Venus is second from the sun, the sixth largest planet, and the brightest object in the sky (not counting the Sun and the Moon). It used to be believed that life could survive on this planet since it is so similar in size and mass to the Earth, but space probes determined that the surface temperature is around 500° C, the atmosphere is crushing, and the clouds contain sulfuric acid. Definitely not livable terrain.
Venus is also sometimes considered to be Earth's sister planet because they are so similar in many ways. Their chemical compositions are similar and they are about the same size (Venus is only a little smaller than Earth). In fact, its diameter is 95% that of Earth's, and its mass is 80% Earth's mass. Also, both have very few craters, which indicates that it is rather young. They were probably formed about the same time and out of the same nebula.
But compared to Earth, Venus' rotation is strange. First, its "day" is a little longer than its year. Second, a Venus day is equal to 243 Earth days. Third, the way Venus rotates around itself and around the Sun makes it so that the same face is always towards Earth when the two planets are closest together. Fourth, Venus rotates on an axis that is almost perpendicular to its orbit. (see diagram at right). At 178° inclination, Venus spins with retrograde motion. Also, Venus' orbit is almost perfectly circular. The distance from the Sun never varies by more than 2 million km.
Venus has a diameter of 12,104 km, which can be compared to Earth's diameter of 12,750 km. Also, Venus is almost perfectly spherical. (see diagram at left) On another note, Venus is like Mercury because it is considered an inferior planet. Thus, if you look at Venus through a telescope on Earth you can see different phases. When Galileo observed this interesting fact, Copernicus' heliocentric theory of the solar system gained more supportive evidence.
Before we ventured out to Venus, Venus was a mystery of planet. Nothing definite was known about the conditions below its permanent cloud layer. Early this century, Nobel prize winning chemist Svante Arrhenius (1859-1927) imagined this world full of primitive vegetables. In the 1950s F.I. Whipple and D. H. Menzel postulated that Venus had seas and maybe even primitive life-forms. The other theory was that the planet was terribly hot, with no water at all. As you probably gathered by know, the latter theory proved to be correct. Based on information of fly-by vehicles, orbiters, and soft-landers, it has been determined that the surface is extremely hot. The first successful probe was the Mariner 2, which landed in 1962. This probe proved the surface temperature was extremely hot (482° C; 900° F).
After Mariner 2 visited Venus, many more spacecraft followed to visit Venus. More than 20 in all have so far ventured out that way, including Pioneer Venus, the Soviet Venera 7, the Venera 9, and the Magellan. The Venera 9 returned with the first photographs of the surface, as shown in the picture to the left. The Magellan provided detailed maps of Venus' surface using radar.
The main atmosphere is actually the layer from 200 to 400 kilometers above the ground. Here the temperature is only about 27° C. If you go down 100 kilometers, the temperature would fall to 100° C. Then at 60 kilometers, a position within the cloud layers, the temperature is back up to 13° C and the pressure is about half what you would find at sea-level on Earth. At 50 kilometers above the ground, the pressure is up to one atmosphere. If you keep going down, the temperature rises. Below 47 kilometers, the clouds become a haze until you get to 30 kilometers. At the planet's surface the pressure is more than 90 atmospheres and the temperature is almost 500° C.
Rain in the atmosphere is made up of sulfuric acid, but these drops probably evaporate before reaching the surface (consider the heat!). This sulfur dioxide content actually varies tremendously. In 1978 there was a huge increase, and since then the level has been decreasing slowly. This infers that there was a volcanic eruption on the surface, which sent sulfur dioxide and haze into the atmosphere, maybe even up to 70 kilometers above the surface. There, sulfuric acid particles were formed, adding to the already present particles.
There are also atmospheric winds, which decrease when they get close to the surface. At the cloud tops, the winds blow at 100 meters/second, at 50 kilometers the winds blow at about 50 meters/second, and at the surface only a few meters/second. Although the winds at the surface are much weaker, even this wind has huge force because of the thick atmosphere.
Venus probably once had large amounts of water like Earth. At first, conditions were probably less forbidding. In the early stages of the Solar System, the Sun was about 30 % less luminous than now, allowing Venus and Earth to evolve similarly. When the Sun got more powerful, Venus' surface temperature rose and any water was boiled and evaporated. So, now Venus is quite dry. Earth would have turned out the same way if it had been a little closer to the sun.
When the Magellan visited Venus, its imaging radar showed that much of the surface of Venus is covered by lava flows. There are also several shield volcanoes, such as Sif Mons which is shown at the right. It has been found that Venus is still volcanically active, but only in a few hot spots; actually, most of the volcanoes on Venus haven't erupted for a few hundred million years.
There aren't any small craters on Venus because small meteoroids would burn up in Venus' dense atmosphere before reaching the surface. The craters that are there seem to have come in groups, probably because large meteoroids that reach the surface would break up in the atmosphere. The surface that we see on Venus right now is only about 800 million years old because the eruptions of volcanoes wiped out the surface that was there before, along with any large craters that hit earlier on.
The Magellan spacecraft (1990-94) brought back a lot of interesting information. For example, it was found that Venus has some rather unique features. Things you don't expect to find on a planet that is described as being a sister planet to Earth. Magellan showed us such interesting features as circular formations called arachnoids (see left). pancake volcanoes (right top) and collapsed lava domes called coronae (right bottom). Arachnoids are concentric rings of ridges of unknown origin. Pancake volcanoes looks more like breakfast than a mountain range formed from thick lava. And coronae are the remains of lakes of magna covers with a blister of crust. Some believe that arachnoids are in fact the early stages of coronae.
Inside, Venus is very much like Earth: with an iron core that is about 6000 km across and a mantle of molten rock that takes up most of the planet. The gravity data collected by the Magellan shows that Venus' crust is stronger and thicker than it was thought to be. Like Earth, Venus suffers from a convection in the mantle that causes surface stress. On Earth, the stress is relieved by shifting of large plates. On Venus, surface stress is relieved within many small regions.