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.

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