7. Venus: The veiled planet


The phases of Venus

The phases of Venus

. When fully illuminated, Venus looks small and far away; its apparent size is about seven times larger when the crescents are narrowest and Venus is nearest the Earth. After observing similar phases with his small telescope in 1610, Galileo wrote “Cynthiae figuras aemulatur mater amorum”, or “The mother of love (Venus) emulates the figure of Cynthia (Moon)”. The phases and variation in the apparent size of Venus provided important early evidence that the planets revolve around the Sun rather than the Earth. (Lowell Observatory photograph.)


Surface rocks on Venus

Surface rocks on Venus

. On 5 March 1982 the Venera 14 lander touched down on Venus at 13 degrees south latitude and 310 degrees east longitude, where it survived for just one hour before succumbing to the planet’s heat. That was long enough to radio back these photographs of the surface of Venus, which include part of the lander and a mechanical arm at the bottom. The thin, plate-like slabs of rock could be due to molten lava that cooled and cracked. The composition and texture of these rocks is similar to terrestrial basaltic lava. (Courtesy of Iosif Shklovskii.)


Wind speeds and cloud layers of Venus

Wind speeds and cloud layers of Venus

. At all altitudes in the thick atmosphere of Venus, the dominant retrograde winds blow westward with a speed that increases with height. From a gentle breeze near the ground, the wind speed increases to 100 meters per second at great heights. Space probes that penetrated the planet's clouds and balloons that floated in its atmosphere have been able to detect three distinct layers in the opaque sulfurous clouds. The top layer contains small droplets of sulfuric acid; the middle layer contains larger but fewer particles. The bottom layer is the densest and contains the largest particles; it is comparable to a bad city smog in visibility. Beneath the lowest layer, the atmosphere is hot enough to vaporize all particles, so it is relatively clear down there.


Winds blow crater ejecta

Winds blow crater ejecta

. A radar image of Adivar Crater is surrounded by a streamlined, horseshoe or parabolic shapes (top and bottom right) and a bright, jet-like streak (center left) that extend over the surrounding plains. These unusual features, seen only on Venus, are believed to result from the interaction of ejected debris and high-speed winds in the upper atmosphere, blowing from the east (right). The crater is 30 thousand meters in diameter, and is named for the Turkish educator and author Halide Adivar; it is located at 9 degrees north latitude and 76 degrees east longitude, just north of the western Aphrodite highland. (Courtesy of JPL and NASA.)


Hadley cells

Hadley cells

. Incident solar energy drives a Hadley-cell circulation of the atmosphere, which keeps heat from building up at any one location. Air rises in warm regions near the equator on the sunlit side of Venus, where the planet is heated most by the Sun. Some of the warm air flows towards cooler zones near the poles, and sinks and returns to warm equatorial regions again. Strong winds also blow around the planet in the direction that Venus rotates, from east to west.


Interaction with the solar wind

Interaction with the solar wind

. Even though Venus has no appreciable magnetic field, the solar wind is prevented from reaching the surface by Venus's dense atmosphere and by electrical currents induced in its conducting ionosphere. The planet has a well-developed bow shock, but it does not have belts of trapped particles. Both the ionosphere and the extended corona of hot gas, derived from the upper atmosphere, help to divert the solar wind.


Surface terrain

Surface terrain

. The side-looking radar images and altimetry data from Magellan have been merged to obtain this three-dimensional view of the terrain on Venus. It includes radar-dark, lowland plains and radar-bright highlands. A hot upwelling of magma is believed to have formed 200-thousand-meter wide Nagavonyi Corona in the foreground; just behind the corona is a shield volcano that is 2 thousand meters high. Nagavonyi is a Ganda (Uganda) crop goddess; the distant highlands include Phoebe Regio, a region named for the Greek Titaness (Courtesy of JPL and NASA.)


Surface elevations for Earth, Venus and Mars

Surface elevations for Earth, Venus and Mars

. Most places on Earth (center) stand near one of two prevailing levels, the high-standing continents or the low-lying ocean floors. In contrast, the surface of Venus (right) is unusually smooth and flat; about 60 percent of its terrain lies within 500 meters of the mean planetary radius of 6,051.9 thousand meters. A small percentage of its terrain consists of elevated highlands that are comparable in height to many continents on Earth. The surface features on Mars (left) are spread over a broader range of elevation than most of those on Venus; but the Martian surface elevations are not double-peaked as on Earth.


