| Revision as of 16:23, 11 July 2005 editKeithlard (talk | contribs)379 edits →Magellan studies the geology of Venus: add JSC Magellan image← Previous edit | Revision as of 17:33, 11 July 2005 edit undoKeithlard (talk | contribs)379 edits →[]es: copyeditNext edit → | ||
| Line 77: | Line 77: | ||
| ==]es== | ==]es== | ||
| Flow of hot material from a planet's interior to its surface is the |
Flow of hot material from a planet's interior to its surface is the principal mode of heat loss. Internal geological heat comes from four processes: | ||
| *Energy from the original accretion of the planet or moon, | *Energy from the original accretion of the planet or moon, | ||
| *Heat produced by the disintegration of ] elements in the planet's interior, | *Heat produced by the disintegration of ] elements in the planet's interior, | ||
| Line 85: | Line 85: | ||
| On Earth, a combination of factors give rise to heat loss. In the case of some bodies, like ] moon ], gravitational forces from Jupiter and ] produce enormours tidal agitations, giving rise to the largest volcanoes in the ]. | On Earth, a combination of factors give rise to heat loss. In the case of some bodies, like ] moon ], gravitational forces from Jupiter and ] produce enormours tidal agitations, giving rise to the largest volcanoes in the ]. | ||
| Although Venus bears a strong resemblance to Earth, it seems that the ] so active in Earth's geology do not exist on Venus. Still, it is believed that 80% of the geographic features on its surface are the product of some type of volcanic process. | Although Venus bears a strong resemblance to Earth, it seems that the ] so active in Earth's geology do not exist on Venus. Still, it is believed that 80% of the geographic features on its surface are the product of some type of volcanic process. | ||
| Differences can be seen in volcanic deposits. In many cases, volcanic activity is localized to a fixed source, and deposits are |
Differences can be seen in volcanic deposits. In many cases, volcanic activity is localized to a fixed source, and deposits are found in the vicinity of this source. This kind of volcanism is called "centralized volcanism," in that volcanoes and other geographic features form distinct regions. The second type of volcanic activity is not radial or centralized; lava taps cover wide expanses of the surface. These eruptions are catalogued as "flow type" volcanoes. | ||
| The second type of volcanic activity is radial or centralized, but in this case lava taps cover wide expanses of the surface. These eruptions are catalogued as "fluid type". | |||
| <!-- this section needs a good bit of rewriting, based on a better source: http://history.nasa.gov/JPL-93-24/ch9.htm --> | |||
| Volcanoes less than 20 km in diameter are very abundant on Venus and they may number hundreds of thousands or even millions. Many appear as flattened domes or 'pancakes', thought to be formed in a similar way to ]es on Earth. These volcanoes are between 1 and 15 km of diameter and less than 1 km in height. It is common to find groups of hundreds of these volcanoes in areas called shield fields. | |||
| On Earth, volcanos are mainly of two types: ]es and ] or stratum-volcanos. The shield volcanoes, such as the ] volcanos, receive ] from the depths of the Earth in zones called '']s'' (''puntos calientes''). The type of ] from these volcanos is relatively fluid and permits the escape of gases. Composed volcanos, such as ] and ], are associated with tectonic plates. In this type of volcano, the water of the ] lowers next to the plate that slides in the ] under the Earth's ], and in this manner facilitates a better one ''derretimiento'' of the same one producing a gummier lava that complicates the exit of the gases, and for that reason, composed volcanos have violent eruptions. | On Earth, volcanos are mainly of two types: ]es and ] or stratum-volcanos. The shield volcanoes, such as the ] volcanos, receive ] from the depths of the Earth in zones called '']s'' (''puntos calientes''). The type of ] from these volcanos is relatively fluid and permits the escape of gases. Composed volcanos, such as ] and ], are associated with tectonic plates. In this type of volcano, the water of the ] lowers next to the plate that slides in the ] under the Earth's ], and in this manner facilitates a better one ''derretimiento'' of the same one producing a gummier lava that complicates the exit of the gases, and for that reason, composed volcanos have violent eruptions. | ||
Revision as of 17:33, 11 July 2005
Template:SpanTransWeek es:Geología de Venus
Venus has striking surface characteristics, which are as beautiful as they are unusual. The majority of what we know today about its surface stems from radar observations, mainly images sent by the Magellan probe from August 16, 1990 until the end of its sixth orbital cycle in September 1994. 98% of the of the planet's surface was mapped, of which 22% comprises of stereoscopic images.
