Geology of Venus - AbsoluteAstronomy.com

Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

is a planet with striking surface characteristics. Most of what is known about its surface stems from radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

observations, mainly images sent by the Magellan

Magellan probe

The Magellan spacecraft, also referred to as the Venus Radar Mapper, was a 1,035-kilogram robotic space probe launched by NASA on May 4, 1989, to map the surface of Venus using Synthetic Aperture Radar and measure the planetary gravity...

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

Atmosphere of Venus

The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K , while the pressure is 93 bar. The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible...

and presents clear evidence of former violent volcanic activity. It has shield

Shield volcano

A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...

and composite volcanoes similar to those found on Earth.

Relative to the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, Mars

Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

or Mercury

Mercury (planet)

Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, Venus has few small impact crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s. This is likely a result of the planet's dense atmosphere, which burns up smaller meteor

METEOR

METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...

s. 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

Corona (planetary geology)

In planetary geology, a corona is an oval-shaped feature. Coronae appear on both the planet Venus and Uranus's moon Miranda and may be formed by upwellings of warm material below the surface.-Coronae on Venus:...

(Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

for crown

Crown (headgear)

A crown is the traditional symbolic form of headgear worn by a monarch or by a deity, for whom the crown traditionally represents power, legitimacy, immortality, righteousness, victory, triumph, resurrection, honour and glory of life after death. In art, the crown may be shown being offered to...

s, based on their appearance), tesserae (large regions of highly deformed terrain, folded and fractured in two or three dimensions), and arachnoids

Arachnoid (astrogeology)

In astrogeology, an arachnoid is a large structure of unknown origin, and they have been found only on the surface of the planet Venus. Arachnoids get their name from their resemblance to spider webs. They appear as concentric ovals surrounded by a complex network of fractures, and can span 200...

(for those features resembling a spider's web). Long rivers of lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

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

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

(some 40 million kilometres 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 450°C (842°F). This is mostly caused by the greenhouse effect

Greenhouse effect

The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

created by an atmosphere composed mainly of carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

(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 500 km/h (311 mph). 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

After the Moon, Venus was the second object in the solar system

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

to be explored by radar from the Earth. The first studies were carried out in 1961 at NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Goldstone Observatory

Goldstone Deep Space Communications Complex

The Goldstone Deep Space Communications Complex — commonly called the Goldstone Observatory — is located in California's Mojave Desert. Operated by ITT Corporation for the Jet Propulsion Laboratory, its main purpose is to track and communicate with space missions. It includes the Pioneer...

, part of the Deep Space Network

Deep Space Network

The Deep Space Network, or DSN, is a world-wide network of large antennas and communication facilities that supports interplanetary spacecraft missions. It also performs radio and radar astronomy observations for the exploration of the solar system and the universe, and supports selected...

. At successive inferior conjunctions, Venus was observed both by Goldstone and the National Astronomy and Ionosphere Center

Arecibo Observatory

The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...

in Arecibo

Arecibo

Arecibo may refer to:*Arecibo, Puerto Rico, a municipality located by the Atlantic Ocean*Arecibo Observatory, a very sensitive radio telescope located approximately south-southwest from the city of Arecibo...

. These studies confirmed earlier measurements during transits of the meridian

Meridian (astronomy)

This article is about the astronomical concept. For other uses of the word, see Meridian.In the sky, a meridian is an imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the zenith, through the south point on the...

, which had revealed in 1963 that the rotation of Venus was retrograde

Retrograde motion

Retrograde motion is motion in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such...

(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

Rotation period

The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars...

of Venus was 243.1 days, and that its axis of rotation was almost perpendicular to its orbital plane

Orbital plane (astronomy)

All of the planets, comets, and asteroids in the solar system are in orbit around the Sun. All of those orbits line up with each other making a semi-flat disk called the orbital plane. The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded...

. It was also established that the radius

Radius

In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

of the planet was 6052 kilometres (3,760.5 mi), some 70 kilometres (43.5 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, which were christened Alpha Regio

Alpha Regio

Alpha Regio is a region of the planet Venus extending for about 1500 kilometers centered at 22°S, 5°E.It was discovered and named by Dick Goldstein in 1964. The name was approved by the International Astronomical Union's Working Group for Planetary System Nomenclature between 1976 and 1979...

