At first glance, if Earth had a twin, it would be Venus. The two planets are
similar in size, mass, composition, and distance from the Sun. But there the
similarities end. Venus has no ocean. Venus is covered by thick, rapidly
spinning clouds that trap surface heat, creating a scorched greenhouse-like
world with temperatures hot enough to melt lead and pressure so
intense that standing on Venus would feel like the pressure felt 900 meters
deep in Earth's oceans. These clouds reflect sunlight in addition to trapping
heat. Because Venus reflects so much sunlight, it is usually the brightest
planet in the sky.
The atmosphere consists mainly of carbon dioxide (the same gas that produces
fizzy sodas), droplets of sulfuric acid, and virtually no water vapor -
not a great place for people or plants! In addition, the thick atmosphere
allows the Sun's heat in but does not allow it to escape, resulting in surface
temperatures over 450 °C, hotter than the surface of the planet Mercury
which is closest to the Sun. The high density of the atmosphere results in a
surface pressure 90 times that of Earth, which is why probes that have landed
on Venus have only survived several hours before being crushed by the
incredible pressure. In the upper layers, the clouds move faster than hurricane-
force winds on Earth.
Venus sluggishly rotates on its axis once every 243 Earth days, while it
orbits the Sun every 225 days - its day is longer than its year! Besides that,
Venus rotates retrograde, or "backwards," spinning in the opposite direction
of its orbit around the Sun. From its surface, the Sun would seem to
rise in the west and set in the east.
Earth and Venus are similar in density and chemical compositions, and
both have relatively young surfaces, with Venus appearing to have been
completely resurfaced 300 to 500 million years ago.
The surface of Venus is covered by about 20 percent lowland plains, 70
percent rolling uplands, and 10 percent highlands. Volcanism, impacts,
and deformation of the crust have shaped the surface. No direct evidence
of currently active volcanoes has been found, although large variations of
sulfur dioxide in the atmosphere lead some scientists to suspect that volcanoes
may be active.
Although no rainfall, oceans, or strong winds exist to erode surface features,
some weathering and erosion does occur. The surface is brushed by gentle
winds, no stronger than a few kilometers per hour, enough to move grains
of sand, and radar images of the surface show wind streaks and sand dunes.
In addition, the corrosive atmosphere probably chemically alters rocks.
Impact cratering is also affected by the dense atmosphere: craters smaller
than 1.5 to 2 km across do not exist on Venus, largely because small meteors
burn up in Venus? dense atmosphere before they can reach the surface.
More than 1,000 volcanoes or volcanic centers larger than 20 km in diameter
dot the surface of Venus. There may be close to a million volcanic centers
that are over 1 km in diameter. Much of the surface is covered by vast
lava flows. In the north, an elevated region named Ishtar Terra is a lava-filled
basin larger than the continental United States. Near the equator, the
Aphrodite Terra highlands, more than half the size of Africa, extend for
almost 10,000 km. Volcanic flows have also produced long, sinuous chan-nels
extending for hundreds of kilometers.
With few exceptions, features on Venus are named for accomplished
women from all of Earth's cultures.
Venus' interior is probably very similar to that of Earth, containing an iron
core about 3,000 km in radius and a molten rocky mantle covering the
majority of the planet. Recent results from the Magellan
that Venus' crust is stronger and thicker than had previously been thought.
Venus has no satellites and no intrinsic magnetic field, but the solar wind
rushing by Venus creates a pseudo-field around the planet.