Venus' cloudtops in UV (left) and Venus' surface imaged with radar (right).
Venus, Earth, and Mars are approximately at the same distance from the Sun. This means they formed out of the same material and had approximately the same initial temperatures 4.5 billion years ago. Long ago these three planets probably had moderate enough temperatures suitable for life. However, Venus is now much too hot for life and Mars is too cold for life. What happened to these two planets and why are they so different from the comparative paradise here on Earth? This section explores these three planets in more detail in order to answer this important question and what it might say for the future of the Earth.
Venus is about 95% the size of the Earth and has 82% of the Earth's mass. Like the Earth, Venus has a rocky crust and iron-nickel core. But the similarities stop there. Venus has a thick atmosphere made of 96% carbon dioxide (CO2), 3.5% nitrogen (N2), and 0.5% other gases. At Venus' surface, the air pressure is 91 times the Earth's surface atmospheric pressure. Venus' surface atmospheric pressure is the same as what you would feel if you were 1 kilometer below the ocean surface on the Earth. The deepest free-divers can get down to around 160 meters (and divers breathing special mixtures of gases can get down to 730 meters). If you want to send someone to Venus, that person would need to be in something like a diving bell.
The Venus explorer would also need a very powerful cooling system: the surface temperature is 737 K (= 477° C)! This is hot enough to melt lead and is over twice as hot as it would be if Venus did not have an atmosphere. Why does Venus have such a thick atmosphere and why is it so hot on its surface?
Venus is so hot because of a huge greenhouse effect that prevents heat from escaping to space. Plant nurseries use greenhouses to keep their plants warm during the winter months. Energy in the form of visible light from the Sun passes through the glass walls and glass roofs of a greenhouse and heats up the plants and soil inside the greenhouse. The air in contact with the plants and soil gets warmed up. The glass walls and roofs prevent the hot air from escaping to the outside. The same sort of thing happens to the interior of your car when you leave it out in the Sun with the windows rolled up.
On a planet, certain gases like carbon dioxide or water vapor in the atmosphere prevent heat energy in the form of infrared light from leaking out to space. These so-called ``greenhouse gases'' allow visible light from the Sun to pass through and heat up the surface. The surface gets warm enough to emit infrared light. Some of the infrared light is absorbed by the greenhouse gases and radiated back toward the surface, keeping the surface warm. On Venus, the super-abundance of CO2 in its atmosphere is responsible for the huge greenhouse effect. Venus' greenhouse effect probably started from the presence of a lot of water vapor, but Venus is now a very dry place.
Venus' water was always in the gaseous form and could reach high enough in the atmosphere for ultraviolet light from the Sun to hit it. Ultraviolet light is energetic enough to break apart, or dissociate, water molecules into hydrogen and oxygen. The very light hydrogen atoms were able to escape into space and the heavier oxygen atoms combined with other atoms. Venus' water was eventually zapped away. The Earth's ozone layer prevents the same thing from happening to the water here.
Ordinary hydrogen has only one proton in the nucleus, while the isotope deuterium has one proton + one neutron. Therefore, deuterium is about twice as heavy as ordinary hydrogen and will stay closer to the surface on average. Gases higher up in the atmosphere are more likely to escape to space than those close to the surface.
On Earth the ratio of ordinary hydrogen to deuterium (H/D) is 1000 to 1, while on Venus the proportion of deuterium is about ten times greater---the H/D ratio is 100 to 1. The H/D ratio on Venus and Earth are assumed to have been originally the same, so something caused the very light hydrogen isotopes on Venus to preferentially disappear. An easy explanation for it is the ultraviolet dissociation of water.
A summary flowchart of what happened on Venus is given on the Earth-Venus-Mars summary page. Water vapor started the greenhouse heating. Carbon dioxide was baked out of the rocks, further aggravating the greenhouse effect. A runaway greenhouse started. The end result was all of the carbon dioxide in the atmosphere and the water dissociated away. The flowchart on the Earth-Venus-Mars page up to the last arrow occurred several billion years ago. The diamond at the end describes the current state: CO2 maintains the extremely hot temperature.
Go back to previous section --
Go to next section
last updated: June 1, 2007