Major surface features on Venus

Major surface features on Venus

. Most of Venus lies at roughly the same radius, in the lowland, volcanic plains (white areas). The highland massifs (dark areas) include Aphrodite Terra, an elongated feature extending along the equator between 70 degrees and 210 degrees east longitude. All of the elevated regions near the equator mark the sites of extensive volcanism, such as Maat Mons (Fig. 2.27) and Sapas Mons (Figs. 7.16, 7.17). The other main elevated region is Ishtar Terra in the north at about 0 degrees longitude (center top). Ishtar Terra is roughly the size of Australia; the elevated plateau in its western part, Lakshmi Planum, is bounded on all sides by mountain (Fig. 7.18), including Maxwell Montes. This mountain's 11-thousand-meter height above the average radius exceeds by two thousand meters the height of Mount Everest above sea level.


Lava flows

Lava flows

. This Magellan image shows a lava-filled canyon produced when Venus’s crust was pulled apart and magma rose within the gap. The lowland plainss contain many similar canyon systems, typically about 10 thousand meters wide and up to 1000 thousand meters long, apparently filled with solidified lava flow. This region, located near the equator between Asteria Regio and Phoebe Regio, is about 10 thousand meters wide and stretches 600 thousand meters to the north (top). (Courtesy of JPL and NASA.)


Sinuous Channel

Sinuous Channel

. This segment of a sinuois lava channel in Lavinia Planitia is approximately 200 thousand meters long and just 2 thousand meters wide. Such channel-like features are common in the lowland plains of Venus. They meander across the plains, like terrestrial rivers, but apparently formed by fluid lava that melts its way across the surface. The very flat surface of Venus may explain why the channels remain within narrow boundaries without forming any lava lakes or tributaries. (Courtesy of JPL and NASA.)


Stuart Crater

Stuart Crater

. This beautiful crater exhibits asymmetric radar-bright ejecta attributed to an oblique impact and interaction with the dense, thick atmosphere (Section 2.2). Its rim, which is 67 thousand meters in diameter, encircles a bright floor that may have unique physical properties. However, the lava that surrounds them has not modified most craters on Venus, including this one. This indicates that the craters formed after the volcanic lowland plains. This impact crater’s name honors Mary Stuart, Queen of Scots (1542-1587). (Courtesy of NASA.)


Volcanoes on Venus

Volcanoes on Venus

. Radar-reflectivity and topographic altitudes from Magellan have been combined to construct this three-dimensional perspective view of the surface of Venus. Lava flows extend for hundreds of thousands of meters in the foreground at the base of the volcano Sapas Mons (center). Another volcano, Maat Mons, appears on the horizon; a close-up view of Maat Mons was shown in Fig. 2.27. The volcanic origin of these peaks is suggested by the fiery orange and yellow colors, but there can be no fires on Venus without oxygen. The artificial tints are based on color images taken by the Veneras 13 and 14 landers, simulating the color of sunlight that filters through the thick atmosphere. Sapas is named after a Phoenician goddess, Maat is the name of an ancient Egyptian goddess of truth and justice, and Mons is the Latin term for “mountain”. (Courtesy of JPL and NASA.)


Surface of Venus

Surface of Venus

. A radar system on board the Magellan spacecraft has mapped the surface of Venus to a resolution of 120 meters, producing this global view of the planet's terrain. A vast equatorial system of highlands and ridges runs from the continentlike feature Aphrodite Terra (left of center) through the bright highland Atla Regio (just right of center) to Beta Regio (far right and north). This image is centered at 180 degrees longitude, and drawn using a sinusoidal projection that does not distort the area at different latitudes. A more traditional projection of the Magellan global data was shown in Fig. 2.12. Dark areas are correspond to terrain that is smooth at the saale fo the radar wavelength of 0.13 meters; bright areas are rough. (Courtesy of JPL and NASA.)


