Size / / /

Maybe this world is another planet's hell.

--Aldous Huxley

Venus was once thought to be a haven for life. Its size, mass, and density are similar to those of the Earth. Those similarities, coupled with the fact that it is closer to the warming rays of the Sun, must mean that it is a tropical paradise, a virtual Garden of Eden; at least, that was the perception up until the early 20th century. In fact, Arrhenius, a Swedish chemist, suggested in 1918 that Venus must be hot and humid, with dense luxuriant plant growth. In some theories, Venus was Earth as it was 200 million years ago: a land of dinosaurs. In other theories, Venus was a water world. Though regarded by many as inferior to his Martian series, Edgar Rice Burroughs's Venus series depicts the planet as a lush ocean world. Isaac Asimov's Lucky Starr series also describes Venus as a watery world.

Reality hit close to home when the first space probes returned information about our "twin." Venus is actually a far cry from the Garden of Eden originally imagined. In fact, it has a greater likeness to the biblical Hell.

On Venus' Surface

The atmosphere of Venus is hot, corrosive, and thick. It's made up predominantly of carbon dioxide (with trace amounts of argon, carbon monoxide, sulfur dioxide, and water). Temperatures reach levels on the surface that would melt lead -- about 470 degrees celsius. Pressures at the surface reach levels that would be experienced by a submarine at a depth of 3000 feet in our oceans -- about 90 times the atmospheric pressure experienced on Earth at sea level. Sealing off this high-pressure furnace is a veil of sulfuric acid clouds that are whipped around the planet by a jet stream moving at speeds of 350 kilometers per hour. A novel that truly captures the feel of this fiery inferno of a planet is Ben Bova's Venus. To make matters more interesting, Bova has created scientifically plausible aliens for his world: metal-eating creatures that inhabit the subsurface of the planet.

The climate on Venus has often been described as the "greenhouse effect" gone to extremes. This may, in fact, be true. If you were to release all of the carbonates locked up within the rocks of the Earth into the atmosphere, the Earth's weather would be similar to how Venus is today. If you were to lock up all the carbon dioxide of Venus in the form of carbonates, as on Earth, Venus would be closer to being a "twin" of the Earth.

However, there has been some recent speculation that the climate of Venus might not continue to be the ultimate greenhouse effect gone amok. Scientists at the University of Colorado have suggested that Venus could either cool dramatically, or even heat to greater temperatures and pressures in the future. Their research centers on the carbon dioxide in Venus' atmosphere being removed by a reaction between sulfur dioxide and the surface of the planet. With the sulfur dioxide removed, there would be a decrease in the greenhouse effect, in turn resulting in more carbon dioxide being incorporated into the surface. Over time, the lessening of the carbon dioxide would further reduce the greenhouse effect. The alternative is that with increased volcanic activity, the greenhouse gases may increase further, resulting in an even more inhospitable environment on the planet.

Will it ever be possible to colonize Venus? Many scientists will say "never." Others will say that it's possible, but that it is unethical to alter a world to suit our needs. Then there are those that will give overwhelming support to the idea. Some speculative fiction writers, such as Pamela Sargent in her Venus trilogy, believe that it is a possibility. The trilogy follows the terraforming of the planet to suit human colonists. The series has been described as the Venusian equivalent of Kim Stanley Robinson's classic Mars series.

Venus In The Sky

No one knows who first saw Venus, but after the Sun and the Moon, it is the brightest object in the sky, with an apparent magnitude of -4.4. (To compare, the Moon has an apparent magnitude of -12 and the Sun's is -26.) Many ancient cultures thought that Venus was actually two objects, since it was visible both day and night, and they used two names to identify them. Venus was also known as the morning star and the evening star. It was in 500 BC that Pythagoras realized that the evening and morning stars were in fact the same body.

Early observations of this very noticeable neighbor were, to say the least, disappointing. The phases of the planet could be determined, but little else was known. In the early eighteenth century, an astronomer and Catholic deacon named Francesco Bianchini claimed to be able to see markings on the planet's surface, and went on to "map" the planet. Later observers were not able to confirm his observations. In the early twentieth century, Percival Lowell turned his more powerful telescope toward the planet and found that there were radial streaks on Venus; he was convinced that these lines were surface features. As with Bianchini's observations, Lowell's, too, could not be confirmed by later observers. More than likely, Bianchini and Lowell were observing variations in Venus' cloud cover, since visible light telescopes show nothing of the planet's surface.

