Surface Features

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Shown in the lower color insert on page C-7, Venus's surface is well known through the radar altimetry data of the Pioneer, Venera, Mariner, and Magellan missions craft. Though Venus's surface is covered with interesting features, it has little relief. Ninety percent of the surface lies within six-tenths of a mile (1 km) of the average level of the surface, equivalent to a planetary radius of 3,782.2 miles (6,051.5 km). The total elevation range of the planet is only about eight miles (13 km), while on Earth the range is about 12 miles (20 km). Venus also has the longest channel found anywhere in the solar system, the Dali and Diana Chasma system, which is 4,588 miles (7,400 km) long and an average of 1.1 miles (1.8 km) wide. The Nile River on Earth is a close second at 4,184 miles (6,695 km) long, while the Grand Canyon is only 250 miles (400 km) long, and the great Vallis Marineris system on Mars is 2,400 miles (4,000 km) long.

Optical Depth

^^ptical depth (usually denoted r) gives a measure of how opaque a medium is to radiation passing through it. In the sense of planetary atmospheres, optical depth measures the degree to which atmospheric particles interact with light: Values of r less than one mean very little sunlight is scattered by atmospheric particles or has its energy absorbed by them, and so light passes through the atmosphere to the planetary surface. Values of r greater than one mean that much of the sunlight that strikes the planet's outer atmosphere is either absorbed or scattered by the atmosphere, and so does not reach the planet's surface. Values of r greater than one for planets other than Earth also mean that it is hard for observers to see that planet's surface using an optical telescope.

Optical depth measurements use the variable z, meaning height above the planet's surface into its atmosphere. In the planetary sciences, r is measured downward from the top of the atmosphere, and so r increases as z decreases, so that at the planet's surface, r is at its maximum, and z is zero. Each increment of r is written as dr. This is differential notation, used in calculus, meaning an infinitesimal change in r The equation for optical depth also uses the variable K (the Greek letter kappa) to stand for the opacity of the atmosphere, meaning the degree of light that can pass by the particular elemental makeup of the atmosphere. The Greek letter rho (p) stands for the density of the atmosphere, and dz, for infinitesimal change in z, height above the planet's surface.

Mathematical equations can be read just like English sentences. This one says, "Each tiny change in optical depth (dr) can be calculated by multiplying its tiny change in height (dz) by the density of the atmosphere and its opacity, and then changing the sign

All of the features on Venus are named after famous women and include lowland volcanic plains, faults, rolling uplands, and plateaus. The largest of the plateaus, Ishtar Terra and Aphrodite Terra, are on the scale of continents. The surface has relatively few impact craters and has apparently been resurfaced by volcanic activity. Volcanoes on Venus come in several shapes and sizes, from small round domes to wide, shallow pancake-shaped mountains to the large round complex structures called coronae. Aside from the volcanic activity, Venus's thick atmosphere does serve to partially protect its surface from of the result" (this sign change is just another way to say that optical depth t increases as z decreases; they are opposite in sign).

To measure the optical depth of the entire atmosphere, this equation can be used on each tiny increment of height (z) and the results summed (or calculus can be used to integrate the equation, creating a new equation that does all the summation in one step). Optical depth also helps explain why the Sun looks red at sunrise and sunset but white in the middle of the day. At sunrise and sunset the light from the Sun is passing horizontally through the atmosphere, and thus has the greatest distance to travel through the atmosphere to reach an observer's eyes. At midday the light from the Sun passes more or less straight from the top to the bottom of the atmosphere, which is a much shorter path through the atmosphere (and let us remember here that no one should ever look straight at the Sun, since the intensity of the light may damage their eyes).

Sunlight in the optical range consists of red, orange, yellow, green, blue, indigo, and violet light, in order from longest wavelength to shortest (for more information and explanations, see appendix 2, "Light, Wavelength, and Radiation"). Light is scattered when it strikes something larger than itself, like a piece of dust, a huge molecule, or a drop of water, no matter how tiny, and bounces off in another direction. Violet light is the type most likely to be scattered in different directions as it passes through the atmosphere because of its short wavelength, thereby being shot away from the observer's line of sight and maybe even back into space. Red light is the least likely to be scattered, and therefore the most likely to pass through the longest distances of atmosphere on Earth and reach the observer's eye. This is why at sunset and sunrise the Sun appears red: Red light is the color most able to pass through the atmosphere and be seen. The more dust and water in the atmosphere, the more scattering occurs, so the more blue light is scattered away and the more red light comes through, the redder the Sun and sunset or sunrise appear.

impacts; the thick atmosphere burns up small meteorites from friction and slows larger meteorites.

