Venus is the goddess of love and beauty. The planet is named this probably because it is the brightest of the planets known to the ancient astronomers. (With a few exceptions, the surface features on Venus are named for female figures.)

Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon. Like Mercury, it was popularly thought to be two separate bodies: Eosphorus as the morning star and Hesperus as the evening star, but the Greek astronomers knew better.

Since Venus is an inferior planet, it shows phases when viewed with a telescope from the perspective of Earth. Galileo's observation of this phenomenon was important evidence in favor of Copernicus's heliocentric theory of the solar system.

The second planet from the sun is brighter than every other object in the sky except the Sun and the Moon. Venus is bright enough to make people believe that is a U.F.O.! 108 million km from the Sun, Venus' orbit is almost a circle. Venus is only 40 million km away. Like Mercury, Venus sometimes appears as an "evening star" and sometimes as a "morning star." The Greeks thought it was two separate objects one in the evening and one in the morning. At certain times, Venus is so bright that it will cast a visible shadow.

The radar maps of Venus look much like the way the Earth would look if our planet's surface was not being constantly modified by erosion and deposition of sediment. Since there is no water or ice on Venus, and the surface wind speeds are slow, almost nothing even touches the geological features made by the Venusquakes and volcanic eruptions. Now that we have penetrated below the clouds of Venus, we find its surface (right) to be like a history book, revealing the history of hundreds of millions of years of geological activity.

About 75% of the surface of Venus consists of lowland lava plains. These are like the basaltic ocean basins of the Earth, but they were not created quite the same way. There is no subduction zones on Venus, unlike Earth, this planet does not have plate tectonics. Although convection in the mantle caused stress in the crust of Venus, it was never able to take on plate motion.

The formation of the lava plains (left)of Venus resembled that of the lunar maria, which were also the result of lava eruptions and the lack of crustal plate tectonics. Venusian lava plains are much younger than the lunar plains, but they've been distorted by the stresses in the crust. On the other hand, the lunar plains have remained almost unchanged since their formation of more than 3 billion years ago.

Rising above the lowland lava plains are mountains and mountain ranges, as well as two large continents. The largest continental area on Venus, called Aphrodite, is about the size of Africa. Aphrodite lies along the equator for about one-third of the way around the planet. Next in size is the northern highland region Ishtar, which is about the size of Australia. Ishtar contains the highest region on the planet, the Maxwell Mountains, which rise about 11 km above the adjacent lowlands.

One of the first questions astronomers wondered was the age of the surface of Venus. Though, the age of a surface is not the age of the world, a young age can mean that active geology occurs and that it can resurface the world in short time. The age can be calculated from counting craters. The more craters, the older it is. The largest crater on Venus is 275 km wide, a bit bigger than the largest Earth crater but much smaller than the lunar impact basins.

You might think that the thick atmosphere of Venus would protect the itself from impacts, that it would burn up the projectiles before they could even reach the surface. But this is only the case for small projectiles. The crater statistics show that projectiles smaller than about 1 km were stopped by the atmosphere.

Craters (right) that are 10 - 30 km big are often deformed, probably because the incoming projectile breaks apart and explodes in the atmosphere before it can hit the ground. There are also examples of multiple craters that occurred when the projectile broke into several pieces before hitting the surface. But if we just look at the impacts that produce craters with diameters of 30 km or greater, crater counts are as useful on Venus for measuring surface age as they are on airless bodies such as the Moon.

The numbers of craters on the plains of Venus are only about 15% of the lunar mare values, indicating a surface age only about 500 to 600 million years old. These results indicate that Venus is a planet with persistent geological activity, intermediate between that of the Earth's ocean basins and continents.

Almost all of the craters look fresh, with little interference or filling in by either lava or windblown dust. This is one way we know that the rates of erosion or sediment deposition are very low. We have the impression that most of the venusian plains were resurfaced by large-scale volcanic activity a few hundred million years ago, and that relatively little has happened since then. We see a similar situation in the lunar maria, which were all formed within a relatively short time (geologically speaking), but on Venus this period of widespread volcanic activity was much more recent. As planetary scientists study the Magellan data, they are becoming more and more convinced that Venus experienced some sort of planet-wide volcanic commotion about 500 million years ago.

