Planetary Atmospheres

The primary gases present in the atmospheres of Venus, Earth, and Mars are nitrogen, carbon dioxide, oxygen, water, and argon. For Venus and Mars carbon dioxide is by far the most important of these, making up 96% and 95% of the two planets' atmospheres, respectively. The reason that Earth's carbon dioxide content (about 335 parts per million, or 0.0335%) is so different is that the compound is tied up in rocky materials such as limestone, chalk, and calcite, having been dissolved in seawater and deposited in carbonate rocks such as these. Nitrogen is the most abundant gas in Earth's atmosphere (77%), although it is also a major component of the Venusian (3.5%) and the Martian (2.7%) atmospheres.

The presence of oxygen in Earth's atmosphere is a consequence of the presence of living organisms on the planet. The widespread incorporation of carbon dioxide into rocky materials can also be explained on the same basis. Water is present in all three planets' atmospheres but in different ways. On Venus trace amounts of the compound occurs in the atmosphere in combination with oxides of sulfur in the form of sulfuric acid (most of the water that Venus once had has long since disappeared). On Earth most water has condensed to the liquid form and can be found in the massive oceans that cover the planet's surface. On Mars the relatively small amounts of water available on the planet have been frozen out of the atmosphere and have condensed in polar ice caps, although substantial quantities may also lie beneath the planet's surface, in the form of permafrost.

On the basis of solar proximity alone one would expect the temperatures of the four terrestrial plants to decrease as a function of their distance from the Sun. That pattern tends to be roughly true for Mercury, Earth, and Mars, whose average surface temperatures range from 333°F (167°C) to 59°F (15°C) to -67°F (-55°C), respectively. But the surface temperature on Venus—855°F (457°C)—reflects the powerful influence of the planet's very thick atmosphere of carbon dioxide, sulfur dioxide, and sulfuric acid, all strong greenhouse gases.