The surface of Venus is covered by a dense atmosphere and presents clear evidence of violent volcanic activity in the past, including shield and composite volcanoes similar to those found on Earth.
Unlike the Moon, Mars or Mercury, which have suffered an intense period of craterization, Venus has relatively few small impact craters, but does have more medium to large craters. This is as a result of the planet's dense atmosphere burning up smaller meteorites as they enter into it.
Some other unusual characteristics of the planet include features called coronae (Latin for crowns, based on their appearance), and other figures known as arachnoids, due to their resemblance to arachnids. Long rivers of lava have been discovered, as well as evidence of Eolian erosion and tectonic shifts which have played an essential role in making the surface of Venus as complex as it is today.
Despite the fact that Venus is the planet closest to Earth (some 40 million kilometres at inferior conjunction), and is in many ways similar, the resemblance is superficial: no probe has been able to survive more than a few hours on its surface because the atmospheric pressure is some 90 times that of the Earth's. The temperature on the surface is around 450°C (842°F). This is mostly caused by the greenhouse effect created by an atmosphere composed mainly of carbon dioxide (96.5%).
The observations of probes and telescopes show a Y-shaped pattern of cloud formation near the equator indicating that the upper layers of the amosphere circulate around the planet once every four days, suggesting the presence of winds of up to 500 km/h (311 mph). This is believed to be an important factor in shaping the terrain of Venus.
Knowledge of the surface of Venus before Magellan
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 miles), some 70 kilometres (43 miles) 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 (radio-reflective) regions christened Alpha, Beta, and Maxwell; 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 reconaissance 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 and 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 100 metres (328 feet).
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 1981, the Soviet Venera 13 sent the first color image of Venus's surface and analyzed 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.
In 1983, the Venera 15 and 16 probes further advanced the work of the Pioneer Venus Orbiter, acquiring more precise radar images and altimetry data for the northern latitudes of the planet. The images had a 1-2 kilometer (0.6-1.2 mile) resolution, comparable to those obtained by the best Earth radars. The altimetry data obtained by the Venera missions had a resolution four times better than Pioneer's. 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 miles) above the surface, allowing scientists to study the dynamics of the most active part of Venus's atmosphere.
All of these probes gathered data critical for the success of the Magellan probe, which made the most detailed investigations of Venus's geology.
Magellan studies the geology of Venus
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 miles) wide and 70,000 kilometres (43,496 miles) 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 miles). 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.
Surface characteristics
With the invention of the telescope, Venus became the object of more interesting optic observations. In the past, many astronomers had claimed to see dark marks in the layer of clouds that wraps around Venus, while others even said that they could see part of the surface through holes in the clouds. Another of these assertions is that many astronomers 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. Such is the case of J.H. Schroeter, a respected observer and collaborator of William Herschel, who reported the sights in 1788 and 1790. The description of his report said that it was a matter of a prominent mountain located in the terminator that separates the light hemisphere from the dark one. Despite the controversy, this observation was often cited.
The reality is something else: the surface of Venus is very flat. After 93% of the topography was mapped by a probe, Pioneer Venus, scientists found that the total distance from the lowest point to the highest point on the entire surface was about 13 km (8 miles), while on the Earth the distance from the basins to the Himalayas is about 20 km (12.4 miles).
According to the data of the altimeters of the Pioneer, nearly 51% of the surface is found located inside the 500 meters (1640 feet) of the medium radius of 6,051.9 km (3760 miles); only 2% of the surface is located at greater elevations than the 2 km. (1.2 miles) on the medium radius.
The altimetry experiment of Magellan confirmed the general character of the landscape. According the Magellan data, 80% of the topography is within 1 km (0.6 miles) of the median radius. The most important elevations are in the mountain chains that surround Lakshmi Planum: Maxwell Mount (11 km, 6.8 miles), Akna Mount (7 km, 4.3 miles) and Freyja Mount (7 km, 4.3 miles). Despite the relatively planar landscape of Venus, the altimetry data also found large inclined plains. Such is the case on the southwest side of the Maxwell Mount, which in some parts seems to be inclined some 45°. Inclinations of 30° were registered in Danu Mounts and the Thetis Royal region.