Venus Up Close

Since 1961, there have been twenty-six probes sent to fly by or land on the planet's surface that have greatly enhanced our knowledge of this planetary neighbor. Venera 7, a probe launched by the former Soviet Union, was actually the first spacecraft ever to land on another planet. The most recent mission was the Magellan mission, which was launched on May 4, 1989 and arrived at the planet in August of 1990. During its four year mission it made a detailed map of Venus' surface using a synthetic aperture radar capable of producing images at a resolution as high as 300 meters. Magellan also provided a gravitational profile of the planet that allowed geologists to "view" Venus' interior.

What Magellan found was a geologist's paradise. Venus is the most volcanically active planet in the solar system. The planet features numerous tectono-volcanic structures -- which, despite their name, are different from volcanoes in two ways. First, volcanoes tend to form along older rifts or faults in the crust. Second, whereas volcanoes are mainly just piles of lava, tectono-volcanic structures are formed by faulting that occurs over rising magma. Three different tectono-volcanic structures can be found on Venus.

  • Coronas. There are about 300 of these known to exist on Venus. They tend to be large, with a round to oval shape and a distinct ring of faults or ridges. They may have either a flat, raised, or down-dropped center and an outer moat-like trough. They range in size from about 100 kilometers to nearly 1000 kilometers across; the average is about 250 kilometers across. Coronas are thought to form over small mantle plumes. With later cooling, the uplift sinks, resulting in the collapsing of the center seen in many older coronas.

  • Arachnoids. These are the smaller cousins of coronas. So far about 250 have been mapped on Venus. They are thought to form in the same manner as coronas, but being smaller, they are formed over smaller plumes that have less uplift. Along with coronas, arachnoids are seldom found in the lowlands, but most frequently just above lowland plains.

  • Novas. Another tectono-volcanic structure is the nova, which displays a starburst-like pattern of faults and a broad dome-like uplift. Sizes range from 50 to 300 kilometers across, with most being about 200 kilometers across. Scientists have mapped about 50 novas. They tend to be on the higher plains where they lie over mantle plumes.

Venus also shows evidence of lava flows. There are three types of lava flows on Venus: the first are the large, flood-like flows or fluctus. Flow fields are about 100 to 700 km in length, and their widths can be highly variable, with some reaching spans of over 300 kilometers.

Lava channels are another type of lava flow; there are about 300 lava channels on Venus. Most are within lava flood fields or on other volcanoes. They are about 0.5 to 1.5 km wide and usually are less than 400 km long. One, however, is over 6800 km long, the longest such feature in the solar system. In fact, it is longer than the longest river on Earth, the Nile, which is 6695 kilometers long.

Calderas are another type of lava flow. They are round or oval pits with steep walls and low rims. They are thought to form when the ground falls over an emptied magma chamber. They are often associated with shield volcanoes and most are between 40 and 80 kilometers in size. All of them lie in the Venusian highlands, suggesting that shallow magma chambers do not form in the lowlands.

Venus also has true volcanoes. In fact, there are more volcanoes on Venus than anywhere else in the solar system. Most of the volcanoes are shields, but a few are not. These non-shield volcanoes produce thick viscous lava. Volcanoes are classified into three types: the most common are the pancake domes, which are widely scattered across the surface of Venus. Pancake domes are rarely found in the vicinity of shield volcanoes, but instead lie near the corona structures of the lowland plains. The smaller relative of the pancake dome is a tick-like structure that is thought to be essentially a modified dome.

Another type of volcano has thick or banded flows. Since most basaltic lava is thin and fluid, the lava from these volcanoes is probably of granite or quartz origin. Another theory is that the lava that is thick and viscous is gas-rich basalt. The fan-shaped or banded flows, another type of non-shield volcano, are relatively rare on Venus and most are located in a southern lowland plain.

Most of the volcanoes on Venus are shields. There are over 100,000 smaller shields, most less than 20 kilometers across. They tend to cluster into fields, which are widely scattered on Venus but occur mostly in the lowland plains. Few are in the higher elevations. At medium sizes, true shields are rare. Of the 270 volcanoes between 20 and 100 kilometers in size, only 70 are true shields.