, Beta Regio

Beta Regio

Beta Regio is a region of the planet Venus known as a volcanic rise. Measuring about 3000 km, it constitutes a prominent upland region of Venus centered at ....

, and Maxwell Montes

Maxwell Montes

Maxwell Montes is a mountain massif on the planet Venus, part of which contains the highest point on the planet's surface.- General description :...

. Improvements in radar techniques later resulted in an image resolution of 1–2 kilometres.

Since the beginning of the age of space exploration

Space exploration

Space exploration is the use of space technology to explore outer space. Physical exploration of space is conducted both by human spaceflights and by robotic spacecraft....

, Venus has been considered as a site for future landings. Launch window

Launch window

Launch window is a term used in spaceflight to describe a time period in which a particular launch vehicle must be launched. If the rocket does not launch within the "window", it has to wait for the next window....

s occur every 19 months, and from 1962 to 1985 every window was used to launch reconnaissance probes.

In 1962, Mariner 2

Mariner 2

Mariner 2 , an American space probe to Venus, was the first space probe to conduct a successful planetary encounter . The first successful spacecraft in the NASA Mariner program, it was a simplified version of the Block I spacecraft of the Ranger program and an exact copy of Mariner 1...

flew over Venus, becoming the first man-made object to visit another planet. In 1965, Venera 3

Venera 3

Venera 3 was a Venera program space probe that was built and launched by the Soviet Union to explore the surface of Venus. It was launched on November 16, 1965 at 04:19 UTC from Baikonur, Kazakhstan....

became the first space probe to actually land

Lander (spacecraft)

A lander is a spacecraft which descends toward and comes to rest on the surface of an astronomical body. For bodies with atmospheres, the landing is called atmospheric reentry and the lander descends as a re-entry vehicle...

on another world, although it was a crash-landing. In 1967, Venera 4

Venera 4

Venera 4 ) was a probe in the Soviet Venera program for the exploration of Venus. Venera-4 was the first successful probe to perform in-place analysis of the environment of another planet. It was also the first probe to land on another planet...

became the first probe to send data from the interior of Venus's atmosphere, while Mariner 5

Mariner 5

Mariner 5 was a spacecraft of the Mariner program that carried a complement of experiments to probe Venus' atmosphere by radio occultation, measure the hydrogen Lyman-alpha spectrum, and sample the solar particles and magnetic field fluctuations above the planet...

measured the strength of Venus's magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

at the same time. Finally, in 1970, Venera 7

Venera 7

The Venera 7 was a Soviet spacecraft, part of the Venera series of probes to Venus. When it landed on the Venusian surface, it became the first man-made spacecraft to successfully land on another planet and to transmit data from there back to Earth.*Launch date/time: 1970 August 17 at 05:38...

made the first controlled landing on Venus. In 1974, Mariner 10

Mariner 10

Mariner 10 was an American robotic space probe launched by NASA on November 3, 1973, to fly by the planets Mercury and Venus. It was launched approximately two years after Mariner 9 and was the last spacecraft in the Mariner program...

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

Venera 9

Venera 9 was a USSR unmanned space mission to Venus. It consisted of an orbiter and a lander. It was launched on June 8, 1975 02:38:00 UTC and weighed 4,936 kg...

transmitted the first images of the surface of Venus and made gamma ray

Gamma ray

Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

observations of rocks at the landing site. Later in that same year, Venera 10

Venera 10

Venera 10 was a USSR unmanned space mission to Venus. It consisted of an orbiter and a lander. It launched on June 14, 1975 03:00:31 UTC.-Orbiter:The orbiter entered Venus orbit on October 23, 1975...

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

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

. 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

Atmospheric pressure

Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

at the surface was 90 times that of Earth's, confirming earlier radar observations.
In 1982, the Soviet Venera 13

Venera 13

Venera 13 was a probe in the Soviet Venera program for the exploration of Venus.Venera 13 and 14 were identical spacecraft built to take advantage of the 1981 Venus launch opportunity and launched 5 days apart, Venera 13 on 1981-10-30 at 06:04:00 UTC and Venera 14 on 1981-11-04 at 05:31:00 UTC,...

sent the first colour image of Venus's surface and analysed the X-ray fluorescence

X-ray fluorescence

X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...

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

Seismology

Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...

activity in the planet's crust

Crust

Crust may refer to:* Crust * The Crust, television seriesPhysical sciences:* Crust , at least continent-wide structure* Soil crust, local biology-sensitive structureFood:* Crust, dense surface layer of bread...

