I. MAGELLAN IMAGES VENUS
In size, density and composition, Venus is almost identical to Earth, yet its radar images picture a new, unearthly world whose nature and history turn out to be quite different.
Impact craters on Venus have lava-filled interiors and asymmetric fluid-like ejecta; small craters sometimes appear in clusters and very small meteoroids never make it to the ground.
Vertical motions associated with upwelling hot spots have buckled, crumpled, fractured and stretched the surface of Venus.
Venus was resurfaced by rivers of outpouring lava about 500 million years ago, and tens of thousands of volcanoes now pepper its surface.
Venus contains unique volcanic features, termed pancake domes, arachnoids, and coronae.
A. Unveiling Venus
The Global Perspective
Click Here to view a Magellan radar image of Venus
The surface of Venus cannot be seen; it can only be sensed by radar, whose pulses can be used to create images. A radar image is essentially a map of topography and surface roughness or radar brightness. The altitudes are inferred from the length of time it takes for a radar pulse to reach a particular part of the surface and return an echo. In radar images, the smooth areas are dark, somewhat like a wet road seen in the headlights of a car at night, while the rougher areas are bright.
Radar images obtained from the orbiting Magellan spacecraft have been assembled to map about 99 percent of the surface. The entire world has been mapped with a clarity and fine detail that is not available for much of the Earth. (High-resolution charts of the ocean floor probably remain military secrets.)
In size, density and composition, Venus is almost identical to Earth, yet its radar images picture a new, unearthly world whose nature and history turn out to be quite different. Venus is a smorgasbord of volcanism, a greenhouse gone wild! Its surface is fractured and crumpled by upwelling heat, blackened by massive outpourings of lava, scarred by beautiful outflows surrounding ancient impact crater, and punctuated by unique volcanic constructs never seen before.
On Venus we see a transformed surface without the erosive effects of time. There is no water on Venus, and the hot, heavy turgid atmosphere lies on the ground without strong winds or seasons. Since there is no erosion, everything is totally exposed and largely preserved - at least between periods of intense volcanic activity or internal upheaval. So, the radar images show the entire structural history of the surface, for at least 500 million years. (In contrast, erosion from water, wind and glaciers have ground away the outer layers of the oldest mountains on Earth. ) In places, we can therefore detect pristine features or the long-term evolution of the surface of Venus; in others we see a hopeless superposition of recurrent episodes of surface change.
The radar maps show that most of Venus is an extraordinarily smooth world whose surface is quite different from ours. Without its water, the Earth would appear to have two main levels: the ocean floors and the continents. In contrast, the surface of Venus is largely at one level, and more than 80 percent of the surface lies within one kilometer of the average planetary radius, 6052 kilometers (Fig. 4.10). These low-lying areas are called plains, or the Greek planitiae. They have been smoothed by a coating of lava.
Some Magellan radar images of the landscape on Venus have been artificially colored in electric orange and searing yellow, perhaps to remind us of its volcanic origin. As suggested from Russian lander observations, an orange tint approximates the color of light that filters through the thick atmosphere, but there can be no fires on Venus without oxygen. Moreover, the black sky in these images is unreal; it would be appropriate for an airless body like the Moon, but then we do not know what the sky on Venus actually looks like from the surface.
Molten material has burned paths in the preexisting lava deposits, creating long, narrow channels that meander thousands of kilometers across the planet's surface and remain only about a kilometer wide over their entire length. These riverlike features were formed not by water, but by lava that remained liquid over distances that are longer than the Nile, the longest river on Earth. The high surface temperature on Venus probably kept the lava liquid, and prevented the cooling flow front from damming up the flow behind it. The very flat surface of Venus may explain why the channels remained within narrow boundaries without forming any lava lakes or tributaries.
