before sunrise or three hours
after sunset.
When viewed through a telescope, the planet exhibits
phases like the moon. Full Venus appears the smallest
because it is on the far side of the sun from earth.
Maximum brilliance (a stellar magnitude of -4.4or 15
times the brightest star) is seen in the crescent phase.
The phases and positions of Venus in the sky repeat with
the synodic period of 1.6 years. Transits across the face
of the sun are rare, occurring in pairs at intervals of a
little more than a century. The next two will be in 2004
and 2012.Exploration
Venus's complete cloud cover and deep atmosphere make it
difficult to study from earth, and most knowledge of the
planet has been obtained through the use of space
vehicles, particularly those carrying probes that descend
through the atmosphere. The first flyby was that of
Mariner 2, launched by the United States in 1962,
followed by Mariner 5 in 1967 and Mariner 10 in 1974. The
Soviet Union has developed several entry probes, some
combined with flybys or orbiters: Veñera 4 and 5 (1967),
6 (1969), 7 (1970), 8 (1972), 9 and 10 (1975), 11 and 12
(1978), 13 and 14 (1981), and 15 and 16 (1983); Vega 1
and 2, sent toward Halley's comet in 1984, also flew by
Venus and released descent capsules. Several of these
probes successfully reached the planet's surface. The U.S.
sent two Pioneer Venus missions in 1978. Pioneer Venus 2
sent four probes to the surface, while the remaining
craft explored the upper atmosphere. Pioneer Venus 1, an
orbiter, continues to measure the upper atmosphere. The
Magellan probe, launched toward Venus in 1989, began
transmitting radar images of the planet in 1990.
Atmosphere
The surface temperature on Venus is highly uniform and is
about 459° C (732 K/858° F); the surface pressure is 96
bars (compared with 1 bar for earth); the atmosphere of
the planet consists of nearly all carbon dioxide (CO2).
The cloud base is at 50 km (31 mi), and the cloud
particles are mostly concentrated sulfuric acid. The
planet has no detectable magnetic field.
That 97 percent of Venus's atmosphere is CO2 is not as
strange as it might seem; in fact, the crust of earth
contains almost as much in the form of limestone. About 3
percent of the Venusian atmosphere is nitrogen gas (N2).
By contrast, 78 percent of earth's atmosphere is nitrogen.
Water and water vapor are extremely rare on Venus. Many
scientists argue that Venus, being closer to the sun, was
subjected to a so-called runaway greenhouse effect, which
caused any oceans to evaporate into the atmosphere. The
hydrogen atoms of the water molecules could have been
lost to space and the oxygen atoms to the crust. Another
possibility is that Venus had very little water to begin
with.
The sulfuric acid of the clouds also has its analogue on
earth in a very thin haze in the stratosphere. On earth,
sulfuric acid is carried down in the rain and reacts with
surface materials; indeed, this so-called acid rain is
damaging parts of the environment. On Venus the acid
evaporates at the cloud base and can only remain in the
atmosphere. The upper parts of the clouds, visible from
earth and from Pioneer Venus 1, extend as haze 70 to 80
km (44 to 50 mi) above the planet surface. The clouds
contain a pale yellow impurity, better detected at near-ultraviolet
wavelengths. Variations in the sulfur dioxide content of
the atmosphere may indicate active volcanism on the
planet.
Certain cloud patterns and weather features can be
discerned in the cloud tops that give some information
about wind motion in the atmosphere. The upper-level
winds circle the planet at 360 km/hr (225 mph). These
winds cover the planet completely, blowing at virtually
every latitude from equator to pole. Tracking the motions
of descending probes has shown that, despite the scale of
these high-speed, upper-level winds, well more than half
of Venus's tremendously dense atmosphere, near the planet's
surface, is almost stagnant. From the surface up to 10 km
(6 mi) altitude, wind speeds are only about 3 to 18 km/hr
(2 to 11 mph). The high-speed winds probably result from
the transfer of momentum from Venus's slow-moving,
massive lower atmosphere to higher altitudes where the
atmosphere is less massive, so that the same momentum
results in a much higher velocity.
The upper atmosphere and ionosphere have been studied in
great detail by Pioneer Venus 1, which passes through
them once each day. On earth this region is very hot; on
Venus it is not, even though Venus is closer to the sun.
Surprisingly, the night side of Venus is extremely cold.
(Day-side temperatures are 40° C/104° F, compared to
night-side temperatures of -170° C/-274° F.) Scientists
suspect that strong winds blow from the day side toward
the near vacuum that is caused by the low temperatures on
the night side. Such winds would carry along light gases,
such as hydrogen and helium, which are concentrated in a
night-side “bulge.”
On earth the ionosphere is isolated from the solar wind
by the magnetosphere. Venus lacks a magnetic field of its
own, but the solar wind seems to generate an induced
magnetosphere, probably by a dynamo action involving its
own magnetic field.
Surface Features
Venus rotates very slowly on its axis, and the direction
is retrograde (opposite to that of earth). Curiously,
Venus's synodic year is almost exactly five solar days
long, with the result that the same side always faces
earth when the two planets are closest. At such times,
the side facing earth can be viewed and mapped by earth-based
radio telescopes.
In contrast to the very large antenna needed for earth-based
radar mapping of Venus, a modest instrument of Pioneer
Venus 1 was able to conduct a nearly global survey.
Combined with data from the Soviet probes and from earth-based
radar, the survey shows that the surface of Venus is
primarily a rolling plain interrupted by two continent-sized
highland areas, which have been named Ishtar Terra and
Aphrodite Terra after two manifestations of the goddess
Venus. Aphrodite Terra, although not as high as Ishtar
Terra, extends nearly halfway around the equatorial
region; it occupies the planet's far side as viewed from
earth at closest approach.
The more powerful radar aboard the Magellan spacecraft
has revealed huge active volcanoes, large solidified lava
flows, and a vast array of meteorite craters. The largest
impact crater yet observed is almost 160 km (100 mi)
across—the smallest about 5 km (3mi). Although the
probe's radar could resolve even smaller craters, if any
were present, Venus's dense atmosphere apparently
prevents smaller meteorites from impacting the surface of
the planet.
The global survey and other probes have also revealed
evidence that a great deal of tectonic activity has taken
place on Venus, at least in the past. Such evidence
includes ridges, canyons, a troughlike depression that
extends across 1400 km (870 mi) of the surface, and a
gigantic volcanic cone whose base is more than 700 km (435
mi) wide. The Soviet probes have sent back photographs of
the areas in which they set down, and have also measured
the natural radioactivity of the rocks. The radioactivity
resembles that of granite and strongly suggests that the
material of Venus is differentiated, or chemically
separated, by volcanic activity. Angular rocks that are
visible in the Soviet pictures also suggest the existence
of geologic activity that would counteract the forces of
erosion.
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