.

In 1983, Venera 15

Venera 15

Venera 15 was a spacecraft sent to Venus by the Soviet Union. This unmanned orbiter was to map the surface of Venus using high resolution imaging systems...

and 16

Venera 16

Venera 16 was a spacecraft sent to Venus by the Soviet Union. This unmanned orbiter was to map the surface of Venus using high resolution imaging systems...

acquired more precise radar images and altimetry data for the northern latitudes of the planet. This was the first use of synthetic aperture radar

Synthetic aperture radar

Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...

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 meteor craters were actually unusual volcanic features. The features of coronas, arachnoids, tessera and genuine meteorite 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 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 caused by the return of Halley's comet, the Soviet Union launched two Vega

Vega program

The Vega program was a series of Venus missions which also took advantage of the appearance of Comet Halley in 1986. Vega 1 and Vega 2 were unmanned spacecraft launched in a cooperative effort among the Soviet Union and Austria, Bulgaria, Hungary, the German Democratic Republic, Poland,...

probes to Venus. Vega 1 and 2

Vega 2

Vega 2 is a Soviet space probe part of the Vega program. The spacecraft was a development of the earlier Venera craft. They were designed by Babakin Space Center and constructed as 5VK by Lavochkin at Khimki...

each sent an instrumented helium balloon to a height of 50 kilometres (31.1 mi) above the surface, allowing scientists to study the dynamics of the most active part of Venus's atmosphere.

Magellan studies the geology of Venus

Launched May 4, 1989 aboard the space shuttle Atlantis

Space Shuttle Atlantis

The Space Shuttle Atlantis is a retired Space Shuttle orbiter in the Space Shuttle fleet belonging to the National Aeronautics and Space Administration , the spaceflight and space exploration agency of the United States...

, 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 70000 kilometres (43,496.1 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

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 (111.8 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

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

, optical observations of Venus became possible, although it soon became apparent that its surface is permanently hidden by dense cloud. In 1643, Francesco Fontana

Francesco Fontana

Francesco Fontana was an Italian lawyer and astronomer.He created woodcuts showing the Moon and the planets as he saw them through a self-constructed telescope...

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

William Herschel

Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

, who reported several sightings from 1789 onwards of a bright circular point of light near the southern terminator

Terminator (solar)

A terminator, twilight zone or "grey line" is a moving line that separates the illuminated day side and the dark night side of a planetary body...

of Venus, thought to be reflected light from a very tall mountain range or peak, around 43 kilometres (26.7 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.

The surface of Venus is comparatively flat. When 93% of the topography

Topography

Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...

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 (8.1 mi), while on the Earth the distance from the basins

Oceanic basin

Hydrologically, an oceanic basin may be anywhere on Earth that is covered by seawater, but geologically ocean basins are large geologic basins that are below sea level...

to the Himalayas is about 20 kilometres (12.4 mi).

According to data from the Pioneer altimeter

Altimeter

An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...

s, nearly 51% of the surface is located within 500 metres (1640 feet) of the median radius of 6052 km (3,760.5 mi); only 2% of the surface is located at elevations greater than 2 kilometres (1.2 mi) from the median radius.

The altimetry experiment of Magellan confirmed the general character of the landscape. According to the Magellan data, 80% of the topography is within 1 kilometre (0.621372736649807 mi) of the median radius. The most important elevations are in the mountain chains that surround Lakshmi Planum

Lakshmi Planum

Lakshmi Planum is a plateau feature on the surface of Venus on the Western Ishtar Terra. It is named after Lakshmi, the Hindu goddess of wealth.It is roughly above the mean planetary radius...

: Maxwell Montes

Maxwell Montes

Maxwell Montes is a mountain massif on the planet Venus, part of which contains the highest point on the planet's surface.- General description :...

(11 km, 6.8 mi), Akna Montes

Akna Montes

Akna Montes are a mountain range on Venus centered at 68.9°N, 318.2°E and stretching 830 km long.The Akna range is a north-south trending ridge belt that forms the western border of the elevated smooth plateau of Lakshmi Planum. The Lakshmi plateau plains are formed by extensive volcanic eruptions...

(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 inclined some 45°. Inclinations of 30° were registered in Danu Montes and Themis Regio.

About 75% of the surface is composed of bare rock.