Divisions of Venus
Based on the altimeter data of the Pioneer Venus probe, the topography of the planet is divided into three topographical provinces: lowlands, deposition plains, and highlands. The Magellan data support these divisions. The most important provinces of the highlands are Aphrodite Terra, Ishtar Terra, and Lada Terra, as well as the regions of Beta, Phoebe and Themis. The regions Alpha, Bell, Eistla and Telhus form a less important group of highlands.
Impact craters
Earth-based radar studies made it possible to identify some topographic patterns related to craters, and the soundings Venera 15 and 16 identified almost 150 such features of probable impact origin. Global coverage from Magellan subsequently made it possible to identify nearly 900 impact craters. This is a very low number, considering the size of the planet's surface. The difference in this figure, with respect to Mercury, the moon and Mars (as well as several of the outer planets' moons) is that tectonic processes on Venus have given it a dense atmosphere which has filtered meteorites, eliminating the smallest.
The Venera and Magellan data agree: there are very few impact craters with a diameter less than 30 km, and data from Magellan show an absence of any craters less than 2 km in diameter. The craters of Venus present several peculiarities: in the first place, they seem to be relatively new, and do not seem to have suffered deterioration from subsequent meteor impacts. The impact craters also display great lava taps of high radar reflectivity, which demonstrates that they are young.
Analysis of impact crater images, accounting for—among other characteristics—crater superposition, distribution and density in the surface, is an important tool for understanding the geological history of the planet.
Volcanoes
Flow of hot material from a planet's interior to its surface is the principal mode of heat loss. Internal geological heat comes from four processes:
- Energy from the original accretion of the planet or moon,
- Heat produced by the disintegration of radioactive elements in the planet's interior,
- Heat from internal movements within the planet, and
- Heat produced by tidal interactions with the mass of nearby bodies.
On Earth, a combination of factors give rise to heat loss. In the case of some bodies, like Jupiter's moon Io, gravitational forces from Jupiter and Europa produce enormours tidal agitations, giving rise to the largest volcanoes in the solar system.
Although Venus bears a strong resemblance to Earth, it seems that the tectonic plates so active in Earth's geology do not exist on Venus. Still, it is believed that 80% of the geographic features on its surface are the product of some type of volcanic process.
Differences can be seen in volcanic deposits. In many cases, volcanic activity is localized to a fixed source, and deposits are found in the vicinity of this source. This kind of volcanism is called "centralized volcanism," in that volcanoes and other geographic features form distinct regions. The second type of volcanic activity is not radial or centralized; lava taps cover wide expanses of the surface. These eruptions are catalogued as "flow type" volcanoes.
Volcanoes less than 20 km in diameter are very abundant on Venus and they may number hundreds of thousands or even millions. Many appear as flattened domes or 'pancakes', thought to be formed in a similar way to shield volcanoes on Earth. These volcanoes are between 1 and 15 km of diameter and less than 1 km in height. It is common to find groups of hundreds of these volcanoes in areas called shield fields.
On Earth, volcanos are mainly of two types: shield volcanoes and cones or stratum-volcanos. The shield volcanoes, such as the hawaiian volcanos, receive magma from the depths of the Earth in zones called hot spots (puntos calientes). The type of lava from these volcanos is relatively fluid and permits the escape of gases. Composed volcanos, such as Mount Saint Helens and Mount Pinatubo, are associated with tectonic plates. In this type of volcano, the water of the oceanic bark lowers next to the plate that slides in the zone of subduction under the Earth's crust, and in this manner facilitates a better one derretimiento of the same one producing a gummier lava that complicates the exit of the gases, and for that reason, composed volcanos have violent eruptions.
In Venus, the morphology (with large and thin strained of lava), apparent absence of tectonic of plates and water they do that the volcanos seem to those of Hawaii. Nevertheless, the size of the volcanos of Venus is different: in the Land the volcanos in shield can have scores of kilometers of wide and only to 8 km of height (Mauna Praises, if their base located in the marine bed is considered), in Venus, the amplitude of these volcanos comes cover hundreds of kilometers but they are quite flat, with a height average of 1,5 km.
Los domos de Venus son entre 10 y 100 veces más amplios que los terrestres y en el radar exhiben fracturas que indican que han sido formadas por la tensión creada por el ascenso de lava viscosa en el interior de la corteza o por el hundimiento de la cámara magmática. Estos domos masivos reciben el nombre informal de “domos panqueques” y por lo general están asociados con coronae y tesserae. Su presencia es una fuerte evidencia de la existencia de lavas de composición química evolucionada (Figura 9 y 11).