Although most of Venus's terrain consists of smooth, volcanic plains, about 10 percent of the planet's surface consists of highlands that tower above the plains. There are two large-scale elevated regions that punctuate the smoothed-out surface (Fig. 4.11, Fig. 4.12). They are Aphrodite Terra in the equatorial regions, and Ishtar Terra in the far north; Aphrodite and Ishtar are respectively the Greek and Babylonian goddesses of love. Aphrodite Terra is over 10,000 kilometers long and covers a quarter of the planet's circumferance at the equator, while Ishtar Terra fills about half the planet's circumferance at its high northern latitudes and is about the size of Australia.
Aphrodite Terra is part of a globe-circling chain of elevated regions, collectively referred to as the equatorial highlands (Fig. 4.13); they extend from Beta Regio across quasi-circular features, known as coronae, to the largest area Aphrodite. The equatorial highlands also contain a network of long fractures, or rift valleys, where subsurface forces are pulling the crust apart and cracking it open. The linear rift zones in the equatorial highlands can extend for thousands of kilometers, but are cracked apart by just a few kilometers. (In contrast, rifts that split open the Earth's continents can open up to make way for its widest oceans.)
Ishtar Terra contains a high plateau, Lakshmi Planum, that is ringed by towering mountain ranges. The loftiest peak is Maxwell Montes - the only feature on Venus named after a man, the physicist James Clerk Maxwell (See Focus 4C, Naming features on Venus). Maxwell rises to Himalayan altitudes, standing over 11 kilometers above the surrounding terrain, and has crumpled flanks formed by compression somewhat like the great mountain belts on Earth.
Because of the heat and weakness of the rocks on Venus, the mountains on Venus have the gradual slopes of gentle hills, though the highest peaks rival Mount Everest in height. Radar images of tall mountains on Venus are also unusually bright, apparently due to highly reflective material located only at high elevations. This may be caused by a weathering process associated with the temperature and pressure conditions at these heights.
B. Craters on Venus
Click here to view a radar image of the crater Isabella
Venus is peppered with impact craters, though not as liberally as the Moon. Large impact craters on Venus are relatively scarce when compared with the closely-spaced, overlapping craters in the lunar highlands. At one time Venus was probably as heavily cratered as the Moon, but the relatively small number and wide spacing of the craters on Venus indicate that the surface we now see is much younger.
Of course, all satellites and planets, including the Moon and Venus, are roughly the same age, about 4.6 billion years old, but their surfaces differ in age depending upon the amount of external erosion and internal activity. When the Moon's cratering rate is scaled to Venus, the relative paucity of craters on its surface indicates an average age of 500 million years; this makes the surface of Venus practically new-born compared with the Moon's which was heavily bombarded 4 billion years ago.
The relative youth of Venus's surface is not by itself surprising. The Earth's ocean floor is being continually renewed by volcanic outflow, and is nowhere more than 200 million years old. And Jupiter's moon Io is so volcanically active that no impact craters are found on its surface. Other objects like the Moon, Mercury and many satellites retain their ancient heavily-cratered terrain because they are not internally active.
What is unexpected is that the spatial distribution of craters of all sizes on Venus is nearly everywhere the same, with no great concentration anywhere, and that very few craters on Venus have been significantly modified by external volcanic flows or by surface deformation (Fig. 4.14). The uniform distribution and pristine nature of the impact craters may be the result of global resurfacing by widespread volcanism that wiped the face of Venus clean about 500 million years ago. After that, volcanic activity declined significantly but did not cease.
Some catastrophic internal event seems to have triggered a wholesale inundation of the surface of Venus, perhaps when the crust became weakened by the intense heat and was dragged down into the hotter underlying material. (Such a global catastrophe is quite different from the slow, gradual changes that have transformed much of the Earth's surface.) What we are probably seeing now is cratering by meteorites since the catastrophic volcanic resurfacing 500 million years ago; this explains the random distribution of craters and the fact that very few of them have been flooded by the external flow of lava.