Based on altimeter data from the Pioneer Venus probe, supported by 'Magellan' data, the topography of the planet is divided into three provinces: lowlands, deposition plains, and highlands.

Highlands

This unit covers about 10% of the planet's surface, with altitudes greater than 2 km.

The most important provinces of the highlands are Aphrodite Terra

Aphrodite Terra

Aphrodite Terra is a highland region on Venus, near the equator. It is about the same size as Africa, and much rougher than Ishtar Terra.The surface appears buckled and fractured which suggests large compressive forces. There are also numerous extensive lava flows. Channels cross this terrain and...

, Ishtar Terra

Ishtar Terra

Ishtar Terra is one of two main highland regions on the planet Venus. It is the smaller of the two "continents", and is located near the north pole. It is named after the Akkadian goddess Ishtar....

, and Lada Terra, as well as the regions Beta Regio

Beta Regio

Beta Regio is a region of the planet Venus known as a volcanic rise. Measuring about 3000 km, it constitutes a prominent upland region of Venus centered at ....

, Phoebe Regio and Themis Regio. The regions Alpha Regio

Alpha Regio

, Bell Regio, Eistla Regio and Tholus Regio form a less important group of highlands.

Deposition plains

Deposition plains have altitudes averaging 0 to 2 km and cover more than half of the planet's surface.

Lowlands

The rest of the surface is lowlands and generally lies below zero altitude. Radar reflectivity data suggest that at a centimeter scale these areas are smooth, as a result of gradation (accumulation of fine material eroded from the highlands).

Impact craters

Earth-based radar surveys made it possible to identify some topographic patterns related to craters

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

, and the Venera 15 and Venera 16 probes identified almost 150 such features of probable impact origin. Global coverage from Magellan subsequently made it possible to identify nearly 900 impact craters.

Compared to Mercury

Mercury (planet)

, the Moon

Moon

and other such bodies, Venus has very few craters. In part, this is because Venus's dense atmosphere burns up smaller meteor

METEOR

METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...

ites before they hit the surface. The Venera and Magellan data are in agreement: there are very few impact craters with a diameter less than 30 kilometres (18.6 mi), and data from Magellan show an absence of any craters less than 2 kilometres (1.2 mi) in diameter. The small craters are irregular and appear in groups, thus pointing to the deceleration and the breakup of impactors. However, there are also fewer of the large craters, and those appear relatively young; they are rarely filled with lava, showing that they were formed after volcanic activity in the area ceased, and radar data indicates that they are rough and have not had time to be eroded down.

Compared to the situation on bodies such as the Moon, it is more difficult to determine the ages of different areas of the surface on Venus, on the basis of crater counts, due to the small number of craters at hand. However, the surface characteristics are consistent with a completely random distribution, implying that the surface of the entire planet is roughly the same age, or at least that very large areas are not very different in age from the average.

Taken together, this evidence suggests that the surface of Venus is young. The impact crater distribution appears to be most consistent with models that call for a near-complete resurfacing of the planet. Subsequent to this period of extreme activity, process rates declined and impact craters began to accumulate, with only minor modification and resurfacing since.

A young surface all created at the same time is a different situation compared with any of the other terrestrial planets.

Global resurfacing event

It is hypothesized that Venus underwent some sort of global resurfacing about 300–500 million years ago, however no Venusian rock has ever been dated. Although it is exciting to imagine the entire crust of Venus sinking into the mantle, it is probably more reasonable to discuss massive lava flows lasting thousands to millions of years.

One possible explanation for this event is that it is part of a cyclic process on Venus. On Earth, plate tectonics allows heat to escape from the mantle. However, Venus has no evidence of plate tectonics, so the theory is that the interior of the planet heats up (due to the decay of radioactive elements) until material in the mantle is hot enough to force its way to the surface. The subsequent resurfacing event covers most or all of the planet with lava, until the mantle is cool enough for the process to start over.

There are several other attributes of Venus that this model can help explain. Venus's lack of a magnetic field is puzzling, as Venus is similar to Earth in size, and presumably composition. However, it can be explained by a core that is not losing heat. Also, Venus has a much higher deuterium

Deuterium

Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

to hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

ratio in its atmosphere than do the Earth or comets. Atmospheric escape

Atmospheric escape

Atmospheric escape is the loss of planetary atmospheric gases to outer space.- Thermal escape mechanisms :One classical thermal escape mechanism is Jeans escape. In a quantity of gas, the average velocity of a molecule is determined by temperature, but the velocity of individual molecules varies...

is one of the very few processes that differentiate between the deuterium and hydrogen. The extremely high ratio implies that there were large amounts of water in Venus's atmosphere more recently than the beginning of the solar system, and that a massive eruption would release large quantities of water (as well as other compounds, for example the sulfur that leads to the sulfuric acid clouds of Venus).

