Geology of Venus

Venus is a planet with striking surface characteristics. Most of what we know today about its surface stems from radar observations, mainly images sent by the Magellan probe between August 16, 1990 and the end of its sixth orbital cycle in September 1994. Ninety-eight per cent of the planet's surface was mapped, 22% of it in three-dimensional stereoscopic images.

The surface of Venus is covered by a dense atmosphere and presents clear evidence of former violent volcanic activity. It has shield and composite volcanoes similar to those found on Earth.

Relative to the Moon, Mars or Mercury, Venus has few small impact craters. This is likely a result of the planet's dense atmosphere, which burns up smaller meteors. Venus does have more medium-to-large-size craters, but still not as many as the Moon or Mercury.

Some other unusual characteristics of the planet include features called coronae (Latin for crowns, based on their appearance), tesserae (large regions of highly deformed terrain, folded and fractured in two or three dimensions, believed to be unique to Venus), and arachnoids (for those features resembling a spider's web). Long rivers of lava have been discovered, as well as evidence of Aeolian erosion and tectonic shifts which have played an essential role in making the surface of Venus as complex as it is today.

Although Venus is the planet closest to Earth (some 40 million kilometres at inferior conjunction), and is similar in size, 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 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%).

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 images from the surface reveal no evidence of wind erosion.

Knowledge of the surface of Venus before Magellan
Main article: Exploration of Venus

After the Moon, Venus was the second object in the solar system to be explored by radar from the Earth. The first studies were carried out in 1961 at NASA's Goldstone Observatory, part of the Deep Space Network. At successive inferior conjunctions, Venus was observed both by Goldstone and the National Astronomy and Ionosphere Center in Arecibo. These studies confirmed earlier measurements during transits of the meridian, which had revealed in 1963 that the rotation of Venus was retrograde (it rotates in the opposite direction to that in which it orbits the Sun). The radar observations also allowed astronomers to determine that the rotation period of Venus was 243.1 days, and that its axis of rotation was almost perpendicular to its orbital plane. It was also established that the radius of the planet was 6,052 kilometres (3761 mi), some 70 kilometres (43 mi) less than the best previous figure obtained with terrestrial telescopes.

Interest in the geological characteristics of Venus was stimulated by the refinement of imaging techniques between 1970 and 1985. Early radar observations suggested merely that the surface of Venus was more compacted than the dusty surface of the Moon. The first radar images taken from the Earth showed very bright (radar-reflective) highlands, which were christened Alpha Regio, Beta Regio, and Maxwell Montes. Improvements in radar techniques later resulted in an image resolution of 1–2 kilometres.

Since the beginning of the age of space exploration, Venus has been considered as a site for future landings. Launch windows occur every 19 months, and from 1962 to 1985 every window was used to launch reconnaissance probes.

In 1962, Mariner 2 flew over Venus, becoming the first man-made object to visit another planet. In 1965, Venera 3 became the first space probe to actually land on another world, although it was a crash-landing. In 1967, Venera 4 became the first probe to send data from the interior of Venus's atmosphere, while Mariner 5 measured the strength of Venus's magnetic field at the same time. Finally, in 1970, Venera 7 made the first controlled landing on Venus. In 1974, Mariner 10 swung by Venus on its way to Mercury and took ultraviolet photographs of the clouds, revealing extraordinarily high wind speeds in the Venusian atmosphere.

In 1975, Venera 9 transmitted the first images of the surface of Venus and made gamma ray observations of rocks at the landing site. Later in that same year, Venera 10 sent further images of the surface.

In 1978, the Pioneer 12 probe (also known as Pioneer Venus 1 or Pioneer Venus Orbiter) circled Venus and provided data for the first altimetry and gravity maps of the planet between 63 and 78 degrees of latitude. The altimetry data had an accuracy of 150 kilometers.

That same year, Pioneer Venus 2 launched four probes into Venus's atmosphere which determined, when combined with data from prior missions, that the surface temperature of the planet was approximately 460°C (860°F), and that the atmospheric pressure at the surface was 90 times that of Earth's, confirming earlier radar observations.

In 1982, the Soviet Venera 13 sent the first colour image of Venus's surface and analysed the X-ray fluorescence of an excavated soil sample. The probe operated for a record 127 minutes on the planet's hostile surface. Also in 1982, the Venera 14 lander detected possible seismic activity in the planet's crust.