Following impact, large objects leave craters that at first sight resemble those on the Moon. That is, large craters on Venus contain
central peaks, flat floors, distinct circular rims, inner terraced walls, smooth (radar-dark) floors and rough (radar-bright) ejecta beyond the rims (Fig. 4.15). Compared to Earth, there is little weathering or surface erosion on dry Venus, despite the planet's dense atmosphere, so its craters retain a look of freshness, as on the Moon. But the dense atmosphere on Venus affects both the incoming meteoroid and its ejected debris, creating features that are unlike any other craters in the solar system.
Because of the dense atmosphere and greater gravity, the ejecta that is thrown out of a crater on Venus does not go as far as it does on the Moon. The drag of the atmosphere and the greater downward pull of gravity on Venus confines the ejecta to narrow, deep, rough and radar-bright regions near the craters (Fig. 4.16). In contrast, the Moon has no atmosphere and less gravity, so debris flys far from impact in ballistic trajectories that can nearly circle the Moon.
The bright apron of debris that surrounds large craters on Venus often has a lobate, petal-like appearance with an unexpected asymmetry (Fig. 4.17). Material that is ejected from the crater becomes entrained in the hot, thick atmosphere, transforming it into a turbulent, fluid-like substance. The material flows and spreads out from the crater, creating patterns that resemble flowers or butterflies, rather than hurtling away from it to great distances. (On Mars, crater ejecta sometimes cause lobate flows, but they are attributed to the melting of underground ice - an entirely different cause.) Moreover, when the impact on Venus is oblique, the atmospheric wake of the incoming meteoroid prevents the ejecta from scattering back in the direction from which the meteoroid came, so the radar-bright ejecta is missing in this region.
Remarkably, craters smaller than 15 kilometers in diameter often appear in clusters and have noncircular rims and multiple hummocky floors. These crater clusters are attributed to the breakup and dispersion of the incoming meteoroid by the dense atmosphere. In one case, four little craters are clustered together around a common center, as if they all arrived at the same time along the same trajectory (Fig. 4.18). They probably originated when a larger projectile fragmented in Venus's thick atmosphere; the fragments would continue on the same trajectory and cause the close group of craters.
Venus exhibits no craters smaller than a few kilometers across. Any incoming object that would make a smaller crater is completely destroyed before reaching the ground. The dense atmosphere heats up and burns the smaller incoming projectiles. So there are relatively few small craters and fine dust particles on Venus. By way of comparison, the Moon has no atmosphere, and the small meteorites pulverize the surface into a thick layer of dust.
C. Tectonic Processes on Venus
Click here to view a 3-D perspective view of Venusian terrain composed of radar images merged with altimiter data from the Magellan spacecraft
The surface of Venus has also been deformed by internal forces, which have buckled, crumpled, fractured and stretched the crust, like a face seamed and thickened by age (Fig. 4.19). These crustal deformations, and the internal changes affecting them, are known as tectonics, the Greek word for building. They are due to heat that is pent-up within the hot interior of Venus.
All planets have heat to get rid of. The heat is either left over from their formation, by the collision of smaller bodies, or is still being generated by the decay or transmutation of radioactive elements deep inside the planet. Because Venus and the Earth are about the same size, and composed of the same rocky stuff, they should have a comparable amount of internal heat, so it was once thought that their surfaces would result from similar tectonic events.
On Earth, processes associated with the outward flow of heat move the continents, create new ocean floors, set off earthquakes and ignite volcanoes. The outer solid regions of the Earth are broken into a mosaic of large plates, thousands of kilometers across, which move laterally across its surface. They are continuously being formed on one side, where the surface cracks open at mid-ocean ridges, and destroyed at the other side by diving within the planet's innards in deep-ocean trenches or by grinding together to raise mountains. The plates are rigid, so any movement on one side also happens at the other; in between the plates move sideways carrying the continents with them.