More evidence is needed to put the theory of global resurfacing of Venus on firm ground. However, several different indications support it, and it is hard to explain the crater pattern of Venus without something at least vaguely resembling this idea.

Volcanoes

The surface of Venus is dominated by volcanism

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

. Although Venus is superficially similar to Earth, it seems that the tectonic plates

Tectonic Plates

Tectonic Plates is a 1992 independent Canadian film directed by Peter Mettler. Mettler also wrote the screenplay based on the play by Robert Lepage. The film stars Marie Gignac, Céline Bonnier and Robert Lepage.-Plot summary:...

so active in Earth's geology do not exist on Venus. About 80% of the planet consists of a mosaic of volcanic lava

Lava

plains, dotted with more than a hundred large isolated shield volcano

Shield volcano

es, and many hundreds of smaller volcanoes and volcanic constructs such as coronae

Corona (planetary geology)

. These are geological features believed to be almost unique to Venus: huge, ring-shaped structures 100–300 kilometres (60–180 mi) across and rising hundreds of metres above the surface. The only other place they have been discovered is on Uranus's

Uranus

Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

moon Miranda

Miranda (moon)

-External links:* at * at The Nine8 Planets* at Views of the Solar System* * from the...

. It is believed that they are formed when plumes of rising hot material in the mantle

Mantle (geology)

The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

push the crust upwards into a dome shape, which then collapses in the centre as the molten lava cools and leaks out at the sides, leaving a crown-like structure: the corona.

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; flood basalt

Flood basalt

A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...

s cover wide expanses of the surface, similar to features such as the Deccan Traps

Deccan Traps

The Deccan Traps are a large igneous province located on the Deccan Plateau of west-central India and one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt that together are more than thick and cover an area of and a volume of...

on Earth. These eruptions result in "flow type" volcanoes.

Volcanoes less than 20 kilometres (12.4 mi) 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 volcano

Shield volcano

es on Earth. These pancake dome

Pancake dome

A pancake dome, also known as a lava dome, is an unusual type of volcano found on the planet Venus. They are widely scattered on that planet and often form groups or clusters, though with smaller numbers of pancake domes in each group than is typical for the more common shield volcanos...

volcanoes are up to 15 kilometres (9.3 mi) in diameter and less than 1 kilometre (0.621372736649807 mi) 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 volcano

Shield volcano

es and composite or stratovolcano

Stratovolcano

A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...

es. The shield volcanoes, for example those in Hawaii

Hawaii

Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

, eject magma

Magma

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

from the depths of the Earth in zones called hot spot

Hotspot (geology)

The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...

s. The lava from these volcanos is relatively fluid and permits the escape of gases. Composite volcanos, such as Mount Saint Helens and Mount Pinatubo

Mount Pinatubo

Mount Pinatubo is an active stratovolcano located on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is west of the dormant and...

, are associated with tectonic plates. In this type of volcano, the oceanic crust

Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...

of one plate slides beneath the other in a subduction zone, together with an inflow of seawater, producing a gummier lava that restricts the exit of the gases, and for that reason, composite volcanoes tend to erupt more violently.

On Venus, where there are no tectonic plates or seawater, volcanoes are of the shield type. Nevertheless, the morphology of the volcanos of Venus is different. On the Earth, shield volcanoes can be a few tens of kilometres wide and up to 10 kilometres high (6.2 mi) in the case of Mauna Kea

Mauna Kea

Mauna Kea is a volcano on the island of Hawaii. Standing above sea level, its peak is the highest point in the state of Hawaii. However, much of the mountain is under water; when measured from its oceanic base, Mauna Kea is over tall—significantly taller than Mount Everest...

, measured from the sea floor. On Venus, these volcanos can cover hundreds of kilometres in area, but they are relatively flat, with an average height of 1.5 kilometre (0.93205910497471 mi).