In 1983, Venera 15 and 16 acquired more precise radar images and altimetry data for the northern latitudes of the planet. This was the first use of synthetic aperture radar on Venus. The images had 1–2 kilometre (0.6–1.2 mile) resolution. The altimetry data obtained by the Venera missions had a resolution four times better than Pioneer's. 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 probes to Venus. Vega 1 and 2 each sent an instrumented helium balloon to a height of 50 kilometres (31 mi) above the surface, allowing scientists to study the dynamics of the most active part of Venus's atmosphere.

Magellan studies the geology of Venus
3D view of Venus's Eistla Regio produced from Magellan radar data.

Launched May 4, 1989 aboard the space shuttle Atlantis, the Magellan probe was first placed into low Earth orbit, before firing its upper stage motor to send it on a trajectory toward Venus. On August 10, Magellan arrived at Venus and began to take images with radar. Each day it made 7.3 Venus orbits, imaging a strip 17–28 kilometres (11–17 mi) wide and 70,000 kilometres (43,496 mi) long. Covering the whole planet required 1,800 strips, which were combined into a single mosaic image.

The first images of Venus were received on August 16, 1990, and routine mapping operations began on September 15, 1990. The first mapping cycle (Cycle 1) lasted 243 terrestrial days—the time it takes Venus to rotate on its own axis under the probe's orbital plane. Cycle 1 was completed successfully on May 15, 1991, mapping 84% of the Venusian surface.

Cycle 2 began immediately afterwards and lasted until January 15, 1992. In each cycle, the probe was inclined at a different "look angle", producing stereoscopic data which enabled scientists to compile a three-dimensional map of the surface—a technique known as synthetic aperture radar.

Cycle 3 was due to finish on September 14, 1992, but was terminated a day early due to problems with onboard equipment. In total, radar coverage of 98% of the surface of Venus was obtained, with 22% of the images in stereo. Magellan produced surface images of unprecedented clarity and coverage, which are still unsurpassed.

Cycles 4, 5 and 6 were devoted to collecting gravimetric data, for which Magellan was aerobraked to its lowest possible stable orbit, with a periapsis or closest approach of 180 kilometres (112 mi). At the end of Cycle 6 its orbit was reduced further, entering the outer reaches of the atmosphere. After carrying out a few final experiments, Magellan successfully completed its mission on October 11, 1994, and was de-orbited to burn up in Venus's atmosphere.

Topography

With the invention of the telescope, optical observations of Venus became possible, although it soon became apparent that its surface is permanently hidden by dense cloud. In 1643, Francesco Fontana was the first of several astronomers claiming to see dark markings on these clouds, while others even said that they could see part of the surface through holes in the clouds. Astronomers also claimed to have seen brilliant points in certain spots on the disk of the planet, suggesting an enormous mountain whose top was higher than the clouds. The most famous such observations were made by Johann Hieronymus Schröter, a respected observer and collaborator of William Herschel, who reported several sightings from 1789 onwards of a bright circular point of light near the southern terminator of Venus, thought to be reflected light from a very tall mountain range or peak, around 43 kilometres (28 mi) high. Herschel disputed these observations and held them to be attributable to imperfections in Schröter's telescope. Many other observers claimed to see irregularities in the terminator of Venus, and the debate continued into the 20th century until radar observations were able to penetrate the clouds and reveal that, in fact, no such giant mountains exist.[1]

The surface of Venus is comparatively flat. When 93% of the topography was mapped by Pioneer Venus, scientists found that the total distance from the lowest point to the highest point on the entire surface was about 13 kilometres (8 mi), while on the Earth the distance from the basins to the Himalayas is about 20 kilometres (12.4 mi).

According to data from the Pioneer altimeters, nearly 51% of the surface is located within 500 metres (1640 feet) of the median radius of 6,052 km (3760 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 (0.6 mi) of the median radius. The most important elevations are in the mountain chains that surround Lakshmi Planum: Maxwell Montes (11 km, 6.8 mi), Akna Montes (7 km, 4.3 mi) and Freya Montes (7 km, 4.3 mi). Despite the relatively flat landscape of Venus, the altimetry data also found large inclined plains. Such is the case on the southwest side of Maxwell Montes, which in some parts seems to be 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, Ishtar Terra, and Lada Terra, as well as the regions Beta Regio, Phoebe Regio and Themis Regio. The regions 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
Danilova crater in relief

Earth-based radar surveys made it possible to identify some topographic patterns related to craters, 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.
Danilova, Aglaonice and Saskja craters

Crater counts are an important source for estimating the age of the surface of a planet. Over time, bodies in the solar system are randomly impacted, so the more craters a surface has, the older it is. Compared to Mercury, the Moon and other such bodies, Venus has very few craters. In part, this is because Venus's dense atmosphere burns up smaller meteorites 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 mi), and data from Magellan show an absence of any craters less than 2 kilometres (1.2 mi) in diameter. 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.