Otras características únicas son la existencia de novas ("novae") y aracnoides.
La formación de las novas se da cuando grandes cantidades de magma llegan hasta la superficie sin erupcionar formando diques extrusivos que son brillantes a las imágenes de radar. Estos diques se organizan en lineamientos simétricos que denotan el área de levantamiento, aunque también se puede producir una depresión causada por la subsidencia del material magmático. Si tales lineamientos, sean éstos grabens o crestas, se irradian desde un punto central reciben el nombre de novas (nombre usado para enfatizar la semejanza a la figura de una estrella que ha explotado). Se han identificado cerca de 50 de estas estructuras.
Cuando los lineamientos de grabens, fisuras, y crestas se extienden varios radios desde la circunferencia de un punto central reciben el nombre de aracnoides, indicativo de su apariencia. Se identificaron cerca de 250 aracnoides.
Tectonic activity
Despite the fact that Venus shows no evidence of it currently, the planet's surface shows various geographic patterns associated with plate tectonic activity. Features such as faults, folds, volcanoes, large mountains and rift valleys are indicative of crust plates that move over the planet's molten interior.
The active volcanism of Venus has generated chains of folded mountains, rift valleys and terrain that is known as tesserae, a word meaning "floor tiles" in Greek. Tesserae exhibit the effects of eons of compression and tensional deformation.
Unlike on earth, the deformations on Venus are directly related to the dynamic forces within the planet's mantle. Gravitationnal studies suggest venus lacks an asthenosphere-- a layer of lower viscosity that that facilitates the movement of tectonic plates. The absence of this layer suggest that the deformation of Venus' surface can be interpreted in terms of convective movements within the plante.
La deformación tectónica sobre Venus se evidencia en una variedad de escalas, las más pequeñas que han sido identificadas están relacionadas con fracturas lineales o fallas. En muchas zonas estas fallas están presentan un alineamiento paralelo en forma de red. También se encuentran pequeñas crestas montañosas discontinuas parecidas a las encontradas en la Luna y Marte. La presencia de tectónica extensiva manifiesta la existencia de fallas normales (donde la roca sobre el plano de la falla se hunde respecto a la roca sobre la misma) y fracturas superficiales. Las imágenes de radar muestran que este tipo de deformación por lo general está concentrada en cinturones ubicados en zonas ecuatoriales y de altas latitudes en el sur del planeta. Estas zonas abarcan cientos de kilómetros de ancho y parecen estar enlazadas por todo el planeta formando una estructura global asociada con la aparición de volcanes.
Los rifts venusianos, formados por la extensión de la litosfera son depresiones de decenas a cientos de metros de ancho y con extensiones de hasta 1.000 km como algunos de la Tierra. Los rifts en Venus por lo general van asociados con grandes elevaciones volcánicas con forma de domos como en Beta Regio, Atla Regio y la parte occidental de Eistla Regio. Estas tierras altas parecen ser el resultado de enormes plumas (corrientes de elevación) del manto que han causado la elevación, fracturas, creación de fallas y vulcanismo.
La cadena montañosa más alta de Venus, Maxwell Montes en Ishtar Terra, fue formada por un proceso de compresión, extensión y movimientos laterales. Otro tipo de accidente geográfico encontrado en las tierras bajas, consiste en cinturones lineales ubicados a distancias muy próximas que se elevan a varios kilómetros sobre la superficie con amplitudes de cientos de kilómetros y longitudes de miles de kilómetros. Existen dos concentraciones importantes de estos cinturones: uno se ubica en Lavinia Planitia en altas latitudes del hemisferio sur, y el segundo se encuentra adyacente a Atalanta Planitia en las altas latitudes del hemisferio norte.
Los tesserae, que son terrenos de complejas crestas, se encuentran fundamentalmente en Aphrodite Terra, Alpha Regio, Tellus Regio y la parte oriental de Ishtar Terra (Fortuna). Estas regiones contienen la superposición y cortes de grabens de diferentes unidades geológicas lo que significa que son las partes más antiguas del planeta.
Algunos científicos creen que los tesserae pueden ser análogos a los continentes terrestres. Otros suponen que son regiones producidas por un manto en movimiento descendiente que provocó las fracturas y plegamientos para formar una espesa corteza basáltica o sitios de antiguas plumas del manto que crearon grandes volúmenes de lava sobre la superficie de Venus.
Magnetic field
Planets' magnetic fields are formed by cores of ferrous liquid by the rotational movements produced by their melting.