Sapas Mons

Sapas Mons

. Located in an equatorial highland called Atla Regio, the volcano Sapas Mons is about 400 thousand meters in diameter and only 1.5 thousand meters high; it is named after a Phoenician goddess. The sides of the volcano are composed of numerous overlapping lava flows from flank eruptions similar to terrestrial volcanoes such as the Hawaiian shield volcanoes. The summit contains two smooth, radar-dark mesas, as well as groups of pits, some as large as one thousand meters across. They may have formed when underground chambers of magma were drained, causing the surface to collapse above them. A 20-kilometer-diameter impact crater northeast (upper right) of the volcano is partly buried in lava flows. (Courtesy of NASA.)


Mountain ranges

Mountain ranges

. Venus and Earth are the only planets in our solar system that have mountain belts. The highland massif of Ishtar Terra in the northern hemisphere of Venus includes a huge plateau, named Lakshmi Planum. It is about the size of Africa, rises 3.5 thousand meters above the surrounding terrain, and is bordered by the Akna, Danu, Freyja and Maxwell mountains, each about one million meters in extent. Maxwell Montes stands 11 thousand meters above the mean radius. Akna and Freyja are the respective names of the Yucatan goddess of birth and the Norse mother of Odin. Danu is the Celtic mother of god. Maxwell is named after the British physicist James Clerk Maxwell, while Cleopatra is the Egyptian queen who had affairs with Julius Ceaser and Mark Anthony. The Blackfoot Indian woman, Sacajawea, guided the Lewis and Clark expedition to the Pacific Northwest, and Claudine Collette was a French novelist. (Adapted from a USGS map using Arecibo, Pioneer Venus and Venera 15 and 16 radar data.)


Atla Regio

Atla Regio

. Fractures, seen as bright, thin lines, criss-cross the volcanic deposits in part of the Atla region of Venus. The fractures are not buried by the lava, indicating that the convulsive tectonic activity post-dates most of the volcanic activity. This Magellan radar image is approximately 350 thousand meters across, and centered at 9 degrees south latitude and 199 degrees east longitude. Several circular volcanoes, surrounded by radar-bright lobes , are also present. This region is named Atla after a Norse giantess, mother of Heimdall. (Courtesy of JPL and NASA.)


Bereghiny Planitia

Bereghiny Planitia

. This high-resolution Magellan mosaic shows an area in the low-lying plan Bereghinya Planitia, named for the Slavic water spirit. The image is 1.84 million meters wide, and centered at 45 degrees north latitude and 11 degrees east longitude. Its fractured surface is attributed to plumes of magma that rise from inside Venus, pushing against the planet’s crust but not breaking through it. The most prominent features are the circular and oval structures that are sometimes called arachnoids for the web-like appearance of their fractures (Fig. 7.12). Also visible are lava flows, impact craters, and volcanic domes. (Courtesy of NASA.)


Alpha Regio

Alpha Regio

. This Magellan image mosaic reveals the radar-bright, highly fractured southeastern portion of Alpha Regio (left), forming an intricate pattern of ridges and valleys. The complexly deformed, high-standing terranis are called tesserae. The image, which is approximately 600 thousand meters across, also shows dark, smooth volcanic planes that border the eastern edge of Alpha Regio. Within these dark plains are eight rounded, pancake domes up to 35 thousand meters across and up to 1 thousand meters high (right center and bottom right). Alpha is the first letter of the Greek alphabet. (Courtesy of NASA.)


Ovda Regio

Ovda Regio

. The surface of Venus is laced with fractures, ranging from elaborate networks of fine cracks to giant canyons millions of meters long. Episodes of fracturing and folding have created the high-standing domes and ridges shown in this highland area known as Ovda Regio; it forms the western part of Aphrodite Terra. The low-lying valleys between these ridges were then flooded by lava, creating this surrealistic radar image of bright, closely-packed islands in a dark sea. An impact crater 60 thousand meters across (top left) is superimposed on the older flood lava. An extensive fracture system extends outward like spokes from the circular feature at the bottom right; these fractures also cut across older volcanic features. This Magellan radar image is 600 thousand meters wide. The region is named Ovda after a Titaness having supernatural powers. (Courtesy of NASA.)


Arachnids

Arachnids

. An enlarged view of a Magellan mosaic of Bereghinya Planita (Fig. 7.9), showing circular structures of up to 230 thousand meters in diameter. Their central domes are surrounded by concentric and radial fractures, like a spider in its web. Such features have been informally dubbed arachnids for their web-like appearance. They are similar in form, but usually smaller than, the circular volcanic structures known as corona (Fig. 7.13). (Courtesy of NASA.)


