atmospheric circulation, composition, pressure, and
temperature. The orbiter provided radar images of the sur-
face, as well as a detailed global topographic map with a
resolution of about 150 km. Major topographic regions such
as Aphrodite Terra and Bell Regio were mapped, as were
the 11 km high Maxwell Montes. The spacecraft was also
used to map the gravity field of Venus.
The Soviets followed with 4 more soft landers between
1978 and 1981, with three of the landers returning surface
panoramas and surface compositional information. The last
two soft landers (Venera 13and 14 ) returned color panora-
mas (see Section 2) and drilled into the surface for samples.
The next two Soviet missions were orbiters,Venera 15and
16 , and returned synthetic-aperture radar (SAR) images of
the northern hemisphere of Venus in 1983, with resolutions
of about 5–10 km. This rich data set revealed new types of
features on the surface of Venus, including tessera terrain
and coronae (discussed later).Vega 1and 2 in 1984 carried
balloon probes into the atmosphere and were the Soviet
Union’s last missions to Venus.
NASA’sMagellanmission to Venus was launched in 1989
from the space shuttle and arrived at Venus in August of- It obtained SAR images and altimetry of the surface
between 1990 and 1994, mapping over 98% of the surface.
The spacecraft also obtained high-resolution gravity field
measurements, especially after the orbit was lowered and
circularized in 1993. The 120 m resolution SAR images and
1–10 km resolution altimetry data completely unveiled the
surface of Venus and provided a global data set that could
be used to test models of the interior and surface evolution
of the planet.
In 2005, the European Space Agency will launch its first
mission to Venus:Venus Express. The mission focused on
studying the composition and the circulation of the atmo-
sphere of Venus.