Surface Features

Earth-based observatories and Venus-orbiting spacecraft have provided global-scale information on the nature of the planet's surface. All have used radar systems to penetrate the thick Venusian clouds.

The entire surface of the planet is dry and rocky. Because there is no sea level in the literal sense, elevation is commonly expressed as a planetary radius—i.e., as the distance from the centre of the planet to the surface at a given location. Another method, in which elevation is expressed as the distance above or below the planet's mean radius, is also used. Most of the planet consists of gently rolling plains. In some areas the elevations change by only a few hundred metres over distances of hundreds of kilometres. Globally, more than 80 percent of the surface deviates less than 1 km (0.6 mile) from the mean radius. At several locations on the plains are broad, gently sloping topographic depressions, or lowlands, that may reach several thousand kilometres across; they include Atalanta Planitia, Guinevere Planitia, and Lavinia Planitia.

Two striking features are the continent-sized highland areas, or terrae—Ishtar Terra in the northern hemisphere and Aphrodite Terra along the equator. In addition to the two main terrae are several smaller elevated regions,

(Top) Global topographic map of Venus derived from laser altimetry data gathered by the Magellan spacecraft. Selected major topographic features and spacecraft landing sites are labeled. The most prominent features are the two continent-sized highland areas—Ishtar Terra in the northern hemisphere and Aphrodite Terra along the equator. Encyclopedia Britannica, Inc. (Bottom) A close-up view of western Ishtar Terra reveals gently rolling plains, the dominant feature of Venus's surface. The highlands of Ishtar Terra are centred on the lava-covered plateau Lakshmi Planum (upper right). Courtesy of NASA/JPL/Caltech wigiiuae

240° E 570° E 300" E 330" E O" 30" E 00" E 00" E 130" E 160" E 180" E 210* E 240" E

Vosiü Sipos— Ishtar Terra

" Fortuna Toracra

Asteria Spía* Regio Wegío

Venera 14 Venera 11

Vnga 1, Atía \ Regio Aphrodtte Terra

including Alpha Regio, Beta Regio, and Phoebe Regio.

Ishtar is the smaller of the two main terrae and is roughly the size of Australia. It extends from about latitude 45°N to 75° N and from about longitude 300°E to 75° E. Ishtar possesses the most spectacular topography on Venus, comprising several distinct physiographic provinces. The dominant feature of western Ishtar is Lakshmi Planum, a high, flat, lava-covered plateau. Lakshmi is bounded on most sides by mountain ranges and has been likened to the Plateau of Tibet.

The eastern portion of Ishtar is geologically complex, consisting largely of tessera terrain. Fortuna Tessera, the main feature of eastern Ishtar, appears extraordinarily rugged and highly deformed in radar images, displaying many different trends of parallel ridges and troughs that cut across one another at a wide range of angles. The geologic processes that formed Ishtar are not well understood, but they probably included thickening of the Venusian crust in response to motions in the planet's mantle.

Aphrodite Terra is twice the size of Ishtar Terra and is comparable in area to South America. It extends from about longitude 60° E to 150° E. The topography of Aphrodite, more complex than that of Ishtar, is characterized by a number of distinct mountain ranges and several deep, narrow troughs. Its western extremity consists of two large curving ridges that partially surround a broad circular region of low-lying rugged terrain. Most of Aphrodite is formed by two broad upland regions, Ovda Regio in the central part and Thetis Regio farther east. Ovda spans about 4,000 km (2,500 miles) from north to south, Thetis about 3,000 km (1,900 miles). Both are composed primarily of tessera terrain. At its eastern extremity Aphrodite Terra merges into a complex of rift valleys and other tectonic features. The geologic processes that formed Aphrodite remain to be established, but they probably included thickening of the Venusian crust in response to motions in the planet's mantle.

Features Due to Tectonic Activity

Many of the surface features on Venus can be attributed to tectonic activity— that is, to deformational motions within the crust. These include mountain belts, plains deformation belts, rifts, coronae, and tesserae.

