Venus is a rocky inner planet. Like Earth, it has an inner and an outer core. Both cores are made of iron-nickel. The inner core of Venus is solid. It has a radius of about 640 miles (1030 km), or 17 percent of the planet's total radius. The outer core is liquid. The outer core is about 1,100 miles (1,800 km) thick, or 30 percent of the radius.
Above the outer core is a mantle almost 2,000 miles (3,200 km) thick. The mantle takes up about 52 percent of the planet's radius. The mantle is mostly silicates, with some metallic oxides. Over the mantle is a crust 20 miles (32 km) thick. The crust is made of rock, silicates, and metals. The crust takes up less than one percent of the radius. Above the crust, a heavy atmosphere surrounds the planet. The atmosphere rises about 42 miles (67 km) above the surface. The atmosphere is mostly carbon dioxide. On Earth, the air contains water vapor, which forms clouds. Venus's atmosphere has very little water vapor. On Venus the clouds are made of deadly sulfuric acid.
Surface Features
Venus and Earth have similar surface features. Venus, however, has no surface water. At one time, people thought Venus had water even though they couldn't see its surface through the thick clouds. They assumed that, like the clouds on Earth, the clouds on Venus were made of water vapor. To have water vapor clouds, Venus would need large oceans. However, recent space probes have shown that the surface of Venus is dry and dusty.
Venus is mostly rocks and sand. About 80 percent of the surface is planitiae. Planitiae are wide, flat areas. The planitiae were made by flowing lava that has solidified.
The rest of the surface is lowlands and giant plateaus. The plateaus rise several miles above the lowlands. Two of these plateaus are as large as continents on Earth. One, called Ishtar Terra, is nearly the size of the United States. The other, Aphrodite Terra, is even larger.
There are also enormous volcanoes on the plateaus. No one knows for sure if any are still active. They are very big. Some peaks rise nearly seven miles (11 km) above the plains. These peaks are higher than Earth's highest mountain, Mt. Everest. The base of one volcano, Rhea Mons, is as big as the entire state of New Mexico. Another, Sapas Mons, is about 250 miles (400 km) across and almost a mile (1.6 km) high. Many lava flows cover the areas near volcanoes.
Parts of the plateaus rise into huge mountain ranges. Aphrodite Terra is a large plateau in the northern hemisphere. It has a mountain range higher than any on Earth.
In the past, magma frequently poured out from the inside of the planet. It covered large areas and left lava flows hundreds of miles long. In some places the magma bubbled up. These bubbles left large domes of lava on the surface. Space probes have discovered circles in the surface. These circles are called coronae. They were formed when hot magma from the mantle pushed the ground up from beneath. This caused the surface to bulge. When the magma drained away, the bulge collapsed. What was left was a circular basin ringed with ridges. Some volcanoes may have been created by magma pushing its way up through the softened ground of a corona. As more and more lava poured through the hole, a giant peak formed.
Another unusual volcanic feature in an arachnoid. An arachnoid is a small corona with cracks that radiate from the center. Coronae and arachnoids have not been found on any planet other than Venus.
Meteorites have also left their mark on Venus. The landscape has many large impact craters. The largest, crater mead, is 168 miles (270 km) across. No crater is less than about a mile and a quarter wide. Small comets, asteroids, or meteoroids were crushed to powder by the planet's very heavy atmosphere! Most of the craters are well preserved and show little sign of erosion. They aren't even partially covered by lava flows. This may mean that Venus was completely covered by a tremendous lava flow late in its history. Craters have been pocking this fresh surface ever since. No other massive flows took place to cover the newer craters.
Atmosphere
Above the planet's surface is a heavy atmosphere. It extends about 42 miles (67 km) above the surface. The atmosphere is about 96 percent carbon dioxide. The atmosphere is fairly clear to a height of about 17 miles (27 km) above the surface. Above 17 miles is a blanket of haze and clouds made of sulfuric acid.
There are three layers of clouds. The bottom layer extends from 17 to 27 miles (32 to 52 km) in altitude. This layer is the thickest of the three layers. However, it thins out to a haze closer to the surface. The sulfuric acid drops in these clouds sometimes condense into sulfuric acid rain. The rain never reaches the ground. It evaporates in the intense heat and rises up as vapor. The middle layer has fewer, but larger sulfuric acid droplets. The middle layer lies between 32 and 36 miles (52 to 58 km) in altitude. The top layer is a heavy mist of sulfuric acid. The sulfuric acid is stronger than the acid in a car battery. The clouds of the upper atmosphere create a haze that covers from view the lower atmosphere. The tops of these clouds are 42 miles above the planet's surface. That's six times higher than where the clouds of Earth end.