Venus is also home to about 150 large shield volcanoes. They range from 100 to 600 kilometers across, with heights averaging from 0.3 to 5.0 kilometers. The largest, Sif Mons, is 700 kilometers in diameter and 5.5 kilometers in height. It is comparable in size to the largest volcano in the solar system, Olympus Mons of Mars.

The interesting thing about Venusian volcanism is that it is not based on plate tectonics, as Earth's volcanism is. Plate tectonics is the idea that the crust of the Earth is divided into a series of rigid plates that float over a viscous underlayer in the mantle. It is where these plates meet that much of Earth's severe geological activity, such as earthquakes and volcanic explosions, can be observed. Since Venus does not have plate tectonics, the volcanic hot spots of Venus do not appear in linear chains as they do on Earth. Instead, the volcanism on Venus is probably due to mantle hot spots.

The volcanic action on Venus is also different from the kinds of volcanic activity on Earth. On Venus, volcanoes seem to involve fluid lava flows, rather than explosions as on Earth. There are several reasons for this. First, Venus has a higher atmospheric pressure, which requires the lava to reach a higher gas content for it to explode. Second, the main driving force of volcanic explosions on Earth is water, a substance that is in very short supply on Venus. The lack of subduction zones (a boundary in plate tectonics whereby one plate bends down into the mantle at an angle of about 45 degrees under another plate) on Venus also reduces the likelihood of explosive volcanoes.

Although plate tectonics as we understand it on Earth does not exist on Venus, there are nevertheless tesseras on Venus, which are complex ridges in the terrain. These features are found on some of the Venus Plateau highlands. These ridges are created by a shortening of the crust. That is, when the crust is pushed together, the surface folds, buckles, or breaks. This process is related to the process by which mountains are built on Earth, but without continental drift, there has to be another driving force causing the process. That force is still unknown, but the mere existence of tesseras is an indication of an active surface.

Since Venus' surface is not replenished through a system of plate tectonics, it is somewhat difficult to explain the cratering observed on the planet. On Venus, the craters appear to be very random, and there are no old craters. The best explanation for this phenomenon is that Venus has a thick crust. With a thick crust, heat builds up and cannot escape as easily into space. Therefore, every 750 million years the surface of Venus will actually turn itself inside out. Then the process starts over again with a slow buildup of heat under an ever-thickening crust.

Venus also has some other strange features. For example, it has a retrograde turn on its axis; that is, it spins on its axis in a direction opposite to that of the Earth and the other planets of the solar system. One of the best explanations for this is that early in the formation of the solar system, there was a lot of debris flying around in space. Earth was hit by one of the larger objects, which loosened up what would later become our Moon. It is not inconceivable that Venus had a similar encounter and, instead of creating a satellite, it absorbed the additional mass, kinetic energy, and angular momentum, resulting in a new spin.

Adding to the strangeness of Venus is the fact that it rotates very slowly, taking about 243 days to complete a cycle, while its trip around the Sun only takes 225 days. Despite the fact that Venus probably is made up of a partly molten metallic core, a rocky mantle, and a crust, it lacks a magnetic field because of its slow rotation.

Venus also has a bizarre inclination to its orbit. In fact, its north pole is located 87 degrees below the ecliptic, thus putting the north pole within 3 degrees of the south ecliptic pole.

Our strange "twin" is hardly a twin at all. Those features that made early scientists think that it was our planetary twin are actually the only similarities that exist between Venus and the Earth. Venus is as alien a world as any science fiction writer could conjure up in his or her imagination. In fact, with the exception of Bova's Venus, there never has been an imaginary planet on par with the hellish conditions that we now know exist on Venus. Reality is truly stranger than fiction.


Reader Comments

Peter Jekel is the Director of Infectious Disease Prevention in one of the largest Health Department Districts in Ontario. He has lived in Bracebridge, Ontario with his family for the past 16 years. His previous publications in Strange Horizons can be found in our Archive.

Peter Jekel runs the Infectious Diseases Program for one of the largest Health Department Districts in Ontario, Canada. He lives with his family, made up of wife, daughter, dog, 3 cats, 20 fish, and a rabbit. His previous publications for Strange Horizons can be found in our Archive.
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