Belts and Rifts

Found in the terrae, Venus's mountain belts are in some ways similar to ones on Earth such as the Himalayas of Asia and the Andes of South America. Among the best examples are those that encircle Lakshmi Planum, which include Freyja, Akna, and Danu Montes.

The tallest mountain range on Venus is Maxwell Montes, which is particularly

Akna Montes, a mountain belt on Venus bordering Lakshmi Planum in Ishtar Terra, in a radar image obtained by the Magellan spacecraft. North is up. NASA/Goddard Space Flight Center broad and comparable in size to the Himalayas, rising to about 11 km (7 miles) above the planet's mean radius. First observed as a bright feature in Earth-based radar observations of the planet made in the 1960s, the region initially was dubbed Maxwell after the British physicist James Clerk Maxwell, whose formulation of the laws that relate electricity and magnetism is the basis of radar. Subsequent Earth-based and spacecraft radar observations revealed its mountainous nature.

A major feature of Maxwell Montes is Cleopatra, a circular depression near its eastern margin that has a diameter of slightly more than 100 km (60 miles)

and a depth of more than 2.5 km (1.6 miles). Suspected after its discovery of being a volcanic caldera, Cleopatra was later generally recognized to be an impact crater.

In radar images Maxwell Montes is one of the brightest features on Venus. This high radar reflectivity, responsible for the region's early discovery, is due in part to its very rugged nature. It apparently is also due to the highest elevations on Venus being coated with some as-yet-unidentified material, perhaps an iron-containing mineral such as pyrite or magnetite, that is unusually reflective at radar wavelengths.

Venus's mountain belts typically consist of parallel ridges and troughs with spacings of 5-10 km (3-6 miles). They probably developed when broad bands of the lithosphere were compressed from the sides and became thickened, folding and thrusting surface materials upward. Their formation in some respects thus resembles the building of many mountain ranges on Earth. On the other hand, because of the lack of liquid water or ice on Venus, their appearance differs in major ways from their counterparts on Earth. Without the flow of rivers or glaciers to wear them down, Venusian mountain belts have acquired steep slopes as a result of folding and faulting. In some places the slopes have become so steep that they have collapsed under their own weight. The erosional forms common in mountainous regions on Earth are absent.

Although plains deformation belts are similar in some ways to mountain belts, they display less pronounced relief and are found primarily in low-lying areas of the planet, such as Lavinia Planitia and Atalanta Planitia. Like mountain belts, they show strong evidence for parallel folding and faulting and may form primarily by compression, deformation, and uplift of the lithosphere. Within a given lowland, it is common for deformation belts to lie roughly parallel to one another, spaced typically several hundred kilometres apart.

Rifts are among the most spectacular tectonic features on Venus. The best-developed rifts are found atop broad, raised areas such as Beta Regio, sometimes radiating outward from their centres like the spokes of a giant wheel. Beta and several other similar regions on Venus appear to be places where large areas of the lithosphere have been forced upward from below, splitting the surface to form great rift valleys. The rifts are composed of innumerable faults, and their floors typically lie 1-2 km (0.6-1.2 miles) below the surrounding terrain. In many ways the rifts on Venus are similar to great rifts elsewhere, such as the East African Rift on Earth or Valles Marineris on Mars; volcanic eruptions, for example, appear to have been associated with all these features. The Venusian rifts differ from Earth and Martian ones, however, in that little erosion has taken place within them owing to the lack of water.

Coronae and Tesserae

Coronae (Latin: "garlands" or "crowns") are landforms that apparently owe their origin to the effects of hot, buoyant blobs of material, known from terrestrial geology as diapirs, that originate deep beneath the surface of Venus. Coronae evolve through several stages. As diapirs first rise through the planet's interior and approach the surface, they can lift the rocks above them, fracturing the surface in a radial pattern. This results in a distinctive starburst of faults and fractures, often lying atop a broad, gently sloping topographic rise. (Such features are sometimes called novae, a name given to them when their evolutionary relationship to coronae was less certain.)