The planet's surface temperature rises to nearly 900¬°F (484¬°C). That's hot enough to melt lead! No other object orbiting the Sun in our solar system is that hot. Carbon dioxide is the cause of the furnace-like temperatures. Sunlight heats up the surface. The surface absorbs the heat and radiates it back in the form of infrared radiation. This radiation rises, but carbon dioxide and the sulfuric acid clouds keep it from escaping into outer space. The heat builds in the atmosphere, under the clouds. This is known as the greenhouse effect.
Heat rising from the surface causes strong winds at the higher altitudes. They circle the planet every four days at speeds of up to 220 miles per hour (350 kph). Venus's winds all blow from east to west. In the turbulent atmosphere, lightning discharges as often as 25 times each second.
Slow winds also blow across the surface of the planet with the force of a fast-moving river. They push an atmosphere much heavier than Earth's. The air is so thick, in fact, that it bends light at an incredible angle. On the surface of Venus, the horizon would seem to bend up towards the sky! The atmosphere is very heavy. The pressure on the surface is 90 times greater than Earth's surface pressure. The air pressure is as great as the water pressure pushing on a submarine half a mile under Earth's ocean. On Earth, the air pressure at sea level is 14 pounds per square inch. On Venus, the air pressure at ground level is almost 1,300 pounds per square inch. Several Russian space probes were crushed by the pressure before they ever landed on Venus!
Orbit and Rotation
Orbit
Venus orbits the Sun in 224 days and 17 hours. Venus's orbital velocity is 78,246 miles per hour (126,000 kph). The orbit covers a distance of 422 million miles (684 million km). Its average distance from the Sun is 67.2 million miles (108.2 million km). Venus revolves around the Sun in a more nearly circular orbit than any of the other planets. Venus's orbital plane is inclined at an angle of 3.4¬° to the ecliptic. The orbit of Venus comes within about 25.7 million miles (41.4 million km) of Earth at its closest approach. At that point, it is the nearest large body to Earth (except for the Moon). Venus is then only about 100 times farther from Earth than the Moon. Evening and Morning Star
Because Venus is so bright, it usually appears in the evening sky before any of the other stars or planets. It's the last to disappear in the morning. Venus is so bright, you can sometimes see it in the day, and it casts shadows at night! For almost seven months, Venus can be seen in the western sky. It appears to follow the Sun as it sets. Finally, it appears to catch up to the Sun and is lost to sight. After about eight weeks, it reappears on the eastern horizon, just before dawn. Venus is then visible as the morning star for about nine months. Next it appears that the Sun catches up to it, and Venus disappears behind the Sun. Seven weeks later, Venus can be seen again as the evening star, which completes a 584-day cycle.
Like the Moon
Venus revolves around the Sun inside the orbit of Earth. So, it exhibits phases, just as the Moon does. Unlike the Moon, though, Venus doesn't stay the same distance from Earth during its orbit. That makes Venus seem to grow and shrink in size as it goes through its phases. As a crescent, it looks seven times larger than when it's full. Venus is full when it's farthest from us, at the other side of the Sun. When we see its crescent phase, it is nearer to us than the Sun.
The dark side of Earth's Moon is not completely dark. It receives some reflected light from the Earth. This light is called Earth shine. The night side of Venus is also never pitch black. The dark side is faintly lighted by a sheen called ashen light. On Venus, the light might be from solar radiation striking atomic particles in its atmosphere. This creates an aurora-like effect.
When Venus transits the Sun, it appears as a black dot covering only 1/1,000 of the Sun. This transit occurs very rarely. The last transit took place in 1882, and the next one will take place on June 8, 2004.
Rotation
Venus has a retrograde rotation. It also rotates slowly. It takes 243 days to rotate once on its axis. It takes Venus over 224 days to orbit the Sun. Venus takes longer to complete one rotation than it does to orbit the Sun. That means a Venusian day is longer than a Venusian year!
Venus doesn't rotate fast enough to have any effect on its shape. Venus spins so slowly that the force of gravity at its equator is the same as at its poles. So, it doesn't have an equatorial bulge. Venus is almost a perfect sphere. Venus's axial tilt is 267¬°. With such a tilt, the planet's axis is nearly straight up and down to its orbital plane. Since Venus's equator is almost directly aligned with the plane of its orbit, the planet has almost no seasonal variation.
Moons and Rings
Venus does not have any moons or a ring system.
Magnetic Field
Unlike Earth, Venus has almost no magnetic field. This discovery was made by the Mariner 2 space probe. Earth has a magnetic field because it has an outer core made of liquid metal. This metal conducts electricity. As the Earth rotates, the liquid moves in the core. This creates an electric current, which in turn generates a magnetic field.
Venus also has a liquid metal outer core. Venus doesn't rotate fast enough to create much movement in the liquid of the core. Very little electric current is created. A stronger current is needed to produce a magnetic field.
Gravity
Venus is slightly smaller than Earth. Its mass is also slightly less. Its gravitational pull therefore is also slightly less than Earth's. On the surface of Venus, gravity is 0.88 times the gravity on Earth's surface.