The dome

Lava dome

|250px|thumb|right|Image of the [[rhyolitic]] lava dome of [[Chaitén Volcano]] during its 2008–2009 eruption.In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano...

s of Venus (commonly called pancake dome

Pancake dome

s) are between 10 and 100 times larger than those formed on Earth. They are usually associated with "coronae" and tesserae. The pancakes are thought to be formed by highly viscous, silica-rich lava erupting under Venus's high atmospheric pressure. Domes called scalloped margin dome

Scalloped margin dome

A scalloped margin dome is a volcanic dome that has experienced collapse and mass wasting such as landslides on its perimeter. The margins of these domes have headscarps or 'scallops' separated by ridges that are a consequence of adjoining scallops. Sometimes debris or slumping can be found at...

s (commonly called ticks because they appear as domes with numerous legs), are thought to have undergone mass wasting events such as landslides on their margins. Sometimes deposits of debris can be seen scattered around them.

Other unique features of Venus's surface are novae (radial networks of dike

Dike (geology)

A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...

s or graben

Graben

In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....

s) and arachnoid

Arachnoid (astrogeology)

s. A nova is formed when large quantities of magma are extruded onto the surface to form radiating ridges and trenches which are highly reflective to radar. These dikes form a symmetrical network around the central point where the lava emerged, where there may also be a depression caused by the collapse of the magma chamber

Magma chamber

A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...

.

Arachnoids are so named because they resemble a spider's web, featuring several concentric ovals surrounded by a complex network of radial fractures similar to those of a nova. It is not known whether the 250 or so features identified as arachnoids actually share a common origin, or are the result of different geological processes.

Tectonic activity

Despite the fact that Venus appears to have no tectonic plates as such, the planet's surface shows various features usually associated with tectonic activity. Features such as faults

Geologic fault

In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from the action of tectonic forces...

, folds

Fold (geology)

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

, volcano

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es, large mountains and rift valleys are caused on Earth by plates moving over relatively weak parts of the planet's interior.

The active volcanism of Venus has generated chains of folded mountains, rift valleys, and terrain known as tesserae, a word meaning "floor tiles" in Greek. Tesserae exhibit the effects of eons of compression and tensional deformation.

Unlike those on Earth, the deformations on Venus are directly related to dynamic forces within the planet's mantle

Mantle (geology)

. Gravitational studies suggest that Venus lacks an asthenosphere

Asthenosphere

The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...

—a layer of lower viscosity

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

that facilitates the movement of tectonic plates. The absence of this layer suggests that the deformation of the Venusian surface can be explained by convective

Convection

Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

movements within the planet.

The tectonic deformations on Venus occur on a variety of scales, the smallest of which are related to linear fractures or faults. In many areas these faults appear as networks of parallel lines. Small, discontinuous mountain crests are found which resemble those on the Moon

Moon

and Mars

Mars

. The effects of extensive tectonism are shown by the presence of normal faults, where the crust has sunk in one area relative to the surrounding rock, and superficial fractures. Radar imaging shows that these types of deformation are concentrated in belts located in the equatorial zones and at high southern latitude

Latitude

s. These belts are hundreds of kilometres wide and appear to interconnect across the whole of the planet, forming a global network associated with the distribution of volcanoes.

The rifts of Venus, formed by the expansion of the lithosphere

Lithosphere

The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

, are groups of depressions tens to hundreds of metres wide and extending up to 1,000 kilometres in length. The rifts are mostly associated with large volcanic elevations in the form of domes, such as those at Beta Regio

Beta Regio

Beta Regio is a region of the planet Venus known as a volcanic rise. Measuring about 3000 km, it constitutes a prominent upland region of Venus centered at ....

, Atla Regio and the western part of Eistla Regio. These highlands seem to be the result of enormous mantle plume

Mantle plume

A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...

s (rising currents of magma) which have caused elevation, fracturing, faulting, and volcanism.

The highest mountain chain on Venus, Maxwell Montes

Maxwell Montes

Maxwell Montes is a mountain massif on the planet Venus, part of which contains the highest point on the planet's surface.- General description :...

in Ishtar Terra

Ishtar Terra

Ishtar Terra is one of two main highland regions on the planet Venus. It is the smaller of the two "continents", and is located near the north pole. It is named after the Akkadian goddess Ishtar....

