Although Venus is Earth's closest neighbor, very little is known about the planet because it is permanently covered by thick clouds. In 1962, Soviet and American space probes, coupled with Earth-based radar and infrared spectroscopy, began slowly unraveling some of the mystery surrounding Venus. Twenty-eight years later, the Magellan spacecraft, sent by the United States, arrived at Venus in Aug. 1990 and began radar-mapping the planet's surface in greater detail.
According to the latest findings, Venus's atmosphere exerts a pressure at the surface 94.5 times greater than Earth's. Walking on Venus would be as difficult as walking a half-mile beneath the ocean. Because of a thick blanket of carbon dioxide, a “greenhouse effect” exists on Venus. Venus intercepts twice as much of the Sun's light as does Earth. The light enters freely through the carbon dioxide gas and is changed to heat radiation in molecular collisions. But carbon dioxide prevents the heat from escaping. Consequently, the temperature of the surface of Venus is over 800°F (427°C), hot enough to melt lead.
The atmospheric composition of Venus is about 96% carbon dioxide, 4% nitrogen, and minor amounts of water, oxygen, and sulfur compounds. There are at least four distinct cloud and haze layers that exist at different altitudes above the planet's surface. The haze layers contain small aerosol particles, possibly droplets of sulfuric acid. A concentration of sulfur dioxide above the cloud tops has been observed to be decreasing since 1978. The source of sulfur dioxide at this altitude is unknown; it may be injected by volcanic explosions or atmospheric overturning.
Measurements of the Venusian atmosphere and its cloud patterns reveal nearly constant high-speed zonal winds, about 220 mph (100 meters per second) at the equator. The winds decrease toward the poles so that the atmosphere at cloud-top level rotates almost like a solid body. The wind speeds at the equator correspond to Venus's rotation period of four to five days at most latitudes. The circulation is always in the same direction—east to west—as Venus's slow retrograde motion. Earth's winds blow from west to east, the same direction as its rotation.
Venus is round, very different from the other planets and from the Moon. Venus has neither polar flattening nor an equatorial bulge. The diameter of Venus is 7,519 mi (12,100 km). Venus has a retrograde axial rotation period of 243.1 Earth days. The surface atmospheric pressure is 1,396 psi (95 Earth atmospheres). The planet's mean distance from the Sun is 67.2 million miles (108.2 million kilometers). The period of its revolution around the Sun is 224.7 days.
The highest point on Venus is the summit of Maxwell Montes, 6.71 mi (10.8 km) above the mean level, more than a mile higher than Mount Everest. There is some evidence that this huge mountain is an active volcano. The lowest point is in the rift valley, Diana Chasma, 1.8 mi (2.9 km) below the mean level. This point is about one-fifth the greatest depth on Earth, in the Marianas Trench.
Venus has an extreme lowland basin, Atalanta Planitia, which is about the size of Earth's North Atlantic Ocean basin. The smooth surface of the Atalanta Planitia resembles the mare basins of the Moon.
There are only two highland, or continental, masses on Venus: Ishtar Terra and Aphrodite Terra. Ishtar Terra is 6.8 mi (11 km) at its highest points (the highest peaks on Venus) and those of Aphrodite Terra rise to about 3.10 mi (5 km) above the planet. Ishtar Terra is about the size of the continental United States and Aphrodite Terra is about the size of Africa.
The unmanned NASA spacecraft Magellan was launched on May 4, 1989, from the shuttle Atlantis and arrived at Venus Aug. 10, 1990, to map most of the planet. Despite some problems with its radio transmissions, the results of the radar mapping delighted scientists and provided them with the sharpest images ever taken of the planet's surface. Images taken from Magellan show ten times more detail than ever seen before.
The radar images provided scientists with compelling evidence that the planet has been dominated by volcanism on a global scale. The photos also showed that the planet's second-highest mountain, Maat Mons, rising 5 mi (8 km) above the Venusian plains, appears to be covered with fresh lava and is possibly an active volcano.
Magellan discovered the longest known channel in the solar system on Venus. It is 4,200 mi (6,800 km) long and averages slightly over a mile (1.8 km) wide. Its origin is puzzling to scientists because high-temperature lava is unlikely to have caused such a long-distance flow on the surface, and there are no known substances that could remain liquid long enough under the planet's atmospheric pressure and temperature to have carved out this snakelike feature. The channel is slightly longer than the Nile River, the longest river on Earth.
Magellan ended its radar and emissions mapping in Sept. 1992 after covering 98% of the planet's surface. The spacecraft continued to gather data until Oct. 1994, when it was intentionally crashed into the planet's surface. The European Space Agency (ESA) launched the Venus Express in Nov. 2005 to study the atmosphere and subsurface.
Venus is the brightest of all the planets and is often visible in the morning or evening, when it is frequently referred to as the Morning Star or Evening Star. At its brightest, it can sometimes be seen in full daylight with the naked eye, if one knows where to look.
A rare transit of Venus took place on June 8, 2004. This is when Venus crosses in front of the Sun and can be seen from Earth. Transits occur in pairs eight years apart (June 6, 2012, is the date of the second transit in the current pair), and then the next duo of transits happens either 1051/2 or 1211/2 years later. The next pair of transits will begin on Dec. 11, 2117.