There is no evidence for a system of sliding plates on Venus. If the crust was spreading sideways, than the terrain should get older as you move away from the source of crustal spreading; but the crater data indicate that the large-scale surface of Venus is everywhere about the same age. Moreover, there is no evidence for such a system of plates on Venus. There is no long, connected system of ridges on Venus (like that at bottom of the Earth's oceans), and there are no volcanic arcs that are similar to the Earth's deep-ocean trenches. The outer solid shell of Venus therefore seems to consist of one rigid plate, not many shifting plates as on Earth. This probably also explains the one-level topography on Venus that is unlike Earth's two-level topography.
There is no water on the surface of Venus, and the planet may also be exceptionally dry inside, with little or no water to lubricate plate motion. The outer shell of Venus is probably seized up tight, like a car engine without oil, so there can be no large-scale, global horizontal motions. In addition, the surface of Venus is now halfway to the melting temperature of rock, so its outer solid regions may be ductile and pliable, unable to form rigid slabs that are stiff enough to slide around in large intact pieces.
An alternative explanation for the lack of plate tectonics on Venus is that an episode of catastrophic resurfacing may have released much of the planet's pent-up heat long ago. In that case, the interior of Venus could now be relatively cool, when compared to that of the Earth, and Venus may not now have enough internal heat to form plates and drive them sideways.
Whatever the exact explanation, Venus must have had vast churning reservoirs of hot material beneath its crust, and probably still does. When the molten rock becomes swollen by heat, it becomes lower in density and rises through the cooler, overlying high-density material, carrying heat upward like bubbles in a pot of boiling water. The upward-flowing magma wants to puncture a hole in the overlying material, or break on through to the other side, and release all that pent-up heat. When subjected to these forces, the crust's motion is mainly up and down, or predominantly vertical, rather than sideways, and the surface wrinkles, rises or cracks open largely in place.
As the surface moves up in some locations and down in others, the associated stresses pull the surface apart in some places and push it together in others. Over time, these vertical motions occur in different places, creating stresses in different directions, probably producing a strange criss-crossed pattern of fractured terrain that is only found on Venus (Fig. 4.20). These far-ranging systems of intersecting ridges and grooves are called tesserae, the Greek word for tiles. Repeated tectonic activity has deformed some of the high-lying tesserae into a chaotic terrain with multiple linear and curvilinear structures at a variety of scales (Fig. 4.21).
The greatest tectonic deformation on Venus is seen in its four mountain ranges - Akna, Freyja, Maxwell and Danu Montes (Fig. 4.22). They surround the highland plateau, Lakshmi Planum, and reach as high as 11 kilometers above the surrounding terrain. The belts of mountains, with their banded ridges and narrow valleys, resemble mountain ranges on Earth, but the mountains on Venus are almost certainly not related to the colliding continents that formed the Alps and Himalayas on Earth. The mountains of Venus seem instead to be related to upwelling and volcanism.
With the surface temperature so high, rock behaves plastically like Silly Putty, and over long times it acts like a fluid, so its a wonder that there are any high-standing mountains on Venus. They would tend to sink down, spread out and collapse over time under the relentless force of gravity, like the curved shoulders and sagging breasts of an aging woman. When left alone, a mountain on Venus should collapse under its own weight in about 10 million years.
How does a mountain taller than Mount Everest remain so high when its rocks are halfway to their melting point? Something has to be holding it up. On Earth, any large topographic feature like a mountain is compensated for, and held up, by a low-density root projecting downward into the denser interior; this balances the mass excess of the mountain and the combination is in equilibrium, like an iceberg floating on the ocean.
Such a compensation can be detected by its gravitational effect on a satellite passing overhead, in much the same way that mass concentrations, called mascons, were discovered on the Moon. A satellite slows down over low-density regions where the pull of gravity is lower, and speeds up over high-density ones with greater gravitational pull.
Measurements by the Magellan spacecraft indicate that the strength of gravity matches the planet's topography, so the highest spots exert the strongest gravity with larger gravitational effects than terrestrial mountains. This suggests that the mountains on Venus have a much deeper compensation than those on Earth, and that the highland areas on Venus are connected to the deep interior of the planet. Upwelling plumes of molten material, which are hotter and less dense than their solid surroundings, seem to be doing the compensating and holding up the mountains and highlands of Venus.