Even though Venus has a core of iron, the planet does not have a detectable magnetic field. One of the reasons for this could be the planet's peculiar rotational movement. Venus's very slow rotation (some 243 Earth days) is probably the reason for the absence of the field; there is no other explanation.
Lava flows and canals
A diferencia de las coladas de lava terrestres, en Venus éstas se producen a una escala superior. Los flujos de lava venusianos alcanzan con frecuencia longitudes de cientos de kilómetros y incluso más de 1.000 en su longitud total. La amplitud de estos flujos puede alcanzar de unos pocos hasta algunas decenas kilómetros.
Todavía no se sabe por qué las coladas de lava en Venus son tan grandes. Las elevadas temperaturas de 475°C que reinan en Venus disminuyen la velocidad de enfriamiento de las lavas, pero no lo suficiente para presentar semejante diferencia en longitud con respecto a las coladas terrestres.
Las coladas de lava en Venus parecen ser en su mayoría de composición basáltica, por lo tanto, relativamente más fluidas. Dentro de las lavas basálticas, en la Tierra se conocen de dos tipos: lava aa y lava cordada. La lava aa presenta una textura rugosa en forma de pequeños bloques fragmentados. La lava cordada, como su nombre lo indica, se presenta como una capa de cuerdas o de estilo almohadillado.
La rugosidad del terreno se representa en el brillo de las imágenes de radar (las superficies más suaves son más oscuras) y sirven para determinar las diferencias de las lavas aa y las cordadas. Estas variaciones también pueden reflejar las diferencias en edad y el estado de preservación. Los canales y los tubos de lava (canales que se han enfriado y se ha creado un techo encima) son muy comunes en Venus.
La mayor parte de los campos de flujos están asociados a volcanes. Los volcanes centrales están rodeados por extensas coladas que forman el edificio del volcán. Por otra parte, también se relacionan a cráteres de fisura, coronas, densos cúmulos de domos volcánicos, conos, pozos y canales.
Gracias a Magellan se identificaron más de 200 canales y complejos de valles. Los canales fueron clasificados como canales simples, canales complejos o canales compuestos.
Los canales simples se caracterizan por estar formados por un largo y único canal principal. La categoría incluye a los rilles similares a los observados en la Luna, y un nuevo tipo llamado canali, que corresponden a canales individuales muy largos que mantienen su amplitud a lo largo de su longitud. El canali más largo que ha sido identificado tiene una longitud de más de 7.000 km.
Los canales complejos incluyen redes anastomosadas además de redes de distribución. Este tipo de canales ha sido observado en asociación con varios cráteres de impacto y en importantes inundaciones de lava asociadas a campos de flujo muy importantes.
Los canales compuestos están constituidos por segmentos simples y complejos. El mayor de estos canales presenta una red anastomosada y colinas modificadas como las presentes en Marte.
A pesar de los innumerables cráteres encontrados en su superficie, no se han encontrado indicios de que el agua fuera el origen de éstos. De hecho no hay evidencia de que el agua fuera estable en los últimos 600 millones de años en la atmósfera y superficie de Venus que tiene entre 200 y 600 millones de años.
Con respecto a la formación de los espectaculares canales, hay dos candidatos: lava y los fluidos de los deyectos de impacto. Las características de estas corrientes de lava son muy inusuales, tal vez la caliente superficie de Venus ayude a la erosión térmica. Por otra parte, es probable que existan fluidos de lava con muy baja viscosidad como basaltos con un alto contenido de hierro y magnesio o incluso lavas de sulfuro o carbonato. La interacción de los deyectos de impacto han creado grandes fluidos que se extienden por cientos de kilómetros y tienen morfologías típicas de canales.
Surface processes
Water does not exist on Venus, and thus the only erosive process to expect is the interaction produced by the atmosphere with the surface. This interaction is made present in the ejecta of impact craters, which has been expelled along the surface of Venus. The material ejected during meteorite impact is lifted to the upper atmosphere, where winds transport the material toward the west. As the material is deposited on the surface, the material forms parabolic-shaped patterns. This type of deposit can be established on top of various geologic features or lava flows. Therefore, these deposits are the youngest structures on the planet. Images from Magellan reveal the existence of more than 60 of these parabolic-shaped deposits that are associated with crater impacts.