Once a diapir has neared the surface and cooled, it loses its buoyancy. The initially raised crust then can sag under its own weight, developing concentric faults as it does so. The result is a circular-to-oval pattern of faults, fractures, and ridges. Volcanism can occur through all stages of corona formation. During the late stages it tends to obscure the radial faulting that is characteristic of the early stages.

Coronae are typically a few hundred kilometres in diameter. Although they may have a raised outer rim, many cor-onae sag noticeably in their interiors and also outside their rims. Hundreds of cor-onae are found on Venus, observed at all stages of development. The radially fractured domes of the early stages are

Oblique view of coronae in the Sedna Planitia lowlands of Venus, generated by computer from data collected by the Magellan spacecraft's radar imaging system. The topographic rise left of centre is a corona in an early evolutionary stage characterized by raised crust that is fractured in a radial pattern. The depression at the far right represents a corona in a later stage, in which the raised crust has sagged at the centre, with concentric fractures added to the radial ones. The image is highly exaggerated in its vertical direction—the more mature corona, for example, is about 100 km (62 miles) across but actually only about 1 km deep. NASA/JPL/Caltech comparatively uncommon, while the concentric scars characteristic of mature coronae are among the most numerous large tectonic features on the planet.

Tesserae (Latin: "mosaic tiles") are the most geologically complex regions seen on Venus. Gravity data suggest that the thickness of the crust is fairly uniform over much of the planet, with typical values of perhaps 20-50 km (1230 miles). Possible exceptions are the tessera highlands, where the crust may

Aine Corona and other volcanic features in a region on Venus to the south of Aphrodite Terra, shown in an image obtained from radar data gathered by the Magellan spacecraft. Aine Corona is the central large circular structure bounded by numerous arc-shaped concentric faults. It measures about 200 km (125 miles) across. Also visible are two flat-topped pancake domes, one to the north of the corona and a second inside its western border, and a complex fracture pattern in the upper right of the image. NASA/JPL

be significantly thicker. Several large elevated regions, such as Alpha Regio, are composed largely of tessera terrain. Such terrain appears extraordinarily rugged and highly deformed in radar images, and in some instances it displays several different trends of parallel ridges and troughs that cut across one another at a wide range of angles. The deformation in tessera terrain can be so complex that sometimes it is difficult to determine what kinds of stresses in the lithosphere were responsible for forming it. In fact, probably no single process can explain all tessera formation.

Tesserae typically appear very bright in radar images, which suggests an extremely rough and blocky surface at scales of metres. Some tesserae may be old terrain that has been subjected to more episodes of mountain building and faulting than have the materials around it, each one superimposed on its predecessor to produce the complex pattern observed.

Volcanic Features

Along with intense tectonic activity, Venus has undergone much volcanism. The largest volcanic outpourings are the huge lava fields that cover most of the rolling plains. These are similar in many respects to fields of overlapping lava flows seen on other planets, including Earth, but they are far more extensive. Individual flows are for the most part long and thin, which indicates that the erupting lavas were very fluid and hence were able to flow long distances over gentle slopes. Lavas on Earth and the

Highlands of tessera terrain rising from the plains region known as Leda Planitia in Venus's northern hemisphere, in an image produced from radar data collected by the Magellan spacecraft. Having an extraordinarily rugged appearance in radar images, the terrain displays several different patterns of ridges and troughs crisscrossing in various directions. Tesserae are the most geologically complex terrains known on Venus and may be the result of numerous consecutive episodes of mountain building. NASA/JPL

Moon that flow this readily typically consist of basalts, and so it is probable that basalts are common on the plains of Venus as well.

