7. Venus: The veiled planet

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Basic data

Physical properties of Venus*

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Mass 4.870 x 1024 kilograms = 0.815 ME
Mean radius 6.0519 x 106 meters = 0.949 RE
Mean mass density 5,244 kilograms per cubic meter
Rotation period 243.025 Earth days, retrograde
Orbital period 224.7 Earth days
Mean distance from Sun 1.08157 x 1011 meters = 0.723 AU
Age 4.6 x 109 years
Atmosphere 96 percent carbon dioxide, 3.5 percent nitrogen
Surface pressure 92 bars
Surface temperature 735 degrees kelvin
Magnetic field strength less than 3 x 10-9 tesla or 10-5 BE
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* The symbols ME, RE, and BE denote respectively the mass, radius and magnetic field strength of the Earth.

The veiled planet

The view from Earth

Fig. .. 

When viewed through a telescope, Venus brightens and fades, and also changes in apparent size, during its dance around the Sun. As noticed by Galileo Galilei (1564-1642) in 1610, the planet exhibits a complete sequence of Moon-like phases. Its apparent illumination goes from a full round disk to a narrow crescent and back to rotundity again every19 months. This was one of the earliest indications that the planets move around the Sun rather than the Earth. Venus also appears to grow when it approaches us in its orbit and shrinks as it recedes. When Venus is furthest from the Earth, on the opposite side of the Sun, it is fully illuminated and smallest. As the planet comes closer to Earth, it looks partly illuminated and larger.

Venus rotates backwards, at an exceptionally slow rate

Although no human eye has ever seen the surface of Venus, radio waves can penetrate its obscuring veil of clouds and touch the landscape hidden beneath. By bouncing pulses of radio radiation off the surface, radar astronomers discovered in 1967 that Venus spins in the backward direction, opposite to that of its orbital motion. That is, unlike the other terrestrial planets, Venus does not rotate in the direction in which it orbits the Sun. The radar observations also showed that Venus spins with a period longer than any other planet, at 243.025 Earth days. This rotation period is even longer than the planet's 224.7 Earth-day period of revolution around the Sun, so the day on Venus is longer than its year.

Penetrating the clouds

A hot and heavy atmosphere

Venus's thick, carbon-dioxide atmosphere traps the Sun's heat, raising the ground temperature by the greenhouse effect to almost three times what it would be without an atmosphere, and to about as hot as a self-cleaning oven. An airless body at Venus’s distance from the Sun would be warmed by solar radiation to a surface temperature of only about 230 degrees kelvin, below the freezing point of water, but the greenhouse effect raises the surface temperature to a sizzling 735 degrees kelvin. That is hot enough to boil the ground dry, and to incinerate any humans that might visit the planet. The massive atmosphere imposes a pressure that is 92 times that at sea level on Earth. It would crush you out of existence. The surface pressure is comparable to that experienced by a submarine 500 fathoms, or 1,000 meters, below the surface of our terrestrial oceans.

Fig. .. 

Some scientists thought that the high temperatures and pressures would melt, flatten and chemically weather the surface into a featureless plain. However, the surface photographs showed fresh-appearing rock without eroded edges.

Clouds of concentrated sulfuric acid

What accounts for the unbroken layer of pale yellow clouds that covers Venus? A detailed study of the sunlight reflected from the uppermost clouds indicates that the reflecting cloud particles have a spherical shape, implying that the particles are liquid droplets rather than ice crystals. Water and other plausible liquids were ruled out because they have the wrong reflecting and refracting properties. Baffled astronomers found the answer in the 1970s. A combination of spectroscopy and polarimetry, or how the cloud droplets polarize light, showed that the clouds of Venus are composed of concentrated sulfuric acid! That is the same sulfuric acid that is commonly used in car batteries.

Circulation of the atmosphere

Fig. .. 

The wind speed increases with altitude, rising to about 100 meters per second in the clouds at about 70 thousand meters in height. The high-flying clouds race around the planet once every four Earth days, from east to west in the same backwards direction that the planet rotates. So, the top of the atmosphere is blown around Venus more than 50 times faster than the planet rotates; such a rapid motion is sometimes called super-rotation. These high-speed zonal (east-west) winds are driven by the rotation of the solid planet beneath them, but the exact mechanism for maintaining the flow is not well understood.

Fig. .. 

The atmosphere and winds have transformed the impact craters on Venus, which are unlike those seen on any other world. The dense atmosphere affects the impact debris, changing it into fluid-like flows, and the material ejected during impact is moved by the winds. Some fresh craters are surrounded by radar-bright haloes, streamlined hoods and tail-like wind streaks that act like wind vanes, pointing downwind at the time of impact. The wind streaks indicate that the winds just above the surface were blowing toward the equator from the northern and southern hemisphere.

Fig. .. 

