Knowledge of the surface of Venus before Magellan

Tweet

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. The studies carried out were similar to the earlier measurement of 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 6,052 kilometres (3761 mi), some 70 kilometres (43 mi) 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 christened Alpha Regio, Beta Regio, and Maxwell Montes; improvements in radar techniques later achieved an image resolution of 1–2 kilometres. Since the beginning of the age of space exploration, Venus had been considered as a site for future landings. Launch windows occur every 19 months, and from 1962 to 1985, every window was utilized 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, albeit 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 the 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 would send further images of the surface In 1978, the Pioneer 12 probe (also known as Pioneer Venus 1 or Pioneer Venus Orbiter ) circled Venus and completed 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, confirming earlier radar observations. In 1983, the Venera 15 and 16 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 a 1–2 kilometre (0.6–1.2 mile) resolution. The altimetry data obtained by the Venera missions had a resolution four times better than Pioneer's. Venera-15 and 16 returned images of far greater 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 remander of the 24-hour orbit permitted the transmission of the 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 were processed into image strips and maps by SIMD math co-processors on a computer at the Institute of Radio Engineering and Electronics in Moscow. From Venera-15 and 16, most of the basic geomorphology of Venus were discovered. Soviet geologists discovered that many objects identified as meteor crators previously were actually unusual volcanic features. The features of Coronas, Arachnoids, Tessera and genuine meteroite 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 of plate boundaries. The rarity of meteorite 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 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 50 kilometres (31 mi) above the surface, allowing scientists to study the dynamics of the most active part of Venus's atmosphere. In 1981, 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 1981, the Venera 14 lander detected possible seismic activity in the planet's crust. Launched May 4, 1989 aboard the space shuttle Atlantis, the Magellan probe was first placed into low Earth orbit, before firing its upper stage motor to send it on a trajectory toward Venus. On August 10, Magellan arrived at Venus and began to take images with radar. Each day it made 7.3 Venus orbits, imaging a strip 17–28 kilometres (11–17 mi) wide and 70,000 kilometres (43,496 mi) long. Covering the whole planet required 1,800 strips, which were combined into a single mosaic image. The first images of Venus were received on August 16, 1990, and routine mapping operations began on September 15, 1990. The first mapping cycle (Cycle 1) lasted 243 terrestrial days—the time it takes Venus to rotate on its own axis under the probe's orbital plane. Cycle 1 was completed successfully on May 15, 1991, mapping 84% of the Venusian surface. Cycle 2 began immediately afterwards and lasted until January 15, 1992. In each cycle, the probe was inclined at a different "look angle", producing stereoscopic data which enabled scientists to compile a three-dimensional map of the surface—a technique known as synthetic aperture radar. Cycle 3 was due to finish on September 14, 1992, but was terminated a day early due to problems with onboard equipment. In total, radar coverage of 98% of the surface of Venus was obtained, with 22% of the images in stereo. Magellan produced surface images of unprecedented clarity and coverage, which are still unsurpassed. Cycles 4, 5 and 6 were devoted to collecting gravimetric data, for which Magellan was aerobraked to its lowest possible stable orbit, with a periapsis or closest approach of 180 kilometres (112 mi). At the end of Cycle 6 its orbit was reduced further, entering the outer reaches of the atmosphere. After carrying out a few final experiments, Magellan successfully completed its mission on October 11, 1994, and was de-orbited to burn up in Venus's atmosphere. Topography-----------: With the invention of the 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 43 kilometres (28 mi) 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 (James, 2003). The reality is quite different: the surface of Venus is comparatively very flat. When 93% of the topography was mapped by Pioneer Venus, scientists found that the total distance from the lowest point to the highest point on the entire surface was about 13 kilometres (8 mi), while on the Earth the distance from the basins to the Himalayas is about 20 kilometres (12.4 mi). According to the data of the altimeters of the Pioneer, nearly 51% of the surface is found located within 500 metres (1640 feet) of the median radius of 6,052 km (3760 mi); only 2% of the surface is located at greater elevations than 2 kilometres ( 1.2 mi) from the median radius. The altimetry experiment of Magellan confirmed the general character of the landscape. According the Magellan data, 80% of the topography is within 1 kilometre (0.6 mi) of the median radius. The most important elevations are in the mountain chains that surround Lakshmi Planum: Maxwell Montes (11 km, 6.8 mi), Akna Montes (7 km, 4.3 mi) and Freya Montes (7 km, 4.3 mi). Despite the relatively flat landscape of Venus, the altimetry data also found large inclined plains. Such is the case on the southwest side of Maxwell Montes, which in some parts seems to be incli