, was formed by processes of compression, expansion, and lateral movement. Another type of geographical feature, found in the lowlands, consists of ridge belts elevated several metres above the surface, hundreds of kilometres wide and thousands of kilometres long. Two major concentrations of these belts exist: one in Lavinia Planitia near the southern pole, and the second adjacent to Atalanta Planitia near the northern pole.

Tesserae are found mainly in Aphrodite Terra

Aphrodite Terra

, Alpha Regio

Alpha Regio

, Tellus Regio and the eastern part of Ishtar Terra

Ishtar Terra

Ishtar Terra is one of two main highland regions on the planet Venus. It is the smaller of the two "continents", and is located near the north pole. It is named after the Akkadian goddess Ishtar....

(Fortuna Tessera). These regions contain the superimposition and intersection of graben

Graben

In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....

s of different geological units, indicating that these are the oldest parts of the planet. It was once thought that the tesserae were continents associated with tectonic plates like those of the Earth; in reality they are probably the result of floods of basaltic lava forming large plains, which were then subjected to intense tectonic fracturing.

Magnetic field and structure

Venus's crust appears to be 50 kilometres (31.1 mi) in thickness, and composed of silicate rocks

Silicate minerals

The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...

. Venus's mantle

Mantle (geology)

is approximately 3000 kilometres (1,864.1 mi) thick, but its composition is unknown. Since Venus is a terrestrial planet

Terrestrial planet

A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

, it is presumed to have a core made of semisolid iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

and nickel

Nickel

Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

with a radius

Radius

of approximately 3000 kilometres (1,864.1 mi).

Pioneer Venus Orbiter data indicates that Venus does not have a significant magnetic field

Magnetic field

. The magnetic field of a planet is produced by a dynamo in its core. A dynamo requires a conducting liquid, rotation, and convection

Convection

Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

. Venus is thought to have an electrically conductive core, and although its rotation period is very long (243.7 Earth days), simulations show that this is adequate to produce a dynamo (Stevenson 2003). This implies that Venus lacks convection in its core. Convection occurs when there is a large difference in temperature between the inner and outer part of the core, but since Venus has no plate tectonics to let off heat, it is possible that it has no inner core, or that its core is not currently cooling.

Lava flows and channels

Lava flows on Venus are often much larger than Earth's, up to several hundred kilometres long and tens of kilometres wide. It is still unknown why these lava fields or lobate flows reach such sizes, but it is suggested that they are the result of very large eruptions of basaltic, low-viscosity lava spreading out to form wide, flat plains.

On Earth, there are two known types of basaltic lava: Aa and Pāhoehoe. Aa lava presents a rough texture in the shape of broken blocks (clinker

Clinker

Clinker may refer to:* Clinker , construction method for wooden boats* Clinker , waste from industrial processes* Clinker , a kilned then quenched cement product* Clinker brick, rough dark coloured bricks...

s). Pāhoehoe lava is recognized by its pillowy or ropy appearance. Rough surfaces appear bright in radar images, which can be used to determine the differences between Aa and Pāhoehoe lavas. These variations can also reflect differences in lava age and preservation. Channels and lava tubes (channels that have cooled down and over which a dome has formed) are very common on Venus. Two planetary Astronomers from the University of Wollongong in Australia, Dr Graeme Melville and Prof. Bill Zealey, researched these lava tubes, using data supplied by NASA, over a number of years and concluded that they were widespread and up to ten times the size of those on the Earth. Melville and Zealey said that the gigantic size of the Venusian lava tubes (tens of metres wide and hundreds of kilometres long) may be explained by the very fluid lava flows together with the high temperatures on Venus, allowing the lava to cool slowly.

For the most part, lava flow fields are associated with volcanoes. The central volcanoes are surrounded by extensive flows that form the core of the volcano. They are also related to fissure craters, coronae, dense clusters of volcanic domes, cones, wells and channels.

Thanks to Magellan, more than 200 channels and valley complexes have been identified. The channels were classified as simple, complex, or compound. Simple channels are characterized by a single, long main channel. This category includes rill

Rill

A Rill can be a:*1.) natural fluvial topographic feature;*2.) functional constructed channel to carry a water supply from a water source some distance away;*3.) aesthetic garden water feature.-Natural:...

s similar to those found on the Moon

Moon

, and a new type, called canali, consisting of long, distinct channels which maintain their width throughout their entire course. The longest such channel identified (Baltis Vallis

Baltis Vallis

Baltis Vallis is a sinuous channel on Venus ranging from one to three kilometers wide and more than 7,000 kilometers long, slightly longer than the Nile and the longest known channel of any kind in the solar system. It is thought that it once held a river of lava known as a lava channel.The channel...