Careful assembly of a geologic map has allowed scientists to determine the order of geological events. Liz Rosenberg and George McGill, geologists at the University of Massachusetts at Amherst, assembled one for the vicinity of Pandrosos Dorsa. There they found graben cutting lava flows, so the lava flows had to have formed before the graben. Graben are long low areas formed by crustal extension. When the planet's crust is stretched, faults form that allow blocks to

Formation of a Graben

Graben, long, low areas bounded by faults, are formed by crustal extension.

fall down relative to the original surface elevation, as shown in the figure above. Repeated graben in the American southwest form the Basin and Range region.

The complete sequence of geological events, pieced together by many scientists, is as follows: Belts of folds and fractures formed early in Venusian history, followed by tessera, then plains and shield volcanoes, then grabens, and then late belt fracturing. The tessera are areas with regular fracturing. (For more on names for planetary landforms, see the sidebar "Fossa, Sulci, and Other Terms for Planetary Landforms," on page 142.)

Craters

Venus's surface has about 1,000 craters, indicating that it was most recently resurfaced between 300 and 500 million years ago. Venus's surface is therefore one of the youngest in the solar system, along with the Earth and some active moons such as Io and Europa. While larger craters on Venus have a similar range of shapes to those on other planets, from bowl-shaped to complex, smaller craters are frequently irregular or complex. Venus's thick atmosphere affects the speed and integrity of bolides entering its atmosphere and creates a different range of sizes and shapes of craters than are found on other terrestrial planets. The odd shapes of smaller craters are interpreted as resulting from bolide breakup while passing through the thick atmosphere.

Complex craters account for about 96 percent of all craters on Venus with diameters larger than about nine miles (15 km). Complex craters are thought to be formed by the impact of a large intact bolide with a speed that has not been strongly effected by its passage through the dense Venusian atmosphere. Complex craters are characterized by circular rims, terraced inner wall slopes, well-developed ejecta deposits, and flat floors with a central peak or peak ring. Barton crater (named after Clara Barton, founder of the Red Cross), shown below in a radar image, is a good example of a complex crater. Barton crater has two rings and a central peak and is 31 miles (50 km) in diameter.

The largest known crater on Venus is Mead crater, 168 miles (280 km) in diameter. Mead is named for Margaret Mead, the American anthropologist who lived from 1901 to 1978. Mead is a good example of a complex crater, as shown in the radar image on page 145. The crater shows multiple rings but no central peak. The peak may be

Barton crater, 31 miles (50 km) in diameter, is an excellent example of a complex crater on Venus. (NASA/Magellan/WD

Fossa, Sulci, and Other Terms for Planetary Landforms

O n Earth the names for geological features often connote how they were formed and what they mean in terms of surface and planetary evolution. A caldera, for example, is a round depression formed by volcanic activity and generally encompassing volcanic vents. Though a round depression on another planet may remind a planetary geologist of a terrestrial caldera, it would be misleading to call that feature a caldera until its volcanic nature was proven. Images of other planets are not always clear and seldom include topography, so at times the details of the shape in question cannot be determined, making their definition even harder.

To avoid assigning causes to the shapes of landforms on other planets, scientists have resorted to creating a new series of names largely based on Latin, many of which are listed in the following table, that are used to describe planetary features. Some are used mainly on a single planet with unusual features, and others can be found throughout the solar system. Chaos terrain, for example, can be found on Mars, Mercury, and Jupiter's moon Europa. The Moon has a number of names for its exclusive use, including lacus, palus, rille, oceanus, and mare. New names for planetary objects must be submitted to and approved by the International Astronomical Union's (IAU) Working Group for Planetary System Nomenclature.

Feature astrum, astra catena, catenae chaos chasma, chasmata colles corona, coronae crater, craters dorsum, dorsa facula, faculae fluctus

Nomenclature for Planetary Features

Description radial-patterned features on Venus chains of craters distinctive area of broken terrain a deep, elongated, steep-sided valley or gorge small hills or knobs oval-shaped feature a circular depression not necessarily created by impact ridge bright spot flow terrain