It appears that, like the Earth, Venus is a planet that has experienced widespread volcanism, and we can see many different types of volcanic features. In the lowland plains, volcanic eruptions are the main way the surface is renewed, with large flows of highly fluid lava destroying old craters and generating a fresh surface every 500 million years or so. In addition, there are numerous younger volcanic mountains and other structures associated with surface hot spots--places where convection in the planet's mantle transports the interior heat to the surface.

The largest individual volcano on Venus, called Sif Mons, is about 500 km across and 3 km high--broader but lower than the Hawaiian volcano Mauna Loa. At its top is a volcanic crater or caldera about 40 km across, and its slopes show individual lava flows up to 500 km long. Thousands of smaller volcanoes pixelate the surface which corresponds to cones or domes about the size of a shopping mall parking lot. Most of these seem similar to terrestrial volcanoes.

All of this volcanism is the result of eruption of lava onto the surface of the planet. But the hot magma rising from the inside of a planet does not always make it to the surface. On both the Earth and Venus, this magma can collect to produce bulges in the crust. Many of the granite mountain ranges on Earth, such as the Sierra Nevada in California, involve these collections of magma that push up on the material above them.

These features are common on Venus, and in the lack of plate tectonics or surface erosion, they are much more visible than they are on Earth. Their characteristic visible expression is a large circular or oval feature called a corona. Coronae are typically several hundred kilometers across, with slightly raised interiors surrounded by a depressed ring or moat. They are a unique feature of Venusian geology. They are not seen on any other planet or satellite in the solar system.

The successful Soviet Venera landers discovered an extraordinarily inhospitable planet, with a surface pressure of 90 bars and a temperature hot enough to melt lead and zinc. Despite these unpleasant conditions, the spacecraft were able to photograph their surroundings and collect surface samples for chemical analysis before their instruments gave up. They found that the rock in the landing areas is igneous, mainly basalts. Each picture shows a flat, desolate landscape with a variety of rocks, which some may be ejecta from impacts. Other areas show flat, layered lava flows.

The Sun cannot shine directly through the heavy, solid clouds, but the surface is fairly well illuminated by diffused light. The illumination is about the same as that on Earth under a very heavy overcast, but with a strong red tint because the massive atmosphere blocks shorter-wavelength colors of light. The weather at the bottom of this deep atmosphere remains perpetually hot and dry, with calm winds. Because of the heavy clouds and atmosphere, the surface is practically the same all over.

The most abundant gas in the atmosphere of Venus is carbon dioxide, which is about 96% of the atmosphere. The second most is nitrogen. The predominance of carbon dioxide over nitrogen is not unusual because Earth's atmosphere would also be mostly carbon dioxide if this gas were not locked up in marine sediments. The proportions of major gases are very similar for Venus and Mars. With its surface pressure of 90 bars, the Venus' atmosphere is more than 10,000 times the size that of Mars.

In addition to these gases, we have found that sulfur dioxide in Venus' middle atmosphere plays a role in the chemistry of the Venusian clouds. The atmosphere of Venus is very dry. The absence of water is one of the main ways that Venus differs from the Earth.

The Venusian atmosphere has a huge troposphere that goes up to at least 50 km above the surface. In the troposphere, the gas is heated from below and circulates slowly, rising near the equator and descending over the poles. With no rapid rotation to break up this constant flow, the atmospheric circulation is very stable. Also, the size and mass of the atmosphere maintains stability.

In the upper troposphere, between 30 and 60 km above the surface, there is a thick cloud layer composed of sulfuric acid droplets. Sulfuric acid is formed from sulfur dioxide and water. In the atmosphere of the Earth, sulfur dioxide is one of the gases emitted by volcanoes, but it is quickly washed out by rainfall. In the dry atmosphere of Venus, this rather unpleasant substance is apparently stable. Below 30 km, the Venusian atmosphere is void of any clouds.

The atmospheric balloon instruments sent by the VEGA spacecraft in 1985 floated for 46 hours in the middle of Venus' cloud layer, at an altitude of 53 km. At that altitude, its instruments found that the conditions were almost Earth-like. The pressure there is 0.5 bar and the temperature a comfortable 305 K, just a little warmer than the room in which you are in. If it had enough oxygen and less of nasty sulfuric acid clouds, this would not be a bad place to visit. It certainly beats the conditions on our "sister" planet.