) has a length of more than 6800 kilometres (4,225.3 mi), about one-sixth of the circumference of the planet.

Complex channels include anastomosed

Anastomosis

An anastomosis is the reconnection of two streams that previously branched out, such as blood vessels or leaf veins. The term is used in medicine, biology, mycology and geology....

networks, in addition to distribution networks. This type of channel has been observed in association with several impact craters and important lava floods related to major lava flow fields. Compound channels are made of both simple and complex segments. The largest of these channels shows an anastomosed web and modified hills similar to those present on Mars

Mars

.

Although the shape of these channels is highly suggestive of fluid erosion, there is no evidence that they were formed by water. In fact, there is no evidence of water anywhere on Venus in the last 600 million years. While the most popular theory for the channels' formation is that they are the result of thermal erosion by lava, there are other hypotheses, including that they were formed by heated fluids formed and ejected during impacts.

Surface processes

Water is almost nonexistent on Venus, and thus the only erosive

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

process to be found (apart from thermal erosion by lava flows) is the interaction produced by the atmosphere with the surface. This interaction is present in the ejecta of impact craters expelled onto the surface of Venus. The material ejected during a meteorite

Meteorite

A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

impact is lifted to the upper atmosphere, where winds transport the material toward the west. As the material is deposited on the surface, it forms parabola

Parabola

In mathematics, the parabola is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface...

-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 parabola-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 metre per second. Given the density of the lower Venusian 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 yardang

Yardang

A yardang is a streamlined hill carved from bedrock or any consolidated or semiconsolidated material by the dual action of wind abrasion, dust and sand, and deflation. Yardangs become elongated features typically three or more times longer than wide, and when viewed from above, resemble the hull of...

s. 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 sides 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 crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s follow a trajectory in the direction of the equator. This tendency suggests the presence of a system of circulation of Hadley cell

Hadley cell

The Hadley cell, named after George Hadley, is a circulation pattern that dominates the tropical atmosphere, with rising motion near the equator, poleward flow 10–15 kilometers above the surface, descending motion in the subtropics, and equatorward flow near the surface...

s between 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. Radar reflection from the oldest lava flows, covered by the newest flows, present distinct intensities. 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 in the presence of carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

and sulfur dioxide

Sulfur dioxide

Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

(see carbonate-silicate cycle

Carbonate-silicate cycle

The carbonate–silicate geochemical cycle is the naturally occurring reversible chemical reaction with summary equationCaSiO3+CO2CaCO3+SiO2.Equilibrium of the carbonate-silicate reaction is generally shifted in the favor of carbonate formation under near surface temperature and pressure conditions,...

for details). These two gases are the planet's first and third most abundant gases, respectively; the second most abundant gas is inert nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

. The reactions probably include the deterioration of silicate

Silicate

A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

s by carbon dioxide to produce carbonate

Carbonate

In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

s and quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

, as well as the deterioration of silicates by sulfur dioxide to produce anhydrate calcium sulfate

Calcium sulfate

Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...

and carbon dioxide.

One of the most interesting characteristics of radar images is the diminishing of reflection at high altitudes, exhibiting extremely low values beyond a radius of 6054 kilometres (3,761.8 mi). This change is related to the diminishing of emission and temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

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 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

Dielectric constant

The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...

. Yet another theory says that the layer one metre above the surface is formed by sheets of a conductive material such as pyrite

Pyrite

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

. 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 behaviour on the surface of Venus are perovskite

Perovskite

A perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers. Perovskites take their name from this compound, which was first discovered in the Ural mountains of...

and pyrochlore

Pyrochlore

Pyrochlore 2Nb2O6 is a solid solution between the niobium end member , and the tantalum end member .-Occurrence:...

s.

Despite these theories, the existence of ferroelectric minerals on Venus has not been confirmed. Only in situ exploration will lead to an explanation of such unresolved enigmas.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Knowledge of the surface of Venus before Magellan

Magellan studies the geology of Venus

Topography

Highlands

Deposition plains

Lowlands

Impact craters

Global resurfacing event

Volcanoes

Tectonic activity

Magnetic field and structure

Lava flows and channels

Surface processes

See also

Resources available online

Publications

Related books

External links