Feature

Description

fossa, fossae

narrow, shallow, linear depression

labes

landslide

labyrinthus, labyrinthi

complex of intersecting valleys

lacus

small plain on the Moon; name means "lake"

lenticula, lenticulae

small dark spots on Europa (Latin for freckles); may

be domes or pits

linea, lineae

a dark or bright elongate marking, may be curved or straight

macula, maculae

dark spot, may be irregular

mare, maria

large circular plain on the Moon; name means "sea"

mensa, mensae

a flat-topped hill with cliff-like edges

mons, montes

mountain

oceanus

a very large dark plain on the Moon; name means "ocean"

palus, paludes

small plain on the Moon; name means "swamp"

patera, paterae

an irregular crater

planitia, planitiae

low plain

planum, plana

plateau or high plain

reticulum, reticula

reticular (netlike) pattern on Venus

rille

narrow valley

rima, rimae

fissure on the Moon

rupes

scarp

sinus

small rounded plain; name means "bay"

sulcus, sulci

subparallel furrows and ridges

terra, terrae

extensive land mass

tessera, tesserae

tile-like, polygonal terrain

tholus, tholi

small dome-shaped mountain or hill

undae

dunes

vallis, valles

valley

vastitas, vastitates

extensive plain

(continues)

Fossa, Sulci, and Other Terms for Planetary Landforms

(continued)

The IAU has designated categories of names from which to choose for each planetary body, and in some cases, for each type of feature on a given planetary body. On Mercury, craters are named for famous deceased artists of various stripes, while rupes are named for scientific expeditions. On Venus, craters larger than 12.4 miles (20 km) are named for famous women, and those smaller than 12.4 miles (20 km) are given common female first names. Colles are named for sea goddesses, dorsa are named for sky goddesses, fossae are named for goddesses of war, and fluctus are named for miscellaneous goddesses.

The gas giant planets do not have features permanent enough to merit a nomenclature of features, but some of their solid moons do. Io's features are named after characters from Dante's Inferno. Europa's features are named after characters from Celtic myth. Guidelines can become even more explicit: Features on the moon Mimas are named after people and places from Malory's Le Morte d'Arthur legends, Baines translation. A number of asteroids also have naming guidelines. Features on 253 Mathilde, for example, are named after the coalfields and basins of Earth.

obscured by the flat, radar-bright material filling the crater's center. This bright filling may be melted surface material from the energy of the impactor, or it may be volcanic magma produced deeper in Venus from impact-related internal changes, which then erupted into the crater.

Irregular craters make up about 60 percent of the craters with diameters less than about nine miles (15 km). The second class of Venusian craters, irregular craters, are thought to form as the result of the impact of bolides that have been fragmented during their passage through the atmosphere. Irregular craters are characterized by irregular rims and hummocky or multiple floors. The dense atmosphere also slows smaller meteorites sufficiently so that they cannot produce craters when they strike the surface. There are virtually no impact craters smaller than two miles (3 km) on the surface ofVenus.

Venus's very thick atmosphere also may be responsible for an unusual feature of its craters.Venus's craters commonly have distinc-

Mead crater, at 168 miles (280 km) in diameter, is Venus's largest known crater. (NASA/Magellan)

tive flows out from their edges, different from craters on other planets. When an asteroid or meteorite strikes a planetary surface, the impact is immensely powerful and hot, and a plume of vaporized material, consisting of part of the planetary surface as well as the impacting body, expands rapidly upward, much like the blast from an explosion. On the Moon, Mars, or other bodies with thin atmospheres, the vapor cloud rapidly expands and becomes less dense. On Venus, the atmosphere may be so dense itself that it contains the vapor cloud, which then condenses and runs out of the crater along the ground, rather than rising into the Venusian atmosphere and dispersing. Both laboratory experiments and computer modeling suggest this may be the case, as their results match what is seen on Venus, but proving a theory like this is nearly impossible, unless high-speed photos could be taken of the process actually occurring on the planet.

Howe crater, shown in the lower color insert on page C-8 in a combined altimetry and radar image, is two miles (37 km) in diameter. Howe crater is named for Julia Howe, the American biographer and poet. The crater in the background to the left of Howe is Danilova, 30 miles (48 km) in diameter and named for the Russian ballet dancer Maria Danilova. In the right background lies Aglaonice crater, named for an ancient Greek astronomer, with a diameter of 39 miles (63 km).All three of these craters show the run-out flows from their rims that are distinctive of craters on Venus.

Beneath and between Venus's scanty impact record stand immense floods of volcanism and a series of fascinating structural features, including numerous volcanoes, lava plains, and plateaus. Over them all are printed the effects of crustal compression and extension, in the form of faults and folds.