The high surface temperature of Venus was discovered by radio astronomers in the late 1950's and confirmed by Mariner 2 and by the early Venera probes. It is difficult to understand how this planet could be so hot. Although Venus is closer to the Sun than Earth is, its surface is hundreds of degrees hotter than you might think from the extra sunlight it gets. Scientists wondered what could be making the surface of Venus so hot (700 K!). The answer turned out to be the venerable greenhouse effect.

The greenhouse effect works on Venus just as it does on the Earth. But since Venus has so much more Carbon dioxide--almost a million times more--the effect is bigger. Sunlight that comes through the atmosphere of Venus heats the surface, but the Carbon dioxide acts as a "blanket", making it very difficult for the infrared radiation to escape back into space. Because of this, the surface heats up until it is releasing enough energy to balance the energy it gets from the Sun.

Has Venus always had a huge atmosphere and high surface temperature, or did it evolve to these conditions from a climate that once was nearly Earth-like? The answer to this question is interesting if you look at the increasing levels of Carbon dioxide in the Earth's atmosphere. Are we in any danger of transforming our own planet into a place like Venus?

A possible evolution of Venus from an Earth-like beginning to its present state: Venus began with moderate temperatures, water oceans, and much of its carbon dioxide dissolved in the ocean, as how the Earth is today. Then reasonable additional heating, by a small rise in the engery output of the Sun or by an increase in atmospheric Carbon dioxide. It turns out that even a small amount of extra heat can lead to evaporation from the oceans and the release of gas from surface rocks.

This means a further increase in the atmospheric Carbon dioxide and water, raising the greenhouse effect and then leading to still more heating and the release of even more Carbon dioxide and water. Unless some other processes stops this, the temperature will then continue to rise. This situation is called the runaway greenhouse effect.

The runaway greenhouse is not just a bigger and badder greenhouse effect, it is a process in which an atmosphere evolves from a state in which the greenhouse effect is small, such as on the Earth, to once where it is a major factor, as it is now on Venus. Once the larger greenhouse conditions continue to develop, the planet creates a new, much hotter equilibrium close its surface. Reversing the situation is difficult and most likely impossible.

Note that in this scenario, if large bodies of water are available, the runaway greenhouse leads to their evaporation, creating an atmosphere of hot water vapor, which itself is a major contributor to the greenhouse effect. Water vapor in the atmosphere is not stable in the presence of ultraviolet light. It tends to split the molecules of water into their elemental parts--oxygen and hydrogen. The lighter element hydrogen can escape from the atmospheres of the terrestrial planets, leaving the oxygen behind to combine with surface rocks. The loss of water is then an irreversible process. Once the water is gone, it cannot be brought back. There is evidence that shows this happened to the water that was once on Venus.

While we don't know when the greenhouse effect turns into a runaway greenhouse effect, Venus stands as clear proof that a planet can't continue heating forever without a major effect on its oveans and atmosphere. As a conclusion, we and future Earthlings will probably want to pay close attention to this.

More spacecraft have been sent to Venus than to any other planet. Although the 1962 U.S. Mariner 2 flyby was the first, the Soviet Union launched most of the missions to Venus, which was their major focus of their planetary exploration program. The early Soviet Venera entry probes were crushed by the high pressure of the atmosphere before they could even reach the surface of Venus, but in 1970 Venera 7 became the first probe to land and send data back to Earth. It lasted for 23 minutes before yielding to the hot surface temperature. In 1985 two instrumented balloons were sent into the planet's atmosphere by the Soviet VEGA flyby missions, which then continued on to intercept Comet Haley.

However, a better understanding of Venus required astronomers to make a global study of its surface, a task difficult because of the cloud layers surrounding the planet. So, they use a radar instrument to probe through the screening layer.

The first global radar map was somewhat made by the U.S. Pioneer 12 orbiter in the late 70s, followed by better maps from the Soviet Venera 15 and 16 radar orbiters in the early 80s. However, most of our information on the geology of Venus is originated from the U.S. Magellan spacecraft, which mapped Venus between 1991 and 1993. With its powerful imaging radar, Magellan was able to study the surface at resolution of 100 m, much higher than that of previous missions, giving us our first detailed look at the surface of Venus.