The atmosphere redistributes heat from one part of Venus to another, thereby moderating temperature differences. Most of the sunlight falling on Venus is either reflected by the clouds or absorbed in them. And because the Sun's rays fall directly on the equator and obliquely at the poles, the equatorial clouds are initially warmer than the polar ones. But this temperature difference generates winds that transfer heat in a single large Hadley cell.

Fig. .. 

Energetic ultraviolet sunlight ionizes some of the atoms and molecules in the outer atmosphere above the clouds, forming an electrified layer similar to the Earth's ionosphere, and this layer helps shield the ground from the solar wind. The ions provide conduction paths for electrical currents that produce forces that counter the wind. As a result, the solar wind slows down and is deflected around the planet in a bow shock, and the interplanetary magnetic field is draped back to form a magnetotail. The Pioneer Venus Orbiter found that the solar wind’s interaction with Venus changes on times scales of hours to years, depending on the vagaries of the wind, with a bow shock that expands and contracts in step with the 11-year cycle of solar magnetic activity.

Unveiling Venus with radar

Magellan and its predecessors

No human eye has ever gazed on the surface of Venus; it can only be sensed by radio transmissions. Radar, an acronym for radio detection and ranging, uses its own source of radio radiation, and does not need sunlight to probe the planet, gathering data day or night. Only radar is capable of piercing the thick clouds of sulfuric acid that blanket Venus.

Fig. .. 

Magellan’s radar images revealed a rich and varied landscape with stunning and unprecedented clarity, describing a surface whose nature and history turned out to be quite different from those of the Earth. The surface of Venus is covered by massive, global outpourings of lava, punctuated by unique volcanic constructs never seen before, scarred by sparse, pristine impact craters surrounded by beautiful outflows, and fractured, stretched, crumpled and split open by upwelling magma. Even hardened professional astronomers were inspired with a sense of wonder at these discoveries.

A smoothed-out world

Fig. .. 

Radar data from the Pioneer Venus Orbiter and Magellan showed that Venus is an extraordinarily smooth world, largely at one low level and quite different from the Earth. About 85 percent of the surface lies within one thousand meters of the average planetary radius, 6,051.9 thousand meters. A coating of lava has smoothed these vast low-lying plains. Without its water, the topography of the Earth occurs at two distinct elevations, which correspond to the continents and ocean floors.

Fig. .. 

Although most of Venus's terrain consists of smooth, low-lying, volcanic plains, about 15 percent of the planet's surface consists of highlands that tower above the plains, rising an average 4 thousand to 5 thousand meters above the mean planetary radius. There are two large-scale elevated regions that punctuate the smoothed-out surface; they are Aphrodite Terra in the equatorial region and Ishtar Terra in the far north.

Aphrodite Terra is over 10 million meters long and covers a quarter of the planet's circumference at the equator. It contains tall volcanoes, long lava flows and deep faults and fractures. Western Aphrodite is built from the massifs Ovda and Thetis Regiones; the eastern part of Aphrodite is occupied by Atla Regio. Ishtar Terra fills about half the planet's circumference at its high northern latitudes and is about the size of Australia. It consists of an elevated plateau encircled by narrow mountain belts.

Volcanic plains

Planet-wide covering of lava

Fig. .. 

Spreading lava has flooded and filled the low-lying regions of Venus, creating extensive smooth plains that cover about 85 percent of Venus’s surface. The volcanic nature of these lowland plains, each designated by the term planitia, is demonstrated in the Magellan images. You can practically see the molten rock spreading like heavy cream across these plains, often running for hundreds of thousands of meters down gentle slopes. The magma has risen from within canyons as the crust pulled apart, cooling and solidifying into lava flows that look like frozen river currents.

Fig. .. 

In other places, the molten material has burned paths in the preexisting lava deposits, following a narrow, sinuous smoothly curving course. They can meander for millions of meters across the planet's surface. Many end in outflows that look like river deltas. These river-like channels were formed not by water, but by lava that was hot enough to carve through solid rock, remaining hot and 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 molten rock behind it.

A relatively young surface

Fig. .. 

Venus, like all planets, has been subjected to a continual rain of meteorite bombardment over the aeons. The plains of Venus are uniformly peppered with impact craters, the scars of this bombardment, though nowhere near as liberally as on the surfaces of the Moon, Mars and Mercury. On Venus the craters of a given size are far fewer in number and more widely spaced than on the Moon. At one time Venus was probably as heavily pockmarked with large craters as the Moon’s ancient surface is, but the scarcity of the craters now on Venus indicates that the surface we now see is much younger than the lunar surface. The average surface age on Venus is 750 million years.

Highland massifs

Towering volcanoes

Fig. ..  Fig. .. 