The ejection material, transported by the wind, is responsible for the process of renovation of the surface at speeds, according to the measurements of the Venera soundings, of approximately one meter per second. Given the density of Venus' lower atmosphere, the winds are more than sufficient to provoke the erosion of the surface and the transportation of fine-grained material. In the regions covered by ejection deposits one may find wind lines, dunes, and yardangs. The wind lines are formed when the wind blows ejection material and volcano ash, depositing it on top of topographic obstacles such as domes. As a consequence, the leeward side of domes are exposed to the impact of small grains that remove the surface cap. Such processes expose the material beneath, which has a different roughness, and thus different characteristics under radar, compared to formed sediment.
The dunes are formed by the depositing of particulates that are the size of grains of sand and have wavy shapes. Yardangs are formed when the wind-transported material carves the fragile deposits and produces deep furrows.
The line-shaped patterns of wind associated with impact craters follow a trajectory in the direction of the equator. This tendency suggests the presence of a system of circulation of Hadley cells between the medium latitudes and the equator. Magellan radar data confirm the existence of strong winds that blow toward the east in the upper surface of Venus, and meridional winds on the surface.
Meteor impacts on Venus have occurred for the last hundreds of millions of years. The superposition of lava flows can be noted. The oldest lava flows, covered by the newest flows, present distinct intensities on radar reflection. The oldest flows reflect less than the plains that surround the flows. Data from Magellan show that the most recent flows are similar to aa and pāhoehoe. However, the oldest lava flows are darker and look like deposits in arid regions of the Earth that have suffered meteor impacts.
Chemical and mechanical erosion of the old lava flows is caused by reactions of the surface with the atmosphere under the presence of carbon dioxide and sulphur dioxide. These two gases are the first and third most abundant gases, respectively; the second most abundant gas is inert nitrogen. The reactions probably include the deterioration of silicates by carbon dioxide to produce carbonates and quartz, as well as the deterioration of silicates by sulphur dioxide to produce anhydrate calcium sulphate and carbon dioxide.
One of the most interesting characteristics of radar images is the diminishing of reflection at high altitudes, exhibiting values extremely low beyond a radius of 6,054 kilometers. This change is related to the diminishing of emission, and is related to the diminishing of temperature at high altitudes.
There are various hypotheses for the unusual characteristics of Venus' surface. One idea is that the surface consists of loose ground with spherical hollows that that produce an efficient reflection of radar. Another idea is that the surface is not smooth and is covered by material that has an extremely high dielectric constant. Yet another theory says that la capa one meter above the surface is formed by sheets of a conductive material such as pyrite. Last, a recent model supposes the existence of a small proportion of ferroelectric mineral.
Ferroelectric minerals exhibit a unique property at high temperatures: the dielectric constant increases abruptly, yet as the temperature increases further, the dielectric constant returns to its normal values. The minerals that could explain this behavior on the surface of Venus are perovskite and pyrochlores.
Despite these theories, the existence of ferroelectric minerals on Venus has not been confirmed. Only in situ exploration will permit the explanation of such unresolved enigmas.
See also
References
- This article draws heavily on the corresponding article in the Spanish-language Misplaced Pages, which was accessed in the version of July 9, 2005.
Publications
- The Face of Venus. The Magellan Radar Mapping Mission, by Ladislav E. Roth and Stephen D. Wall. NASA Special Publication, Washington, D.C. June 1995 (SP-520).
- "Estrella del atardecer", El Universo, Enciclopedia de la Astronomía y el Espacio, Editorial Planeta-De Agostini, págs. 161-167. Tomo 1 (1997).
- Ciencias de la Tierra. Una Introducción a la Geología Física, de Edward J. Tarbuck y Frederick K. Lutgens. Prentice Hall (1999).
- Fotografías:
Related books
- Surface Modification on Venus as Inferred from Magellan Observations on Plains, de R. E. Ardvison, R. Greeley, M. C. Malin, R. S. Saunders, N. R. Izenberg, J. J. Plaut, E. R. Stofan, y M. K. Shepard. Geophisics Research 97, 13.303. (1992)
- The Magellan Imaging Radar Mission to Venus, de W. T. K. Johnson. Proc. IEEE 79, 777. (1991)
- Planetary Landscapes, 3rd Edition, de R. Greeley. Chapman & Hall. (1994)
External links
- Past missions - Mariner 10
- The Voyage of Mariner 10
- Magellan mission to Venus
- Online resources of the Magellan mission to Venus
- Guide for the interpretation of the images taken by Magellan