Faults and Folds

Venus's surface has ample evidence of strong crustal deformation in the form of linear faults and folds. In places the faults are aligned roughly in parallel; in other places they are randomly oriented into what is called chaos terrain; in still others, the fault sets intersect at angles and form patterns similar to a tiled floor. This final pattern, called tesserae, may form shapes from narrow diamonds to squares. The polygons in tesserated areas have diameters of a half to 15 miles (1 to 25 km), with an average of about 1.3 miles (2 km).

Tesserae regions are thought to be the oldest visible crust on Venus. They stand higher than the surrounding volcanic plains, as shown in the radar image on page 147, and comprise about 15 percent of the surface ofVenus.The right-hand side of the image shows a large region of tesserae, while on the left a bright volcanic flow has approached the tessera across the plain. The age sequence in this image is therefore, from oldest to youngest, the bright, highly fractured tesserar highlands, the dark lowland volcanic seas, and the recent bright volcanic flow. This image was taken in the Eistla Regio in Venus's Northern Hemisphere, at about 1° south latitude and 37° east longitude.

Debra Buczkowski and George McGill, scientists at the University of Massachusetts at Amherst, have used detailed radar maps ofVenus from the Magellan mission to more clearly understand these early Venusian features. By examining the patterns of radar reflection the xtf&F

pif • - . . . XT1 , linear crustal features can be identified as folds (which rise above the surrounding surface and indicate that the crust was compressed) or graben (which fall beneath the surrounding surface and indicate extension). In some regions, in particular around a volcano called Imini Mons, radial ridges surround the volcano and are superimposed on an earlier linear set of folds (called wrinkle ridges) along with a few graben. The researchers found that the pressure of magma in a chamber beneath the volcano, combined with a regional compressive state in the crust, is sufficient to create radial ridges around the volcano in addition to the regional linear sets. Using computer models such as these based on careful examination of regional geological radar images, scientists can actually recreate some of the tectonic settings of the planet's distant past.

Tesserae (right) and a volcanic flow (left) are both clearly visible in this radar image of Venus's surface. Tesserae regions are thought to be some of the oldest crust on the planet, while the younger volcanic flows flow onto them. (NASA/Magellan)

Plateaus and Mountains

Venus has two giant plateaus, similar in size to terrestrial continents: the Aphrodite Terra and the Ishtar Terra.The Ishtar Terra is the largest of Venus's plateaus. At 3,200 miles (5,000 km) wide Ishtar Terra is larger than the continental United States. The plateau is surrounded by steep flanks and mountain ranges, and the western half of Ishtar Terra is an unusually smooth and even area named the Lakshmi Planum.The Lakshmi Planum itself is about the size ofTibet.

The four major mountain ranges of Venus—Maxwell Montes, Frejya Montes, Akna Montes, and Danu Montes—all surround the Lakshmi Planum. Maxwell Montes is on the east coast, Akna Montes on the west coast, Frejya Montes to the north, and Danu Montes in a portion of the south.The center of Ishtar Terra is split by the Maxwell Montes, mountains 7.5 miles (12 km) high.The Maxwell Montes are the highest mountains on Venus and 1.5 times as high as Mt. Everest on Earth.

The Magellan image shown in the figure below is 180 miles (300 km) in width and 138 miles (230 km) in height and is centered at 55° north latitude, 348.5° longitude, in the eastern Lakshmi Planum.This part of the Lakshmi Planum is relatively flat and composed of many

The Lakshmi Planum shows bright and dark volcanic flows (radar brightness corresponds to surface roughness) and dark patches that are thought to be formed by impactors that break up in the atmosphere before forming craters. (NASA/Mage//an/JPL)

The Lakshmi Planum shows bright and dark volcanic flows (radar brightness corresponds to surface roughness) and dark patches that are thought to be formed by impactors that break up in the atmosphere before forming craters. (NASA/Mage//an/JPL)

lava flows. On top of the lava flows are three dark splotches, created by impactors that broke up in the thick atmosphere before creating a crater.

The terrae on Venus are distinct and striking features that are intriguing to structural geologists. Though there are hypotheses for the formation of the large plateaus that involve large-scale mantle flow, there is no one leading idea.

Volcanoes

The surface ofVenus displays more than 1,000 volcanic centers.There are about 170 large shield volcanoes similar to Hawaii on Earth, up to several hundred miles across and 2.5 miles (4 km) high. Many of the almost 1,000 smaller shield volcanoes appear in clusters.These smaller shield volcanoes appear in a number of unusual shapes. Some flat-topped, steep-sided circular shield volcanoes intermediate in size have been dubbed "pancake domes." Other shield volcanoes of intermediate size have radiating faults that resemble the legs on an insect, leading to their being informally called "ticks."