Of the many types of lava-flow features seen on the Venusian plains, none are more remarkable than the long, sinuous canali. These meandering channels usually have remarkably constant widths,

: ' "r ■■ ■

1 •!§»

v , •

Canali, or lava channels, in Venus's Lo Shen Valles region, north of the equatorial elevated terrain Ovda Regio, shown in a radar image from the Magellan spacecraft. Collapsed source areas for some of the meandering lava flows are visible in the image. NASA/Goddard Space Flight Center which can be as much as 3 km (2 miles). They commonly extend as far as 500 km (300 miles) across the surface; one is 6,800 km (4,200 miles) long. Canali probably were carved by very low-viscosity lavas that erupted at sustained high rates of discharge. In a few instances segments of canali appear to proceed uphill, which suggests that crustal deformation took place after the channels were carved and reversed the gentle downward surface slopes to upward ones. Other channel-like volcanic features on Venus include sinuous rilles that may be collapsed lava tubes and large, complex compound valleys that apparently result from particularly massive outpourings of lava.

In many locations on Venus, volcanic eruptions have built edifices similar to the great volcanoes of Hawaii on Earth or those associated with the Tharsis region on Mars. Sif Mons is an example of such a volcano. Located at the western end of the elevated region Eistla Regio, south of Ishtar Terra, it is about 2 km (1.2 miles) high and has a base 300 km (200 miles) in diameter. There are more than 100 others distributed widely over the planet. Known as shield volcanoes, they reach heights of several kilometres above the surrounding plains and can be hundreds of kilometres across at their base. Made up of many individual lava flows piled on

Sif Mons, in a low-angle computer-generated view based on radar data from the Magellan spacecraft. In this radar image, lava flows having rougher surfaces appear brighter than smoother flows and are therefore presumably more recent. NASA/JPL

one another in a radial pattern, they develop when a source of lava below the surface remains fixed and active at one location long enough to allow the volcanic materials it extrudes to accumulate above it in large quantities. Like those found on the rolling plains, the flows constituting the shield volcanoes are generally very long and thin and are probably composed of basalt.

When a subsurface source of lava is drained of its contents, the ground above it may collapse, forming a depression called a caldera. Many volcanic calderas are observed on Venus, both atop shield volcanoes and on the widespread lava plains. They are often roughly circular in shape and overall are similar to calderas observed on Earth and Mars. The summit region of Sif Mons, for example, exhibits a caldera-like feature 40-50 km (25-30 miles) in diameter.

Along with the extensive lava plains and the massive shield volcanoes are many smaller volcanic landforms. Enormous numbers of small volcanic cones are distributed throughout the plains. Particularly unusual in appearance are so-called pancake domes, which are typically a few tens of kilometres in diameter and about 1 km (0.6 mile) high and are remarkably circular in shape. Flat-topped and steep-sided, they appear to have formed when a mass of thick lava was extruded from a central vent and spread outward for a short distance in all directions before solidifying. The lavas that formed such domes clearly were

Volcanic pancake domes in the elevated region Eistla Regio on Venus, in a radar image produced from Magellan spacecraft data. The two larger domes, each about 65 km (40 miles) across, have broad flat tops less than 1 km (0.6 miles) high. They apparently were formed from unusually thick lava that oozed to the surface and spread in all directions. NASA

Merged pancake domes on the eastern edge of the Alpha Regio highland area of Venus, in an oblique view generated by computer from radar data gathered by the Magellan spacecraft. The volcanic features, each about 25 km (15 miles) in diameter and about750 metres (0.5 mile) high, are thought to have been formed from the extrusion of extremely viscous lava onto the surface. The vertical scale of the image is exaggerated to bring out topological detail. NASA/ JPL/Caltech much more viscous than most lavas on Venus. Their composition is unknown, but—given the knowledge of lavas on Earth—they are likely to be much richer in silica than the basalts thought to predominate elsewhere on the planet.

Volcanic edifices are not uniformly distributed on Venus. Although they are common everywhere, they are particularly concentrated in the Beta-Atla-Themis region, between longitudes 180° and 300° E. This concentration may be the consequence of a broad active upwelling of the Venusian mantle in this area, which has led to enhanced heat flow and formation of magma reservoirs.

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook

Post a comment