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Venus is a planet with striking surface characteristics. Most of what is known about its surface stems from radar observations, mainly images sent by the ''Magellan'' probe between August 16, 1990 and the end of its sixth orbital cycle in September 1994. Ninety-eight percent of the planet's surface was mapped, 22% of it in three-dimensional stereoscopic images. The surface of Venus is covered by a dense atmosphere and presents clear evidence of former violent volcanic activity. It has shield and composite volcanoes similar to those found on Earth. Relative to the Moon, Mars or Mercury, Venus has few small impact craters. This is likely a result of the planet's dense atmosphere, which burns up smaller meteors. Venus does have more medium-to-large-size craters, but still not as many as the Moon or Mercury. Some other unusual characteristics of the planet include features called ''coronae'' (Latin for crowns, based on their appearance), ''tesserae'' (large regions of highly deformed terrain, folded and fractured in two or three dimensions), and ''arachnoids'' (for those features resembling a spider's web). Long rivers of lava have been discovered, as well as evidence of Aeolian erosion and tectonic shifts which have played an essential role in making the surface of Venus as complex as it is today. Although Venus is the planet closest to Earth (some at inferior conjunction), and is similar in size, the resemblance is superficial: no probe has been able to survive more than one hour on its surface because the atmospheric pressure is some 90 times that of Earth's. The temperature on the surface is around . This is mostly caused by the greenhouse effect created by an atmosphere composed mainly of carbon dioxide (96.5%). Ultraviolet surveys of Venus show a Y-shaped pattern of cloud formation near the equator indicating that the upper layers of the atmosphere circulate around the planet once every four days, suggesting the presence of winds of up to . These winds exist at high altitudes, but the atmosphere at the surface is relatively calm, and most images from the surface reveal little evidence of wind erosion. ==Knowledge of the surface of Venus before ''Magellan''== (詳細はtelescope, optical observations of Venus became possible, although it soon became apparent that its surface is permanently hidden by dense cloud. In 1643, Francesco Fontana was the first of several astronomers claiming to see dark markings on these clouds, while others even said that they could see part of the surface through holes in the clouds. Astronomers also claimed to have seen brilliant points in certain spots on the disk of the planet, suggesting an enormous mountain whose top was higher than the clouds. The most famous such observations were made by Johann Hieronymus Schröter, a respected observer and collaborator of William Herschel, who reported several sightings from 1789 onwards of a bright circular point of light near the southern terminator of Venus, thought to be reflected light from a very tall mountain range or peak, around high. Herschel disputed these observations and held them to be attributable to imperfections in Schröter's telescope. Many other observers claimed to see irregularities in the terminator of Venus, and the debate continued into the 20th century until radar observations were able to penetrate the clouds and reveal that, in fact, no such giant mountains exist. After the Moon, Venus was the second object in the Solar System to be explored by radar from the Earth. The first studies were carried out in 1961 at NASA's Goldstone Observatory, part of the Deep Space Network. At successive inferior conjunctions, Venus was observed both by Goldstone and the National Astronomy and Ionosphere Center in Arecibo. These studies confirmed earlier measurements during transits of the meridian, which had revealed in 1963 that the rotation of Venus was retrograde (it rotates in the opposite direction to that in which it orbits the Sun). The radar observations also allowed astronomers to determine that the rotation period of Venus was 243.1 days, and that its axis of rotation was almost perpendicular to its orbital plane. It was also established that the radius of the planet was , some less than the best previous figure obtained with terrestrial telescopes. Interest in the geological characteristics of Venus was stimulated by the refinement of imaging techniques between 1970 and 1985. Early radar observations suggested merely that the surface of Venus was more compacted than the dusty surface of the Moon. The first radar images taken from the Earth showed very bright (radar-reflective) highlands, which were christened Alpha Regio, Beta Regio, and Maxwell Montes. Improvements in radar techniques later resulted in an image resolution of 1–2 kilometres. Since the beginning of the age of space exploration, Venus has been considered as a site for future landings. Launch windows occur every 19 months, and from 1962 to 1985 every window was used to launch reconnaissance probes. In 1962, ''Mariner 2'' flew over Venus, becoming the first man-made object to visit another planet. In 1965, ''Venera 3'' became the first space probe to actually land on another world, although it was a crash-landing. In 1967, ''Venera 4'' became the first probe to send data from the interior of Venus's atmosphere, while ''Mariner 5'' measured the strength of Venus's magnetic field at the same time. Finally, in 1970, ''Venera 7'' made the first controlled landing on Venus. In 1974, ''Mariner 10'' swung by Venus on its way to Mercury and took ultraviolet photographs of the clouds, revealing extraordinarily high wind speeds in the Venusian atmosphere. In 1975, ''Venera 9'' transmitted the first images of the surface of Venus and made gamma ray observations of rocks at the landing site. Later in that same year, ''Venera 10'' sent further images of the surface. In 1978, the ''Pioneer 12'' probe (also known as ''Pioneer Venus 1'' or ''Pioneer Venus Orbiter'') circled Venus and provided data for the first altimetry and gravity maps of the planet between 63 and 78 degrees of latitude. The altimetry data had an accuracy of 150 kilometers. That same year, ''Pioneer Venus 2'' launched four probes into Venus's atmosphere which determined, when combined with data from prior missions, that the surface temperature of the planet was approximately 460 °C (860 °F), and that the atmospheric pressure at the surface was 90 times that of Earth's, confirming earlier radar observations. In 1982, the Soviet ''Venera 13'' sent the first colour image of Venus's surface and analysed the X-ray fluorescence of an excavated soil sample. The probe operated for a record 127 minutes on the planet's hostile surface. Also in 1982, the ''Venera 14'' lander detected possible seismic activity in the planet's crust. In 1983, ''Venera 15'' and ''16'' 〔http://www.mentallandscape.com/V_RadarMapping.htm〕 acquired more precise radar images and altimetry data for the northern latitudes of the planet. This was the first use of synthetic aperture radar on Venus. The images had 1–2 kilometre (0.6–1.2 mile) resolution. The altimetry data obtained by the Venera missions had a resolution four times better than ''Pioneers. Venera 15 and 16 returned images of far higher quality than earth-based radar images, showing relief and texture absent from range-doppler imaging. From a highly eccentric polar orbit, the spacecraft recorded survey strips from the north pole down to 30 degrees latitude during a 16-minute pass. The remainder of the 24-hour orbit permitted the transmission of 8 megabytes of information. Venus rotates 1.48 degrees every 24 hours, allowing the entire polar cap to be scanned during the mission, from November 11, 1983 to July 10, 1984. This collection of radio holograms was processed into image strips and maps by SIMD math co-processors on a computer at the Institute of Radio Engineering and Electronics in Moscow. Most of the basic geomorphology of Venus was established based on data from Venera 15 and 16. Soviet geologists discovered that many objects previously identified as impact craters were actually unusual volcanic features. The features of coronas, arachnoids, tessera and genuine impact craters were identified for the first time. No evidence of plate tectonics was seen, and Soviet scientists argued with Americans about this until Magellan verified their theory, that the entire planet was missing any features indicating plate boundaries. The rarity of impact craters showed that the surface of Venus was surprisingly young, only about 100 million years old. This suggested intense volcanic activity and resurfacing. In 1985, during the euphoria caused by the return of Halley's Comet, the Soviet Union launched two ''Vega'' probes to Venus. ''Vega 1'' and ''2'' each sent an instrumented helium balloon to a height of above the surface, allowing scientists to study the dynamics of the most active part of Venus's atmosphere. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Geology of Venus」の詳細全文を読む スポンサード リンク
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