Together the activity of these many volcanoes have produced more than 200 large flow fields, areas completely covered with multiple lava flows.Widespread volcanism resurfaced the planet and is thought to have ended 300—500 million years ago. So much resurfacing occurred that most of the planet's impact craters were buried, and now the planet has fewer than 1,000 craters on its surface.

Volcanic plains make up about 80 percent of Venus's surface. An example of the flat, featureless resurfaced volcanic plains is shown in the Magellan radar image on page 150. A small grouping of volcanoes on the left create a shield volcano. Scalloped dome volcanoes appear on the right; the scalloped look arises from faulting around the edges of the volcano. The farthest right feature is a volcanic caldera. The image area is 303 by 193 miles (489 by 311 km).

Many of the small shield volcanoes have ridges both beneath them and cutting through their shields. The shields form a thin, widespread veneer.Analyses of data from Venera 9 and 10 and Vega 1 and 2 landing sites indicate that the shield plains with wrinkle ridges probably consist of tholeiitic lava, the same kind of dark, easily flowing lava that is erupted at Hawaii on Earth. Because the volcanic lava on Venus apparently flowed so easily, some scientists think it contained water, which reduces the viscosity of lava. There may be a water-bearing mineral

Venus's lava plains are flat and among the youngest surface features on the planet. (NASA/Magellan/JPL)

shallow in Venus's lithosphere that melts easily, erupting low-viscosity melts across Venus's surface.The lava could have contained little silica and much calcium carbonate, a combination that produces a lava that flows easily at the high surface temperatures of the planet. Maat Mons is the largest volcano on Venus and stands five miles (8 km) above sur-roundings.The volcano has dark flows to its east, probably representing low-viscosity lava that cooled with a smooth surface. Other flows from the volcano are bright, probably indicating a rough flow that reflects radar well. The differences in reflectivity may represent different magma compositions, an important clue to internal processes in Venus.

Photos from the surface of Venus taken by Venera 14 show many thin horizontal layers of rock near the landing site. Each layer is only a couple of inches in thickness.Though some scientists have suggested these may be sedimentary layers, perhaps a more likely possibility is that the layers are volcanic tuffs. Tuffs are consolidated layers of volcanic ash, laid down by air falls. Along with the compositional data on the lava flows sent back by Venus landers, these thin volcanic layers show that volcanic activity on Venus is similar to that on the Earth, though apparently more voluminous.

Venus is probably still volcanically active, and some scientists regularly search for radar data that show hot fresh volcanic flows on the planet.Whether Venus's surface is resurfaced continuously or periodically, humankind has a chance to see active volcanism on a planet other than Earth.

Coronae and Arachnoids

Coronae are large, roughly circular features, some with domes in their centers and some with depressed features like calderas. They were first recognized by Soviet radar images from early missions, a great accomplishment considering the technology available. Coronae all have roughly circular rims, sometimes in series, and sometimes accompanied by domes, plateaus, depressions, moats, chasms, or volcanic flows.Arachnoids are also complex combinations of faulting and volcanism. They are smaller than coronae, at 30—140 miles (50—230 km) in diameter, with a central volcanic feature surrounded by a complex network of fractures. There are about 210 coronae and 270 arachnoids in total on Venus.

The combinations of faulting and volcanism differ among coronae. Quetzalpetlatl is among the largest of the coronae, with a diameter of 500 miles (800 km).There appears to have been abundant volcanism at the site, and the formation has a moat and a rim at its edge. Another corona, Heng-O, has a diameter of 680 miles (1,100 km) and outer rim heights of 0.25—1 mile (0.4—1.6 km) but no recent volcanic activity.

Artemis, by far the largest corona, has a diameter of 1,615 miles (2,600 km), large enough to reach from Colorado to California, were it on Earth. Artemis contains complex systems of fractures, numerous flows and small volcanoes, and at least two impact craters, the larger of which is located in the lower left (southwest) quadrant of the feature as shown in the radar image on page 152. The fractures that define the edge of Artemis, called Artemis Chasma, form steep troughs with raised rims approximately 75 miles (120 km) wide and with as much as 1.6 miles (2.5 km) of relief from the rim crest to the bottom of the trough. Lava plains are tilted away in all directions.The stripes in the image are simply missing data. The coronae appear to

Artemis is by far Venus's largest corona, consisting of a complex system of faults and volcanic features 1,615 miles (2,600 km) in diameter. (NASA/Magellan/JPL)

have started to form long before the regional plains and to have continued long after.