Fotla Corona

Fotla Corona

. Large circular and oval structures with diameters of 200 thousand to 1 million meters are called coronae. The one shown at the center of this Magellan image is known as Fotla Corona, about 200 thousand meters across and named after the Celtic fertility goddess. It is located in a vast plain to the south of Aphrodite Terra, and is centered at 59 degrees south latitude and 164 degrees east longitude. Molten rock rising from the interior of the planet most likely explains the corona’s circular shape, raised topography, complex fractures, and associated volcanism. Just north (top) of this corona is a flat-topped pancake dome, about 35 thousand meters in diameter, thought to have formed by the eruption of sluggish, pasty lava. Another pancake dome is located inside the western (left) part of the corona. There is also a smooth, flat region in the center of the corona, probably a relatively young lava flow. Complex fracture patterns like the one in the north-east (top-right) of the image are often observed in association with coronae. (Courtesy of JPL and NASA.)


Tessera

Tessera

. Beccause there is little soil and no egetation or erosion to confuse us, we can see tectonic paterrns on Venus much more easily than on our own planet. This region, covering an area of 37 thousand meters by 80 thousand meters long, is located at 30 degrees north latitude and 333 degrees east longitude, on the low rise separating Sedna Planitia and Guinevere Planitia. It shows a criss-crossed pattern of radar-bright lines which appear to be faults or fractures. The orthogonal system of ridges and grooves is formed in elevated terrain (by 1 or 2 thousand meters), and spaced at regular intervals of 1 to 20 thousand meters. Known as tessarae, from the Latin word for “tiles”, the features suggest repeated episodes of intense surface fracturing that may be unrelated to volcanic activity. This type of terrain is not seen on any other planet. It covers about 8 percent of Venus's surface. Although tessarae are the oldest visible landform on Venus, they are young by planetary standards, having formed within the last billionb years. Sedna is an Eskimo goddess whose fingers became seals and whales, and Guinevere is the legendary Queen of the British King Arthur. (Courtesy of NASA.)


Chaotic tessera terrain

Chaotic tessera terrain

. Many episodes of surface deformation apparently created this complicated version of the more ordered tessera shown in Fig. 7.22. The complex pattern of ridges and valleys has was probably formed by several episodes of faulting, folding, shearing, compression and extension. The chaotic tessera terrain is often found at relatively high elevations up to 3 thousand meters above the surrounding plains; the Magellan image shown here is located at about 25 degrees south latitude and 1 degrees east longitude within Alpha Regio. The largest ridges and troughs are about 10 thousand meters wide and less than 70 thousand meters long. Alpha is the first letter of the Greek alphabet. (Courtesy of NASA.)


Vertical motions

Vertical motions

. The surface features on Venus have been formed by vertical, up and down motions driven by molten rock welling up from the planet's interior. When rising bubbles of hot rock press against the crust, they can form domed, cracked features known as coronae. Larger rising plumes support many of the highlands of Venus, while sinking regions can lead to mountain formation. When the molten rock breaks through the surface it forms towering volcanoes.


Impact craters, terrain type, and terrain ages

Impact craters, terrain type, and terrain ages

. Impact craters (top diagram) are randomly scattered all over Venus. Most are pristine (white dots). Those modified by lava (red dots) or by faults (triangles) are concentrated in places such as Aphrodite Terra. Areas with a low density of craters (blue background) are often located in highlands. Higher crater densities (yellow background) are usually found in the lowland plains. The terrain type (middle diagram) is predominnanately volcanic plains (blue). Within the plains are deformed areas such as tessarae (pink) and rift zones (white), as well as volcanic features such as coronae (peach), lava floods (red) and volcanoes of various sizes (orange). Volcanoes are not concentrated in chains as they are on Earth, indicting that plate tectonics does not operate. Terrain age data (bottom diagram) indicate that volcanoes and coronae tend to clump along equatorial rift zones, which are younger (blue) than the rest of the Venusian surface (green). Tesserae, ridges and plains are older (yellow). In genreal, however, the surface lacks the extreme variation in age that is found on Earth and Mars.


Summary diagram

Summary diagram

. Summary diagram