 Venus is the closest planet to earth, and about the same size and mass. It is covered in clouds, which makes it shine brightly in the evening sky, but these clouds have hidden its surface from our telescopes until relatively recently.The surface of Venus is very different from that of the Earth. It has no oceans, and is surrounded by a heavy atmosphere composed mainly of carbon dioxide, with virtually no water vapor. Its clouds are composed of sulfuric acid droplets. At ground level, the atmospheric pressure is about 92 times that of the Earth's at sea-level! Venus is very hot. Its surface temperature of about 482° C (900° F) is mostly due to a strong greenhouse effect caused by the thick atmosphere of carbon dioxide. Sunlight passes through the atmosphere to heat the surface of the planet, but heat from the ground cannot radiate out of the thick atmosphere until it has sufficient energy, an energy that makes the temperature high. Venus rotates on its axis very slowly. A Venusian day is 243 Earth days, and is longer than its year of 225 days. Oddly, Venus rotates from east to west. To a person standing on the planet, the sun would rise in the west and set in the east. Many of our discoveries about Venus have happened in the last 50 years or so, after we learned how to see through the clouds with radar, and were able to send unmanned spacecraft to the planet to scan its surface. More than twenty spacecraft have visited Venus, including the Galileo craft while on its way to Jupiter. Four of the most successful missions in revealing the Venusian surface were NASA's Pioneer Venus mission (1978), the Soviet Union's Venera missions (1982-1984), and NASA's Magellan radar mapping mission (1990-1994).  This image is part of the first color panoramic view from the surface of Venus. It was transmitted by a TV camera on the Soviet Venera 13 lander, which parachuted to the surface on March 1, 1982. Despite the harsh conditions, the Venera 13 lander survived long enough to send back pictures and perform an analysis of the soil. Part of the lander itself is visible in the lower right portion of the image on the right. An earlier Soviet Venus lander, Venera 7 (1970), was the first spacecraft to return data from the surface of another planet.
The Magellan spacecraft was launched on May 4, 1989, arriving at Venus in late 1990. It was inserted into a polar orbit (north-south rather than east-west), dipping as low as 300 km above the surface on each pass around the planet. Before radio contact with Magellan was lost in 1994, almost 98% of the surface was photographed at resolutions better than 100 m, as the planet turned beneath the orbiting craft. Topographic maps were made from the data received.Magellan found many interesting surface features, including the large circular lava domes, typically 25-kilometers across, that are depicted below. Volcanism is thought to have created the domes, although the precise mechanism remains unknown. 
The Venusian surface consists of vast plains covered by lava flows, and mountain regions and highlands deformed by geological activity. Magellan pictures of highland regions above 2.5 kilometers are very bright, which is a characteristic of moist soil. However, liquid water does not exist on the surface, so this can't be the explanation. One theory is that the bright areas might be metallic iron pyrite (also know as "fools gold"), which could exist in stable form at higher altitudes. Venus is covered by many impact craters, volcanos, and volcanic features, including the domes shown here. At least 85% of the surface is covered with volcanic rock. Hugh lava flows, extending for hundreds of kilometers, have flooded the lowlands, creating vast plains. More than 100,000 small volcanos and hundreds of large ones cover the surface. Lava flows extend for hundreds, sometimes thousands, of kilometres. |