On older bodies such as the Moon, crater counts can also be used to determine the ages of different areas of the surface; on the Moon, there are more craters in the older hills. On Venus, there are only a few such craters, which makes this analysis difficult. 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 allow the escape of heat 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 to hydrogen ratio in its atmosphere than do the Earth or comets. Atmospheric escape 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
Main article: Volcanism on Venus
Computer generated perspective view of pancake domes in Venus's Alpha Regio
Arachnoid surface feature on Venus

The surface of Venus is dominated by volcanism. Although Venus is superficially similar to Earth, it seems that the tectonic plates so active in Earth's geology do not exist on Venus. About 80% of the planet consists of a mosaic of volcanic lava plains, dotted with more than a hundred large isolated shield volcanoes, and many hundreds of smaller volcanoes and volcanic constructs such as coronae. 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 moon Miranda. It is believed that they are formed when plumes of rising hot material in the mantle 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 basalts cover wide expanses of the surface, similar to features such as the Deccan Traps on Earth. These eruptions result in "flow type" volcanoes.

Volcanoes less than 20 kilometres (12 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 volcanoes on Earth. These pancake dome volcanoes are up to 15 kilometres (9 mi) in diameter and less than 1 kilometre (0.6 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 volcanoes and composite or stratovolcanoes. The shield volcanoes, for example those in Hawaii, eject magma from the depths of the Earth in zones called hot spots. The lava from these volcanos is relatively fluid and permits the escape of gases. Composite volcanos, such as Mount Saint Helens and Mount Pinatubo, are associated with tectonic plates. In this type of volcano, the oceanic crust 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, 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 kilometres (0.9 mi).

The domes of Venus (commonly called pancake domes) 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 domes (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 dikes or grabens) and arachnoids. 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.

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, folds, volcanoes, large mountains and rift valleys are caused on Earth by plates moving over the planet's molten 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. Gravitational studies suggest that Venus lacks an asthenosphere—a layer of lower 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 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 and 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 latitudes. 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, 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, Atla Regio and the western part of Eistla Regio. These highlands seem to be the result of enormous mantle plumes (rising currents of magma) which have caused elevation, fracturing, faulting, and volcanism.

The highest mountain chain on Venus, Maxwell Montes in Ishtar Terra, 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 kilometres 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, Alpha Regio, Tellus Regio and the eastern part of Ishtar Terra (Fortuna Tessera). These regions contain the superimposition and intersection of grabens 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.[2]
Cutaway diagram of possible internal structure.

Magnetic field and structure

Venus's crust appears to be 50 kilometres (31 mi) in thickness, and composed of silicious rocks. Venus's mantle is approximately 3,000 kilometres (1860 mi) thick, but its composition is unknown. Since Venus is a terrestrial planet, it is presumed to have a core made of semisolid iron and nickel with a radius of approximately 3,000 kilometres (1860 mi).

Pioneer Venus Orbiter data indicates that Venus does not have a significant 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. 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 originating from Ammavaru caldera (300 km outside the image) overflowed the ridge left of center and pooled to its right.
An anastomosing 2-km-wide lava channel in Sedna Planitia.

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.[2]

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

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 rills similar to those found on the 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) has a length of more than 6,800 kilometres (4,200 mi), about one-sixth of the circumference of the planet.

Complex channels include anastomosed 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.

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 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 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-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 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 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 craters follow a trajectory in the direction of the equator. This tendency suggests the presence of a system of circulation of Hadley cells 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 ʻaʻa 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 and sulfur dioxide (see carbonate-silicate cycle for details). These two gases are the planet's 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 sulfur dioxide to produce anhydrate calcium sulfate 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 6,054 kilometres (3762 mi). This change is related to the diminishing of emission and 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 produce an efficient reflection of radar.[citation needed] Another idea is that the surface is not smooth and is covered by material that has an extremely high dielectric constant.[citation needed] Yet another theory says that the layer one metre above the surface is formed by sheets of a conductive material such as pyrite.[citation needed] Last, a recent model supposes the existence of a small proportion of ferroelectric mineral.[citation needed]

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 and pyrochlores.

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.