The image in the figure on page 153 shows two coronae, the Bahet and Onateh.This large image is a mosaic taken around 49° north latitude and 2° east longitude. Bahet, on the left, is 138 by 90 miles (230 by 150 km). Onateh is larger, at about 210 miles (350 km) in diame-ter.This large image was taken with a resolution of 400 feet (120 m) per pixel, allowing detailed examination of the surface. Each corona is surrounded by rings of troughs and ridges.

There are a number of theories for the formation of coronae. The original theories postulated that a hot, rising plume of mantle material from inside Venus rose and pushed against the bottom of Venus's lithosphere (see the sidebar "Interior Structure of the Terrestrial Planets," on page 82). A hot, buoyant plume would push up on the lithosphere, causing it to rise in a dome. When the plume subsides, the dome will subside, but the surface will show radial fractures and ring-shaped ridges, typical of the coronae seen on Venus.

Other studies using computer-based numerical modeling by Sue Smrekar, Ellen Stofan, and Trudi Hoogenboom, scientists at the Jet Propulsion Laboratory, and Gregory Houseman, a scientist at the University of Leeds, have shown that these features can be made by a process called delamination. In this scenario, the lower lithosphere drips from the upper lithosphere and crust and sinks into the Venusian mantle. During the dripping and sinking process, the surface of the planet is first pulled down (causing radial fractures) and then relaxes back to close to its original height, causing ring-shaped ridges.These researchers have been able to reproduce most of the complex and variable coronae topography by combinations of warm upwellings and the stresses of delamination.

Channels

Channels are common on Venus's plains and are thought to be created by flowing lava. The extremely hot and caustic surface conditions of

Bahet and Onateh coronae appear to be connected by a series of faults. Coronae are thought to be formed by some combination of rising mantle plumes and sinking lithospheric drips. (NASA/Magellan/JPL)

Polygonal terrain is crossed by a portion of the Baltis Vallis channel, believed to have been formed by low-viscosity flowing lava. (NASA/Magellan/JPL)

Venus make flowing water impossible, but they allow lava to remain heated and flow for much longer distances than it can on Earth. Normal silica-rich lava is an unlikely candidate, though, because of the extreme volumes and low viscosities required to form channels of such length.The composition of the fluid that carved Venus's channels remains a mystery.

In the image below, the right-hand side is covered with a polygonal terrain, while running up the left side is a 370-mile (600-km) segment of the largest channel on Venus, the Dali and Diana Chasma sys-tem.The Dali and Diana Chasma system consist of deep troughs that extend for 4,588 miles (7,400 km). The Nile River, Earth's longest river, is 3,470 miles (5,584 km) from Lake Victoria to the Mediterranean Sea.Though much deeper than this channel on Venus, Valles Marineris on Mars is only 2,800 miles (4,200 km) long. The Dali and Diana Chasma system is therefore the longest channel in the solar system.

This dark Venusian channel shows the wide curves of a lava river rather than the tight bends of a water channel. (NASA/Mage/lan/JPL)

This dark Venusian channel shows the wide curves of a lava river rather than the tight bends of a water channel. (NASA/Mage/lan/JPL)

right side of the top edge of the image and winding diagonally toward the bottom left of the image.This image also shows a large region of polygonal terrain, where the surface of Venus has been broken into regular patterns by faulting.

A long, radar-dark sinuous channel that particularly resembles Earth water channels is shown in the Magellan image on page 155.The Venusian channels, however, are far less tightly sinuous than those on Earth, and the Venus channels are commonly associated with dark units that appear to be volcanic flows. Even this narrow, well-defined channel was almost certainly produced by flowing liquid lava.

Though Venus's thick and continuous cloud cover foiled for centuries the attempts of astronomers to see its surface, the development of radar in the 20th century made imaging Venus possible. Through the intense efforts of the Soviets (outlined in the next chapter) and the immensely successful Mariner and Magellan missions, almost the whole planet has now been mapped. Venus has a young volcanic surface and few impact craters.The planet carries an immense number of volcanoes, perhaps more than the total on Earth, including those in oceanic plates. Though it has no plate tectonics, curious features called coronae may indicate patterns of mantle convection that have pulled down or pushed up the crust and created faulted volcanic provinces.Venus is almost certainly still volcanically active.

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