Fifteen percent of the Venus surface comprises highlands, where the largest volcanoes are found, concentrated on or near the equatorial highlands. Large-scale plumes of rising magma have probably pushed up this globe-circling, elevated region from below. When the molten rock pierced the surface, lava flowed out to form towering volcanoes that are perched atop the raised highlands. Some of the volcanoes are found in Beta Regio on the western side of the equatorial highlands. Several rise out of Atla Regio in the eastern end of Aphrodite Terra, including Maat Mons and Sapas Mons.

Fig. .. 

One of the highest volcanoes, Maat Mons, rises 9 thousand meters above the surface, and spreads 200 thousand meters across it. Sapas Mons is shorter and broader. Both peaks are known as shield volcanoes, because they have the shape of a shield or an inverted plate. Similar giant shield volcanoes are found in the Hawaiian Islands, each with a broad base and gentle slopes.

Mountain ranges

Fig. .. 

The region of Venus that most closely resembles terrestrial mountains is Ishtar Terra, located at far northern latitudes. It consists of an elevated plateau, Lakshmi Planum, which is bounded on three sides by mountain belts the Danu, Akna and Freya Montes. Lakshmi just drops off into the surrounding plains on the fourth side, forming an immense cliff. The belts of mountains, with their banded ridges and narrow valleys, resemble mountain ranges on Earth, and the loftiest peak, Maxwell Montes, rises to Himalayan altitudes, standing over 11 thousand meters above the surrounding terrain.

Tectonics on Venus

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The hot rising material has buckled, fractured and stretched the crust on Venus, like a crumpled piece of paper or a face seamed and thickened by age. It has split the crust open and spread it apart, forming rift valleys with steep sides and sunken floors. Some of them are found in Alta Regio alongside its volcanoes. The linear rift zones in the equatorial highlands can extend for millions of meters, but are cracked apart by just a few thousand meters. In contrast, rifts that split open the Earth’s continents can widen up to make way for its biggest oceans.

Fig. ..  Fig. .. 

When a bubble of molten rock, or magma, rises to just below the surface, it presses against the crust, causing the ground to bulge and crack. Circular and radial fractures are created around the edges of the rising dome, forming a network of radar-bright features that resemble a spider. Some of them have therefore been nicknamed arachnoids, from the Greek and modern Latin words for ""spider"". The term coronae, the Latin word for ""crown"" is used for the larger, elevated, circular structures that are also pushed up from below by rising molten rock trying to get out. Both arachnoids and coronae are unique to Venus and have not been found on any other planet.

Coronae have concentric ridges and fractures that are hundreds of thousands of meters across, and large volcanic outpourings have occurred within them. When enough lava spills out into a corona, the upwelling subsides and it is no longer supported from below. The bulge will deflate and buckle the surrounding terrain, producing an annulus of ridges and troughs that often surrounds coronae, like the moat around a castle. Or else, the magma cools and retreats as it ages and the molten rock drains back down the vent from whence it came. Then the dome will collapse like a giant fallen soufflé, creating ring-like fractures and a crumpled, cracked surface.

The increasing pressure of the upwelling magma can stretch the planet's skin until it bursts, like the broken cheese bubbles in a pizza or a split in an overcooked hotdog. Small volcanic domes, known as pancakes, are sometimes formed when pasty, sluggish lava breaks through and flows along the surface like toothpaste. In other places the crust breaks and spreads open and lava flows into the gap like olive oil.

Fig. ..  Fig. .. 

As the surface moves up in some locations and down in others, the associated stresses pull the surface apart or push it together. Over time, these stresses can be created in different directions, producing a regularly spaced, gridded pattern of fractured terrain that is only found on Venus at this scale. Some of the cracked patterns of the tessarae have regular six-sided shapes that can be attributed to global heating and cooling of the surface. Repeated episodes of surface deformation in some highlands have additionally created a chaotic network of ridges, troughs and depressions with linear and curved structures.

Fig. .. 

On Venus, the dominant movement is often vertical, or up and down. Upwelling material pushes against the ground, creating arachnoids, coronae and tessarae, and punctures the surface to form volcanoes. Volcanic rises are held up by the hot rising material, and long mountain ranges may have been built during sinking, downward compression. Moreover, vast regions of the planet consist of flat, lowland plains with no substantial motion, either vertically or horizontally.

Fig. .. 

Although astronomers know virtually nothing about the first 4 billion years on Venus, they have been able to piece together a sequence of events during the last 500 million years. At the beginning of the record, they see complex and extensive surface deformation giving rise to an intensely fractured crust over nearly the entire globe. Widespread lava flooding created the flat lowland plains soon after this episode of tessera formation. After this brief but intense period of global volcanic floods, the style and rate of volcanism changed. Localized volcanoes grew on top of the vast plains and coronae were formed within them, but primarily in the equatorial regions where extensive rifts are also found.

Summary Diagram

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