How is Venus similar to Earth? How is it different? How did the atmosphere of Venus evolve? What is a runaway greenhouse effect?
The thick atmosphere of Venus keeps it shrouded in clouds of mostly carbon dioxide, as seen by this infrared image taken by the Mariner 10 spacecraft. Venus is about the same size as Earth, but closer to the Sun, with a surface temperature high enough to melt lead. It has the odd characteristic of spinning slowly in the opposite direction of most of the bodies in our Solar system. This is probably due the the details of the impacts early in the history of the planet.
This is an artist's rendition of what Venus' sky might look like. The sky would look very different than that of Earth, since there is a high concentration of sulfuric acid in the clouds. The European Space Agency (ESA) Venus Express mission took data of the atmosphere of Venus 2006 - 2014. Their major discoveries included shifting polar vortices, and the possibility of recently active volcanoes.
The Venus Express spacecraft was able to peer through the clouds over the south pole of Venus to expose a swirling polar vortex. A vortex like this is related to hurricanes on Earth and elsewhere, driven by the rotation of the planet. High resolution images show features like a double eye in the vortex.
This diagram depicts the atmosphere of Venus. Notice that it extends much higher in altitude above the planet than does the atmosphere of Earth. The atmosphere of Venus is largely carbon dioxide. The clouds are made of sulfuric acid droplets, which can be highly charged, so lightning is common, probably more common than on Earth. The surface temperature of Venus is about 750 K (860 degrees Fahrenheit) and the pressure is about 90 times that of Earth.
The high pressure, heat and sulfuric acid on the surface of Venus make it very hard to directly explore the surface. The Russian Venera series of spacecraft attempted to do so, and Venera 9 was the first craft to successfully make a soft landing on the surface and send back pictures, in 1975.
This image from Venera 13 was the first color image to return from the planet Venus, in 1982. It is looking straight down at the base of the lander. It lasted for about two hours in the intense conditions on the surface, sending back data. There have been no landers on the surface of Venus since the 1980's.
Most of our information about the surface of Venus is obtained using radar mapping through the clouds. This image of Venus was pieced together from radar mapping by the Magellan spacecraft and earlier missions, as well as the Arecibo radio telescope. The colors are simulated to resemble photos taken by the Venera lander. This image shows the surface of Venus, looking down at its north pole. The bright spot in the lower center is Venus's highest mountain, Maxwell Montes, which is about 6.6 miles high, above the average elevation.
Before exploring Venus, people had the idea that it would be a lush tropical paradise, because of its proximity to the Sun. The surface of Venus turned out to be an extremely hostile environment. Radar mapping reveals features like these volcanic domes, and the Venus Express craft has found evidence of active volcanoes.
The Maat Mons volcano on Venus is about eight kilometers high, the second highest peak on the planet. Lava fields flow for hundreds of kilometers around this volcano. This is a shield volcano, like those found in Hawaii.
Why is the surface of Venus so hot? It has experienced a greenhouse effect so powerful that we call it a "runaway greenhouse effect."
To begin with, the early history of the evolution of the atmosphere was much like that on Earth, with a primary and secondary atmosphere.
Atmospheric evolution of Earth
Atmospheric evolution on Venus diverged from the on Earth, primarily because Venus is much closer to the sun. The greater heat meant that is was too hot for water to condense, so it stayed in the atmosphere as water vapor. No oceans formed. The lack of oceans served to also keep more of the carbon dioxide in the atmosphere, since CO2 dissolves well in water. The high abundance o the greenhouse gases, H2O and CO2, drove the greenhouse effect at a higher rate.
The effect behaved much the same as that on Earth. Some light was reflected off the cloud tops. Some wavelengths of light penetrated the atmosphere better than others. The light lost energy upon absorption and re-emission from the surface, and the re-emitted light was less able to get through the atmosphere, so it was radiated back toward the planet.
Radiation from the Sun broke up water vapor in the atmosphere into its constituent elements, hydrogen and oxygen. The hydrogen was able to reach escape velocity because of the high temperature and escape into space. Also, Venus has a significant electric field, which causes oxygen ions to accelerate and reach escape velocity. This meant that even when the planet cooled off slowly, the water vapor was gone, so no oceans could ever form. The lack of oceans, and the continued volcanic activity drove the greenhouse effect to a runaway state.
Atmospheric evolution of Venus
Venus rotates extremely slowly, rotating only once in about 225 Earth days. Venus also has a retrograde rotation, meaning it rotates in the opposite sense of most other bodies in our Solar system. Since the overall rotation direction arose from the fact that the bodies in the Solar system were formed from a rotating disk of dust and gas, this leads us to believe that Venus' slow retrograde rotation must have been caused by collisions during its formation.
The slow rotation means that Venus does not produce much of a magnetic field; it is about a tenth as strong as that on Earth. The magnetic field is not produced from a dynamo effect, like Earth's. In the atmosphere of Venus, the interaction between ions in the atmosphere and the solar wind induce a magnetic field, which deflects the solar wind, as shown in this artist's depiction of the solar wind deflection of Venus (top), Earth (center) and Mars (bottom).