Venus and Earth are in similar places in the solar system. They are, respectively, the 2nd and 1st largest rocks in the solar system. They have similar sizes, densities, and (probably) compositions. Both have signs of recent geological activity, and both have substantial atmospheres. However, this is where the similarities end. Venus has an extremely slow rotation rate, and it rotates backwards with respect to the rest of the planets. Venus shows no evidence of large-scale plate tectonic activity, there is no liquid water on its surface, and its surface is much hotter than Earth's. The Venusian atmosphere is corrosive, meaning that it has chemicals in it that can destroy metals and other substances.
Venus's high reflectivity makes it bright. After the Sun and the Moon, it is the 3rd brightest object in Earth's sky. Its orbit always keeps it close to the Sun, so we only see it either close to sunrise or sunset, hence the reason it is sometimes referred to as the "morning star" or "evening star".
Investigations of Venus
Venus's thick atmosphere prevents us from taking pictures of its surface in optical light (i.e., light our eyes can see). Instead, we have had to land probes on its surface, and investigate its surface with remote radar.
Seven probes have landed on Venus's surface, all a part of the "Venera" program of the former Soviet Union (Venera is the Russian name for Venus). This program was in operation during the 1970s and early 80s. The probes verified earlier evidence concerning Venus's hot surface: the temperature is 740 kelvin (467 centigrade). At this temperature lead, tin, and zinc would all be a liquid.
At its surface, the Venusian air pressure is 90 atmospheres, or the pressure experienced on Earth when under about 890 meters of water. The thickness of the atmosphere makes it move about sluggishly near the surface.
Runaway Greenhouse Effect
The major atmospheric constituent is carbon dioxide (CO2). Carbon dioxide is transparent to optical light but opaque to infrared light. Any gas exhibiting these properties is called a "greenhouse gas". For example, water, and CFCs (chlorinated fluorocarbons) are also greenhouse gases. Greenhouse gases will allow optical light from our Sun to shine down onto the surface of the planet. The planet's surface is heated as a result. This heat is released as infrared light. However, the same gas that allowed optical light in will not not allow infrared light out. Therefore, the total energy on the surface of the planet increases, which causes an increase in temperature. Some infrared light will leak out, but it is not very much. Eventually, the surface gets hot enough that the total energy coming in will equal the energy leaving, thus resulting in an "equilibrium temperature" that is higher than would be the case in the absence of the greenhouse gas.
A runaway greenhouse effect is when the increasing temperature on a planet's surface aids the mechanism that is increasing the temperature in the first place. For example, any water evaporating from the surface becomes a greenhouse gas, heats the surface, and thus causes MORE water to evaporate, thus more heating. The cycle cannot be ended until all the water has disappeared.
At the top of Venus's atmosphere, energetic ultraviolet light provides the energy needed for the building and destruction of some molecules in Venus's atmosphere. For example, water molecules can be destroyed via:
H2O + uv -> 2 H + O (free atoms)
Water molecules can also combine with sulfur dioxide, SO2, and free oxygen to form sulfuric acid, H2SO4. Sulfuric acid is a very efficient greenhouse gas. Even though only trace amounts (i.e., much less than 1%) exist in Venus's atmosphere, it still contributes to the high temperature of the surface.
The concentration of sulfur dioxide, SO2, in Venus's atmosphere varies over time. Perhaps volcanic activity on the surface periodically injects new sulfur dioxide into the atmosphere. Currently we have no other evidence for present volcanic activity.
The high atmospheric pressure at Venus's surface results in slow wind speeds, only a few (0-6) km/hr. At the top of the atmosphere the wind speeds are much higher, stretching the clouds into long streamers. A given cloud system is seen to move around Venus in only 4 days, indicative of a wind speed of 360 km/hr.
The surface illumination on Venus resembles that of the Earth on a heavily clouded day.
The landscape of Venus has been imaged using radar from Earth and from remote probes: the Pioneer Venus spacecraft in 1978-80, and the Magellan spacecraft in 1990. Radar waves were beamed to the planet, and then their echo was recorded. The characteristics of the echo give us a three-dimensional image of the planet's surface.
Notable in these images is the lack of ocean basins and continents on Venus. Only 10% of Venus's surface can be characterized as highlands, compared with 45% of the Earth's surface. Place names on Venus are taken from real and mythical women. The largest two uplands are the Aphrodite Terra (along the equator) and Ishtar Terra (near the north pole); "terra" refers to a rough or mountainous upland.
The Magellan spacecraft found about 1,000 crates on Venus, ranging in size from about 2 to 280 km in diameter. Smaller craters do not form because smaller meteors (diameter less than 500 m) will vaporize in Venus's atmosphere. The absence of larger craters indicates that the Venusian surface is younger than that of the Moon; the use of crater counts yields an age of about 500 million years.
However, radar images of these craters indicate that they have not been weathered or eroded much. The implication is that Venus retains its surface craters for a very long time. In comparison, Earth has about 150 craters across its surface, and most of these are heavily worn down by weathering and geologic processes.
Surface evidence for tectonic action
The change of shape of the solid crust of a planet results in a number of different landforms, from mountains to valleys. The collective term for this type of deformation activity is tectonics. There are small-scale tectonic features everywhere on Venus, but no large-scale features like continents. We surmize that Venus's surface does not have well-defined plates like the Earth. Large mountain ranges, like Ishtar Terra, are the result of tectonic activity. This activity must be fairly recent because gravity will flatten out, via collapse, large mountains on both Earth and Venus unless these mountains are constantly being renewed.
The largest mountain range on Venus compares with that of the Earth: the Maxwell mountains rise to a height of 11 km, whereas the Himalayas rise to 8 km.
Besides mountain ranges, the other evidence for tectonic action is the existence of large circular formations of cracks or stretches of the surface. Presumably, the land has been pushed up from underneath by hot rock rising from Venus's center. These features are called "coronae". Examples include Artemis Corona (the largest, at a diameter of 2,000 km), Heng-o Corona, Pavlova Corona, and Demeter Corona.
Lava plains cover 80% of the Venusian surface. Sheets of lava have spread from cracks and fissures in the surface to blanket most of the planet. Few craters have lava-filled basins, so this volcanic action must have been more intense in the past. The evidence points to a burst of volcanic activity about 500 million years ago, before the time of implantation of the craters we see today. Generally, on the Earth and other planets, geologic processes occur gradually over many, many millions of years. On Venus, the evidence is for an abrupt period of surface reconstruction.
There are many thousands of volcanoes on Venus, ranging in size from a few 10s to a few 100s of km across, and up to about 5 km in height. We do not know if any of these are currently active.
Seven Venera landers successfully transmitted data from Venus's surface. Eventually, the high pressure and temperature destroyed the spacecraft. The longest-lived lander lasted for just under 2 hours. Images form the landers extended from the foot of the spacecraft all the way to the horizon. These images showed us a very flat surface, covered with loose, fine-grained rock, and some flat, plate-like rock. These are all most likely lava plain.