Volcanoes and broad highland regions probably arise over plumes of hot material welling up from deep within the planet. The hot, low-density molten rock behaves like a sluggish fluid, rising upward with glacial slowness in spite of its heat. Eventually, the magma spreads out beneath the surface, pushes it up, and sometimes punches holes in the crust, like a welder's torch, providing a conduit that permits lava to flow out from volcanoes and fissures in the planet. The associated upwelling can also cause the crust to rupture and spread, forming rift valleys with steep sides and sunken floors. (The Earth has similar hot spots and rift valleys - one hot spot formed the Hawaiian Islands with its large shield volcanoes like Mauna Loa, and Africa is now being split apart. )
D. Volcanoes on Venus
Click here for a view of a volcano on Venus
Tens of thousands of volcanoes, ranging from Hawaii-sized edifices to more numerous domes of a few kilometers across, have been identified on the face of Venus. The small domes pop up everywhere on the surface, while the larger shield volcanoes are generally located on top of broad, regional rises. The planet exhibits every type of volcanic edifice known on Earth, and some that have never been seen before. And it has apparently been completely resurfaced by rivers of outpouring lava. So, Venus has been volcanic to a degree unmatched on Earth!
One of the highest volcanoes, Maat Mons, has none of the reflective material found on most other high peaks. Its radar-dark peak may therefore be relatively young, perhaps one that erupted so recently that weathering has not yet occurred (Fig. 4.23). But there is no definite evidence of currently active volcanoes on Venus; and even if there were, the extreme surface temperature and lack of water make it unlikely that they would be explosive. (By way of comparison, the melted rocks in terrestrial volcanoes contain an abundance of water that vaporizes with explosive energy when released.)
Most of the Cytherean volcanoes have the low slopes and round shapes of shield volcanoes on Earth. They are built up from runny lava that spreads out over great distances with the ease of spilt olive oil, forming low round structures of gentle slope. Major shield volcanoes on Venus are hundreds of kilometers across and only a few kilometers high (Fig. 4.24).
On the other hand, a smaller number of flows appear to be built from lava that was as stiff and thick as batter. In places, the sluggish lava has oozed onto the hot, flat surface, forming volcanic domes as round and flat as pancakes (Fig. 4.25). Each one has a dark feature almost precisely at the center, suggesting a vent from which the pasty lava flowed, like pancake batter on a hot griddle. Some of them even have little craters or pits on them that resemble bubbles that have burst in the batter. So, depending on the internal conditions when the magma formed, the resulting lava has the consistency and viscosity of either motor oil or toothpaste, and this helps determine the size and shape of the resulting volcanic formations.
Volcanism on Venus also produces larger, elevated circular structures, hundreds of kilometers across, that are termed arachnoids, for their spiderlike appearance, and coronae, the Latin word for crown (Fig. 4.26, Fig. 4.27). Unlike the much smaller pancakes, which are due to lava extruded onto the surface, coronae are caused by molten rock that pushes up the ground from underneath. These uniquely Chytherean landforms are probably the tops of plumes that are smaller and weaker than those that hold up the mountains and highland regions on Venus. When the hot rock in these lesser plumes rises up to the surface, it presses against it, causing the ground to bulge and fracture and forming a circular dome that will lift higher as time goes on.
The increasing pressure may stretch the planet's skin until it bursts, like the broken cheeze bubbles in a pizza or a split in an overcooked hotdog. Lava will then spill out on the surface, and the weight of the corona, no longer supported from below, may buckle the surrounding terrain, creating a trench that resembles the moat around a castle.
Or else, the plume may just get old and die down, and the molten rock will drain back down the vent from whence it came. Then the dome will collapse like a giant souffle, creating a ring of fractures and a crumpled, cracked surface.
Several Russian Venera spacecraft provided additional evidence for volcanic activity on Venus when they landed on the surface to take pictures of the terrain. It has a sweeping flow to it, containing flat slab-like rocks with sharp edges as well as loose material (Fig. 4.28). The fresh-appearing rocks and soil suggest that active volcanic processes may be replacing the old eroded surface. As the molten lava spreads across the surface and cools, it may produce the thin, fractured, layer of rock that we see in the Venera photographs.
Chemical analysis of the surface made by the Venera landers indicates that its composition is mainly basaltic, such as the lavas on the Earth's ocean floors and the Moon's mare. This basalt was identified at several landing sites, suggesting that volcanoes once poured lava over much of Venus's surface and perhaps this process continues today.
There is abundant evidence for volcanoes and lava flows on Venus, but the current observations have not yet determined whether the planet is geologically alive or dead today. Variations in the observed amounts of atmospheric sulfur dioxide have, for example, been attributed to volcanoes that are now belching forth gases, and Maat Mons could be an active volcano; but simple atmospheric circulation might also cause the sulfur dioxide variations and no one has caught a volcano in the act of erupting on Venus.
Although uncertainties about current volcanic activity have not yet been settled, there is a wide variety of evidence suggesting that Venus has been a dynamic, active world dominated by volcanism on a global scale. There are nevertheless no moving plates on Venus. The planet therefore seems to be in an arrested state of development, perhaps because Venus lost its oceans long ago. We therefore now turn out attention to the water planet, Earth.
Focus 4C Naming features on Venus
Major topographic provinces on Venus are termed Chasmata, or canyons, Montes, or mountains, Planitiae, or low plains, Regiones, for areas of moderate relief, and Terrae, to describe extensive highlands. Features found only on Venus include Tesserae, or tiles, and ovid-shaped features called Coronae. The surface of Venus also contains numerous impact craters and at least one Planum, for plateau. Each one of these feature types is preceded by the last name of a notable woman in history, mythology or religion. The only exceptions are Maxwell Montes, named in honor of the British physicist James Clerk Maxwell, and Alpha and Beta (Regio), the first two letters of the Greek alphabet.
Different types of features are named for different categories of goddesses or mortal women. Chasmata are named for goddesses of the hunt, Coronae for goddesses of fertility, Planitiae for mythological heroines, Regiones for Titanesses, Terrae for goddesses of love, and Tesserae for goddesses of fate or fortune. Smaller features such as impact craters are designated by the last names of famous women who have been dead at least three years; an example is the jazz singer Billie Holiday. (Hurricanes on Earth are also named for women, but by their first names.)
The surface features shown in Fig. 4.12 are named:
| NAME |
ATTRIBUTE |
| Aphrodite Terra | Greek goddess of love |
Atlanta Planitia | Greek huntress associated with golden apples |
| Guinevere Planitia | British, wife of Arthur |
| Gula Mons | Babylonian Earth mother, creative force |
| Ishtar Terra | Babylonian goddess of love |
/TR>
| Lada Terra | Slavic goddess of love |
/TR>
| Lakshmi Planum | Indian goddess of love and war |
| Maat Mons | Ancient Egyptian goddess of truth and justice |
| Metis Regio | Greek Titaness |
| Niobe Planitia | Greek, changed into stone while weeping for her 12 children killed by Artemis and Apollo |
| Ovda Regio | Titaness with supernatural powers |
| Phoebe Regio | Greek Titaness |
| Sapas Regio | Phoenician goddess |
| Sedna Planitia | Eskimo, her fingers became seals and whales |
| Sif Mons | Teutonic goddess, Thor's wife |
| Tellus Tessera | Greek Titaness, Roman goddess of the Earth |
| Tethus Regio | Greek Titaness |
| Themis Regio | Greek Titaness |
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These names can be suggested by anyone, but eventually they have to be cleared and approved by the International Astronomical Union.
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